Anatomy

ANATOMY(Ανατομή) means in its literal sense the dissection or separation of parts by cutting, but in its usual acceptation it is employed to denote the science the province of which is to determine the construction, the form, and the structure of organised bodies, i.e., of bodies which either are or have been living. It is therefore a department of the science of Biology. It resolves itself into two great divisions— Animal Anatomy or Zootomy, the object of which is to investigate the structure of animals; and Vegetable Anatomy or Phytotomy, the object of which is to elucidate the structure of plants. As Vegetable Anatomy will be treated of in the article Botany, it does not require to be considered here. Animal Anatomy, again, naturally resolves itself into two divisions: one in which the construction, form, and structure of two or more animals are compared with each other, so as to bring out their features of resemblance or dissimilarity,—this is called Comparative Anatomy; the other, in which the construction, form, and structure of parts in a single animal are considered, which is termed Special Anatomy. The special anatomy of an animal may be studied from various points of view: (a) with reference to the succession of forme which it exhibits at various periods from its first appearance as an embryo to the assumption of its adult characters; this is termed Developmental or Embryological Anatomy; (b) with reference either to its form and structure, or to the investigation of the laws by which these are determined, termed Morphological Anatomy; (c) with reference to the function, use, or purpose performed by a part or structure in an animal, termed Teleological or Physiological Anatomy; (d) with reference merely to the relative position of different parts or structures, termed Topographical Anatomy; (e) with reference to the structure and general properties of the tissues or textures which enter into the construction of the parts or organs of animals; to th⅛ branch of study have been applied the terms General Anatomy, Anatomy op Textures, Histology, and, from the microscope being so largely employed in the examination of the textures, Microscopic or Minute Anatomy; (f) with reference to the changes induced by disease in the organs or tissues, termed Morbid or Pathologicae Anatomy. From its manifold aspects anatomy forms the basis of the Biological Sciences. As a knowledge of the laws of motion is essential, and must be constantly recurred to at every step before any true progress can be made in the investigation of the physical sciences, so must the structure of animal bodies be constantly appealed to by the zoologist in all attempts at classification; by the physiologist in all inquiries into the functions performed by the organs and textures in a state of health, and into the special adaptation of parts to particular uses; and by the physician in considering the alterations or disturbance of the functions of parts in the course of disease. To describe the anatomy of the multitudinous forms of animal life from these different points of view would require, not one, but several voluminous treatises, and would much exceed the compass of a single article. Moreover, it is advisable that the anatomy of the different classes of the animal kingdom should be considered under their respective heads,—e.y., that of the Crabs under Crustacea, that of Reptiles under Reptilia, &c. It is intended to devote this article more particularly to the description of the Special Anatomy of the Human Body in a state of health; in other words, to make it a short treatise on Human Anatomy or Anthro-potomy, which, as forming a department of the general science of Comparative Anatomy, is interesting not only to men of science generally, but, from its intimate connnection with the several divisions of the art of healing, and with the study of the functions of the human body, possesses the highest importance to the physician, surgeon, and physiologist.

Previous to entering on the consideration of the Anatomy of the Human Body, it may be well to take a historical view of the progress of the science from its origin to the present time.

HISTORY OF ANATOMY.

In tracing the history of the origin of anatomy, it may be justly said that more learning than judgment has been displayed. Some writers claim for it the highest antiquity, and pretend to find its first rudiments alternately in the animal sacrifices of the shepherd kings, the Jews, and other ancient nations, and in the art of embalming as practised by the Egyptian *priests, Even the descriptions of wounds in the Iliad have been supposed adequate to prove that in the time of Homer mankind had distinct notions of the structure of the human body. Of the first it may be said that the rude information obtained by the slaughter of animals for sacrifice does not imply profound anatomical knowledge; and those who adduce the second as evidence are deceived by the language of the poet of the Trojan war, which, distinguishing certain parts by their ordinary Greek epithets, as afterwards used by Hippocrates, Galen, and all anatomists, has been rather too easily supposed to prove that the poet had studied systematically the structure of the human frame.

With not much greater justice has the cultivation of anatomical knowledge been ascribed to Hippocrates, who, because he is universally allowed to be the father of medicine, has also been thought to be the creator of the science of anatomy. Of the seven individuals of the family of the Heracleidae who bore this celebrated name, the second, who was son of Heraclides and Phenarita, and grandson of the first Hippocrates, was indeed distinguished as a , physician of great observation and experience, and the first who appreciated the value of studying accurately the phenomena, effects, and terminations of disease. It does not appear, however, notwithstanding the vague and general panegyrics of Riolan, Bartholin, Le Clerc, and Portal, that the anatomical knowledge of this illustrious person was either accurate or profound. Of the works ascribed to Hippocrates, five only are genuine. Most of them were written either by subsequent authors of the same name, or by one or other of the numerous impostors who took advantage of the zealous munificence of the Ptolemies, by fabricating works under that illustrious name. Of the few which are genuine, there is none expressly devoted to anatomy; and of his knowledge on this subject the only proofs are to be found in the exposition of his physiological opinions, and his medical or surgical instructions. From these it appears that Hippocrates had some accurate notions on osteology, but that of the structure of the human body in general his ideas were at once superficial and erroneous. In his book on injuries of the head, and in that on fractures, he shows that he knew the sutures of the cranium and the relative situation of the bones, and that he had some notion of the shape of the bones in general, and of their mutual connections. Of the muscles, of the soft parts in general, and of the internal organs, his ideas are confused, indistinct, and erroneous. The term φλeβ<s he seems, in imitation of the colloquial Greek, to have used generally to signify a blood-vessel, without being aware of the distinction of vein and artery; and the term αρτηρία, or air-holder, is restricted to the windpipe. He appears to have been unaware of the existence of the nervous chords; and the term nerve is used by him, as by Grecian authors in general, to signify a sinew or tendon. On other, points his views are so much combined with peculiar physiological doctrines, that it is impossible to assign them the character of anatomical facts; and even the works in which these doctrines are contained are with little probability to be ascribed to the second Hippocrates. If, however, we overlook this difficulty, and admit what is contained in the genuine Hippocratic writings to represent at least the sum of knowledge possessed by Hippocrates and his immediate descendants, we find that he represents the brain as a gland, from which exudes a viscid fluid; that the heart is muscular and of pyramidal shape, and has two ventricles separated by a partition, the fountains of life—and two auricles, receptacles of air; that the lungs consist of five ash-coloured lobes, the substance of which is cellular and spongy, naturally dry, but refreshed by the air; and that the kidneys are glands, but possess an attractive faculty, by virtue of which the moisture of the drink is separated, and descends into the bladder. He distinguishes the bowels into colon and rectum (ô αpχos).

The knowledge possessed by the second Hippocrates was transmitted in various degrees of purity to the descendants and pupils, chiefly of the family of the Heracleidøe, who succeeded him. Several of these, with feelings of grateful affection, appear to have studied to preserve the written memory of his instructions, and in this manner to. have contributed to form part of that collection of treatises which have long been known to the learned world under the general name of the Hippocratic writings. Though composed, like the genuine remains of the physician of Cos, in the Ionian dialect, all of them differ from these in being more diffuse in style, more elaborate in form, and in studying to invest their anatomical and medical matter with the fanciful ornaments of the Platonic philosophy. Hippocrates had the merit of early recognising the value of facts apart from opinions, and of those facts especially which lead to general results; and in the few genuine writings which are now extant it is easy to perceive that he has recourse to the simplest language, expresses himself in terms which, though short and pithy, are always precise and perspicuous, and is averse to the introduction of philosophical dogmas. Of the greater part of the writings collected under his name, on the contrary, the general character is verboseness, prolixity, and a great tendency to speculative opinions. For these reasons, as well as for others derived from internal evidence, while the Aphorisms, the Epidemics, and the works above mentioned, bear distinct marks of being the genuine remains of Hippocrates, it is impossible to regard the book πep ∖ Φwγlo<s Ανθρωπον as entirely the composition of that physician; and it appears more reasonable to view it as the work of some one of the numerous disciples to whom the author had communicated the results of his observation, which they unwisely attempted to combine with the philosophy of the Platonic school and their own mysterious opinions.

Among those who aimed at this distinction, the most fortunate in the preservation of his name is Polybus, the son-in-law of the physician of Cos. This person, who must not be confounded with the monarch of Corinth immortalised by Sophocles in the tragic story of Oedipus, is represented as a recluse, severed from the world and its enjoyments, and devoting himself to the study of anatomy and physiology, and to the composition of works on these subjects. To him has been ascribed the whole of the book on the Nature of the Child and most of that On Man; both physiological treatises interspersed with anatomic^ sketches. His anatomical information, with which wt are specially concerned, appears to have been rude and inaccurate, like that of his preceptor. He represents the large vessels of the body as consisting of four pairs; the first proceeding from the head by the back of the neck and spinal chord to the hips, lower extremities, and outer ankle; the second, consisting of the jugular vessels (αi σφaγiτιδες), proceeding to the loins, thighs, hams, and inner ankle; the third proceeding from the temples by the neck to the scapula and lungs, and thence by mutual intercrossings to the spleen and left kidney, and the liver and right kidney, and finally to the rectum; and the fourth from the forepart of the neck to the upper extremities, the fore-part of the trunk, and the organs of generation.

This specimen of the anatomical knowledge of one of the most illustrious of the Hippocratic disciples differs not essentially from that of Syennesis, the physician of Cyprus, and Diogenes, the philosopher of Apollonia, two authors for the preservation of whose opinions we are indebted to Aristotle. They may be admitted as representing the state of anatomical knowledge among the most enlightened men at that time, and they only show how rude and erroneous were their ideas on the structure of the animal body. It may indeed, without injustice, be said that the anatomy of the Hippocratic school is not only erroneous, but fanciful and imaginary, in often substituting mere supposition and assertion for what ought to be matter of fact. From this censure it is impossible to exempt even the name of Plato himself, for whom some notices in the Timaeus on the structure of the animal body, as taught by Hippocrates and Polybus, have procured a place in the history of the science.

Amidst, the general obscurity in which the early history of anatomy is involved, only two leading facts may be admitted with certainty. The first is, that previous to the time of Aristotle there was no accurate knowledge of anatomy; and the second, that all that was known was derived from the dissection of the lower animals ’ only. By the appearance of Aristotle this species of knowledge, which was hitherto acquired in a desultory and irregular manner, began to be cultivated systematically and with a definite object; and among the services which the philosopher of Stagira rendered to mankind, one of the greatest and most substantial is, that he was the founder of Comparative Anatomy, and was the first to apply its facts to the elucidation of zoology. The works of this ardent and original naturalist show that his zootomical knowledge was extensive and often accurate; and from several of his descriptions it is impossible to doubt that they were derived from frequent personal dissection. Aristotle, who was born 384 years before the Christian era, or in the first year of the 99th Olympiad, was, at the age of 39, requested by Philip to undertake the education of his son Alexander. During this period it is said he composed several works on anatomy, which, however, are now lost. The military expedition of his royal pupil into Asia, by laying open the animal stores of that vast and little known continent, furnished Aristotle with the means of extending his knowledge, not only of the animal tribes, but of their structure, and of communicating more accurate and distinct notions than were yet accessible to the world. A sum of 800 talents, and the concurrent aid of numerous intelligent assistants in Greece and Asia, were intended to facilitate his researches in composing a system of zoological knowledge; but it has been observed that the number of instances in which he was thus compelled to trust to the testimony of other observers led him to commit errors in description which personal observation might have enabled him to avoid.

The first three books of the History of Animals, a treatise consisting of ten books, and the four books on the Parts of Animals, constitute the great monument of the Aristotelian Anatomy. From these we find that Aristotle was the first who corrected the erroneous statements of Polybus, Syennesis, and Diogenes regarding the bloodvessels, which they made, as we have seen, to arise from the head and brain. These he represents to be two in number, placed before the spinal column, the larger on the right, the smaller on the left, which, he also remarks, is by some called αoriα (αορτή), the first time we observe that this epithet occurs in the history. Both he represents to arise from the heart, the larger from the largest upper cavity, the smaller or aorta from the middle cavity, but in a different manner and forming a narrower canal. He also distinguishes the thick, firm, and more tendinous structure of the aorta from the thin and membranous structure of vein. In describing the distribution of the latter, however, he confounds the vena cava and pulmonary artery, and, as might be expected, he confounds the ramifications of the former with those of the arterial tubes in general. While he represents the lung to be liberally supplied with blood, he describes the brain as an organ almost destitute of this fluid. His account of the distribution of the aorta is wonderfully correct. Though he does not notice the coeliac, and remarks that the aorta sends no direct branches to the liver and spleen, he had observed the mesenteric, the renal, and the common iliac arteries. It is nevertheless singular that though he remarks particularly that the renal branches of the aorta go to the substance and not the pelvis (κοιλία) of the kidney, he appears to mistake the ureters for branches of the aorta. Of the nerves (νεύρα) he appears to have the most confused notions. Making them arise from the heart, which he says has nerves (tendons) in its largest cavity, he represents the aorta to be a nervous or tendinous vein (νευρώδης φλεβς). By and by, afterwards saying that all the articulated bones are connected by nerves, he makes them the same as ligaments.

He distinguishes the windpipe or air-holder (αρτηρία) from the oesophagus, because it is placed before the latter, because food or drink passing into it causes distressing cough and suffocation, and because there is no passage from the lung to the stomach. He knew the situation and use of the epiglottis, seems to have had some indistinct notions of the larynx, represents the windpipe to be necessary to convey air to and from the lungs, and appears to have a tolerable understanding of the structure of the lungs. He repeatedly represents the heart, the shape and site of which he describes accurately, to be the origin of the blood-vessels, in opposition to those who made them descend from the head; yet, though he represents it as full of blood and the source and fountain of that fluid, and even speaks of the blood flowing from the heart to the veins, and thence to every part of the body, he says nothing of the circular motion of the blood. The diaphragm he distinguishes by the name διάζωμα, and υπόζωμα. With the liver and spleen, and the whole alimentary canal, he seems well acquainted. The several parts of the quadruple stomach of the ruminating animals are distinguished and named; and he even traces the relations between the teeth and the several forms of stomach, and the length or brevity, the simplicity or complication, of the intestinal tube. Upon the same principle he distinguishes the jejunum (η vηστις), or the empty portion of the small intestines in animals (το εvτεpov λεπτόν), the caecum (τυφλόν τι και ογκώδες), the colon (το κώλον), and the sigmoid flexure (στεvurτεpov καϊ εiλιγμεvov). The modern epithet of rectum is the literal translation of his description of the straight progress (ευθύ) of the bowel to the anus (ποωκτός). He knew the nasal cavities and the passage from the tympanal cavity of the ear to the palate, afterwards described by Eustachius. He distinguishes as “partes similares” those structures, such as bone, cartilage, vessels, sinews, blood, lymph, fat, flesh, which, not confined to one locality, but distributed throughout the body generally, we now term the tissues or textures, whilst he applies the term “partes dissimilares” to the regions of the head, neck, trunk, and extremities.

Next to Aristotle occur the names of Diodes of Carystus, and Praxagoras of Cos, the last of the family of the Asclepiadae. The latter is remarkable for being the first who distinguished the arteries from the veins, and the author of the opinion that the former were air-vessels.

Hitherto anatomical inquiry was confined to the examination of the bodies of brute animals. We have, indeed, no testimony of the human body being submitted to exam in a-tion previous to the time of Erasistratus and Herophilus; and it is vain to look for authentic facts on this point before the foundation of the Ptolemaic dynasty of sovereigns in Egypt. This event, which, as is generally known, succeeded the death of Alexander, 320 years before the Christian era, collected into one spot the scattered embers of literature and science, which were beginning to languish in Greece under a weak and distracted government and an unsettled state of society. The children of her divided states, whom domestic discord and the uncertainties of war rendered unhappy at home, wandered into Egypt, and found, under the fostering hand of the Alexandrian monarchs, the means of cultivating the sciences, and repaying with interest to the country of Thoth and Osiris the benefits which had been conferred on the infancy of Greece by Thales and Pythagoras. Alexandria became in this manner the repository of all the learning and knowledge of the civilised world; and while other nations were sinking under the effects of internal animosities and mutual dissensions, or ravaging the earth with the evils of war, the Egyptian Greeks kept alive the sacred flame of science, and preserved mankind from relapsing into their original barbarism. These happy effects are to be ascribed in an eminent degree to the enlightened government and liberal opinions of Ptolemy Soter, and his immediate successors Philadelphus and Euergetes. The two latter princes, whose authority was equalled only by the zeal with which they patronised science and its professors, were the first who enabled physicians to dissect the human body, and prevented the prejudices of ignorance and superstition from compromising the welfare of the human race. To this happy circumstance Herophilus and Erasistratus are indebted for the distinction of being known to posterity as the first anatomists who dissected and described the parts of the human body. Both these physicians flourished under Ptolemy Soter, and probably Ptolemy Philadelphus, and were indeed the principal supports of what has been named in medical history the Alexandrian School, to which their reputation seems to have attracted numerous pupils. But though the concurrent testimony of antiquity assigns to these physicians the merit of dissecting the human body, time, which wages endless war with the vanity and ambition of man, has dealt hardly with the monuments of their labours. As the works of neither have been preserved, great uncertainty prevails as to the respective merits of these ancient anatomists; and all that is now known of their anatomical researches is obtained from the occasional notices of Galen, Oribasius, and some other writers. From these it appears that Erasistratus recognised the valves of the heart, and distinguished them by the names of tricuspid and sigmoid; that he studied particularly the shape and structure of the brain, and its divisions, and cavities, and membranes, and likened the convolutions to the folds of the jejunum: that he first formed a distinct idea of the nature of the nerves, which he made issue from the brain; and that he discovered lymphatic vessels in the mesentery, first in brute animals, and afterwards, it is said, in man. He appears also to have distinguished the nerves into those of sensation and those of motion.

Of Herophilus it’is said that he had extensive anatomical knowledge, acquired by dissecting not only brutes but human bodies. Of these he probably dissected more than any of his predecessors or contemporaries. Devoted to the assiduous cultivation of anatomy, he appears to have studied with particular attention those parts which were least understood. He recognised the nature of the pulmonary artery, which he denominates arterious vein; he knew the vessels of the mesentery, and showed that they did not go to the vena portae, but to certain glandular bodies; and he first applied the name of twelve-inch or duodenum, (δωδεκαδάκτυλο?) to that part of the alimentary canal which is next to the stomach. Like Erasistratus, he appears to have studied carefully the configuration of the brain; and though, like him, he distinguishes the nerves into those of sensation and those of voluntary motion, he adds to them the ligaments and tendons. A tolerable description of the liver by this anatomist is preserved in the writings of Galen. He first applied the name of choroid or vascular membrane to that which is found in the cerebral ventricles; he knew the straight venous sinus which still bears his name; and to him the linear furrow at the bottom of the fourth ventricle is indebted for its name of calamus scriptorius.

The celebrity of these two great anatomists appears to have thrown into the shade for a long period the names of all other inquirers; for, among their numerous and rather celebrated successors in the Alexandrian school, it is impossible to recognise a name which is entitled to distinction in the history of anatomy. In a chasm so wide it is not uninteresting to find, in one who combined the characters of the greatest orator and philosopher of Home, the most distinct traces of attention to anatomical knowledge. Cicero, in his treatise 'De N^atura Deorum, in a short sketch of physiology, such as it was taught by Aristotle and his disciples, introduces various anatomical notices, from which the classical reader may form some idea of the state of anatomy at that time. The Roman orator appears to have formed a pretty distinct idea of the shape and connections of the windpipe and lungs; and though he informs his readers that he knows the alimentary canal, he omits the details through motives of delicacy, [n imitation of Aristotle, he talks of the blood being conveyed by the veins (venae), that is, blood-vessels, through the body at large; and, like Praxagoras, of the air inhaled by the lungs being conveyed through the arteries.

Aretaeus, though chiefly known as a medical author, makes some observations on the lung and the pleura, maintains the glandular structure of the kidney, and describes the anastomosis or communications of the capillary extremities of the vena cava with those of the portal vein.

The most valuable depository of the anatomical knowledge of these times is the work of Celsus, one of the most judicious medical authors of antiquity. He left, indeed, no express anatomical treatise; but from the introductions to the 4th and 8th books of his work, De Medicina, with incidental remarks in the 7th, the modern reader may form very just ideas of his anatomical attainments. From these it appears that Celsus was well acquainted with the windpipe and lungs and the heart; with the difference between the windpipe and oesophagus (stomachus), which leads to the stomach (ventri-cul·us); and with the shape, situation, and relations of the diaphragm. He enumerates also the principal facts relating to the situation of the liver, the spleen, the kidneys, and the stomach. He appears, however, to have been unaware of the distinction of duodenum or twelve-inch bowel, already admitted by Herophilus, and represents the stomach as directly connected by means of the pylorus with the jejunum or upper part of the small intestine.

The 7th and 8th books, which are devoted to the consideration of those diseases which are treated by manual operation, contain sundry anatomical notices necessary to explain the nature of the diseases or mode of treatment. Of these, indeed, the merit is unequal; and it is not wonderful that the ignorance of the day prevented Celsus from understanding rightly the mechanism of the pathology of hernia. He appears, however, to have formed a tolerably just idea of the mode of cutting into the urinary bladder; and even his obstetrical instructions show that his knowledge of the uterus, vagina, and appendages was not contemptible. It is in osteology, however, that the information of Celsus is chiefly conspicuous. He enumerates the sutures and several of the holes of the cranium, and describes at great length the superior and inferior maxillary bones and the teeth. With a good deal of care he describes the vertebrae and the ribs, and gives very briefly the situation and shape of the scapula, humerus, radius, and ulna, and even of the carpal and metacarpal bones, and then of the diffèrent bones of the pelvis and lower extremities. He had formed a just idea of the articular connections, and is desirous to impress the fact that none is formed without cartilage. From his mention of many minute holes (multa et tenuia foramina), in the recess of the nasal cavities, it is evident that he was acquainted with the perforated plate of the ethmoid bone; and from saying that the straight part of the auditory canal becomes flexuous, and terminates in numerous minute cavities (multa et tenuia foramina diducitur), it is inferred by Portal that he knew the semicircular canals.

Though the writings of Celsus show that he cultivated anatomical knowledge, it does not appear that the science was much studied by the Romans; and there is reason to believe that, after the decay of the school of Alexandria, it languished in neglect and obscurity. It is at least certain that the appearance of Marinus during the reign of Nero is mentioned by authors as an era remarkable for anatomical inquiry, and that this person is distinguished by Galen as the restorer of a branch of knowledge which had been before him suffered to fall into undeserved neglect. From Galen also we learn that Marinus gave an accurate account of the muscles, that he studied particularly the glands, and that he discovered those of the.mesentery. He fixed the number of nerves at seven; he observed the palatine nerves, which he rated as the fourth pair; and described as the fifth the auditory and facial, which he regards as one pair, and the hypoglossal as the sixth.

Not long after Marinus appeared Ruffus of Ephesus, a Greek physician, who in the reign of Trajan was much attached to physiology, and as a means of cultivating, this science studied Comparative Anatomy, and made sundry experiments on living animals. Of the anatomical writings of this author there remains only a list or catalogue of names of different regions and parts of the animal body. He appears, however, to' have directed attention particularly to the tortuous course of the uterine vessels, and to have recognised even at this early period the Fallopian tube. He distinguishes the nerves into those of sensation and those of motion. He knew the recurrent nerve. His name is further associated with the ancient experiment of compressing in the situation of the carotid arteries the pneumogastric nerve, and thereby inducing insensibility and loss of voice.

Of all the authors of antiquity, however, none possesses so just a claim to the title of anatomist as Claudius Galenus, the celebrated physician of Pergamus, λvho was born about the 130th year of the Christian era, and lived under the reigns of Hadrian, the Antonines, Commodus, and Severus. He° was trained by his father Nicon (whose memory he embalms as an eminent mathematician, architect, and astronomer) in all the learning of the day, and initiated particularly into the mysteries of the Aristotelian philosophy. In an order somewhat whimsical he afterwards studied philosophy successively in the schools of the Stoics, the Academics, the Peripatetics, and the Epicureans. When he was seventeen years of age, his father, he informs us, was admonished by a dream to devote his son to the study of medicine; but it was fully two years after that Galen entered on this pursuit, under the auspices of an instructor whose name he has thought proper to conceal. Shortly after he betook himself to the study of anatomy under Satyrus, a pupil of Quintus, and of medicine under Stra-tonicus, a Hippocratic physician, and Aeschrion, an empiric. He had scarcely attained the age of twenty when he had occasion to deplore the loss of the first and most affectionate guide of his studies; and soon after he proceeded to Smyrna to obtain the anatomical instructions of Pelops, who, though mystified by some of the errors of Hippocrates, is commemorated by his pupil as a skilful anatomist. After this he appears to have visited various cities distinguished for philosophical or medical teachers; and, finally, to have gone to Alexandria with the view of cultivating more accurately and intimately the study of anatomy under Heraclianus. Here he remained till his twenty-eighth year, when he regarded himself as possessed of all the knowledge then attainable through the medium of teachers. He now returned to Pergamus to exercise the art which he had so anxiously studied, and received, in his twenty-ninth year, an unequivocal testimony of the confidence which his fellow-citizens reposed in his skill, by being intrusted with the treatment of the wounded gladiators; and in this capacity he is said to have treated wounds with success which were fatal under former treatment. A seditious tumult appears to have caused him to form the resolution of quitting Pergamus and proceeding to Rome, at the age of thirty-two. Here, however, he remained only five years; and returning once more to Pergamus, after travelling for some time, finally settled in Rome as physician to the Emperor Commodus. The anatomical writings ascribed to Galen, which are numerous, are to be viewed not merely as the result of personal research and information, but as the common depository of the anatomical knowledge of the day, and as combining all that he had learnt from the several teachers under whom he successively studied with whatever personal investigation enabled him to acquire. It is on this account not always easy to distinguish what Galen had himself ascertained by personal research from that which was known by other anatomists. This, however, though of moment to the history of Galen as an anatomist, is of little consequence to the science itself; and from the anatomical remains of this author a pretty just idea may be formed both of the progress and of the actual state of the science at that time.

The osteology of Galen is undoubtedly the most perfect of the departments of the anatomy of the ancients. He names and distinguishes the bones and sutures of the cranium nearly in the same manner as at present. Thus, he notices the quadrilateral shape of the parietal bones; he distinguishes the squamous, the styloid, the mastoid, and the petrous portions of the temporal bones; and he remarks the peculiar situation and shape of the sphenoid bone. Of the ethmoid, which he omits at first, he afterwards speaks more at large in another treatise. The malar he notices under the name of zygomatic 4 bone; and he describes at length the upper maxillary and nasal bones, and the connection of the former with the sphenoid. He gives the first clear account of the number and situation of the vertebrae, wh⅛h he divides into cervical, dorsal, and lumbar, and distinguishes from the sacrum and coccyx. Under the head Bones of the Thorax, he enumerates the sternum, the ribs (αi 7rλ∈υpai), and the dorsal vertebrae, the connection of which with the former he designates as a variety of diarthrosis. The description of the bones of the extremities and their articulations concludes the treatise.

Though in myology Galen appears to less advantage than in osteology, he nevertheless had carried this part of anatomical knowledge to greater perfection than any of his predecessors. He describes a frontal muscle, the six muscles of the eye, and a seventh proper to animals; a muscle to each ala nasi, four muscles of the lips, the thin cutaneous muscle of the neck, which he first termed platysma myoides, or muscular expansion, two muscles of the eyelids, and four pairs of muscles of the lower jaw—the temporal to raise, the masseter to draw to one side, and two depressors, corresponding to the digastric and internal pterygoid muscles. After speaking of the muscles which move the head and the scapula, he adverts to those by which the windpipe is opened and shut, and the intrinsic or proper muscles of the larynx and hyoid bone. Then follow those of the tongue, pharynx, and neck, those of the upper extremities, the trunk, and the lower extremities successively; and in the course of this description he swerves so little from the actual facts that most of the names by which he distinguishes the principal muscles have been retained by the best modern anatomists. It is chiefly in the minute account of these organs, and especially in reference to the minuter muscles, that he appears inferior to the moderns.

The angiological knowledge of Galen, though vitiated by the erroneous physiology of the times and ignorance of the separate uses of the arteries and veins, exhibits, nevertheless, some accurate facts which show the diligence of the author in dissection. Though, in opposition to the opinions of Praxagoras and Erasistratus, he proved that the arteries in the living animal contain not air but blood, it does not appear to have occurred to him to determine in what direction the blood flows, or whether it was movable or stationary. Representing the left ventricle of the heart as the common origin of all the arteries, though he is misled by the pulmonary artery, he nevertheless traces the distribution of the branches of the aorta with some accuracy. The vena azygos also, and the jugular veins, have contributed to add to the confusion of his description, and to render his angiology the most imperfect of his works.

In neurology we find him to be the author of the dogma that the brain is the origin of the nerves of sensation, and the spinal chord of those of motion; and he distinguishes the former from the latter by their greater softness or less consistence. Though he admits only seven cerebral pairs, he has the merit of distinguishing and tracing the distribution of the greater part of both classes of nerves with great accuracy. His description of the brain is derived from dissection of the lower animals, and his distinctions of the several parts of the organ have been retained by modem anatomists. His mode of demonstrating this organ, which indeed is clearly described, consists of five different steps. In the first the bisecting membrane— i.e. the falx (∕riyvιγ^ διχοτομούσα)—and the connecting blood-vessels are removed; and the dissector, commencing at the anterior extremity of the great fissure, separates the hemispheres gently as far as the torcular, and exposes a smooth surface (τ⅛v χώραν τυλώδη πως oυσαv), the mesolobe of the moderns, or the middle band. In the second he exposes by successive sections the ventricles, the choroid plexus, and the middle partition.

The third exhibits the pineal body (σώμα κωvoeιδ∈s) or conarium, concealed by a membrane with numerous veins, meaning that part of the plexus which is now known by the name of velum interpositum, and a complete view of the ventricles. The fourth unfolds the third ventricle (τiς aXkη τρίτη κοιλία), the communication between the two lateral ones, the arch-like body (σώμα t∕<αλιδoαδ∈s) fornix, and the passage from the third to the fourth ventricle. In the fifth he gives an accurate description of the relations of the third and fourth ventricle, of the situation of the two pairs of eminences, nates (γλoυτd) and testes (8iδvμia or δpχαs), the scolecoid or worm-like process, anterior and posterior, and lastly the linear furrow, called by Herophilus calamus scriptorius.

In the account of the thoracic organs equal accuracy may be recognised. He distinguishes the pleura by the name of inclosing membrane (fμηv υπεζωκως, membrana succingens), and remarks its similitude in structure to that of the peritoneum, and the covering which it affords to all the organs. The pericardium also he describes as a membranous sac with a circular basis corresponding to the base of the heart, and a conical apex; and after an account of the tunics of the arteries and veins, he speaks shortly of the lung, and more at length of the heart, which, however, he takes some pains to prove not to be muscular, because it is harder, its fibres are differently arranged, and its action is incessant, whereas that of muscle alternates with the state of rest; he gives a good account of the valves and of the vessels; and notices especially the bony ring formed in the heart of the horse, elephant, and other large animals.

The description of the abdominal organs, and of the kidneys and urinary apparatus, is still more minute, and in general accurate. Our limits, however, do not permit us to give any abstract of them; and it is sufficient in general to say that Galen gives correct views of the arrangement of the peritoneum and omentum, and distinguishes accurately the several divisions of the alimentary canal and its component tissues. In the liver, which he allows to receive an envelope from the peritoneum, he admits, in imitation of Erasistratus, a proper substance or parenchyma, interposed between the vessels, and capable of removal by suitable dissection. His description of the organs of generation is rather brief, and is, like most of his anatomical sketches, too much blended with physiological dogmas.

This short sketch may communicate some idea of the condition of anatomical knowledge in the days of Galen, who indeed is justly entitled to the character of rectifying and digesting, if not of creating, the science of anatomy among the ancients. Though evidently confined, perhaps entirely by the circumstances of the times, to the dissection of brute animals, so indefatigable and judicious was he in the mode of acquiring knowledge, that many of his names and distinctions are still retained with advantage in the writings of the moderns. Galen was a practical anatomist, and not only describes the organs of the animal body from actual dissection, but gives ample instructions for the proper mode of exposition. His language is in general clear, his style as correct as in most of the authors of the same period, and his manner is animated. Few passages in early science are indeed so interesting as the description of the process for demonstrating the brain and other internal organs which is given by this patient and enthusiastic observer of nature. To some it may appear absurd to speak of anything like good,anatomical description in an author who writes in the Greek language, or anything like an ,interesting and correct manner in a writer who flourished at à period when taste was depraved or extinct and literature corrupted,—when the philosophy of Antoninus and the mild virtues of Aurelius could do little to soften the iron sway of Lucius Verus and Com-modus; but the habit of faithful observation in Galen seeme to have been so powerful that, in the description of material objects, his genius invariably rises above the circumstances of his age. Though not so directly connected with this subject, it is nevertheless proper to mention that he appears to have been the first anatomist who can be said, on authentic grounds, to have attempted to discover the uses of organs by vivisection and experiments on living animals. In this manner he ascertained the position and demonstrated the action of the heart; and he mentions two instances in which, in consequence of disease or injury, he had an opportunity of observing the motions of this organ in the human body. In short, without eulogising an ancient author at the expense of critical justice, or commending his anatomical descriptions as superior to those of the moderns, it must be admitted that the anatomical writings of the physician of Pergamus form a remarkable era in the history of the science; and that by diligence in dissection and accuracy in description he gave the = science a degree of importance and stability which it has retained through the lapse of many centuries.

The death of Galen, which took place at Pergamus in the ninetieth year of his age and the 193d of the Christian era, may be regarded as the downfall of anatomy in ancient times. After this period we recognise only two names of any celebrity in the history of the science—those of Soranus and Oribasius, with the more obscure ones of Meletius and Theophilus, the latter the chief of the imperial guard of Heraclius.

Soranus, who was an Ephesian, and flourished under the emperors Trajan and Hadrian, distinguished himself by his researches on the female organs of generation. He appears to have dissected the human subject; and this perhaps is one reason why his descriptions of these parts are more copious and more accurate than those of Galen, who derived his knowledge from the bodies of the lower animals. He denies the existence of the hymen, but describes accurately the clitoris. Soranus the anatomist must be distinguished from the physician of that name, who was also a native of Ephesus.

Oribasius, who was born at Pergamus, is said to have been at once the friend and physician of the Emperor Julian, and to have contributed to the elevation of that apostate to the imperial throne. For this he appears to have suffered the punishment of a temporary exile under Valens and Valentinian; but was soon recalled, and lived in great honour till the period of his death. By Le Clerc, Oribasius is regarded as a compiler; and indeed his anatomical writings bear so close a correspondence with those of Galen that the character is not altogether groundless. In various points, nevertheless, he has rendered the Galenian anatomy more accurate; and he has distinguished himself by a good account of the salivary glands, which were overlooked by Galen.

To the same period generally is referred the Anatomical Introduction of an anonymous author, first published in 1618 by Lauremberg, and more recently by Bernard. It is to be regarded as a compilation formed on the model of Galen and Oribasius. The same character is applicable to the treatises of Meletius and Theophilus.

The decline indicated by these languid efforts soon sunk into a state of total inactivity; and the unsettled state of society during the latter ages of the Roman empire was extremely unfavourable, to the successful cultivation of science. The sanguinary conflicts in which the southern countries of Europe were repeatedly engaged with their northern neighbours, between the second and eighth centuries, tended gradually to estrange their minds from scientific pursuits: and the hordes of barbarians by which the Roman empire was latterly overrun, while they urged them to the necessity of making hostile resistance, and adopting means of self-defence, introduced such habits of ignorance and barbarism, that science was almost universally forgotten. While the art of healing was professed only by some few ecclesiastics or by itinerant practitioners, anatomy was utterly neglected; and no name of anatomical celebrity 0ccur3 to diversify the long and uninteresting period commonly distinguished as the Dark Ages.

Anatomical learning, thus neglected by'European nations, is believed to have received a temporary cultivation from the Asiatics. Of these, several nomadic tribes, known to Europeans under the general denomination of Arabs and Saracens, had gradually coalesced under various leaders; and by their habits of endurance, as well as of enthusiastic valour in successive expeditions against the eastern division of the Roman empire, had acquired such military reputation as to render them formidable wherever they appeared. After a century and a half of foreign warfare or internal animosity, under the successive dynasties of the Ommiads and Abbassides, in which the propagation of Islamism was the pretext for the. extinction of learning and civilisation, and the most remorseless system of rapine and destruction, the Saracens began, under the latter dynasty of princes, to recognise the value of science, and especially of that which prolongs life, heals disease, and alleviates the pain of wounds and injuries. The caliph Almansor combined with his official knowledge of Moslem law the successful cultivation of astronomy; but to his grandson Almamun, the seventh prince of the line of the Abbassides, belongs the merit of undertaking to render his subjects philosophers and physicians. By the directions of this prince the works of the Greek and Roman authors were translated into Arabic; and the favour and munificence with which literature and its professors were patronised speedily raised a succession of learned Arabians. The residue of the rival family of the Ommiads, already settled in Spain, was prompted by motives of rivalry or honourable ambition to adopt the same course; and while the academy, hospitals, and library of Baghdad bore testimony to the zeal and liberality of the Abbassides, the munificence of the Ommiades was not less conspicuous in the literary institutions of Cordova, Seville, and Toledo.

Notwithstanding the efforts of the Arabian princes, however, and the diligence of the Arabian physicians, little was done for anatomy, and the science made no substantial acquisition. The Koran denounces as unclean the person who touches a corpse; the rules of Islamism forbid dissection; and whatever their instructors taught was borrowed from the Greeks. Abu-Bekr Al-Rasi, Abu-Ali Ibn-Sina, Abul-Cassem, and Abu-Walid Ibn-Roshd, the Rhazès, Avicenna, Abulcasis, and Averrhoes of European authors, are their most celebrated names in medicine; yet to none of these can the historian with justice ascribe any anatomical merit. Al-Rasi has indeed left descriptions of the eye, of the ear and its meatus, and of the heart; and Ibn-Sina, Abul-Cassem, and Ibn-Roshd give anatomical descriptions of the parts of the human body. But of these the general character is, that they are copies from Galen, sometimes not very just, and in all instances mystified with a large proportion of the fanciful and absurd imagery and inflated style of the Arabian writers. The chief reason of their obtaining a place in anatomical history is, that by the influence which their medical authority enabled them to exercise in the European schools, the nomenclature which they employed was adopted by European anatomists, and continued till the revival of ancient learning restored the original nomenclature of the Greek physicians. Thus, the cervix, or nape of the neck, is nucha; the oesophagus is meri; the umbilical region is sumen or sumac; the abdomen is rnyrach; the peritoneum is siphac; and the omentum, zirbus.

From the general character now given justice requires that we except Abdallatif, the annalist of Egyptian affairs. This author, who maintains that it is impossible to learn anatomy from books, and that the authority of Galen must yield to personal inspection, informs us that the Moslem doctors did not neglect opportunities of studying the bones of the human body imcemeteries; and that he himself, by once examining a collection of bones in this manner, ascertained that the lower jaw is formed of one piece; that the sacrum, though sometimes composed of several, is most generally of one; and that Galen is mistaken when he asserts that these bones are not single.

The era of Saracen learning extends to the 13th century; and after this we begin to approach happier times. The university of Bologna, which, as a school of literature and law, was already celebrated in the twelfth century, became, in the course of the following one, not less distinguished for its medical teachers. Though the misgovernment of the municipal rulers of Bologna had disgusted both teachers and students, and given rise to the foundation of similar institutions in Padua and Naples,—and though the school of Salerno, in the territory of the latter, was still in high repute,—it appears, from the testimony of Sarti, that medicine was in the highest esteem in Bologna, and that it was in such perfection as to require a division of its professors into physicians, surgeons, physicians for wounds, barber-surgeons, oculists, and even some others. Notwithstanding these indications of refinement, however, anatomy was manifestly cultivated rather as an appendage of surgery than a branch of medical science; and, according to the testimony of Guy de Chauliac, the cultivation of anatomical knowledge was confined to Roger, Roland, Jamerio, Bruno, and Lanfranc; and this they borrowed chiefly from Galen.

In this state matters appear to have proceeded with the medical school of Bologna till the commencement of the fourteenth century, when the circumstance of possessing a teacher of originality enabled this university to be the agent of as great an improvement in medical science as she had already effected in jurisprudence. This era, indeed, is distinguished for the appearance of Mondino, under whose zealous cultivation the science first began to rise from the ashes in which it had been buried. This father of modern anatomy, who taught in Bologna about the year 1315, quickly drew the curiosity of the medical profession by well-ordered demonstrations of the different parts of the human body. In 1315 he dissected and demonstrated the parts of the human body in two female subjects; and in the course of the following year he accomplished the same task on the person of a single female. But while he seems to have had sufficient original force of intellect to direct his own route, Riolan accuses him of copying Galen; and it is certain that his descriptions are corrupted by the barbarous leaven of the Arabian schools, and his Latin defaced by the exotic nomenclature of Ibn-Sina and AÎ-Rasi. He died, according to Tiraboschi, in 1325.

Mondino divides the body into three cavities (ventres), the upper containing the animal members, as the head, the lower containing the natural members, and the middle containing the spiritual members. He first describes the anatomy of the lower cavity or the abdomen, then proceeds to the middle or thoracic organs, and concludes with the upper, comprising the head and its contents and appendages. His general manner is to notice shortly the situation and shape or distribution of textures or membranes, and then to mention the disorders to which they are subject. The peritoneum he describes under the name of siphac, in imitation of the Arabians, the omentum under that of zirbus, and the mesentery or eucharus as distinct from both. In speaking of the intestines he treats first of the rectum, then the colon, the left or sigmoid flexure of which, as well as the transverse arch and its connection with the stomach, he particularly remarks; then the caecum or monoculus, after this the small intestines in general under the heads of ileum and jejunum, and latterly the duodenum, making in all six bowels. The liver and its vessels are minutely, if not accurately, examined; and the cava, under the name chilis, a corruption from the Greek κοίλη, is treated at length, with the emulgents and kidneys. His anatomy of the heart is wonderfully accurate; and it is a remarkable fact, which seems to be omitted by all subsequent authors, that his description contains the rudiments of the circulation of the blood. “Postea vero versus pulmonem est aliud orificium venae arterialis, quae portat sanguinem ad pulmonem a corde; quia cum pulmo deserviat cordi secundum modum dictum, ut ei recompenset, cor ei transmittit sanguinem per hanc venam, quae vocatur vena arterialis; est vena, quia portat sanguinem, et arterialis, quia habet duas tunicas; et habet duas tunicas, primo quia vadit ad membrum quod existit in continuo motu, et secundo quia portat sanguinem valde subtilem et choleri-cum.” The merit of these distinctions, however, he afterwards destroys by repeating the old assertion that the left ventricle ought to contain spirit or air, which it generates from the blood. His osteology of the skull is erroneous. In his account of the cerebral membranes, though short, he notices the principal characters of the dura mater. He describes shortly the lateral ventricles, with their anterior and posterior cornua, and the choroid plexus as a blood-red substance like a long worm. He then speaks of the third or middle ventricle, and one posterior, which seems to correspond with the fourth; and describes the infundibulum under the names of lacuna and emboton. In the base of the organ he remarks, first, two mammillary caruncles, the optic nerves, which he reckons the first pair; the oculo-muscular, which he accounts the second; the third, which appears to be the sixth of the modems; the fourth; the fifth, evidently the seventh; a sixth, the nervus vagus; and a seventh, which is the ninth of the moderns. Notwithstanding the misrepresentations into which this early anatomist was betrayed, his book is valuable, and has been illustrated by the successive commentaries of Achillini, Berenger, and Dryander.

Matthew de Gradibus, a native of Gradi, a town in Friuli, near Milan, distinguished himself by composing a series of treatises on the anatomy of various parts of the human body. He is the first who represents the ovaries of the female in the correct light in which they were subsequently regarded by Steno.

Objections similar to those already urged in speaking of Mondino apply to another eminent anatomist of those times. Gabriel de Zerbis, who flourished at Verona towards the conclusion of the 15th century, is celebrated as the author of a system in which he is obviously more ,anxious to astonish his readers by the wonders of a verbose ',and complicated style than to instruct by precise and faithful ^description. In the vanity of his heart he assumed the;title of Medicus Theoricus; but though, like Mondino, he ^derived his information from the dissection of the human ι subject, he is not entitled to the merit either of describing truly or of adding to the knowledge previously acquired. He is superior to Mondino, however, in knowing the olfactory nerves.

Eminent in the history of the science, and more distinguished than any of this age in the history of cerebral anatomy, Alexander Achillini of Bologna, the pupil and commentator of Mondino, appeared at the close of the 15th century. Though a follower of the Arabian school, the assiduity with which he cultivated anatomy has rescued his name from the inglorious obscurity in which the Arabian doctors have in general slumbered. He is known in the history of anatomical discovery as the first who described the two tympanal bones, termed malleus and incus. In 1503 he showed that the tarsus consists of seven bones; he rediscovered the fornix and the infundibulum; and he was fortunate enough to observe the course of the cerebral cavities into the inferior cornua, and to remark peculiarities to which the anatomists of a future age did not advert. He mentions the orifices of the ducts, afterwards described by Wharton. He knew the ileo-caecal valve; and his description of the duodenum, ileum, and colon shows that he was better acquainted -with the site and disposition of these bowels than any of his predecessors or contemporaries.

Not long after, the science boasts of one of its most distinguished founders. James Berenger of Carpi, in the Modenese territory, flourished at Bologna at the beginning of the 16th century. In the annals of medicine his name will be remembered not only as the most zealous and eminent in cultivating the anatomy of the human body, but as the first physician who was fortunate enough to calm the alarms of Europe, suffering under the ravages of syphilis, then raging with uncontrollable virulence. In the former character he surpassed both predecessors and contemporaries; and it was long before the anatomists of the following age could boast of equalling him. His assiduity was indefatigable; and he declares that he dissected above one hundred human bodies. He is the author of a compendium, of several treatises which he names Introductions (Isagogce), and of commentaries or. the treatise of Mondino, in which he not only rectifies the mistakes of that anatomist, but gives minute and in general accurate anatomical descriptions.

He is the first who undertakes a systematic view of the several textures of which the human body ⅛ composed; and in a preliminary commentary he treats successively of the anatomical characters and properties of fat, of membrane in general (panniculum), of flesh, of nerve, of villus or fibre (‰τn), of ligament, of sinew or tendon, and of muscle in general. He then proceeds to describe with considerable precision the muscles of the abdomen, and illustrates their site and connections by woodcuts, which, though rude, are spirited, and show that anatomical drawing was in that early age beginning to be understood. In his account of the peritoneum he admits only the intestinal division of that membrane, and is at some pains to prove that Gentilis, who justly admits the muscular division also, is in error. In his account of the intestines he is the first who mentions Jhe vermiform process of the caecum; he remarks the yellow tint communicated to the duodenum by the gall-bladder; and he recognises the opening of the common biliary duct into the duodenum (quidam porus portans choleram). In the account of the stomach he describes the several tissues of which that organ is composed, and which, after Alman-sor, he represents to be three, and a fourth from the peritoneum; and afterwards notices the rugae of its villous surface. He is at considerable pains to explain the organs of generation in both sexes, and gives a long account of the anatomy of the foetus. He was the first who recognised the larger proportional size of the chest in the male than in the female, and conversely the greater capacity of the female than of the male pelvis. In the larynx he discovered the two arytenoid cartilages. He gives the first good description of the thymus; distinguishes the oblique situation of the heart; describes the pericardium, and maintains the uniform presence of pericardial liquor. He then describes the cavities of the heart; but perplexes himself, as did all the anatomists of that age, about the spirit supposed to be contained. The aorta he properly makes to anse from the left ventricle; but confuses himself with the arteria venalis, the pulmonary vein, and the vena arterialis, the pulmonary artery. His account of the brain is better. He gives a minute and clear account of the ventricles, remarks the corpus striatum, and has the sagacity to perceive that the choroid plexus consists of veins and arteries; he then describes the middle or third ventricle, the infundibulum or lacuna of Mondino, and the pituitary gland; and lastly, the passage to the fourth ventricle, the conarium or pineal gland, and the fourth or posterior ventricle itself, the relations of which he had studied accurately. He rectifies the mistake of Mondino as to the olfactory or first pair of nerves, gives a good account of the optic and others, and is entitled to the praise of originality in being the first observer who contradicts the fiction of the wonderful net, and indicates the principal divisions of the carotid arteries. He enumerates the tunics and humours of the eye, and gives an account of the internal ear, in which,he notices the malleus and incus.

Italy long retained the distinction of giving birth to the first eminent anatomists in Europe, and the glory she acquired in the names of Mondino, Achillini, Carpi, and Massa, was destined to become more conspicuous in the labours of Columbus, Fallopius, and Eustachius. While Italy, however, was thus advancing the progress of science, the other nations of Europe were either in profound ignorance or in the most supine indifference to the brilliant career of their zealous neighbours. The sixteenth century had commenced before France began to acquire anatomical distinction in the names of Dubois, Fernel, and Etienne; and even these celebrated teachers were less solicitous in the personal study of the animal body than in the faithful explanation of the anatomical writings of Galen. The infancy of the French school had to contend with other difficulties. The small portion of knowledge which had been hitherto diffused in the country was so inadequate to eradicate the prejudices of ignorance, that it was either difficult or absolutely impossible to procure human bodies for the purposes of science; and we are assured, on the testimony of Vesalius and other competent authorities, that the practical part of anatomical instruction was obtained entirely from the bodies of the lower animals. The works of the Italian anatomists were unknown; and it is a proof of the tardy communication of knowledge that, while the structure of the human body had been taught in Italy for more than a century by Mondino and his followers, these anatomists are never mentioned by Etienne, who flourished long after.

Such was the aspect of the times at the appearance of Jacques Dubois, who, under the Romanised name of Jacobus Sylvius, according to the fashion of the day, has been fortunate in acquiring a reputation to which his researches do not entitle him. For the name of Jacques Dubois the history of anatomy, it is said, is indebted to his inordinate love of money. At the instance of his brother Francis, who was professor of eloquence in the college of Tournay at Paris, he devoted himself to the study of the learned languages and mathematics; but discovering that these elegant accomplishments do not invariably reward their cultivators with the goods of fortune, Dubois betook himself to medicine. After the acquisition of a medical degree in the university of Montpellier, at the ripe age of fifty-one Dubois returned to Paris to resume a course of anatomical instruction. Here he taught anatomy to a numerous audience in the college of Trinquet; and on the departure of Vidus Vidius for Italy was appointed to succeed that physician as professor of surgery to the Royal College. His character .is easily estimated. With greater coarseness in his manners and language than even the rude state of society in his times can palliate, with much varied learning and considerable eloquence, he was a blind, indiscriminate, and irrational admirer of Galen, and interpreted the anatomical and physiological writings of that author in preference to giving demonstrations from the subject. Without talent for original research or discovery himself, his envy and jealousy made him detest every one who gave proofs of either. We are assured by Vesalius, who was some time his pupil, that his manner of teaching was calculated neither to advance the science nor to rectify the mistakes of his predecessors. A human body was never seen in the theatre of Dubois; the carcases of dogs and other animals were the materials from which he taught; and so difficult even was it to obtain human bones, that unless Vesalius and his fellow-students had collected assiduously from the Innocents and other cemeteries, they must have committed numerous errors in acquiring the first principles. This assertion, however, is contradicted by Riolan, and afterwards by Sprengel and Lauth, the last of whom decidedly censures Vesalius for this ungrateful treatment of his instructor. It is certain that opportunities of inspecting the human body were by no means so frequent as to facilitate the study of the science. Though his mention of injections has led some to suppose him the discoverer of that art, he appears to have made no substantial addition to the information already acquired; and the first acknowledged professor of anatomy to the university of Paris appears in history as one who lived without true honour and died without just celebrity. He must not be confounded with Franciscus Sylvius (De le Boe), who is mentioned by Ruysch and Malacame as the author of a particular method of demonstrating the brain.

Almost coeval may be placed Charles Etienne, a younger brother of the celebrated printers, and son to Henry, who Hellenised the family name by the classical appellation of Stephen (≤τε≠αvoς). It is uncertain whether he taught publicly. But his tranquillity was disturbed, and his pursuits interrupted, by the oppressive persecutions in which their religious opinions involved the family; and Charles Etienne drew the last breath of a miserable life in a dungeon in 1564. Etienne, though sprung of a family whose classical taste has been their principal glory, does not betray the same servile imitation Of the Galenian anatomy with which Dubois is charged. He appears to have been the first to detect valves in the orifice of the hepatic veins. He was ignorant, however, of the researches of the Italian anatomists; and his description of the brain is inferior to that given sixty years before by Achillini. His comparison of the cerebral cavities to the human ear has persuaded Portal that he knew the inferior cornua, the hippocampus, and its prolongations; but this is no reason for giving him that honour to the detriment of the reputation of Achillini, to whom, so far as historical testimony goes, the first knowledge of this fact is due. The researches of Etienne into the structure of the nervous system are, however, neither useless nor inglorious; and the circumstance of demonstrating a canal through the entire length of the spinal chord, which had neither been suspected by contemporaries nor noticed by successors till Senac made it known, is sufficient to place him high in the rank of anatomical discoverers.

The French anatomy of the sixteenth century was distinguished by two circumstances unfavourable to the advancement of the science,—extravagant admiration of antiquity, with excessive confidence in the writings of Galen, and the general practice of dissecting principally the bodies of the lower animals. Both these errors were much amended, if not entirely removed, by the exertions of a young Fleming, whose appearance forms a conspicuous era in the history of anatomy. Andrew Vesalius, a native of Brussels, after acquiring at Louvain the ordinary classical attainments of the day, began at the age of fourteen to study anatomy under the auspices of Dubois. Though the originality of his mind soon led him to abandon the prejudices by which he was environed, and take the most direct course for attaining a knowledge of the structure of the human frame, he neither underrated the Galenian anatomy nor was indolent in the dissection of brute animals. The difficulties, however, with which the practical pursuit of human anatomy was beset in France, and the dangers with which he had to contend, made him look to Italy as a suitable field for the cultivation of the science; and in 1536 we find him at Venice, at once pursuing the study of human anatomy with the utmost zeal, and requested, ere he had attained his twenty-second year, to demonstrate publicly in the university of Padua. After remaining here about seven years, Vesalius went by express invitation to Bologna, and shortly afterwards to Pisa; and thus professor in three universities, he appears to have carried on his anatomical investigations and instructions alternately at Padua, Bologna, and Pisa, in the course of the same winter. It is on this account that Vesalius, though a Fleming by birth and trained originally in the French school, belongs, as an anatomist, to the Italian, and may be viewed as the first of an illustrious line of teachers by whom the anatomical reputation of that country was in the course of the sixteenth century raised to the greatest eminence.

Vesalius is known as the firet author of a comprehensive and systematic view of human anatomy. The knowledge with which his dissections had furnished him proved how many errors were daily taught and learned under the broad mantle of Galenian authority; and he perceived the necessity of a new system of anatomical instruction, divested of the omissions of ignorance and the misrepresentations of prejudice and fancy. The early age at which he effected this object has been to his biographers the theme of boundless commendation; and we are told that he began at the age of twenty-five to arrange the materials he had collected, and accomplished his task ere he had completed his 28th year.

Soon after this period we find him invited as imperial physician to the court of Charles V., where he was occupied in the duties of practice, and answering the various charges which were unceasingly brought against him by the •disciples of Galen. After the abdication of Charles he continued at court in great favour with his son Philip II. To this he seems to have been led principally by the troublesome controversies in which his anatomical writings had involved him. It is painful to think, however, that even imperial patronage bestowed on eminent talents does not insure immunity from popular prejudice; and the fate of Vesalius will be a lasting example of the barbarism of the times, and of the precarious tenure of the safety even of a great physician. On the preliminary circumstances authors are not agreed; but the most general account states that when Vesalius was inspecting, with the consent of his kinsmen, the body of a Spanish grandee, it was observed that the heart still gave some feeble palpitations when divided by the knife. The immediate effects of this outrage to human feelings were the denunciation of the anatomist to the Inquisition; and Vesalius escaped the severe treatment of that tribunal only by the influence of the king, and by promising to perform a pilgrimage to the Holy Land. He forthwith proceeded to Venice, from which he sailed with the Venetian fleet, under James Malatesta, for Cyprus. When he reached Jerusalem, he received from the Venetian senate a message requesting him again to accept the Paduan professorship, which had become vacant by the death of his friend and pupil Fallopius. His destiny, however, which pursued him fast, suffered him not again to breathe the Italian air. After struggling for many days with adverse winds in the Ionian Sea, he was wrecked on the island of Zante, where he quickly breathed his last in such penury that unless a liberal goldsmith had defrayed the funeral charges, his remains must have been devoured by beasts of prey. At the time of his death he was scarcely fifty years of age.

To form a correct estimate of the character and merits of Vesalius, we must not compare him, in the spirit of modern perfection, with the anatomical authors either of later times or of the present day. Whoever would frame a just idea of this anatomist must imagine, not a bold innovator without academical learning,—not a genius coming from a foreign country, unused to the forms and habits of Catholic Europe,—nor a wild reformer, blaming indiscriminately everything which accorded not with his opinion; but a young student scarcely emancipated from the authority of instructors, and whose intellect was still influenced by the doctrines with which it had been originally imbued,—a scholar strictly trained in the opinions of the time, living amidst men who venerated Galen as the oracle of anatomy and the divinity of medicine,—exercising his reason to estimate the soundness of the instructions then in use, and proceeding, in the way least likely to offend authority and wound prejudice, to rectify errors, and to establish on the solid basis of observation the true elements of anatomical science. Vesalius has been denominated the founder of human anatomy; and though we have seen that in this career he was preceded with honour by Mondino and Berenger, still the small proportion of correct observation which their reverence for Galen and Arabian doctrines allowed them to communicate, will not in a material degree impair the original merits of Vesalius. The errors which he rectified and the additions which he made are so numerous, that it is impossible, in such a sketch as the present, to communicate a just idea of them.

Besides the first good description of the sphenoid bone, he showed that the sternum consists of three portions and the sacrum of five or six; and described accurately the vestibule in the interior of the temporal bone. He not only verified the observation of Etienne on the valves of the hepatic veins, but he described well the vena azygos, and discovered the canal which passes in the foetus between the umbilical vein and the vena cava, since named ductus venosus. He described the omentum, and its connections with the stomach, the spleen, and the colon; gave the first correct views of the structure of the pylorus; remarked the small size of the caecal appendix in man; gave the first good account of the mediastinum and pleura, and the fullest description of the anatomy of the brain yet advanced. He appears, however, not to have understood well the inferior recesses; and his account of the nerves is confused by regarding the optic as the first pair, the third as the fifth, and the fi , h as the seventh.

The labours of Vesalius were not limited to the immediate effect produced by his own writings. His instructions and example produced a multitude of anatomical inquirers of different characters and varied celebrity, by whom the science was extended and rectified. Of these we cannot speak in detail; but historical justice requires us to notice shortly those to whose exertions the science of anatomy has been most indebted.

The first tha 4 claims attention on this account is Bartholomeo Eustachi of San Severino, near Salerno, who though greatly less fortunate in reputation than Vesalius, divides with him the merit of creating the science of human anatomy. He extended the knowledge of the internal ear by rediscovering and describing correctly the tube which bears his name; and if we admit that Ingras-sias anticipated him in the knowledge of the third bone of the tympanal cavity, the stapes, he is still the first who described the internal and anterior muscles of the malleus, as also the stapedius, and the complicated figure of the cochlea. He is the first who studied accurately the anatomy of the teeth, and the phenomena of the first and second dentition. The work, however, which demonstrates at once the great merit and the unhappy fate of Eustachius is his Anatomical Engravings, which, though completed in 1552, nine years after the impression of the work of Vesalius, the author was unable to publish. First communicated to the world in 1714 by Lancisi, afterwards in 1744 by Cajetan Petrioli, again in 1744 by Albinus, and more recently at Bonn in 1790, the engravings show that Eustachius had dissected with the greatest care and diligence, and taken the utmost pains to give just views of the shape, size, and relative position of the organs of the human body.

The first seven plates illustrate the history of the kidneys, and some of the facts relating to the structure of the ear. The eighth represents the heart, the ramifications of the vena azygos, and the valve of the vena cava, named from the author. In the seven subsequent plates is given a succession of different views of the viscera of the chest and abdomen. The seventeenth contains the brain and spinal chord; and the eighteenth more accurate views of the origin, course, and distribution of the nerves than had been given before. Fourteen plates are devoted to the muscles.

Eustachius did not confine his researches to the study of relative anatomy. He investigated the intimate structure of organs with assiduity and success. What was too minute for unassisted vision he inspected by means of glasses. Structure which could not be understood in the recent state, he unfolded by maceration in different fluids, or rendered more distinct by injection and exsiccation. The facts unfolded in these figures are so important that it is justly remarked by Lauth, that if the author himself had been fortunate enough to publish them, anatomy would have attained the perfection of the 18th century two centuries earlier at least. Their seclusion for that period in the papal library has given celebrity to many names which would have been known only in the verification of the discoveries of Eustachius.

Eustachius was the contemporary of Vesalius. Columbus and Fallopius were his pupils. Columbus, as his immediate successor in Padua, and afterwards as professor at Rome, distinguished himself by rectifying and improving the anatomy of the bones; by giving correct accounts of the shape and cavities of the heart, of the pulmonary artery and aorta and their valves, and tracing the course of the blood from the right to the left side of the heart; by a good description of the brain and its vessels, and by correct understanding of the internal ear, and the first good account of the ventricles of the larynx.

Fallopius, who, after being professor at Pisa in 1548, and at Padua in 1551, died at the age of forty, studied the general anatomy of the bones; described better than heretofore the internal ear, especially the tympanum and its osseous ring, the two fenestrae and their communication with the vestibule and cochlea; and gave the first good account of the stylo-mastoid hole and canal, of the ethmoid bone and cells, and of the lacrymal passages. In myology he rectified several mistakes of Vesalius. He also devoted attention to the organs of generation in both sexes, and discovered the utero-peritoneal canal which still bears his name.

Osteology nearly at the same time found an assiduous cultivator in John Philip Ingrassias, a learned Sicilian physician, who, in a skilful commentary on the osteology of Galen, corrected numerous mistakes. He gave the first distinct account of the true configuration of the sphenoid and ethmoid bones, and has the merit of first describing the third bone of the tympanum, called stapes, though this is also claimed by Eustachius and Fallopius.

The anatomical descriptions of Vesalius underwent the scrutiny of various inquirers. Those moet distinguished by the importance and accuracy of their researches, as well as the temperate tone of their observations, were Julius Caesar Aranzi, anatomical professor for thirty-two years in the university of Bologna, and Constantio Varoli, physician to Pope Gregory XIII. To the former we are indebted for the first correct account of the anatomical peculiarities of the foetus, and he was the first to show that the muscles of the eye do not, as was falsely imagined, arise from the dura mater, but from the margin of the optic hole. He also, after considering the anatomical relations of the cavities of the heart, the valves, and the great vessels, corroborates the views of Columbus regarding the course which the blood follows in passing from the right to the left side of the heart. Aranzi is the first anatomist who describes distinctly the inferior cornua of the ventricles of the cerebrum, who recognises the objects by which they are distinguished, and who gives them the name by which they are still known {hippocampus); and his account is more minute and perspicuous than that of the authors of the subsequent century. He speaks at large of the choroid plexus, and gives a particular description of the fourth ventricle, under the name of cistern of the cerebellum, as a discovery of his own.

Italy, though rich in anatomical talent, has probably few greater names than that of Constantio Varoli of Bologna. Though he died at the early age of thirty-two, he acquired a reputation not inferior to that of the most eminent of his contemporaries. He is now known chiefly as the author of an epistle, inscribed to Hieronymo Mercuriali, on the optic nerves, in which he describes a new method of dissecting the brain, and communicates many interesting particulars relating to the anatomy of the organ. He observes the threefold division of the inferior surface or base, defines the limits of the anterior, middle, and posterior eminences, as marked by the compartments of the skull, and justly remarks that the cerebral cavities are capacious, communicate with each other, extending first backward and then forward, near the angle of the pyramidal portion of the temporal bone, and that they are .folded on themselves, and finally lost above the middle and inferior eminence of the brain. He appears to have been aware that at this point they communicate with the exterior or convoluted surface. He recognised the impropriety of the term corpus callosum, seems to have known the communication called afterwards foramen Monroianum, and describes the hippocampus more minutely than had been previously done.

Among the anatomists of the Italian school, as a pupil of Fallopius, Eustachius, and Aldrovandus, is generally enumerated Volcher Coiter of Groningen. He distinguished himself by accurate researches on the cartilages, the bones, and the nerves, recognised the value of morbid anatomy, and made experiments on living animals to ascertain the action of the heart and the influence of the brain.

The Frutefull and Necessary Briefe Worke of John Halle (1565), and The Englisheman's Treasure, by Master Thomas Vicary (1586), English works published at this time, are tolerable compilations from former authors, much tinged by Galenian and Arabian distinctions. A more valuable compendium than either is, however, that of John Banister (1578), entitled The Historie of Man, from the most approved Anathomistes in this Present Age.

The celebrity of the anatomical school of Italy was worthily maintained by Hieronymo Fabricio of Acquapen-dente, who, in imitation of his master Fallopius, laboured to render anatomical knowledge more precise by repeated dissections, and to illustrate the obscure by researches on the structure of animals in general. In this manner he investigated the formation of the foetus, the structure of the oesophagus, stomach, and bowels, and the peculiarities of the eye, the ear, and the larynx. The discovery, however, on which his surest claims to eminence rest is that of the membranous folds, which he names valves, in the interior of veins. Several of these folds had been observed by Fcmel, Sylvius, and Vesalius; and in 1547 Cannani observed those of the vena azygos; but no one appears to have offered any rational conjecture on their use, or to have traced them through the venous system at large, until Fabricius in 1574, upon this hypothesis, demonstrated the presence of these valvular folds in all the veins of the extremities.

Fabricius, though succeeded by his pupil Julius Casserius of Piacenza, may be regarded as the last of that illustrious line of anatomical teachers by whom the science was so successfully studied and taught in the universities of Italy. The discoveries which each made, and the errors which their successive labours rectified, tended gradually to give anatomy the character of a useful as well as an accurate science, and to pave the way for a discovery which, though not anatomical but physiological, is so intimately connected with correct knowledge of the shape and situation of parts, that it exercised the most powerful influence on the future progress of anatomical inquiry. This was the knowledge of the circular motion of the blood,—a fact which, though obscurely conjectured by Aristotle, Nemesius, Mondino, and Berenger, and partially taught by Servetus, Columbus, Caesalpinus, and Fabricius, it was nevertheless reserved to William Harvey fully and satisfactorily to demonstrate.

Mondino believed that the blood proceeds from the heart to the lungs through the vena arterialis or pulmonary artery, and that the aorta conveys the spirit into the blood through all parts of the body. This doctrine was adopted with little modification by Berenger, who further demonstrated the existence and operation of the tricuspid valves in the right ventricle, and of the sigmoid valves at the beginning of the pulmonary artery and aorta, and that there were only two ventricles separated by a solid impervious septum. These were afterwards described in greater detail by Vesalius, who nevertheless appears not to have been aware of the important use which might be made of this knowledge. It was the Spaniard Michael Servet or Servetus (born in 1509; burnt in 1553), who in his treatise De Trinitatis Erroribus, published at Haguenau in 1531, first maintained the imperviousness of the septum, and the transition of the blood by what he terms an unknown route, namely, from the right ventricle by the vena arteriosa (pulmonary artery) to the lungs, and thence into the arteria venosa or pulmonary vein and left auricle and ventricle, from which, he adds afterwards, it is conveyed by the aorta to all parts of the body.

Though the leading outlines, not only of the pulmonary or small but even of the great circulation, were sketched thus early by one who, though a philosopher, was attached to the church, it was only in his work De Re Anatomica, published at Venice in 1559, that Columbus formally and distinctly announced the circular course of the blood as a discovery of his own; and maintained, in addition to the impeτviousness of the septum, the fact that the arteria venalis (pulmonary vein) contains, not air, but blood mixed with air brought from the lungs to the left ventricle of the heart, to be distributed through the body at large.

Soon after, views still more complete of the small or pulmonary circulation were given by Andrew Caesalpinus of Arezzo, who not only maintained the analogy between the structure of the arterious vein or pulmonary artery and the aorta, and that between the venous artery or pulmonary veins and veins in general, but was the first to remark the swelling of veins below ligatures, and to infer from it a refluent motion of blood in these vessels. The discoveries of Aranzi and Eustachius in the vessels of the foetus tended at first to perplex and afterwards to elucidate some of these notions. At length it happened that, between the years 1598 and 1600, a young Englishman, William Harvey, pursuing his anatomical studies at Padua under Fabricius of Acquapendente, learnt from that anatomist the existence of the valves in the veins of the extremities, and undertook to ascertain the use of these valves by experimental inquiry. It is uncertain whether he learnt from the writings of Caesalpinus the fact observed by that author, of the tumescence of a vein below the ligature, but he could not fail to be aware, and indeed he shows that he was aware, of the small circulation as taught by Servetus and Columbus. Combining these facts already known, he, by a series of well-executed experiments, demonstrated clearly the existence, not only of the small, but of a general circulation from the left side of the heart by the aorta and its subdivisions, to the right side by the veins. This memorable truth was first announced in the year 1619.

It belongs not to this place either to consider the arguments and facts by which Harvey defended his theory, or to notice the numerous assaults to which he was exposed, and the controversies in which his opponents wished to involve him. It is sufficient to say, that after the temporary ebullitions of spleen and envy had subsided, the doctrine of the circular motion of the blood was admitted by all enlightened and unprejudiced persons, and finally was universally adopted as affording the most satisfactory explanation of many facts in anatomical structure which were either misunderstood or entirely overlooked. The inquiries to which the investigation of the doctrine gave rise produced numerous researches on the shape and structure of the heart and its divisions, of the lungs, and of the blood-vessels and their distribution. Of this description were the researches of Nicolas Steno on the structure of the heart, the classical work of Richard Lower, the dissertation of Pechlin, the treatise of Vieussens, the work of Malpighi on the εtructuro of the lungs, several sketches in the writings of Mayow, and other treatises of less moment. Systematic treatises of anatomy began to assume a more instructive form, and to breathe a more philosophical spirit. The great work of Adrian Spigelius, which appeared in 1627, two years after the death of the author, contains indeed no proof that he was aware of the valuable generalisation of Harvey; but in the institutions of Caspar Bartholin, as republished and improved by his son Thomas in 1651, the anatomical descriptions and explanations are given with reference to the new doctrine. Λ still more unequivocal proof of the progress of correct anatomical knowledge was given in the lectures delivered by Peter Dionis, at the Jardin Royal of Paris, in 1673 and the seven following years, in which that intelligent surgeon gave most accurate demonstrations of all the parts composing the human frame, and especially of the heart, its auricles, ventricles, and valves, and the large vessels connected with it and the lungs. These demonstrations, first published in 1690, were so much esteemed that they passed through seven editions in the space of thirty years, and were translated into English.

The progress of anatomical discovery continued in the meantime to advance. In the course of the 16th century Eustachius, in studying minutely the structure of the vena azygos had recognised in the horse a white vessel full of watery fluid, connected with the internal jugular vein, on the left side of the vertebral column, corresponding accurately with the vessel since named thoracic duct. Fallopius also described vessels belonging to the Ever distinct from arteries and veins; and similar vessels appear to have been noticed by Nicolaus Massa. The nature and properties of these vessels were, however, entirely unknown. On the 23d July 1622 Gaspar Asellius, professor of anatomy at Pavia, while engaged in demonstrating the recurrent nerves in a living dog, first observed numerous white delicate filaments crossing the mesentery in all directions; and though he took them at first for nerves, the opaque white fluid which they shed quickly convinced him that they were a new order of vessels. The repetition of the experiment the following day showed that these vessels were best seen in animals recently fed; and as he traced them from the villous membrane of the intestines, and observed the valves vith which they were EberaUy suppEed, he inferred that they were genuine chyliferous vessels. By confounding them with the lymphatics, he made them proceed to the pancreas and liver,—a mistake which appears to have been first rectified by Francis De le Boe. The discovery of Asellius was announced in 1627; and the foUowing year, by means of the zealous efforts of Nicolas Peiresc, a Eberal senator of Aix, the vessels were seen in the person of a felon who had eaten copiously before execution, and whose body was inspected an hour and a haff after. In 1629 they were publicly demonstrated at Copenhagen by Simon Pauli, and the same year the thoracic duct was observed by Mentel for the first time since it was described by Eustachius. Five years after (1634), John WesEng, professor of anatomy and surgery at Venice, gave the first deEneation of the lacteals from the human subject, and evinced more accurate knowledge than his predecessors of the thoracic duct and the lymphatics. Highmore in 1637 demonstrated unequivocally the difference between the lacteals and the mesenteric veins; and though some perplexity was occasioned by the discovery of the pancreatic duct by Wirsung, this mistake was corrected by Thomas Bartholin; and the discovery by Pecquet in .1647 of the common trunk of the lacteals and lymphatics, and of the course which the chyle foUows to reach the blood, may be regarded as the last of the series of isolated facts by the generalisation of which the extent, distribution, and uses of the most important organs of the animal body were at length developed.

To complete the history of this part of anatomical science one step yet remained,—the distinction between the lacteals and lymphatics, and the discovery of the termination of the latter order of vessels. The honour of this discovery is divided between Jolyffe, an English anatomist, and Olaus Rudbeck, a young Swede. The former, according to the testimony of Glisson and Wharton, was aware of the distinct existence of the lymphatics in 1650, and demonstrated them as such in 1652. It is nevertheless doubtful whether he knew them much before the latter period; and it is certain that Rudbeck observed the lymphatics of the large intestines, and traced them to glands, on the 27th January 1651, after he had, in the course of 1650, made various erroneous conjectures regarding them, and, Eke others, attempted to trace them to the liver. The following year he demonstrated them in presence of Queen Christina, and traced them to the thoracic duct, and the latter to the subclavian vein. Their course and distribution were still more fuUy investigated by Thomas BarthoEn, Wharton, Swammerdam, and Blaes, the last two of whom recognised the existence of valves; while Antony Nuck of Leyden, by rectifying various errors of his predecessors, and adding several new and valuable observations, rendered this part of anatomy much more precise than formerly.

After this period anatomists began to study more minutely the organs and textures. Francis GEsson distinguished himseif by a minute description of the Ever, and a clearer account of the stomach and intestines, than had yet been given. Thomas Wharton investigated the structure of the glands with particular care; and though rather prone to indulge in fanciful generalisation, he developed some interesting views of these organs; while Charleton, who appears to have been a person of great genius, though addicted to hypothesis, made some good remarks on the communication of the arteries with the veins, the foetal circulation, and the course of the lymphatics. But the circumstance which chiefly distinguished the history of anatomy at the beginning of the seventeenth century was the appearance of Thomas Willis, who rendered himself eminent not only by good researches on the brain and nerves, but by many judicious observations on the structure of the lungs, the intestines, the blood-vessels, and the glands. His anatomy of the brain and nerves is so minute and elaborate, and abounds so much in new information, that the reader is struck by the immense chasm between the vague and meagre notices of his predecessors, and the ample and correct descriptions of WilEs. This exceUent work, however, is not the result of his own personal and unaided exertions; and the character of WiUis derives additional lustre from the candid avowal of his obligatione to Wren and Millington, and, above all, to the diligent researches of his fellow-anatomist Richard Lower.

WiUis was the first who numbered the cranial nerves in the order in which they are now usuaUy enumerated by anatomists. His observation of the connection of the eighth pair with the slender nerve which issues from the beginning of the spinal chord is known to aU. He remarked the paraUel lines of the mesolobe, afterwards minutely described by Vicq d’Azyr. He seems to have recognised the communication of the convoluted surface of the brain and that between the lateral cavities beneath the fornix. He described the corpora striata and optic thalami; the four orbicular eminences, with the bridge, which he first named annular protuberance; and the white mammillary eminences, behind the infundibulum. In the cerebellum he remarks the arborescent arrangement of the white and grey matter, and gives a good account of the internal carotids, and the communications which they make with the branches of the basilar artery.

About the middle of the 17 th century Rt. Hooke and Nehemiah Grew employed the simple microscope in the minute examination of plants and animals; and the Dutch philosopher Leeuwenhoeck with great acuteness examined microscopically the solids and fluids of the body, recognised the presence of scales in the cuticle, and discovered the corpuscles in the blood and milk, and the spermatozoa in the seminal fluid. The researches of Malpighi also tended greatly to improve the knowledge of minute structure. He gave the first distinct ideas on the organisation of the lung, and the mode in which the bronchial tubes and vessels terminate in that organ. By the microscope he traced the transition of the arteries into the veins, and saw the movements of the blood corpuscles in the capillaries. He endeavoured to unfold, by dissection and microscopic observation, the minute structure of the brain. He studied the structure of bone, he traced the formation and explained the structure of the teeth; and his name is to this day associated with the discovery of the deeper layer of the cuticle and the Malpighian bodies in the spleen and kidney. In these difficult inquiries the observations of Malpighi are in general faithful, and he may be regarded as the founder of histological anatomy.

Nicolas Steno described with accuracy the lacrymal gland and passages, and rediscovered the parotid duct. Bellini studied the structure of the kidneys, and described the tongue and tonsils with some care; and Drelincourt laboured to investigate the changes effected on the uterus by impregnation, and to elucidate the formation of the foetus. The science might have derived still greater advantages from the genius of Regnier de Graaf, who investigated with accuracy the structure of the pancreas and of the organs of generation in both sexes, had he not been cut off at the early age of thirty-two. Lastly, Wepfer, though more devoted to morbid anatomy, made, nevertheless, some just observations on the anatomical disposition of the cerebral vessels, the glandular structure of the liver, and the termination of the common duct in the duodenum.

The appearance of Frederic Ruysch, who was born in 1638, and became professor of anatomy at Amsterdam in 1665, gave a new impulse to anatomical research, and tended not only to give the science greater precision, but to extend its limits in every direction. The talents of Ruysch are said to have been developed by accident. To repel the audacious and calumnious aspersions with which De Bils attacked De le Boe and Van Horne, Ruysch i published his tract on the valves of the lymphatics, which completely established his character as an anatomist of originality and research. This, however, is the smallest of his services to the science. The art of injecting, which had been originally attempted by Eustachi and Varoli, and was afterwards rudely practised by Glisson, Bellini, and Willis, was at length carried to greater perfection by De Graaf and Swammerdam, the former of whom injected the spermatic vessels with mercury and variously - coloured liquors; while the latter, by employing melted wax with other ingredients, made the first approach to the refinements of modern anatomy. By improving this idea of using substances which, though solid, may be rendered fluid at the period of injecting, Ruysch carried this art to the highest perfection.

By the application of this happy contrivance he was enabled to demonstrate the arrangement of minute vessels in the interior of organs which had escaped the scrutiny of previous anatomists. Scarcely a part of the human body eluded the penetration of his syringe; and his discoveries were proportionally great. His account of the valves of the lymphatics, of the vessels of the lungs, and their minute structure; his researches on the vascular structure of the skin, of the bones, and their epiphyses, and their mode of growth and union; his observations on the spleen, the glans penis, the clitoris, and the womb impregnated and unimpregnated, were but a limited-part of his anatomical labours. He studied the minute structure of the brain; he demonstrated the organisation of the choroid plexus; he described the state of the hair when affected with Polish plait; he proved the vascular structure of the teeth; he injected the dura mater, the pleura, the pericardium, and peritoneum; he unfolded the minute structure of the conglomerate glands; he investigated thatι of the synovial apparatus placed in the interior of the joints; and he discovered several curious particulars relating to the lacteals, the lymphatics, and the lymphatic glands.

Meanwhile, Meibomius rediscovered the palpebral glands, which were known to Casserius; Swammerdam studied the action of the lungs, described the structure of the human uterus, and made numerous valuable observations on the coeca and pancreatoid organs of fishes; and Kerck-ringius laid the foundation of a knowledge of the process of ossification. John Conrad Brunner, in the course of experiments on the pancreas, discovered the glands of the duodenum named after him, and Conrad Peyer described the solitary and agminated glands of the intestinal canal. Leonard Tassin, distinguished for original observation, rendered the anatomical history of the brain more accurate than heretofore, and gave particular accounts of the intestinal tube, the pancreatic duct, and the hepatic ligaments.

That France might not be without participation in the glory of advancing the progress of anatomical knowledge, the names of Duverney and Vieussens are commemorated with distinction. Duverney, born in 1648, and first introduced into public life in 1676 in the Royal Academy of Sciences, decorated with the honorary title of professor of anatomy to the Dauphin, and appointed in 1679 professor at the Jardin Royal, distinguished himself by the first accurate account of the organ of hearing, and by his dissections of several animals at the academy, supplied valuable materials for the anatomical details of the natural history of animals published by that learned body. He appears to have been the first who demonstrated the fact that the cerebral sinuses open into the jugular veins, and to have been aware that the former receive the veins of the brain, and are the venous receptacles of the organ. He understood the cerebral cavities and their mode of communication; distinguishes the posterior pillars of the vault from the pedes hippocampi; recognises the two plates of the septum lucidum; and, what is still more remarkable, he first indicates distinctly the decussation of the anterior pyramids of the medulla oblongata—a fact afterwards verified by the researches of Mistichelli, Petit, and Santorini. He studied the ganglions attentively, and gives the first distinct account of the formation, connections, and distribution of the intercostal nerve. It is interesting to remark that his statement that the veins or sinuses of the spinal chord terminate in the vena azygos was verified by the more recent researches of Dupuytren and Breschet, which show that the vertebral veins communicate by means of the intercostal and superior lumbar veins with the azygos and demi-azygos. His account of the structure of bones, and of the progress of ossification, is valuable. He recognised the vascular structure of the spleen, and described the excretory ducts of the pr03tate gland, the verumontanum, and the anteprostates.

One of the circumstances which at this time tended considerably to the improvement of anatomical science was the attention with which Comparative Anatomy was beginning to be cultivated. In ancient times, and at the revival of letters the dissection of the lower animals was substituted for that of the human body; and the descriptions of the organs of the latter were too often derived from the former. The obloquy and contempt in τzhich this abuse involved the study of animal anatomy caused it to be neglected, or pursued with indifference, for more than two centuries, during which anatomists confined their descriptions, at least very much, to the parts of the human body. At this period, however, the prejudice against Comparative Anatomy began to subside; and animal dissection, though not substituted for that of the human body, was employed, as it ought always to have been, to illustrate obscurities, to determine doubts, and to explain difficulties, and, in short, to enlarge and rectify the knowledge of the structure of animal bodies generally.

For this revolution in its favour, Comparative Anatomy was in a great measure indebted to the learned societies which were established about this time in the different countries of Europe. Among these, the Royal Society of London, embodied by charter by Charles II. in 1663, and the Academy of Sciences of Paris, founded in 1665 by Colbert, are undoubtedly entitled to the first rank. Though later in establishment, the latter institution was distinguished by making the first great efforts in favour of Comparative Anatomy; and Perrault, Pecquet, Duverney, and Mery, by the dissections of rare animals obtained from the royal menagerie, speedily supplied valuable materials for the anatomical naturalist. In England, Nehemiah Grew, Edward Tyson, and Samuel Collins cultivated the same department with diligence and success. Grew has left an interesting account of the anatomical peculiarities of the intestinal canal in various animals; Tyson in the dissection of a porpoise, an opossum, and an ourang outang, adduces some valuable illustrations of the comparative differences between the structure of the human body and that of the lower animals; Collins has the merit of conceiving, and executing on an enlarged plan, a comprehensive system, embodying all the information then extant. With the aid of Tyson and his own researches, which were both extensive and accurate, he composed a system of anatomical knowledge in which he not only gives ample and accurate descriptions of the structure of the human body, and the various morbid changes to which the organs are liable, but illustrates the whole by accurate and interesting sketches of the peculiarities of the lower animals. The matter of this work is so excellent that it can only be ascribed to ignorance that it has received so little attention. Though regarded as a compilation, and though indeed much of the human anatomy is derived from Vesalius, it has the advantage of the works published on the Continent at that time, that it embodies most of the valuable facts derived from Malpighi, Willis, and Vieussens. The Comparative Anatomy is almost all original, the result of personal research and dissection; and the pathological observations, though occasionally tinged with the spirit of the times, show the author to have been endowed with the powers of observation and judicious reflection in no ordinary degree.

About this time also we recognise the first attempts to study the minute constitution of the tissues, by the combination of the microscope and the effects of chemical agents. Bone furnished the first instance in which this method was put in use; and though Gagliardi, who undertook the inquiry, had fallen into some mistakes which it required the observation of Malpighi to rectify, this did not deter Clopton Havers and Nesbitt, in England, and Courtial, Du Hamel, and Delasone, and afterwards Hérissant, in France, from resuming (the same train of investigation. The mistakes into which these anatomists fell belong to the imperfect method of inquiry. The facts which they ascertained have been verified by recent experiment, and constitute no unessential part of our knowledge of the structure of bone.

Ten years after the publication of the work of Collins, Henry Ridley, another English anatomist, distinguished himself by a monograph on the brain, which, though not free from errors, contains, nevertheless, some valuable observations. Ridley is the first who distinguishes by name the restiform processes, or the posterior pyramidal eminences. He recognised the figure of the four eminences in the human subject; he remarked the mammillary bodies; and he discovered the sinus which passes under his name.

Raymond Vieussens, by the publication of his great work on neurography in 1684, threw new light on the configuration and structure of the brain, the spinal chord, and the nerves; and gave a description of the arrangement and distribution of the latter more precise than heretofore. Of the formation and connections of the sympathetic nerve especially he gave viewe which have been generally adopted by subsequent anatomists. His new arrangement of the vessels, published in 1705, contains several curious opinions. His observations oh the structure of the heart, published in 1706, and enlarged in 1715, exhibit the first correct views of the intimate structure of an organ which afterwards was most fully developed by the labours of Lancisi and Senac.

To the same period belong the rival publications of Godfrey Bidloo and William Cowper, the latter of whom, however, stained a reputation otherwise good by publishing as his own the engravings of the former. Cowper further distinguished himself by a minute account of the urethral glands, already known to Columbus and Mery; a good description of the intestinal glands, discovered by Brunner and Peyer; and by demonstrating the communication of the arteries and veins of the mesentery.

The anatomical genius of Italy, which had slumbered since the death of Malpighi, was destined once more to revive in Lancisi, Valsalva, and his illustrious pupils Santorini and Morgagni Valsalva especially distinguished himself by his description of the structure of the ear, which, in possessing still greater precision and minuteness than that of Duverney, is valuable in setting the example of rendering anatomy altogether a science of description. Santorini, who was professor at Venice, was no unworthy friend of Valsalva and Morgagni. His anatomical observations, which relate to the muscles of the face, the brain, and several of the nerves, the ducts of the lacrymal gland, the nose and its cavities, the larynx, the viscera of the chest and belly, and the organs of generation in the two sexes, furnish beautiful models of essays, distinguished for perspicuity, precision, and novelty, above anything which had then appeared. These observations, indeed, which bear the impress of accurate observation and clear conception, may be safely compared with any anatomical writings which have appeared since. Those on the brain are particularly interesting. Morgagni, though chiefly known as a pathological anatomist, did not neglect the healthy structure. His Adversaria, which appeared between 1706 and 1719, and his Epistles, published in 1728, contain a series of observations to rectify the mistakes of previous anatomists, and to determine the characters of the healthy structure of many parts of the human body. Many parts he describes anew, and indicates facts not previously observed. All his remarks show how well he knew what true anatomical description ought to be. In this respect, indeed, the three anatomists now mentioned may be said to have anticipated their contemporaries nearly a century; for, while other authors were satisfied with giving loose and inaccurate or meagre notices of parts, with much fanciful supposition, Valsalva, Santorini, and Morgagni laboured to determine with precision the anatomical characters of the parts which they describe.

The same character is Sue to "Winslow, a native of Denmark, but, as pupil and successor of Duverney, as well as a convert to Catholicism, naturalised in France, and finally professor of anatomy at the Royal Gardens. His exposition of the structure of the human body is distinguished for being not only the first treatise of descriptive anatomy, divested of physiological details and hypothetical explanations foreign to the subject, but for being a close description derived from actual objects, without reference to the writings of previous anatomists. About the same time Cheselden in London, the first Monro in Edinburgh, and Albinus in Leyden, contributed by their several treatises to render anatomy still more precise as a descriptive science. The Osteographia of the first-mentioned was of much use in directing attention to the study of the skeleton and the morbid changes to which it is liable. This work, however, magnificent as it was,, was excelled by that of Albinus, who, in 1747, published engravings descriptive of the bones and muscles, which perhaps will never be surpassed either in accuracy of outline or beauty of execution. The several labours of this author, indeed, constitute an important era in the history of the science. He was the first who classified and exhibited the muscles in a proper arrangement, and applied to them a nomenclature which is still retained by the consent of the best anatomists. He gives a luminous account of the arteries and veins of the intestines, represents with singular fidelity and beauty the bones of the foetus, inquires into the structure of the skin and the cause of its colour in different races; represents the changes incident to the womb in different periods of pregnancy, and describes the relations of the thoracic duct and the vena azygos with the contiguous parts. Besides these large and magnificent works, illustrated by the most beautiful engravings, six books of Academical Annotations were the fruits of his long and assiduous cultivation of anatomy. These contain valuable remarks on the sound structure and morbid deviations of numerous parts of the human body.

Albinus found a worthy successor in his pupil Albert Von Haller, who, with a mind imbued with every department of literature and science, directed his chief attention, nevertheless, to the cultivation of anatomical and physiological knowledge. Having undertaken at an early age (twenty-one) to illustrate, with commentaries, the physiological prelections of his preceptor Boerhaave, he devoted himself assiduously to the perusal of every work which could tend to facilitate his purpose; and as he found numerous erroneous or imperfect statements, and many deficiencies to supply, he undertook an extensive course of dissection of human and animal bodies to obtain the requisite information. During the seventeen years he was professor at Göttingen, he dissected 400 bodies, and inspected their organs with the utmost care. The result of these assiduous labours appeared at intervals in the form of dissertations by himself, or under the name of some one of his pupils, finally published in a collected shape, between 1746 and 1751 (Disputationes Λnatomicce Selectiores y ), and in eight numbers of most accurate and beautiful engravings, representing the most important parts of the human body, e.g., the diaphragm, the uterus, ovaries, and vagina, the arteries of the different regions and organs, with learned and critical explanatory observations. He verified the observations that in the foetus the testicles lie in the abdomen, and showed that their descent into the scrotum may be complicated with the formation of congenital hernia. Some years after, when he had retired from his academical duties at Göttingen, he published, between 1757 and 1765, the large and elaborate work which, with singular modesty, he styled Elements of Physiology. This work, though professedly devoted to physiology, rendered, nevertheless, the most essential services to anatomy. Haller, drawing an accurate line of distinction between the two, gave the most clear, precise, and complete descriptions of the situation, position, figure, component parts, and minute structure of the different organs and their appendages. The results of previous and coeval inquiry, obtained by extensive reading, he sedulously verified by personal observation; and though he never rejected facts stated on credible authorities, he in all cases laboured to ascertain their real value by experiment. The anatomical descriptions are on this account not only the most valuable part of his work, but the most valuable that had then or for a long time after appeared. It is painful, nevertheless, to think that the very form in which this work is composed, with copious and scrupulous reference to authorities, made it be regarded as a compila· tion only; and that the author was compelled to show, by a list of his personal researches, that the most learned work ever given to the physiologist was also the most abundant in original information.

With the researches of Haller it is proper to notice those of his contemporaries, John Frederick Meckel, J. FT. Lieberkühn, and his pupil John Godfrey Zinn. The first, who was professor of anatomy at Berlin, described the Casserian ganglion, the first pair of nerves and its distribution, and that of the facial nerves generally, and discovered the spheno-palatine ganglion. He made some original and judicious observations on the tissue of the skin and the mucous net; and above all, he recognised the connection of the lymphatic vessels with the veins,—a doctrine which, after long neglect, was revived by Fohmann and Lippi. He also collected several valuable observations on the morbid states of the heart and brain. Lieberkühn published in 1745 a dissertation on the villi and glands of the small intestines. Zinn, who was professor of medicine at Göttingen, published a classical treatise on the eye, which demonstrated at once the defects of previous inquiries, and how much it was possible to elucidate, by accurate research and precise description, the structure of one of the most important organs of the human frame. It was republished after his death by Wrisberg. About the same time Weitbrecht gave a copious and minute account of the ligaments, and Μ. Lieutaud, who had already laboured to rectify many errors in anatomy, described with care the structure and relations of the heart and its cavities, and rendered the anatomy of the bladder very precise, by describing the triangular space and the mammillary eminence at its neck.

The study of the minute anatomy of the tissues, which had originally been commenced by Leeuweπhoeck, Malpighi, and Ruysch, began at this period to attract more general attention. De Bergen had already demonstrated the ι general distribution of cellular membrane, and showed that it not only incloses every part of the animal frame, but forms the basis of every organ,—a doctrine which was adopted, and still more fully expanded, by his friend Haller, in opposition to what was asserted by Albinus, who maintains that each part has a proper tissue. William Hunter at the same time gave a clear and ingenious statement of the difference between cellular membrane and adipose tissue, in which he maintained the general distribution of the former, and represented it as forming the serous membranes, and regulating their physiological and pathological properties,—doctrines which were afterwards confirmed by his brother John Hunter. A few years after, the department of general anatomy first assumed a substantial form in the systematic view of the membranes and their mutual con- 1 nections traced by Andrew Bonn of Amsterdam. In his inaugural dissertation De Continuationibus Membranarum, published at Leyden in 1763, this author, after some preliminary observations on membranes in general and their structure, and an exposition of that of the skin, traces its transition into the mucous membranes and their several divisions. He then explains the distribution of the cellular membrane, the aponeurotic expansions, and the periosteum and perichondrium, by either of which, he shows, every bone of the skeleton is invested and connected. He finally gives a very distinct view of the arrangement of the internal membranes of cavities, those named serous and fibro-serous, and the manner of their distribution over the contained organs. This essay, which is a happy example of generalisation, is remarkable for the interesting general views of the structure of the animal body which it exhibits; and to Bonn belongs the merit of sketching the first outlines of that system which it was reserved for the genius of Bichat to complete and embellish. Lastly, Bordeu, in an elaborate essay on the mucous tissue, or cellular organ, as he terms it, brought forward some interesting views of the constitution, nature, and extent of the cellular membrane.

Though anatomy was hitherto cultivated with much success as illustrating the natural history and morbid states of the human body, yet little had been done for the elucidation of local diseases, and the surgical means by which they may be successfully treated. The idea of applying anatomical knowledge directly to this purpose appears to have originated with Bernardin Genga, a Roman surgeon, who published in 1672, at Rome, a work entitled Surgical Anatomy, or the Anatomical History of the Bones and Muscles of the Human Body, with the Description of the Bloodrvessels. This work, which reached a second edition in 1687, is highly creditable to the author, who appears to have studied intimately the mutual relations of different parts. It is not improbable that the example of Genga led Palfyn, a surgeon at Ghent, to undertake a similar task about thirty years after. For this, however, he was by no means well qualified; and the work of Palfyn, though bearing the name of Surgical Anatomy, is a miserable compilation, meagre in details, inaccurate in description, and altogether unworthy of the honour of being republished, as it afterwards was by Antony Petit.

While these two authors, however, were usefully employed in showing what was wanted for the surgeon, others were occupied in the collection of new and more accurate facts. Albinus, indeed, ever assiduous, had, in his account of the operations of Rau, given some good sketches of the relative anatomy of the bladder and urethra; and Cheselden had already, in his mode of cutting into the urinary bladder, shown the necessity of an exact knowledge of the relations of contiguous parts. The first decided application, however, of this species of anatomical research it was reserved for a Dutch anatomist of the 18th century to make. Peter Camper, professor of anatomy at Amsterdam, published in 1760 and 1762 his anatomico-pathological demonstrations of the parts of the human arm and pelvis, of the diseases incident to them, and the mode of relieving them by operation, and explained with great clearness the situation of the blood-vessels, nerves, and important muscles. His remarks on the lateral operation of lithotomy, which contain all that was then known on the subject, are exceedingly interesting and valuable to the surgeon. It appears, further, that he was the first who examined anatomically the mechanism of ruptures, his delineations of which were published in 1801 by Sδmmering. Camper also wrote some important memoirs on Comparative Anatomy, and he was the author of a well-known work on the Relations of Anatomy to the Fine Arts.

The attention of anatomists was now directed to the elucidation of the most obscure and least explored parts of the human frame—the lymphatic vessels and the nerves. Although, since the first discovery of the former by Asellius, Rudbeck, and Pecquet, much had been done, especially by Ruysch, Nuck, Meckel, and Haller, many points, notwithstanding, relating to their origin and distribution in particular organs, and in the several classes of animals, were imperfectly ascertained or entirely unknown. William Hunter investigated their arrangement, and proposed the doctrine that they are absorbents; and John Hunter, who undertook to demonstrate the truth of this hypothesis by experiment, discovered, in 1758, lymphatics in the neck in birds. As the doctrine required the existence of this order of vessels, not only in quadrupeds and birds, but in reptiles and fishes, the inquiry attracted attention among the pupils of Hunter; and William Hewson at length communicated, in December 1768, to the Royal Society of London, an account of the lacteals and lymphatics in birds, fishes, and reptiles, as he had discovered and demonstrated them. The subject was about the same time investigated by the second Monro, who indeed claimed the merit of discovering these vessels in the classes of animals now mentioned. But whatever researches this anatomist may have instituted, Hewson, by communicating his observations to the Royal Society, must be allowed to possess the strongest as well as the clearest claim to discovery. The same author, in 1774, gave the first complete account of the anatomical peculiarities of the lymphatic system in man and other animals, and thereby supplied an important gap in this department. Hewson is the first who distinguishes the lymphatics into two orders—the superficial and the deep—both in the extremities and in the internal organs. He also studied the structure of the intestinal villi, in which he verified the observations of Lieberkühn; and he made many important observations on the corpuscles of the lymph and blood. He finally applied his anatomical discoveries to explain many of the physiological and pathological phenomena of the animal body. Ten years after, John Sheldon, another pupil of Hunter, gave a second history and description of the lymphatics, which, though divested of the charm of novelty, contains many interesting anatomical facts. He also examined the structure of the villi.

Lastly, Cruikshank, in 1786, published a valuable history of the anatomy of the lymphatic system, in which he maintains the accuracy of the Hunterian doctrine, that the lymphatics are the only absorbents; gave a more minute account than heretofore of these vessels, of their coats and valves; and explained the structure of the lymphatic glands. He also injected the villi, and examined them microscopically, verifying most of the observations of Lieberkühn. The origin of the lymphatics he maintains rather by inference than direct demonstration. To these three works, though in other respects very excellent, it is a considerable objection that the anatomical descriptions are much mixed with hypothetical speculation and reasonings on properties, and that the facts are by no means always distinguished from mere matters of opinion. At the same time Haase published an account of the lymphatics of the skin and intestines, and the plexiform nets of the pelvis.

To complete this sketch of the history of the anatomy of the lymphatic system, it may be added that Mascagni, ’. who had been engaged from the year 1777 to 1781 in the same train of investigation, first demonstrated to his pupils several curious facts relating to the anatomy of the lymphatic system. When at Florence in 1782 he made several preparations, at the request of Peter Leopold, Grand Duke of Tuscany; and when the Royal Academy of Sciences at Paris announced the anatomy of this system for their prize essay appointed for March 1784, Mascagni resolved on communicating to the public the results of his researches—the first part of his commentary, with four engravings. Anxiety, however, to complete his preparations detained him at Florence till the close of 1785; and from these causes his work did not appear till 1787. These delays, however, unfavourable as they were to his claims of priority to Sheldon and Cruikshank, were on the whole advantageous to the perfection of his work, which is not only the most magnificent, but also the most complete that ever was published on the lymphatics. In his account of the vessels and their valves he confirms some of Hewson’s observations, and rectifies others. Their origin he proves by inference much in the same manner as Cruikshank; but he anticipates this author in the account of the glands, and he gives the most minute description of the superficial and deep lymphatics, both in the members and in the internal organs.

General accounts of the nerves had been given with various degrees of accuracy by Willis, Vieussens, Winslow, and the first Monro; and the subject had been much rectified and improved by the indefatigable Haller. The first example of minute descriptive neurography was given in 1748 by John Frederick Meckel, whose account of the fifth pair, and of the nerves of the face, will long remain a lasting proof of accuracy and research. The same subject was investigated in 1765 by Hirsch, and in 1777 by Wrisberg. In 1766 Metzger examined the origin, distribution, and termination of the first pair, —a point which was afterwards very minutely treated by Scarpa in his anatomical disquisitions, published in 1780; and the internal nerves of the nostrils were examined in 1791 by Haase. The optic nerve, which had been studied originally by Varoli, and afterwards by Mery, Duverney, Henkel, Moe∏er, Hein, and Kaldschmid, was examined with extreme accuracy, with the other nerves of the organ of vision, by Zinn, in his elaborate treatise. The phrenic nerves and the oesophageal branches of the eighth pair were studied by Haase; the phrenic, the abdominal, and the pharyngeal nerves, by Wrisberg; those of the heart most minutely by Andersch; and the origins, formation, and distribution of the intercostal nerve, by Iwanoff, Ludwig, and Girardi The labours of these anatomists, however, were eclipsed by the splendid works of Walter on the nerves of the chest and belly; and those of Scarpa on the distribution of the 8th pair, and splanchnic nerves in general. In minuteness of description and in beauty of engraving these works have not yet been equalled, and will never perhaps be surpassed. About the same time, Scarpa, so distinguished in every branch of anatomical research, investigated the minute structure of the ganglions and plexuses. The anatomy of the brain itself was also studied with great attention by the second Monro, Malacarne, and Vicq d’Azyr.

Lastly, the anatomy of the gravid uterus, which had been originally studied by Albinus, Roederer, and Smellie, was again illustrated most completely by William Hunter, whose engravings will remain a lasting memorial of scientific zeal and artistic talent.

The perfection which anatomical science attained in the last ten years of the eighteenth and during the present century is evinced not only in the improved character of the systems published by anatomists, but in the enormous advance which has taken place in the knowledge of the minute structure of the animal tissues, of the development of the tissues and organs, and of the modifications in form and structure exhibited by various groups of animals.

The first who gave a good modern system was Sabatier; but his work was speedily eclipsed by the superior merits of the treatises of Sömmering, Bichat, and Portal. The excellent work by Samuel Thomas Sömmering, originally published in the German language, between the years 1791 and 1796; then in the Latin language, between the years 1794 and 1800; and in a second edition in the German language in 1800 and 1801, maintaining the high character which it first possessed for clear arrangement, accurate description, and general precision, was, between the years 1841 and 1844, republished in eight volumes at Leipsic by Bischoff, Henle, Huschke, Theile, Valentin, Vogel, and Wagner, with suitable additions, and a large amount of new and accurate information. In this edition Rudolph Wagner gives, in the first division of the first volume, the life, correspondence, and literary writings of Sömmering; and in the second volume the anatomy of the bones and ligaments. The third volume contains the anatomy of the muscles and the vascular system by Theile. Valentin devotes one volume, the fourth, to the minute anatomy of the nervous system and its parts, as disclosed by careful examination by the microscope; and it must be allowed that the author has been at great pains to present just views of the true anatomy of the brain, the spinal cord, the nervous branches, and the ganglia. In the fifth volume, Huschke of Jena gives the anatomical history of the viscera and the organs of the senses, a department which had been left in some degree incomplete in the original, but for one division of which the author had left useful materials in his large figures already mentioned. In the sixth volume, an entire and complete system of general anatomy, deduced from personal observation and that of other careful observers, the materials being in general new, and in all instances confirmed and rectified, is given by Prof. Henle. The seventh volume contains the history of the process of development in mammalia and man, by Th. L. W. Bischoff. The eighth volume treats of the pathological anatomy of the human body, by Julius Vogel, but contains only the first division, relating to the generalities of the subject. This, which is probably the most accurate as it is the most elaborate system of anatomical knowledge up to the date of its publication in 1844, was translated into the French language by Jourdan, and published in 1846 under the name of Encyclopédie Anatomique. The eighth volume was translated into English in the year 1847,

The Anatomie Générale of Bichat is a monument of his: philosophical genius which will last as long as the structure and functions of the human body are objects of interest. His Anatomie Descriptive is distinguished by clear and natural arrangement, precise and accurate description, and the general ingenuity with which the subject is treated. The physiological observations are in general correct, often novel, and always highly interesting. It is unfortunate, however, that the ingenious author was cut off prematurely during the preparation of the third volume. The later volumes are, however, pervaded with the general spirit by which the others are impressed, and are highly creditable to the learning, the judgment, and the diligence of MM. Roux and Buisson. The system of Portal is a valuable 1 and correct digest of anatomical and pathological know- f ledge, which, in exact literary information, is worthy of i the author of the Histoire de V Anatomie et de la Chirurgie, and, in accuracy of descriptive details, shows that Μ. Portal trusts not to the labours of his predecessors only. Boyer published in 1803 a complete treatise on Descriptive Anatomy. Cloquet formed, on the model of the Anatomie Descriptive of Bichat, a system in which he avails himself of the literature and precision of Sömmering and the details of Portal. An English translation of this work was prepared by Dr Knox. Cruveilhier published in 1834-35 a good general treatise on Descriptive Anatomy, which was translated into English, and published as a part of The Library of Medicine. Cruveilhier’s treatise has passed through several editions. About the same time Blandin published an elementary work on Descriptive Anatomy, and a useful treatise on Topographical Anatomy. But the most elaborate system of human anatomy which has proceeded from the French school is the great treatise of Bourgery, illustrated by numerous large and beautifully-coloured plates of the parts and organs. It consists of two divisions, one on Medical and Physiological Anatomy; the other on Surgical Anatomy.

J. F. Meckel published between 1815 and 1820 a manual of Descriptive Anatomy which combines the philosophical generalisations of Bichat with the precise description and pathological knowledge of Portal. During the succeeding thirty years excellent systematic treatises in the German language were prepared by Rosenmüller, C. F. P. Krause, Frederick Hildebrand (the 4th edition of which was edited in 1830 by the eminent anatomist E. H. Weber), and Fred. Arnold. In 1846 Joseph Hyrtl published a system of Human Anatomy, and in the following year a manual of Topographical and Surgical Anatomy, both of which, but more especially the latter, have gone through several editions. Luschka, the professor of anatomy in Tübingen, has prepared a valuable treatise on Regional Anatomy, in which attention is particularly directed to the relations of the parts which are of interest to the physician and surgeon. The text-book by Hermann Meyer of Zurich is also worthy of mention as a work in which the mechanical construction and uses of parts are described with great care. Henle’s treatise on Human Anatomy, the pubheation of which was commenced in 1855, though the last volume was not completed until 1873, is, however, the most complete work on the subject which has as yet issued from the German press during the latter half of the present century. It is remarkable not only for the elaborate description of the organs and tissues of the body, and the ample references to the labours of other observers, but for r the number and beauty of the wood engravings.

jj In Great Britain systematic treatises on Human Anatomy . were published in the earlier part of the present century by Andrew Fyfe, John Bell, the third Monro, and John i Gordon, all of whom were teachers in the Edinburgh school. In London, Jones Quain prepared an excellent text-book, which, under a succession of editors, who have kept each new edition on a level with the advancing tide of anatomical knowledge, has been much esteemed not only for the clearness of its descriptions, but for the soundness of its information on the various branches of human Systematic Anatomy. The 7th edition, under the editorial superintendence of Professors Sharpey, Allen Thomson, and Cleland, appeared between 1864 and 1867. The passing of the Anatomy Act in 1832, by affording facilities for the pursuit of practical anatomy, gave a great stimulus to its study in this country, and to facilitate the acquisition of a knowledge of the subject many text-books have been published. The most important are Harrison’s Dublin Dissector, and the well-known Demonstrations of Anatomy by Prof. Ellis. The increased importance attached by surgeons to a precise acquaintance with the knowledge of those regions in which operations have most frequently to be performed, has led to the production of valuable special works on their anatomy. The treatise of Allen Burns on the head and neck, those of Sir Astley Cooper and Sir W. Lawrence on hernia, Morton’s Anatomy of the Surgical Regions, the excellent plates on Surgical Anatomy by Joseph Maclise, and the beautiful drawings by Ford from the dissections of Prof. Ellis, with descriptive letterpress, are highly creditable to British anatomists; whilst the treatise on hernia by Scarpa, and Cloquet’s and Hesselbach’s works on the same subject, reflect credit on the Italian, French, and German schools.

But special treatises have also been written on other departments of human descriptive anatomy. Innes, Sandi-fort, and Barclay published works on the muscles generally; and Sir Charles Bell, in his classical treatise on the Anatomy of Expression, described with care the attachments and action of the muscles of the face. Of late years the variations in the usually described arrangements in the muscular system in man have been carefully inquired into, and numerous memoirs have been written, more especially by M i Whinnie, Hallett, W. Gruber, John Wood, W. Turner, and M i Alister. F. 0. Ward published a work on Human Osteology which is characterised by the minuteness and accuracy of its description; G. Μ. Humphry, a treatise in which the physical, physiological, and pathological aspects of the skeleton are dwelt upon; and Luther Holden, a profusely-illustrated work on the same subject, in which the surfaces for muscular attachments are carefully delineated. Sir Charles Bell’s engravings of the arteries, Tiedemann’s more elaborate plates, and Harrison’s admirable description of these vessels, all deserve notice. But the most complete work on the Anatomy of the Arteries which has yet appeared is that by Richard Quain, which consists of eighty-seven large plates, with 543 pages of descriptive letterpress. It will long continue a standard work on the subject.

Numerous treatises on the anatomy of the nervous system have been published. In Germany the brothers Wenzel, Reil, Tiedemann, Gall and Spurzheim, Arnold, and Reichert have prepared works on the descriptive anatomy of the great nerve centres, not only in man but in various animals; and by Tiedemann, Reichert, and Ecker, the development of the brain has been especially studied. In Italy the memoirs of Rolando on the anatomy of the brain, and of Bellingeri on the spinal cord and its nerves, are of importance. From the French school th⅛ writings of Serres, of Foville, of Leuret and Gratiolet, have thrown much new light on the structure of the brain. In Great Britain, Sir Charles Bell, in his great work on the nervous system, developed and established the truth of the separate nature of the nerves of sensation and motion. In 1836, and again in 1847, Samuel Solly published an instructive treatise on the anatomy of the brain. Between 1830 and 1834 Joseph Swan published a valuable series of engravings in illustration of the distribution of the nerves, and Robert Lee has especially investigated the arrangement and distribution of the nerves of the heart and uterus. In the Cyclopaedia of Anatomy and Physiology, under the editorial superintendence of Dr Robert B. Todd, original memoirs, not only on human but comparative anatomy, by eminent writers, have appeared, and have done much to diffuse a knowledge of anatomical science.

The improvement which has been effected in the construction of the compound microscope during the fifty years subsequent to 1822, has contributed in no small degree to enable anatomists to obtain more correct information on the intimate structure of different organs and tissues of the animal body. For the first twenty years of the nineteenth century, opticians and instrument-makers had at intervals endeavoured to render the compound microscope at once an instrument of greater power and more free from sources of error and optical illusion than it had hitherto been possible to obtain it. Two defects, however, still adhered to the compound microscope. The instrument was not achromatic; and a considerable degree of spherical aberration uncorrected rendered the image indistinct.

Between 1812 and 1815 Professor Amici of Modena had attempted to construct an achromatic object-glass of one single lens, but found that this was impracticable. Μ. Selligues of Paris, in 1823, after various trials, found that this could be done by making the object-glass consist of four achromatic compound lenses, each of which was composed of two single lenses. This method was carried into practice and improved by the two MM. Chevalier of Paris. About the same time Dr Goring in London, with the aid of Mr Tulley and Mr Pritchard, constructed compound microscopes upon a similar principle.

By the labours of these practical opticians, and the suggestions of various scientific persons, as Sir John Herschel, Sir Richard Airy, Mr Barlow, one great defect of the compound microscope was obviated. The effects of spherical aberration were in the next place overcome in a very simple manner by the experiments of Mr Joseph Jackson Lister, who had early observed that the combined achromatic object-glasses devised by Selligues were fixed in their cells with the convex side foremost, a most improper position, as it renders the spherical errors very great. This gentleman found, after various trials, that. by placing three or more achromatic glasses with their plane surfaces directed foremost, it was possible to correct completely all spherical aberration.

This fact was made known in the beginning of the year 1830; and by its application the compound microscope was brought to a high degree of perfection as an achromatic instrument in 1831 and 1832, and became the means of affording valuable assistance in anatomical inquiries. The use of the microscope in anatomy, which had in the times of Malpighi, Leeuwenhoeck, William Cowper, Baker, Fontana, Hewson, and the second Monro, been much cultivated, but had afterwards, from the imperfection of the instrument and the illusions to which it not unfrequently gave rise, been neglected, now became so general and so necessary, that since the year 1832 minute structural anatomy has been, if not created anew, at least most thoroughly revised. The amount of knowledge has been enormously increased; that which was already possessed has been rendered greatly more accurate and precise.

It is impossible in this place to name the authors of all the valuable monographs which have appeared during the past forty years, but those who have especially advanced the progress of our knowledge of the minute structure of the tissues and organs may be referred to. Johannes Müller in 1830 published an elaborate commentary on the minute structure of the glands, the first work in which the anatomy of these organs was examined and elucidated in a comprehensive and systematic manner. Ehrenberg explained the structure of numerous infusoria, and disclosed the peculiarities of many other structures, animal, vegetable, and mineral, which had previously eluded the most skilful researches. Francis Kiernan, in 1833, gave the first correct account of the minute anatomy of the liver. Schleiden in 1838, and Schwann in 1839, published most important generalisations on the cellular structure of vegetable and animal organisms. Martin Barry communicated new facts on the structure of the ovum and on the structure of cells generally. John Goodsir laid great emphasis on the office of the nucleus in the nutrition, growth, and reproduction of cells, and on the arrangement of the cells within an organism into departments or territories. Virchow, by his researches into the connective tissues, has still further developed the idea of the cellular structure of the animal organism, and the importance of cells in the performance of physiological and pathological processes. Lionel Beale attributed both to the nucleus and to the substance of the cell immediately surrounding it important functional properties. Max Schultze showed the identity in nature between the sarcode substance of the lower animal organisms and the contents of the cells in the higher animals, and applied to these substances the common term protoplasm, which had previously been introduced by Hugo von Mohl to designate a similar material in the vegetable celL

The minute structure and development of bone has been carefully investigated by J. Goodsir, W. Sharpey, H. Müller, C. Gegenbaur, and A. Kölliker; that of muscle by Bowman, Kölliker, and Sharpey; of nerve by Schwann, Remak, Stilling, Gerlach, Lockhart Clarke, and Deiters; of cartilage by Schwann and Schultze; of thé blood and blood-vessels by Henle, Gulliver, Quekett, Paget, and Wharton Jones; of the mucous membranes by Bowman; of the serous membranes by Henle, Recklinghausen, Ludwig, and Klein; of the teeth by Retzius, A. Nasmyth, J. Goodsir, J. Tomes, R. Owen, Czermak, Huxley, and Waldeyer. The structure of the lungs has been investigated by Addison, Rainey, and Rossignol; of the kidney by Bowman, Henle, and Schweiggerseidel; of the liver by Beale and Hering; of the spleen by Sanders, Gray, Billroth, and W. Müller; of the testicle by A. Cooper, Kölliker, and Henle; of the ovary by Pflüger and Waldeyer; of the thymus by A. Cooper and Simon; of the stomach and intestines by Kölliker, Brinton, and Frey; of the placenta by Eschricht, Reid, Sharpey, Goodsir, Van der Kolk, Virchow, Farre, Priestley, Rolleston, Ercolani, and Turner; of the organs of sense by Henle, Bowman, His, H. Müller, Schultze, Corti, Reissner, and Deiters.

The general results of the labours of these and other investigators have been from time to time incorporated into systematic treatises on microscopic anatomy, of which reference may more especially be made to those prepared by J. Berres, F. Gerber, A. Hill Hassall, A. Kölliker, W. Sharpey, W. Bowman, F. Leydig, Frey, and S. Stricker. Side by side with these inquiries into the structure and development of the tissues, the evolution of the embryo out of the fertilised ovum has been carried on. Purkinje, Von Baer, Coste, Wharton Jones, Valentin, R. Wagner, Rathke, J. Müller, Prevost and Dumas, Martin Barry, Reichert, Bischoff, Kölliker, Vogt, Allen Thomson, Owen, Von Siebold, Dujardin, Milne-Edwards, Claparède, Agassiz, Huxley, Kitchen Parker, and Kowalevsky have all contri buted important memoirs on various branches of embryology.

Comparative Anatomy, which during the 18 th century was diligently cultivated by Daubenton, Pallas, Haller, Buffon, John Hunter, and the second Monro, has become during the present century a subject of increased interest, from its intimate connection with the sciences of zoology, physiology, and geology. It has consequently been studied with great zeal and assiduity, and multitudes of monographs, as well as numerous systematic treatises on the anatomy both of the vertebrata and Invertebrata, have been published.

To name even a tithe of the workers and authors who have added to our knowledge of the facts of comparative anatomy would occupy considerable space. It may suffice to refer to those whose writings have contributed most materially to the advance of the science. In France, Cuvier, Dumeril, the Saint-Hilaires, Blanchard, De Blain-ville, H. and Alphonse Milne-Edwards, Gervais, and Gratiolet; in Germany, Meckel, Tiedemann, Von Baer, Spix, Martius, Bojanus, Otto, Carus, J. Müller, Leuckart, Gegenbaur, and Haeckel; in Sweden and Denmark, Retzius and Eschricht; in Holland and Belgium, Van der Kolk, Vrolik, and Van Beneden; in America, Agassiz, Wyman, and Burmeister; in Great Britain, E. Home, A. Carlisle, R. Grant, Richard Owen, J. Barclay, R. Knox, J. Goodsir, G. Busk, Rymer Jones, W. B. Carpenter, T. H. Huxley, G. J. Allman, W. H. Flower, St George Mivart, and J. Murie are names identified with one or moro branches of the subject.

The investigations into the form and structure of animals have led anatomists to search for parts in one animal which correspond with parts in other animals in their mode of development and arrangement, and to evolve from their researches general doctrines of organic forms. The conception entertained by Goethe of the presence of a pre-maxillary element in the human upper jaw because it exists in other vertebrates, and the announcement of the theory of the vertebrate nature of the skull by Goethe and Oken, directed anatomists into a line of inquiry which has been productive of fruitful results, and has exercised a great influence on the progress and direction of biological science. Geoffroy St Hilaire and C. Martins in France; Spix, Carus, Gegenbaur, and Haeckel in Germany; and Owen, Goodsir, Humphry, Huxley, Parker, and Cleland in Great Britain, have all published important memoirs in this department of anatomical research.

The formation of anatomical museums in connection with universities, and elsewhere, by enabling specimens to be accumulated for observation and comparison, has contributed in no small degree to the progress of anatomical science. Pre-eminent amongst these is the collection originally formed by the genius, energy, and self-devoted-ness of John Hunter, which, under the fostering care of the council of the Royal College of Surgeons of England, has been materially augmented in all its departments by a succession of curators—Clift, Owen, Quekett, and Flower. The aid which has been afforded to anatomists in the publication of their researches, more especially in providing plates and other expensive means of illustration, by the learned societies of Europe, and the circulation which has been given to their memoirs through the Transactions and Proceedings of these societies, and through the Journals devoted to anatomical and physiological science, have materially contributed to the diffusion of a knowledge of discoveries, and to the general advance of the science.

SPECIAL ANATOMY OF THE HUMAN BODY.

Man, zoologically speaking, belongs to the Mammalian class of the Vertebrate sub-kingdom, i.e., his young are brought forth alive, and nourished during infancy on milk secreted in mammary or milk-forming glands. In common with all vertebrate organisms, he possesses a spine or vertebral column and a skull, in which are contained the brain and the spinal marrow, and on the ventral surface of the spinal column are situated the several subdivisions of the alimentary canal.

But man possesses certain special or distinctive anatomical characters. The most noticeable, as seen on an external inspection of his body, is his erect position. He is, indeed, the only living creature that can walk or stand erect, i.e., with the axis of the spine vertical; with the hip and knee joints capable of being fully extended, so that the leg is brought into line with the thigh; with the foot so planted on the ground that it rests on the heel behind and on the roots of the toes in front; with the upper limbs so arranged as to act, not as instruments of progression, but of prehension; and with the head so balanced on the top of the spine that the face and eyes look directly to the front. His bones, joints, and muscles are constructed and arranged so as to enable him to preserve the erect attitude without fatigue. In other vertebrata the axis of the spine is oblique or horizontal, the hip and knee joints are permanently bent at a more or less acute angle, the limbs corresponding to the human upper extremities, are, in the form of legs, wings, or fins, instruments of progression, and the head is articulated with the spine at or near the hinder end of the skulL Owing to the oblique or horizontal attitude of the body in the vertebrata generally, and its erect position in man, the terms which are employed in describing the relative position of different parts are. not used in the same sense by the human and comparative anatomist. Thus, parts which are superior, or above other parts, in the human body, are anterior, or in front, in other vertebrata; and parts which are posterior, or behind other parts in man, are superior to them in other vertebrata. To obviate the confusion which must necessarily arise when comparing the human body with that of other vertebrates, certain descriptive terms have been recommended which, may be employed whether the position of the body be erect or non-erect. Thus,, the aspect of parts directed towards the region where the atlas or first vertebra is situated is atlantal, that directed towards the sacrum is sacral, that towards the back is dorsal, that towards the front is ventral or ûoemal. Quite recently the term prae-axial has been introduced as equivalent to atlantal, and post-axial to sacral.

The body may be considered as divided by an imaginary plane, the mesial plane, into two lateral and similar halves, a right and left, so that it exhibits a bilateral symmetry; and the constituent parts are described as being external or internal to each other, according to their relative position to this plane. For descriptive purposes, also, we may subdivide the body into Axial and Appendiculas portions. The Axial part is the stock or stem of the body, and consists of the Head, the Neck, and the Trunk. The trunk is again subdivided into the chest or Thorax, and the belly or Abdomen; and the abdomen is again subdivided into the abdomen proper and the Pelvis. The axial part contains the organs essential to the preservation of life. In the head is lodged the brain, from which the spinal marrow is prolonged down the spinal canal At the sides of the head are the ears, and opening on to the face are the eyes, nostrils, and mouth. Prolonged down the neck are the gullet and windpipe, with the latter of which is associated the organ of voice. Within the chest lie the heart, lungs, and gullet; and in the abdomen are contained the stomach, intestine, liver, spleen, pancreas, kidneys, and other organs concerned in the urinary and generative functions. The Appendicular part forms the limbs, whieh do not contain organs essential to life. In man the limbs are called Upper and Lower—the former are instruments of prehension, the latter of progression. The subdivisions of the body are not homogeneous in structure, but are built up of several systems of organs, each system being characterised not only by peculiarities in form, appearance, and structure, but by possessing special functions and uses. Thus the bones collectively form the Osseous system; the joints the Articulatory system; the muscles, which move the bones at the joints, the Muscular system; and these several systems collectively constitute the organs of Locomotion. The blood and lymph vessels form the Vascular system; the brain, spinal marrow, and nerves, the Nervous system, with which is intimately associated the organs of Sense; the lungs and windpipe, the Respiratory system; the alimentary canal, with the glands opening into it, the Digestive system; the kidneys, bladder, and urethra, the Urinary system; the testicles, spermatic ducts, and penis in the male, with the ovaries, uterus, and clitoris in the female, the Generative or Reproductive system; the skin, with the hair and nails, the Tegumentary system. These various systems are so arrangé with reference to each other as to form an organic whole.

Anatomy of the Organs of Locomotion.

The organs of locomotion consist of the muscles or active organs, and the bones and joints or passive organs. The anatomy of the bones will first attract our attention.

' Osseous System—Osteology—Skeleton. —The word Skeleton (from σκελλω, to dry) signifies literally the dry or hard parts of the body. When used in a limited sense it is applied merely to the bones, but when used in a wider and more philosophic sense it comprises not only the bones or osseous skeleton, but the cartilages and fibrous membranes which complete the framework of the body. The first evidence of a skeleton in the embryo is the appearance of membranes in many parts of which cartilage is developed, and in course of time this cartilage is converted into bone. In some animals, however, as in the cartilaginous fish, the osseous conversion does not take place, and the skeleton remains permanently cartilaginous; and in the very remarkable fish called Lancelet, or Λmphioxus, the skeleton consists almost entirely of fibrous membrane.

The skeleton serves as a basis of support for the soft parts, as affording surfaces of attachment for muscles and as a protection for many delicate organs. In the vertebrata the osseous skeleton is clothed by the muscles and skin, and is technically called an endo-skeleton. In Invertebrata the skeleton is not unfrequently on the surface of the body, and is termed an exo- or dermo-skeleton. In some vertebrates {e.g., the armadillo, tortoise, and sturgeon), in addition to the proper endo-skeleton, skeletal plates are developed in connection with the integument, so that they possess a dermo-skeleton likewise. In some vertebrates, also, a partial skeleton is formed within the substance of some of the viscera— e.g. y in ruminant animals a bone is situated in the heart; in the walrus and other carnivora, in rodents, bats, and some monkeys, a bone lies in the penis; and in the leopard, jackal, and other carnivora, a cartilaginous style lies in the middle of the tongue. These parts form a splanchno- or visceral skeleton. By some anatomists the teeth, which are unquestionably hard parts of the body, are also referred to the splanchno-skeleton, though they are special modifications of the papillae of the mucous membrane of the gum. In man, the teeth being excluded, there is neither exo-nor splanchno-skeleton, but only an endo-skeleton.

In each of the great subdivisions of the body an endo-skeleton exists, so that we may speak of an Axial Skeleton and an Appendicular Skeleton. The Axial Skeleton consists of the bones of the spine and head, the ribs, and the breastbone; the Appendicular Skeleton, of the bones of the limbs. The number of bones in the skeleton varies at different periods of life. In the adult there are about 200, but in the child they are more numerous; for in the pro cess of consolidation of the skeleton certain bones originally distinct become fused together. In Plates XII., XIII., and XIV., front, back, and side views of the entire skeleton are given, together with figures of the skull and several of its constituent bones.

We shall commence the description of the Axial Skele ton by giving an account of the bones of the spine.

The Spine, Spinal or Vertebral Column, chine, or backbone, consists of a number of superimposed bones which are named Vertebrae, because they can move or turn somewhat on each other. It lies in the middle of the back of the neck and trunk; has the cranium at its summit; the ribs at its sides, which in their turn support the upper limbs; whilst the pelvis, with the lower limbs, is jointed to its lower end. The spine consists in an adult Of twenty-six bones, in a young child of thirty-three, certain of the bones in the spine of the child becoming ankylosed or blended with each other in the adult. These blended bones lose their mobility, and are called false vertebrae; whilst those which retain their mobility are the true vertebrae. In the vertebrata the bones of the spine are arranged in groups, which may be named from their position — vertebrae of the neck or cervical; of the chest, dorsal or thoracic; of the loins, lumbar; of the pelvis, sacral; and of the tail, coccygeal or caudal; and the number of vertebrae in each group maybe expressed in a formula. In man the formula is as follows:—C7D12L5S 5 C0c4 = 33 bones, as seen in the child; but the five sacral vertebrae fuse together into a single bone— the sacrum—and the four coccygeal into the single coccyx. Hence the sacrum and coccyx of the adult are the false, whilst the lumbar, dorsal, and cervical are the true vertebrae.

The vertebrae are irregularly-shaped bones, but as a rule have certain characters in common. Each possesses a body and an arch, which enclose a ring, with certain processes and notches. The Body, or Centrum, is a short cylinder, which by its upper and lower surfaces is connected by means of fibro-cartilage with the bodies of the vertebrae immediately above and below. The collective series of vertebral bodies forms the great column of the spine. The Arch, also called Neural Arch, because it encloses the spinal marrow or nervous axis, springs from the back of the body, and consists of two symmetrical halves united behind in the middle line. Each half consists of an anterior part or pedicle, and a posterior part or lamina. The Rings collectively form the spinal canal. The Processes usually spring from the arch. The spinous process projects backwards from the junction of the two laminae, and the collective series of these processes gives to the entire column the spiny character from which has arisen the term Spine, applied to it. The transverse processes project outwards, one from each side of the arch. The articular processes project, two upwards and two downwards, and are for connecting adjacent vertebrae together. The Notches, situated on the upper and lower borders of the pedicles, form in the articulated spine the intervertebral foramina through which the nerves pass out of the spinal canal.

The vertebrae in each group have characters which specially distinguish them. In man and all mammals, with few exceptions, whatever be the length of the neck, the Cervical Vertebrae are seven in number. The exceptions are the three-toed sloth, which has nine, and Hoffmann’s sloth and the manatee, in which there are only six. In many whales the seven cervicals are fused in the adult into a single bone. In man the body of a cervical vertebra is comparatively small, and its upper surface is transversely concave; the arch has long and obliquely sloping laminae; the ring is large and triangular; the spine is short, bifid, and horizontal; the transverse process consists of two bars of bone, the anterior springing from the side of the body, the posterior from the arch, and uniting externally to enclose a foramen, through which, as a rule, the vertebral artery passes; the articular processes are flat and oblique, and the upper pair of notches are deeper than the lower. The first, second, and seventh cervical vertebrae have characters which specially distinguish them. The first, or Atlas, has no body or spine: its ring is very large, and on each side of the ring is a thick mass of bone, the lateral mass, by which it articulates with the occipital bone above and the second vertebra below. The second vertebra, Axis, or Vertebra dentata, has its body surmounted by a thick tooth-like odontoid process, which is regarded as the body of the atlas displaced from its proper vertebra and fused with the axis. This process forms a pivot round which the atlas and head move in turning the head from one side to the other; the spine is large, thick, and deeply bifid. The seventh, called Vertebra prominens, is distinguished by its long prominent spine, which is not bifid, and by the small size of the foramen at the root of the transverse process. In the human spine the distinguishing character of all the cervical vertebrae is the foramen at the root of the transverse process, but amongst mammals this is not an invariable character, for in the cetacea the transverse process of the atlas is imperforate, and in the horse, ruminants, and many quadrumana, the seventh cervical vertebra has no foramen at the root of its transverse process.

The Dorsal Vertebrae, more appropriately called costal or thoracic, are twelve in number in the human spine; but amongst mammals they range from eleven in the armadillo to twenty-two in the Cape hyrax and Hoffmann’s sloth. They are intermediate in size and position to the cervical and lumbar vertebrae, and are all distinguished by having one or two smooth surfaces on each side of the body for articulation with the head of one or two ribs. . The arch is short and with imbricated laminae; the ring is nearly circular; the spine is oblique, elongated, and bayonet-shaped; the transverse processes are directed back and out, not bifid, and with an articular surface in front for the tubercle of a rib; and the articular processes are flat and nearly vertical. The first, twelfth, eleventh, tenth, and sometimes the ninth, dorsal vertebrae are distinguished from the rest. The first is in shape like the seventh cervical, but has no foramen at the root of the transverse process, and has two articular facets on each side of the body; the ninth has sometimes only one facet at the side of the body; the tenth, eleventh, and twelfth have invariably only a single facet on the side of the body, but the eleventh and twelfth have stunted transverse processes, and the twelfth has its lower articular processes shaped like those of a lumbar vertebra.

The Lumbar Vertebrae in man are five in number, but amongst mammals they range from two in the platypus to eight in the hyrax or agouti. They are the lowest of the true vertebrae, and also the largest, especially in the body. The arch has short and deep laminae; the ring is triangular; the spine is massive and hatchet-shaped; the transverse processes are long and pointed; the articular are thick and strong, the superior pair concave, the inferior convex, and the inferior notches, as in the dorsal vertebrae, are-deeper than the superior. In the lumbar vertebrae and in the lower dorsal an accessory process projects from the base of each transverse process, and a mammillary tubercle from each superior articular process. In man these are small and rudimentary; but in some mammals, as the kangaroo, armadillo, and scaly ant-eater, the mammillary tubercles are large, and in the baboon, dog, cat, and beaver, the accessory processes are well developed. The fifth lumbar vertebra has its body much thicker in front than behind; its spine is less massive, and its lower articular processes are flat.

The Sacrum is composed of five originally separate vertebrae fused into a single bone. In the bandicoot it consists of a single vertebra, whilst it has as many as eight in the armadillo. The relative size and completeness of the sacrum are associated with the development of the haunch bones and of the lower limbs. In whales, where the pelvic bones are rudimentary and there are no hind limbs, there is no sacrum. It forms the posterior wall of the pelvis, is triangular in form, and possesses two surfaces, two borders, a base, and an apex. The anterior or pelvic surface is concave, and is marked by four transverse lines, which indicate its original subdivision into five bones, and by four pairs of foramina, through which are transmitted the anterior sacral nerves. Its posterior surface is convex; in the middle line are tubercles or rudimentary spines, and on each side of these are two rows of tubercles, the inner of which are the conjoined articular and mammillary processes, the outer the transverse processes of the originally distinct vertebrae; in addition, four pairs of foramina are found which transmit the posterior sacral nerves from the sacral canal, which extends through the bone from base to apex, and forms the lower end of the spinal canal. By its borders the sacrum is articulated with the haunch-bones— by its base with the last lumbar vertebra, by its apex with the coccyx. The human sacrum is broader in proportion to its length than in other mammals; this great breadth gives solidity to the lower part of the spine, and, conjoined with the size of the lateral articular surfaces, it permits a more perfect junction with the haunch-bones, and is correlated with the erect position. Owing to the need in woman for a wide pelvis, the sacrum is broader than in man.

The Coccyx consists of only four vertebrae in the human spine. It is the rudimentary tail, but instead of projecting back, as in mammals generally, is curved forwards, and is not visible externally, an arrangement which is also found in the anthropoid apes and in Hoffmann’s sloth. In the spider monkeys as many as thirty-three vertebrae are found in the tail, and in the long-tailed pangolin the number reaches forty-six. Not only is the tail itself rudimentary in man, but the vertebrae of which it is composed are small, and represent merely the bodies of the true vertebrae. As there are no arches, the ring is not formed, and thβ spinal canal does not extend, therefore, beyond the apex of the sacrum. The first coccygeal vertebra, in addition to a body, possesses two processes or horns, which are jointed with two corresponding processes from the last sacral vertebra.

The Human Spine is more uniform in length in persons of the same race than might be supposed from the individual differences in stature, the variation in the height of the body in adults being due chiefly to differences in the length of the lower limbs. The average length of the spine is 28 inches; its widest part is at the base of the sacrum, from which it tapers down to the tip of the coccyx. It diminishes also in breadth from the base of the sacrum upwards to the region of the neck. Owing to the projection of the spines behind and the transverse processes on each side, it presents an irregular outline on those aspects; but in front it is more uniformly rounded, owing to the convex form of the antero-lateral surfaces of the bodies of its respective vertebrae. In its general contour two series of curves may be seen, an antero-posterior and a lateral. The antero-posterior is the more important. In the infant at the time of birth the sacro-coccygeal part of the spine is concave forwards, but the rest of the spine, except a slight forward concavity in the series of dorsal vertebrae is almost straight. When the infant begins to sit up in the arms of its nurse, a convexity forwards in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forwards in the region of the loins. Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are associared with the erect attitude of man. In the human spine alone are the lumbar vertebrae convex forward. A lateral curve, convex to the right, opposite the third, fourth, and fifth dorsal vertebrae, with compensatory curve convex to the left immediately above and below, is due apparently to the much greater use of the muscles of the right arm over those of the left, drawing the spine in that region somewhat to the right. In disease of the spine its natural curvatures are much increased, and the deformity known as humpback is produced. As the spine forms the central part of the axial skeleton, it acts as a column to support not only the weight of the body, but of all that can be carried on the head, back, and in the upper limbs: by its transverse and spinous processes it serves also to give attachment to numerous muscles, and the transverse processes of its dorsal vertebrae are also for articulation with the ribs.

The Thorax, Pectus, or Chest is a cavity or enclosure the walls of which are in part formed of bone and cartilage. Its skeleton consists of the sternum in front, the twelve dorsal vertebrae behind, and the twelve ribs, with their corresponding cartilages, on each side.

The Sternum or Breast Bone is an elongated bone which inclines downwards and forwards in the front wall of the chest. It consists of three parts—an upper, called manubrium or prae-sternum; a middle, the body or mesosternum; and a lower, the ensiform process or xiphi-sternum. Its anterior and posterior surfaces are marked by transverse lines, which indicate not only the subdivision of the entire bone into three parts, but that of the mesosternum into four originally distinct segments. Each lateral border of the bone is marked by seven depressed surfaces for articulation with the seven upper ribs: at each side of the upper border of the prae-sternum is a sinuous depression, where the clavicle, a bone of the upper limb, articulates with this bone of the axial skeleton. The xiphi-sternum remains cartilaginous up to a late period of life, and from its pointed form has been named the ensiform cartilage.

The Ribs or Costae, twenty-four in number, twelve on each side of the thorax, consist not only of the bony ribs, but of a bar of cartilage continuous with the anterior end of each bone, called a costal cartilage, so that they furnish examples of a cartilaginous skeleton in the adult human body; in aged persons these cartilag33 usually become converted into bone. The upper seven ribs are connected by their costal cartilages to the side of the sternum, and are called sternal or true ribs; the lower five do not reach the sternum, and are named asternal or false, and of these the two lowest, from being comparatively unattached in front, are called free or floating. All the ribs are articulated behind to the dorsal vertebrae, and as they are symmetrical on the two sides of the body, the ribs in any given animal are always twice as numerous as the dorsal vertebrae in that animal. They form a series of osseocartilaginous arches, which extend more or less perfectly around the sides of the chest. A rib is an elongated bone, and as a rule possesses a head, a neck, a tubercle, and a shaft. The head usually possesses two articular surfaces, and is connected to the side of the body of two adjacent dorsal vertebrae; the neck is a constricted part of the bone, uniting the head to the shaft; the tubercle, close to the junction of the shaft and neck, is the part which articulates with the transverse process of the vertebra. The shaft is compressed, possesses an inner and outer surface, and an upper and lower border, but from the shaft being somewhat twisted on itself, the direction of the surfaces and borders is not uniform throughout the length of the bone. The ribs slope from their attachments to the spine, at first outwards, downwards, and backwards, then downwards and forwards, and where the curve changes from the backward to the forward direction an angle is formed on the rib. The first, tenth, eleventh, and twelfth ribs articulate each with only a single vertebra, so that only a single surface exists on the head: the surfaces of the shaft of the first rib are almost horizontal; those of the second very oblique; the eleventh and twelfth ribs are rudimentary, have neither neck nor tubercle, and are pointed anteriorly. The ribs are by no means uniform in length: they increase from the first to the seventh or eighth, and then diminish to the twelfth; the first and twelfth are therefore the shortest ribs. The first and second costal cartilages are almost horizontal, but the others are directed upwards and inwards.

In its general form the chest may be likened to a truncated cone. It is rounded at the sides and flattened in front and behind, so that a man can lie either on his back or his belly. Its truncated apex slopes downwards and forwards, is small in size, and allows of the passage of the windpipe, gullet, large veins, and nerves into the chest, and of several large arteries out of the chest into the neck. The base or lower boundary of the cavity is much larger than the apex, slopes downwards and backwards, and is occupied by the diaphragm, a muscle which separates the chest from the cavity of the abdomen. The transverse diameter is greater than the antero-posterior, and the anteroposterior is greater laterally, where the lungs are lodged, than in the mesial plane, which is occupied by the heart.

The Head forms the summit of the axial part of the body. It consists of two portions—the Cranium and the Face.

The Skull, or skeleton of the head, is composed of 22 bones, 8 of which form the skeleton of the cranium, 14 that of the face. Except the lower jaw, which is moveable, the bones are all firmly united by immovable joints. The 8 bones of the cranium are so united together by their edges as to form the walls of a box or cavity, the cranial cavity, in which the brain is lodged. The box of the cranium possesses a base or floor, a vault or roof, an anterior, a posterior, and two lateral walls. The posterior wall is formed by the occipital bone, which also extends for some distance forwards along the middle of the base; in front of the basal part of the occipital is the sphenoid, which also sends a process upwards on each side of the skull; in front of the basal part of the sphenoid is the ethmoid; mounting upwards in front of the ethmoid is the frontal, which forms the forehead, and closes in the front of the cranial box; forming the vault and side walls are the two parietal bones; completing the side walls, and extending for a short distance along the side of the floor, are the two temporal bones; the vertex of the skull is at the junction of the two parietal bones with each other.

The fourteen bones of the face, which are situated below and in front of the cranium, enter into the formation of the walls of cavities which open on the front of the face; thus they complete, along with the frontal, sphenoid, and ethmoid, the walls of the two orbits in which the eye-balls are lodged; along with the ethmoid and sphenoid, the walls of the nostrils; and they form the osseous walls of the mouth. As a general rule, the cranial bones are expanded, and platelike in form. The outer surface of each bone assists in forming the exterior of the cranium, and not unfrequently is marked by ridges or processes for the attachment of muscles. The inner surface, again, is smooth, and pitted with depressions, in which the convolutions of the brain are lodged, and also marked by grooves for the lodgment of dilated veins called blood sinuses, and of arteries termed meningeal The two surfaces of a cranial bone, dense in structure, are called its tables, outer and inner, and aro separated from each other by bone, looser and more spongy in its texture, called diploë. In some localities, more especially in certain of the bones which form the walls of the nostrils, the diploë disappears, and comparatively wide interspaces separate the two tables which contain air, and are called air-sinuses. The margins of the bones are denticulated, and it is by the interlocking of the denticula-tions of adjacent bones that they are jointed together, the joints being named sutures. The bones are pierced by holes or foramina, and similar holes exist between the adjacent margins of some of the bones. These foramina are mostly situated in the floor of the skull, and transmit arteries into the cranial cavity to supply the brain and the inner table with blood, and veins and nerves out of the cavity. The largest of these holes is called foramen magnum. It lies in the occipital bone, immediately above the ring of the atlas; through it the spinal marrow becomes continuous with the brain, and the vertebral arteries pass to supply the brain with blood.

The Occipital, or bone of the Back of the Head (Figs. 6 and 7, and Plate XIII. ), consists of four originally distinct pieces fused into a curved plate-like bone. Its subdivisions are arranged around the foramen magnum—the basilar part, basi-occipital, in front; the condyloid parts, ex-occipi-tals, one on each side; and the tabular part, or supra-occipital, behind. The anterior surface of the supra-occipital is sub t divided into four fossae, in the two upper of which are lodged the occipital lobes of the cerebrum, in the two lower the cerebellum; the upper and lower pairs of fossae are separated by a groove for the lodgment of the lateral venous sinus. The posterior surface is marked by a protuberance and by curved lines for the attachment of muscles; by its margin the supra-occipital articulates wit⅛ the parietal and temporal bones. Each ex-occipital has on its under surface a smooth condyle for articulation with the atlas; in front of the condyle is a foramen which transmits the last or ninth cranial nerve, called hypoglossal, and behind it a foramen for the transmission of a vein sometimes exists. The basi-occipital articulates and, in the adult skull, is fused with the body of the sphenoid (Fig. 7). The upper surface of the basi-occipital is grooved for the lodgment of the medulla oblongata.

Sometimes the part of the supra-occipital situated above the protuberance and upper curved line ossifies as an independent bone, called interparietal. In some mammals, as the sheep, the existence of an interparietal in the young skull is the rule and not the exception.

The Sphenoid or Wedge-shaped bone (Fig. 7, and Plate XII.), lies at the base of the skull; it articulates behind with the occipital; in front it is jointed to the ethmoid and frontal, and by its lateral processes or wings to the frontal, parietal, and temporal bones. From its position, therefore, it binds together all the bones of the cranium, and, moreover, articulates with many of those of the face. For constructive purposes it is the most important bone of the head. It consists of a centrum or body, with which four pairs of processes are connected. The body has a deep depression on its upper surface, compared in shape to a Turkish saddle, in which is lodged the pituitary body; hence it is called pituitary fossa. In front of this fossa is a ridge which marks the place of union of the pre- and post-sphenoidal subdivisions of the body of this bone; the body is grooved laterally for the internal carotid artery and the cavernous blood sinuses, and it is hollowed out in its interior to form the sphenoidal air-sinuses: these airsinuses are partially closed in front by a pair of small bony plates called sphenoidal spongy bones, or bones of Bertin. Behind the pituitary fossa is a pair of processes called posterior clinoid, from which the bone slopes back to the basi-occipital; this slope is called the dorsum selloe, and on it rests the pons Varolii. From the posterior part of each side of the body the great wings, or ali-sphenoids, pass outwards and upwards to the sides of the skull, and each sends off a plate-like process to enter into the formation of the outer wall of the orbit. From the anterior part of each side of the body the lesser wings, orbito-sphenoids, pass outwards, and assist in forming the roof of each orbit; each orbito-sphenoid ends internally in a knob-Eke process caUed anterior clinoid, and at its root is a foramen caUed optic, which transmits the second nerve, or nerve of sight, into the orbit. From the great wings on each side, close to its junction with the body, a pair of pterygoid processes, called internal and external, project downwards, and the internal process ends in a slender hook termed the hamτdar process. The aE-sphenoid is pierced by foramina called rotundum, ovale, and spinosum, the two former of which transmit divisions of the fifth cranial nerve, the last an artery to the membranes of the brain; between the orbito-and ali-sphenoids is a fissure which transmits the third, fourth, sixth, and first divisions of the fifth cranial nerve into the orbit; and at the root of the pterygoid processes is the vidian canal, for the transmission of a nerve of the same name.

The Ethmoid, or Sieve-like bone (Fig. 7, and Plate XIII., fig. 5), is situated between the two orbital plates of the frontal, and in front of the body of the sphenoid. It is cuboidal in shape, and is composed of a central portion and two lateral masses, which are connected together by a thin horizontal plate pierced with holes like a sieve, and caUed cribriform. This cribriform plate forms a part of the floor of the cranial cavity; on it rest the two olfactory bulbs, and the branches of the nerves of smeU, called olfactory or first cranial nerves, pass from the bulbs through the holes in this plate into the nose. The central portion of the bone is a mesial perpendicular plate, mes-ethmoid, and forms a part of the septum which subdivides the nose into the right and left nostrils. Each lateral mass consists of an external smooth plate, os planum, which assists in forming the inner waU of the orbit; and an internal convoluted part, caUed superior and middle spongy bones or turbinals, which enter into the formation of the outer waU of the nostril. These turbinals are associated with the distribution of the nerves of smeU; in the toothed whales, where there are no olfactory nerves, the turbinals are absent, whilst in some mammals, as the crested Seal, they assume a highly convoluted form. The lateral masses are hoUowed out into air-sinuses, caUed ethmoidal ceUs, which communicate with the nostrils and with corresponding sinuses in the sphenoid and frontal bones.

The Frontal, or bone of the Forehead (Figs. 6 and 7, and Plate XIII. ), consists originally of a right and left lateral haff, united by the frontal suture in the middle fine of the forehead. As a rule, this suture disappears in early life, and a single greatly curved bone is formed. The bone is convex forwards, to form the rounded forehead, and presents two eminences, the centres of ossification of the bone; at the root of the nose is an elevation caUed glabella, extending outwards, from which, on each side, is the supra-ciliary ridge, corresponding to the position of the eyebrow. In the crania of some races, e.g., the AustraEan, the forward projection of the glabella and supra-ciliary ridges is considerable; and in the well-known skuU from the vaUey of the Neander it has reached a remarkable size. These ridges and the glabella mark the position of the air-sinuses iu the frontal bone. The upper border of each orbit, which ends internaUy and externally in a process of bone called angular, forms the lower boundary of the forehead. The cerebral surface of the bone is deeply concave, for the reception of the frontal lobes of the brain; the concavity is deepened by the backward projection of two thin plates of bone which form the roofs of the orbits, which platée are separated from each other by the deep notch in which the ethmoid bone is lodged; along the margins of this notch may be seen the openings into the frontal air-sinuses.

The Parietal bones, two in number (Figs. 6 and 7, and Plate XIV. ), form the greater part of the side wall of the skuU, and mount upwards to the vertex, where they unite together along the line of the sagittal suture. Each bone possesses about the centre of its outer surface an eminence, the centre of ossification of the bone, with which a hollow on the cerebral surface, lodging a convolution of the parietal lobe of the brain, corresponds. The bone is quadrilateral in form. Three of its margins are strongly denticulated, for junction with the occipital, frontal, and corresponding parietal; the fourth is scale-like, for union with the temporal, and forms the squamous suture; near the upper margin on the cerebral surface is a groove for the lodgment of the superior longitudinal venous sinus. The anterior inferior angle articulates with the ali-sphenoid, and is marked by a groove for the meningeal artery; the posterior inferior is grooved for the lateral venous sinus, and articulates with the mastoid of the temporal.

The Temporal bones, two in number (Figs. 6 and 7, and Plate XIV. ), are placed at the side and base of the skull, and are remarkable for containing in their interior the organs of hearing. Each bone consists originaUy of four subdivisions—a squamoso-zygomatic, a tympanic, a petro-mastoid, and a styloid—which in course of time fuse together to form an irregular-shaped bone. The squamous part of the squamoso-zygomatic is a thin plate which forms that part of the side of the skull familiarly known as the “temple.” The zygoma extends horizontally forwards as a distinct arched process, to join the malar or cheek-bone. At the root of the zygoma is a smooth fossa, caUed glenoid, which receives the condyle of the lower jaw, and assists in forming the temporo-maxiUary joint. The tympanic portion forms in the foetus a ring, which enlarges subsequently into a curved plate that forms the waU of the external auditory meatus, or passage into the tympanum or middle ear. The tympanic and squamoso-zygomatic parts of the bone fuse together; but a fissure, called Glaserian, situated behind the glenoid fossa, marks their original separation; in this fissure the slender process of the maUeus (one of the bones of the tympanum) is lodged. The petro-mastoid or periotic part of the temporal contains the organ of hearing, and is compEcated in its internal anatomy. It extends forwards and inwards along the floor of the skull, and forms on the exterior of the skuU the large nipple-shaped mastoid process. This process is rough on its outer surface, for the attachment of muscles, and is hoUowed out internaUy into the mastoid cells or air-sinuses, which communicate with the tympanum or middle ear. The petrous-temporal is distinguished by its stony hardness, and has the form of a three-sided pyramid. Its apex lies in relation to the side of the body of the sphenoid; its base corresponds to the tympanic cavity and external meatus; its under surface is rough, and forms a part of the under surface of the skuU; its anterior and posterior surfaces are smooth and in relation to certain parts of the brain. The petrous part of the bone is traversed by a canal which transmits the internal carotid artery and sympathetic nerve into the cranial cavity; in its posterior surface is a passage, internal meatus, down which the seventh cranial nerve proceeds; at the bottom of the meatus the auditory part of that nerve enters the internal ear, whüst the part of the nerve which goes to the muscles of the face traverses a canal in the bone, called aqueduct of Fallopius, which ends externally, between the styloid and mastoid processes, in the stylo-mastoid foramen. The styloid process is a slender part of the bone which projects downwards from the tympanic plate, and is connected with the small cornu of the hyoid bone by the stylo-hyoid ligament. It does not unite with the rest of the bone until a comparatively late period. Between the petrous-temporal and ex-occipital is the jugular foramen, which transmits out of the skull the eighth cranial nerve and the internal jugular vein.

The fourteen bones of the Face are, as a rule, much smaller than those of the Cranium; some have the form of thin scales, others are more irregular in shape. They are named as follows: — Two superior maxillary, two palate, two malar, two nasal, two lachrymal, two inferior turbinal, a vomer, and an inferior maxilla.

The Superior Maxillae, or bones of the Upper Jaw (Figs. 6 and 7, and Plate XIV. ), form the skeleton of a large part of the face, and enter into the formation of the walls of the cavities of the nose, mouth, and orbit; around them the other bones of the face are grouped. The facial surface of each bone presents in front a large foramen for the transmission of the infra-orbital branch of the fifth cranial nerve, and behind, several small foramina for the transmission of nerves to the teeth in the upper jaw. On the same surface is a rough process for articulation with the malar bone. The orbital surface is smooth, forms the floor of the orbit, and possesses a canal in which the infraorbital nerve lies. The nasal surface forms a part of the outer wall and floor of the nostril, and presents a hole leading into a large hollow in the substance of the bone, called the antrum, or superior maxillary air-sinus. The nasal surface articulates with the inferior turbinal and palate bones. The nasal and facial surfaces become continuous with each other at the anterior aperture of the nose, and from them a strong process ascends to join the frontal bone close to the glabella; this process also articulates with the lachrymal and nasal bones. The palatal surface forms a part of the bony roof of the mouth, and presents in front a small hole (the incisive foramen) which communicates with the nose. In the sheep and many other mammals this hole is of large size; the palatal surface is bounded externally by a thick elevated border, in which are the sockets, or alveoli, for the lodgment of the fangs of the teeth; internally this surface articulates by a narrow border with the other superior maxilla and with the vomer, and, posteriorly, with the palate-bone.

The Palate-bone (Fig. 7, and Plate XIV.) lies in contact with the inner surface and posterior border of the superior maxilla, and separates it from the sphenoid. It is in shape not unlike the capital letter L, the horizontal limb forming the hinder part of the bony roof of the mouth by its lower surface, and the back part of the floor -of the nose by its upper. The ascending limb assists in 'forming the outer wall of the nose, and subdivides into an anterior, or orbital, and a posterior, or sphenoidal, process. At the junction of the two limbs is the pyramidal process, which articulates with the lower ends of the pterygoid processes of the sphenoid.

The Vomer (Fig. 7), shaped like a ploughshare, lies vertically in the mesial plane of the nose, and forms a large part of the partition which separates one nostril from the other. It articulates above with the under surface of the body of the sphenoid and the mes-ethmoid; below with the palatal processes of the superior maxillae and palate-bones; in front with the septal cartilage of the nose, whilst the posterior border is free, and forms the hinder edge of the nasal septum.

The Inferior Turbinated is a slightly convoluted bone situated on the outer wall of the nose, where it articulates with the superior maxilla and palate a little below the middle turbinal of the ethmoid.

. The Lachrymal (Fig. 6) is a small scale-like bone, in shape not unlike a finger-nail, placed at the inner wall of the orbit, and fitting between the ethmoid, superior maxilla, and frontal bones. It has a groove on the outer surface, in which is lodged the lachrymal sac.

The Nasal (Fig. 6) is a thin, somewhat elongated bone, which, articulating with its fellow in the middle line, forms with it the bony bridge of the nose; above, it articulates with the frontal, and by its outer border with the ascending process of the superior maxilla.

The Malar bone (Fig. 6), irregular in shape, forms the prominence of the cheek, and completes the outer wall of the orbit. It rests upon the superior maxilla; by its orbital plate it articulates with the great wing of the sphenoid; by its ascending process with the external angular process of the frontal; by its posterior process with the zygomatic process of the temporal, so as to complete the zygomatic arch.

The Inferior Maxilla, Lower Jaw, or Mandible (Figs. 6 and 7, and Plate XIV, fig. 9), is a large horse-shoe shaped bone, which has the distinction of being the only movable bone of the head. It consists originally of two separate halves, which unite during the first year of life into a single bone at the symphysis or chin. A characteristic feature of the human lower jaw is the forward slope of the bone at the chin, for in other mammals the symphysis inclines backwards. In the upper border of this bone are the sockets for the lower series of teeth. At the posterior end of the horse-shoe curve on each side the bone ascends almost vertically, and terminates in two processes—an anterior, or coronoid, which is for the insertion of the temporal muscle, and a posterior, or condyle, which is for articulation with the glenoid fossa of the temporal bone. Where the ascending and horizontal limbs of the bone are continuous, it forms the angle, which is almost a right angle. On the inner surface of the ascending limb is a large foramen, communicating with a canal which traverses the bone below the sockets for the teeth. In this canal are lodged the nerves and blood-vessels for these teeth.

The Hyoid bone lies in the neck, on the same plane as the lower border of the inferior maxilla (Figs. 6 and 7). It is shaped like the letter U, and consists of a body, or ^basi-hyal, from which two long horns, or stylo-hyals, project backwards. At the junction of the body and horns two smaller cornua, or cerato-hyals, project upwards, and are connected with the styloid processes of the temporal bones, or stylo-hyals, by the stylo-hyoid ligaments, or epi-hyals. The hyoid is the bone from which the muscles of the tongue arise, and it is situated immediately above the thyroid cartilage of the larynx, to which it is attached by ligaments.

In its general form the Skull is ovoid, with the long axis extending antero-posteriorly, the frontal and occipital ends rounded, and the sides somewhat flattened. Its average length in the people of the British Islands is a little more than 7 inches; its greatest breadth about 5∣ inches; and its height, from the plane of the foramen magnum to the vertex, about 5∣ inches. Its greatest circumference is about 21 inches. The breadth of the face across the zygomatic arches is about 5 inches. The average capacity of the brain cavity is 92 cubic inches. The British skull is dolicho-cephalic and orthognathic. (See Anthropology.)

The lateral regions of the skull are called the temporal fossoe, and give origin to the temporal muscles. Under cover of each zygomatic arch is the zygomatic fossa. At the bottom of this is a hollow between the superior maxilla and sphenoid, called spheno-maxillary fossa, from which the pterygo-maxillary fissure extends downwards between the pterygoid and superior maxillary; and the sphenomaxillary fissure extends upwards into the orbit. The orbit is a four-walled pyramidal cavity, with the. base directed forward to the face, and the apex backward to the brain cavity. At the apex are the foramina in the sphenoid, through which the nerve of sight and other nerves pass from the brain to the eyeball, muscles, and other soft structures within the orbit.

The nostrils open on the front of the face by a large opening situated between the two superior maxillae, and bounded above by the two nasals. The sides of the opening pass down almost vertically to join the floor, and are not rounded off as in the ape’s skull; from the centre of the floor a sharp process, the nasal spine of the superior maxillae projects forwards, and forms a characteristic feature of the human skull. Attached to the sides of the opening are the lateral cartilages of the nose, which form the wings of the nostrils, and so modify the position of their openings that in the face they look downwards. The nostrils are separated from each other by a vertical mesial partition composed of the mes-ethmoid, vomer, and triangular nasal cartilage, the last-named of which projects forward beyond the anterior surface of the upper jaw, and contributes materially to the prominence of the nose. The outer wall of each nostril presents the convoluted turbinals, which are separated from each other by horizontal passages extending antero-posteriorly; the superior passage or meatus lies between the superior and middle turbinals of the ethmoid, and is continued into the sphenoidal and posterior ethmoidal air-sinuses; the middle meatus lies between the middle and inferior turbinals, and is continued into the frontal, anterior ethmoidal, and maxillary air sinuses. These sinuses are therefore extensions of the nasal chamber or respiratory passage, and correspond with the air cavities which exist in so many of the bones of birds; the inferior meatus lies between the inferior turbinal and floor of the nose; into its anterior part opens the nasal duct which conveys the tears from the front of the eyeball. The posterior openings of the nose are separated from each other by the hinder edge of the vomer, and are placed between the internal pterygoid plates of the sphenoid.

The skull varies in appearance at different periods of life. In infancy the face is small, about ⅜th of the size of the entire head, for the teeth are still rudimentary and the jaws are feeble; the centres of ossification of the cranial bones are prominent; the forehead projects; the skull is widest at the parietal eminences; the air-sinuses, and bony ridges corresponding to them, have not formed. In the adult the face is about half the size of the head, and its vertical diameter greatly elongated, from the growth of the antrum, the nose, and the dental borders of the jaws; and the angle of the lower jaw is almost a right angle. In old age the teeth fall out, the jaws shrink in, their dental borders become absorbed, the angle of the lower jaw, as in infancy, is obtuse; the vertex and floor of the skull also become flattened, and the sides bulge outwards,—changes due to gravitation and the subsidence of the bones by their own weight

The skull of a woman is smaller and lighter, with the muscular ridges and projections due to the air sinuses less strongly marked than in a man, but with the eminences or centres of ossification more prominent. The more feeble air sinuses imply a more restricted respiratory activity and a less active mode of life than in a man. The internal capacity is about 10 per cent, less than that of the male. The face is smaller in proportion to the cranium; the cranium is more flattened at the vertex, and the height is consequently not so great in proportion to the length as in the man. In the female skull, therefore, the infantile characters are less departed from than is the caβe in the male.

Turning now to the Appendicular Skeleton, we shall consider first that of the Superior or Thoracic or Pectoral Extremity, or Upper Limb. The Upper Limb may be subdivided into a proximal part or shoulder, a distal part or hand, and an intermediate shaft, which consists of an upper arm or brachium, and a fore-arm or antibrachium. In each of these subdivisions certain bones are found: in the shoulder, the clavicle and scapula; in the upper arm, the humerus; in the fore-arm, the radius and ulna, the bone of the upper arm in man being longer than the bones of the fore-arm; in the hand, the carpal and metacarpal bones and the phalanges. The scapula and clavicle together form an imperfect bony arch, the Scapular Arch or Shoulder Girdle; the shaft and hand form a free divergent Appendage. The shoulder girdle is the direct medium of connection between the axial skeleton and the divergent part of the limb; its anterior segment, the clavicle, articulates with the upper end of the sternum, whilst its posterior segment, the scapula, approaches, but does not reach, the dorsal spines.

The Clavicle, or Collar Bone (Fig. 9), is an elongated bone which extends from the upper end of the sternum horizontally outwards, to articulate with the acromion process of the scapula. It presents a strong sigmoidal curve, which is associated with the transverse and horizontal direction of the axis of the human shoulder. It is slender in the female, but powerful in muscular males; its sternal end thick and somewhat triangular; its acromial end, flattened from above downwards, has an oval articular surface for the acromion. Its shaft has four surfaces for the attachment of muscles; and a strong ligament, connecting it with the coracoid, is attached to the under surface, near the outer end, whilst near the inner a strong ligament passes between it and the first rib. The clavicle is absent in the hoofed quadrupeds, in the seals and whales, and is feeble in the carnivora; but is well formed, not only in man, but in apes, bats, and in many rodents and Insectivora.

The Scapula, or Shoulder Blade (Fig. 9), is the most important bone of the shoulder girdle, and is present in all mammals. It lies at the upper and back part of the wall of the chest, reaching from the second to the seventh rib. Its form is plate-like and triangular, with three surfaces, three borders, and three angles. The fundamental form of the scapula, as seen in the mole, is that of a three-sided prismatic rod, and its assumption of the plate or blade-like character in man is in connection with the great development of the muscles which rotate the humerus at the shoulder joint. Its costal or ventral surface is in relation to the ribs, from which it is separated by certain muscles: one, called subscapularis, arises from the surface itself, which is often termed subscapular fossa. The dorsum or back of the scapula is traversed from behind forwards by a prominent spine (Pl. XIV., fig. 1, S), which lies in the proper axis of the scapula, and subdivides this aspect of the bone into a surface above the spine, the supra- or prae-spinous fossa, and one below the spine, the infra- or post-spinous fossa. The spine arches forwards, to end in a broad flattened process, the acromion, which has an oval articular surface for the clavicle; both spine and acromion are largely developed in the human Bcapula in correlation with the great size of the trapezius and deltoid muscles, which are concerned in the elevation and abduction of the upper limb. The borders of the scapula, directed upwards, backwards, and downwards, give attachment to several muscles. The angles are inferior, supero-posterior, and supero-anterior. The supero-anterior is the most important; it is truncated, and presents a large, shallow, oval, smooth surface, the glenoid fossa, for articulation with the humerus, to form the shoulder joint. Overhanging the glenoid fossa is a curved beak-like process, the coracoid, which is of importance as corresponding with the separate coracoid bone of birds and reptiles. The line of demarcation between it and the scapula proper is marked on the upper border of the scapula by the supra-scapular notch.

The Humerus, or bone of the Upper Arm (Fig. 9), is a long bone, and consists of a shaft and two extremities. The upper extremity of this bone possesses a convex spheroidal smooth surface, the head, for articulation with the glenoid fossa of the scapula; it is surrounded by a narrow constricted neck, and where the neck and shaft become continuous with each other, two processes or tuberosities are found, to which are attached the rotator muscles arising from the scapular fossae. Between the tuberosities is a groove in which the long tendon of the biceps rests. A line drawn through the head of the humerus perpendicular to the middle of its articular surface, forms with the axis of the shaft of the bone an angle of 40°. The shaft of the humerus is cylindriform above, but flattened and expanded below; about midway down the outer surface is a rough ridge for the insertion of the deltoid muscle, and on the inner surface another rough mark for the insertion of the coraco-brachialis. The demarcation between the cylindriform and expanded parts of the shaft is marked by a shallow groove winding round the back of the bone, in which the musculo-spiral nerve is lodged. The lower extremity of the humerus consists of an articular and a non-articular portion. The articular presents a small head or capitellum for the radius, and a pulley or trochlea for the movements of the ulna in flexion and extension of the limb. The non-articular part consists of two condyloid eminences, internal and external. From the external, or epicondyle, a ridge passes for some distance along the outer border of the bone; it gives origin to the supinator and extensor muscles in the fore-arm. From the internal pτninence, or epitrochlea, a ridge passes up the inner border of the shaft of the bone; this eminence gives origin to the pronator and flexor muscles in the fore-arm. In nearly two per cent of the bodies examined in the anatomy-rooms in the university of Edinburgh, a hooked process has been seen projecting from the shaft of the bone, about 2 inches above the epi-trochlea; this process is connected to the epitrochlea by a fibrous band, so as to form a foramen, which has been called supτa-condyloid. In these cases the median nerve invariably passes through the foramen, and not unfrequently is accompanied by the brachial artery. In the feline carnivora and some other mammals a foramen constantly occurs in this part of the humerus, through which, as a rule, both nerve and artery proceed, though in the common seal it transmits only the nerve.

Before describing the two bones of the fore-arm, the anatomist should note the range of movement which can take place between them. In one position, which is called supine, they lie parallel to each other, the radius being the more external bone, and the palm of the hand being directed forwards; in the other or prone position the radius crosses obliquely in front of the ulna, and the palm of the hand is directed backwards. Not only the bones of the fore-arm, but those of the hand are supposed to be in the supine position when they are described.

The Radius (Fig. 9) is the outer bone of the Fore-arm, and like all long bones possesses a shaft and two extremities. The upper extremity or head has a shallow, smooth cup for moving on the capitellum of the humerus; the outer margin of the cup is also smooth, for articulation with the ulna and annular ligament; below the cup is a constricted neck, and immediately below the neck a tuberosity for the insertion of the biceps. The shaft of the bone possesses three surfaces for the attachment of muscles, and a sharp inner border for the interosseous membrane. The lower end of the bone is much broader than the upper, and is marked posteriorly by grooves for the lodgment of tendons passing to the back of the hand: from its outer border a pointed styloid process projects downwards; its inner border has a smooth shallow fossa for articulation with the ulna, and its broad lower surface is smooth and concave, for articulation with the scaphoid and semilunar bones of the wrist.

The Ulna (Fig. 9) is also a long bone. Its upper end is subdivided into two strong processes by a deep fossa, the greater sigmoid cavity, which possesses a smooth surface for articulation with the trochlea of the humerus. The anterior or coronoid process is marked by an oblique ridge for the insertion of the brachialis anticus, whilst the posterior or olecranon process gives insertion to the large triceps muscle of the upper arm. Immediately below the outer border of the great sigmoid cavity is the small sigmoid cavity for articulation with the side of the head of the radius. The shaft of the bone possesses three surfaces for the attachment of muscles, and a sharp outer border for the interosseous membrane. The lower end, much smaller than the upper, has a pointed styloid process and a smooth articular surface, the outer portion of which is for the lower end of the radius, the lower part for moving on a cartilage of the wrist joint called the triangular fibro-cartilage.

The Hand consists of the Carpus or wrist, of the Metacarpus or palm, and of the free Digits, the thumb and four fingers. Anatomists describe it with the palm turned to. the front, and with its axis in line with the axis of the fore-arm.

The Carpal or Wrist bones (Fig. 9) are eight in number and small in size: they are arranged in two rows, a proximal,— i.e. a row next the fore-arm,—consisting of the scaphoid, semilunar, cuneiform, and pisiform; and a distal, — i.e. a row next the bones of the palm,—consisting of a trapezium, trapezoid, os magnum, and unciform; the bones in each row being named in the order they are met with, from the radial or outer to the ulnar or inner side of the wrist. It is unnecessary to give a separate description of each bone. Except the pisiform or pea-shaped bone, which articulates with the front of the cuneiform, each carpal bone is short and irregularly cuboidal in shape; its anterior (or palmar) surface and its posterior (or dorsal) being rough, for the attachment of ligaments; its superior and inferior surfaces being invariably smooth, for articulation with adjacent bones; whilst the inner and outer surfaces are also smooth, for articulation, except the outer surfaces of the scaphoid and trapezium (the two external bones of the carpus), and the inner surfaces of the cuneiform and unciform (the two internal bones). Occasionally a ninth or supernumerary bone may arise from the subdivision of the scaphoid, semilunar, or trapezoid, into two pieces; more rarely a distinct bone is found in the human wrist intercalated between the trapezoid, os magnum, semilunar, and scaphoid, which corresponds in position to the os intermedium, found constantly in the wrist of the orang, gibbon, the tailed apes, and many rodents and Insectivora.

s The Metacarpal bones, or bones of the Palm of the Hand, are five in number (Fig. 9). They are miniature long bones, and each possesses a shaft and two extremities. The metacarpal of the thumb is the shortest, and diverges outwards from the rest: its carpal extremity is saddle-shaped, for articulation with the trapezium; its shaft is somewhat compressed, and its phalangeal end is smooth and rounded, for the first phalanx of the thumb. The four other metacarpal bones belong to the four fingers: they are almost parallel to each other, and diminish in size from the second to the fifth. Their carpal ends articulate with the trapezoid, os magnum, and unciform: their shafts are three-sided: their phalangeal ends articulate with the first phalanges of the fingers.

! The number of Digits in the hand is five, which is the highest number found in the mammalia. They are distinguished by the names of pollex or thumb, and index, middle, ring, and little fingers. Their skeleton consists of fourteen bones, named phalanges, of which the thumb possesses two, and each of the four fingers three. The phalanx next the metacarpal bone is the first, that which carries the nail is the terminal or ungual phalanx, whilst the intermediate bone is the second phalanx. Each is a miniature long bone, with two articular extremities and an intermediate shaft, except the terminal phalanges, which have an articular surface only at their proximal ends, the distal end being rounded and rough, to afford a surface for the lodgment of the nail.

j The Inferior or Pelvic Extremity, or Lower Limb, consists of a proximal part or haunch, a distal part or foot, and an intermediate shaft subdivided into thigh and leg. Each part has its appropriate skeleton: in the haunch, the pelvic or innominate bone; in the thigh, the femur; in the leg, the tibia and fibula (the thigh-bone in man being

'longer than the leg-bones); in front of the knee, the patella; in the foot, the tarsal and metatarsal bones and phalanges. The bone of the haunch forms an arch or Pelvic Girdle, which articulates behind with the side of the sacrum, and arches forward to articulate with the opposite haunch-bone at the pubic symphysis. It is the direct medium of connection between the axial skeleton and the shaft and foot, which form a free divergent Appendage.

The Os Innominatum, or Haunch-bone, is a large irre- ] gular plate-like bone, which forms the lateral and anterior l boundary of the cavity of the pelvis. In early life it consists of three bones—ilium, ischium, and pubis—which unite about the twenty-fifth year into a single bone. These bones converge, and join to form a deep fossa or cup, the acetabulum or cotyloid cavity, on the outer surface of the bone, which lodges the head of the thigh-bone at the hip-joint. One-fifth of this cup is formed by the pubes, and about two-fifths each by the ischium and ilium. At the bottom of the acetabulum is a depression, to the sides of which the interarticular ligament of the hip-joint is attached. From the acetabulum the ilium extends upwards and backwards, the ischium downwards and backwards, the pubis forwards and inwards. In front of the acetabulum is a large hole, the obturator or thyroid foramen, which is bounded by the ischium and pubes; behind the acetabulum is the deep sciatic notch, which is bounded by the ischium and ilium.

The Ilium (Fig. 10) in man is a broad plate-like bone. In its most simple form, as in the kangaroo, it is a three-sided, prismatic, rod-like bone, one end of which enters into the formation of the acetabulum, whilst the other is free, and forms the iliac crest. In man, notwithstanding its ex panded form, three surfaces may also be recognised, corresponding to the surfaces in the ilium of the kangaroo; and, as in that animal, the lower end aids in forming the acetabulum, while the upper end forms the iliac crest, which, in man, in conformity with the general expansion of the bone, is elongated into the sinuous crest of the ilium. This crest is of great importance, for it affords attachment to the broad muscles which form the wall of the abdominal cavity. One surface of the ilium is external, and marked by curved lines which subdivide it into areas for the origin of the muscles of the buttock; another surface is anterior, and hollowed out to give origin to the iliacus muscle; the third, or internal, surface articulates posteriorly with the sacrum, whilst anteriorly it forms a part of the wall of the true pelvis. The external is separated from the anterior surface by a border which joins the anterior end of the crest, where it forms a process, the anterior superior spine. About the middle of this border is the anterior inferior spine. Between the external and internal surfaces is a border on which are found the posterior superior and inferior spines; between the anterior and internal surfaces is the pectineal border, which forms part of the line of separation between the true and false pelvis.

The Pubis (Fig. 11) is also a three-sided, prismatic, rod-like bone, the fundamental form of which is obscured by the modification in shape of its inner end. In human anatomy it is customary to regard it as consisting of a body and of two branches, a horizontal and a descending ramus. The body and horizontal ramus form the fundamental prismatic rod, and the descending ramus is merely a special offshoot from the inner end of the rod. The outer end of the rod takes a part in the formation of the acetabulum; the inner end is expanded into the body of the pubis, and has a broad margin, or symphysis, for articulation with the corresponding bone on the opposite side of the pelvis. The three surfaces are— a, superior, for the origin of the pectineus muscle; a posterior, which enters into the wall of the true pelvis; and an inferior, which forms the upper boundary of the obturator foramen. The descending ramus is merely a downward prolongation of the inner end of the bone which joins the ischium, and aids in forming the side of the pubic arch. The junction of the outer end of the pubis with the ilium is marked by the pectineal eminence. The superior and posterior surfaces are separated by the sharp pectineal line, which, starting from the spine of the pubis, runs outwards to aid in forming the brim of the true pelvis.

The Ischium (Fig. 11), like the ilium and pubis, has the fundamental form of a three-sided prismatic rod. One extremity (the upper) completes the acetabulum, whilst the lower forms the large prominence, or tuber ischii. The surfaces of the bone are internal or pelvic, external, and anterior. The pelvic and external surfaces are separated from each other by a sharp border, on which is seen the ischial spine. The pelvic and anterior surfaces are separated by a border, which forms a part of the boundary of the obturator foramen; but the margin between the external and anterior surfaces is feebly marked. The tuberosity, a thick, rough, and strong process, gives origin to several powerful muscles: on it the body rests in the sitting posture; an offshoot, or ramus, ascends from it to join the descending ramus of the pubis, and completes both the pubic arch and the margin of the obturator foramen.

By the articulation of the two innominate bones with each other in front at the pubic symphysis, and with the sides of the sacrum behind, the osseous walls of the cavity of the Pelvis are formed. This cavity is subdivided into a false and a true pelvis. The false pelvis lies between the expanded wing-like portions of the two ilia. The true pelvis lies below the two pectineal lines and the base of the sacrum, which surround the upper orifice or brim of the true pelvis, or pelvic inlet; whilst its lower orifice or outlet is bounded behind by the coccyx, laterally by the ischial tuberosities, and in front by the pubic arch. In the erect attitude the pelvis is so inclined that the plane of the brim forms with the horizontal plane an angle of from 60° to 65°. The axis of the cavity is curved, and is represented by a line drawn perpendicularly to the planes of the brim, the cavity, and the outlet; at the brim it is directed upwards and forwards, at the outlet downwards and a little forwards. Owing to the inclination of the pelvis, the base of the sacrum is nearly 4 inches higher than the upper border of the pubic symphysis. The female pelvis is distinguished from the male by certain sexual characters. The bones are more slender, the ridges and processes for muscular attachment more feeble, the breadth and capacity greater, the depth less, the ilia more expanded, giving the greater breadth to the hips of a woman than a man; the inlet more nearly circular, the pubic arch wider, the distance between the tuberosities greater, and the obturator foramen more triangular in the female than in the male. The greater capacity of the woman’s than the man’s pelvis is to afford greater room for the expansion of the uterus during pregnancy, and for the expulsion of the child at the time of birth.

The Femur or Thigh-bone (Fig. 11) is the longest bone in the body, and consists of a shaft and two extremities. The upper extremity or head possesses a smooth convex surface, in which an oval roughened fossa, for the attachment of the inter-articular ligament of the hip, is found; from the head a strong elongated neck passes downwards and outwards to join the upper end of the shaft; the place of junction is marked by two processes or trochanters: the external is of large size, and to it are attached many muscles; the internal is much smaller, and gives attachment to the psoas and iliacus. A line drawn through the axis of the head and neck forms with a vertical line drawn through the shaft an angle of 30°; in a woman this angle is less obtuse than in a man, and the obliquity of the shaft of the femur is greater in the former than in the latter. The shaft is almost cylindrical about its centre, but expanded above and below; its front and sides give origin to the extensor muscles of the leg; behind there is a rough ridge, which, though called linea aspera, is really a narrow surface and not a line; it gives attachment to several muscles. The lower end of the bone presents a large smooth articular surface for the knee-joint, the anterior portion of which forms a trochlea or pulley for the movements of the patella, whilst the lower and posterior part is subdivided into two convex condyles by a deep fossa which gives attachment to the crucial ligaments of the knee. The inner and outer surfaces of this end of the bone are rough, for the attachment of muscles and the lateral ligaments of the knee.

The Patella or Knee-pan (Fig. 11) is a small triangular flattened bone developed in the tendon of the great extensor muscles of the leg. Its anterior surface and sides are rough, for the attachment of the fibres of that tendon; its posterior surface is smooth, and enters into the formation of the knee-joint.

Between the two bones of the leg there are no movements of pronation and supination as between the two bones of the fore-arm. The tibia and fibula are fixed in position; the fibula is always external, the tibia internal.

The Tibia or Shin-bone (Fig. 11) is the larger and more important of the two bones of the leg; the femur moves and rests upon its upper end, and down it the weight of the body in the erect position is transmitted to the foot. Except the femur, it is the longest bone of the skeleton, and consists of a shaft and two extremities. The upper extremity is broad, and is expanded into two tuberosities, the external of which has a small articular facet inferiorly, for the head of the fibula; superiorly, the tuberosities have two smooth surfaces, for articulation with the condyles of the femur; they are separated by an intermediate rough surface, from which a short spine projects, which gives attachments to the inter-articular crucial ligaments and semilunar cartilages of the knee, and lies opposite the intercondyloid fossa of the femur. The shaft of the bone is three-sided; its inner surface is subcutaneous, and forms the shin; its outer and posterior surfaces are for the origin of muscles; the anterior border forms the sharp ridge of the shin, and terminates superiorly in a tubercle for the inser-. tion of the extensor tendon of the leg; the outer border, of the bone gives attachment to the inter-osseous membrane of the leg. The lower end of the bone, smaller than the upper, is prolonged into a broad process, internal malleolus, 1 which forms the inner prominence of the ankle: its under j surface is smooth for articulation with the astragalus; externally it articulates with the lower end of the fibula. l 1

The Fibula, or Splint-bone of the leg (Fig. 11), is a slender long bone with a shaft and two extremities. The upper end or head articulates with the outer tuberosity of the tibia. The shaft is three-sided, and roughened for the origins of muscles; along the inner surface is a slender ridge for the attachment of the interosseous membrane. The lower end has a strong process (external malleolus') projecting downwards to form the outer prominence of the anlde, and possesses a smooth inner surface for articulation with the astragalus, above which is a rough surface for the attachment of ligaments which bind together the tibia and fibula.

The Foot consists of the Tarsus, the Metatarsus, and the five free Digits or Toes, which is the maximum number found in mammals. The human foot is placed in the prone position, with the sole or plantar surface in relation to the ground; the dorsum or back of the foot directed upwards; the axis of the foot at about a right angle to the axis of the leg; and the great toe or hallux, which is the corresponding digit to the thumb, at the inner border of the foot. The human foot, therefore, is a pentadactylous, plantigrade foot.

The bones of the Tarsus, or Ankle (Fig. 11, Tr), are seven in number, and are arranged in two transverse rows,—a proximal, next the bones of the leg, consisting of the astragalus, os calcis, and scaphoid; a distal, next the metatarsus, consisting of the cuboid, ecto-, meso-, and ento-cuneiform. If the tarsal bones be looked at along with those of the metatarsus and toes, the bones of the foot may be arranged in two longitudinal columns,—an outer, consisting of the os calcis, cuboid, and the metatarsal bones and phalanges of the fourth and fifth toes; an inner column consisting of the astragalus, scaphoid, three cuneiform, and the metatarsal bones and phalanges of the first, second, and third toes. The tarsal, like the carpal bones, are short and irregularly cuboidal; the dorsal and plantar surfaces are as a rule rough for ligaments, but as the astragalus is locked in between the bones of the leg and the os calcis, its dorsal and plantar surfaces, as well as the dorsum of the os calcis, are smooth for articulation; similarly, its lateral surfaces are smooth for articulation with the two malleoli. The posterior surface of the os calcis projects backwards to form the prominence of the heel. With this exception, the bones have their anterior and posterior surfaces smooth for articulation. Their lateral surfaces are also articular, except the outer surface of the os calcis and cuboid, which form the outer border; and the inner surface of the os calcis, scaphoid, and ento-cuneiform, which form the inner border of the tarsus. A supernumerary bone is sometimes found in the human tarsus, from a subdivision of either the ento-cunei-foπn, astragalus, os calcis, or cuboid into two parts. In some rodents and other mammals eight is the normal number of bones in the tarsus.

The Metatarsal bones and the Phalanges of the toes agree in number and general form with the metacarpal bones and the phalanges in the hand. The bones of the great toe or hallux are more massive than those of the other digits, and this digit, unlike the thumb or pollex, does not diverge from the other digits, but lies almost parallel to them.

Development and Homologies of the Skeleton.

It will now be advisable to consider briefly the mode of development of the skeleton, and along with the study of its genesis to compare its several parts with each other, in order to ascertain if correspondences in their arrangement and mode of origin exist, even if they differ in the function or office which they perform. When two or more parts or organs correspond with each other in structure, relative position, and mode of origin, we say they are homologous parts, or homologues; whilst parts which have the same function, but do not correspond in structure, relative position, and mode of origin, are analogous parts, or analogues. Homologous parts have therefore a morphological identity with each other, whilst analogous Krts have a physiological agreement. The same parts may be th homologous and analogous, as the fore-limbs of a bat and a bird, both of which, with the same fundamental type of structure, are subservient to flight. In other cases analogous parts are not homologues, as is illustrated by the wing of the insect, which, though subservient to flight, is fundamentally different in structure from the wing of the bat or bird.

In the germinal area of the fertilised vertebrate ovum a longitudinal groove appears which marks the beginning of the cranial cavity and spinal canal of the young embryo. At the bottom of this craniospinal groove a slender rod is formed, called chorda dorsalis or notochord. Each side of the groove then becomes elevated as a thin membrane, to meet behind to enclose a canal in which the brain and spinal marrow are developed. Small dark masses, the primordial or proto- vertebrae, next form on each side of the chorda dorsalis. In these proto-vertebrae, about the sixth or seventh week of intra-uterine life of the human ovum, little masses of cartilage appear, which correspond in number and position to the future spinal vertebrae. The part of the cartilage which forms the body of the future vertebra is developed around the chorda dorsalis, which it encloses in its substance, whilst the cartilaginous neural arch forms in the membrane which closes in the spinal canal. The formation of these cartilaginous vertebrae is completed in the human embryo about the fourth month of intra-uterine life. The bodies of the cartilaginous vertebrae are connected together, by plates or discs of intervening fibro-cartilage, which are also developed around the chorda dorsalis. After the enclosure of the rod-like chorda by the cartilaginous vertebrae and the inter-vertebral discs it disappears, no remains being found in the adult human body, or in that of the higher vertebrates, except perhaps some slight traces in the soft pulpy centres of the inter-vertebral discs; although in the cartilaginous fish it remains as a more or less complete structure throughout life.

In each of the cartilaginous vertebrae bone begins to form and to spread beyond its original point of formation, which is called a centre or nucleus of ossification; the greater part of the body is formed from one of these centres, and each half of the neural arch from another; whilst small ossifie centres arise for the tips of the spinous, transverse, and mammillary processes, and a special plate appears for both the upper and lower smfaces of the body; the fusion of the various centres together to form a complete vertebra takes place between the twentieth and twenty-fifth year. The atlas has a separate centre for each lateral mass and one for the anterior boundary of the ring. The axis, in addition to the ossifie centres found in the vertebrae generally, has one or two for the odontoid process. The seventh cervical vertebra has the anterior bar of its transverse process developed from a separate centre. Each coccygeal vertebra possesses only a single centre, which represents the body of the bone.

At the time when the cranio-spinal canal is being closed in by the development of its membranous walls, the germinal layers of the young embryo grow towards its anterior or ventral surface, and meet in the ventral mesial line, so as to enclose the cavities in which the thoracic and abdominal viscera are developed. In the membranous wall on each side of the thoracic cavity twelve cartilaginous rods, the future ribs, are developed; and, connected with the anterior ends of the seven pairs of upper ribs, the cartilaginous sternum is formed. Each rib ossifies from one centre for its shaft, and one each for the head and tubercle. The sternum ossifies in transverse segments,— one for the prae-stemum, one or sometimes two for each of the four subdivisions of the meso-sternum, and one for the xiphi-sternum. The complete ossification and fusion of the different parts of the sternum into a single bone does not take place until an advanced age.

The axial part of the skeleton, formed by the vertebrae, ribs, and sternum, is built up of a series of thirty-three transverse segments, equal in number, therefore, to the bones of the spine; so that each vertebra, according as it is, or is not, articulated with a pair of ribs and a segment of the sternum, constitutes a complete or incomplete transverse segment. These several segments are serially homologous with each other, but the homology is not so complete in some of the segments as in the others. In the coccygeal, sacral, and lumbar regions of man and most vertebrates, only the vertebral portion of each skeletal segment is represented, though in the abdominal wall of the crocodiles abdominal ribs and a sternum are developed. In the thoracic region the five lowest dorsal vertebrae have five pairs of ribs developed in connection with them; whilst the seven highest vertebrae have not only their corresponding pairs of ribs, but also a sternum, which bone, however, has only six transverse segments. In the cervical region seven. vertebrae are found, but the anterior bar of the transverse process, although fused with the vertebral body, is homologous with a rib, for in man it sometimes develops as a distinct movable rib in connection with the seventh cervical; and in the crocodiles small movable ribs are regularly developed in connection with the different cervical vertebrae. The bodies and neural aχches of the vertebrae are serially homologous with each other; as a rule this is also the case with their processes, but the articular processes of the atlas and the superior pair of the axis, although functionally analogous, are not homologous with the articular processes of the other vertebrae, but with the articular surfaces for the ribs on the bodies of the dorsal vertebrae, for they lie in front of, and not behind, the vertebral notches through which the spinal nerves are . transmitted. The development of the odontoid process of the axis shows it to.be the body of the atlas displaced from its proper bone and fused with the body of the axis.

The development and homology of the skull is a much more difficult problem to solve than that of the spine. The chorda dorsalis extends along the floor of the skull as far forward as the posterior wall of the pituitary fossa. Cartilage is formed around it, without, however, the previous production of proto-vertebrae, and ⅜l∏∙ s cartilage is prolonged forward on each side of the fossa, forming two bars, the trabeculae cranii; these bars then unite, and form the mes-ethmoid cartilage; at the same time the cartilage grows outwards for some distance in the membranous wall of the skull, but it does not mount upwards so as to close it in superiorly, so that the cartilage is limited to the floor of the skull; moreover, the cartilage is not segmented. The roof, side walls, and anterior wall of the cranium retain for a time their primordial membranous structure. This membrane is prolonged downwards into the face proper, where it forms a pair of maxillary lobes or processes, which pass forwards beneath the eyes to form the side parts of the face, and a mid-or frontal-nasal process, into which the cartilaginous mes-ethmoid extends. Immediately below each maxillary lobe four arches, called branchial or visceral, arise in the ventral aspect of the head, and in each of the three first of these arches a rod of cartilage is formed. The arches on opposite sides unite with each other in the ventral mesial line, but those on the same side are separated from each other by intermediate branchial clefts; these clefts all close up in course of time, except the upper part of the first, which remains as the external meatus of the ear, the tympanum, and the Eustachian tube; whilst the interval between the first visceral arch and the maxillary lobes forms the cavity of the mouth. The conversion of the primordial cartilaginous and membranous cranium into the bones of the head takes place by the formation in it of numerous centres of ossification. The basi-, ex-, and so much of the supra-occipital as lies below the superior curved line, are formed from distinct centres in the cartilaginous floor of the skull; whilst the part of the supra-occipital above the curved line arises from independent .centres in the membranous cranium, the whole ultimately fusing together to form the occipital bone. The basi-or post-sphenoid, the pre-with the orbito-sphenoids, the ali-sphenoid with the external pterygoid and the internal pterygoid, also arise in the cartilaginous floor, and they, together with the sphenoidal spongy bones which are formed in the membranous cranium, fuse into the sphenoid bone. The palate is apparently formed by ossification of cartilage continuous with the bar in which the internal pterygoid arises. The central plate and each lateral mass of the ethmoid also arise in the cartilage by distinct centres. The inferior turbinal has also a distinct origin in cartilage. The petro-mastoid part of the temporal arises in cartilage from at least three centres, peri-, pro-, and opisth-otic, and soon blends with the squamous and tympanic elements which arise in the membranous cranium; subsequently the styloid process, which is ossified in the rod of cartilage in the second visceral arch, joins the temporal. The lower end of this same rod forms the lesser cornu of the hyoid; the upper end forms two small bones, the stapes and incus, situated within the cavity of the tympanum. The cartilage of the third visceral arch forms the great cornu and body of the hyoid bone. The name of Meckel’s cartilage is applied to the rod found in the first visceral arch; its upper end is ossified into the malleus, a small bone situated in the tympanic cavity; whilst in the membrane surrounding the rest of the cartilage the lower jaw-bone is formed. The parietal and frontal bones arise altogether in the membranous vault; and the nasal, lachrymal, malar, and superior maxillae arise in connection with the bones which form the face; the vomer is developed in the membrane investing the mes-ethmcid cartilage. The human superior maxilla represents not only the superior maxilla of other vertebrates, but the pre-maxillary bone also; but the two bones become fused together at so very early a period that it is difficult to recognise their original independence. In the deformity of hare-lip and cleft palate, they are not unfrequently separated by a distinct fissure.

Since the time when 0ken and Goethe propounded the theory that the skull was built up of several vertebrae, the vertebral structure of the skull has led to much discussion amongst anatomists. Every one admits that the skull is in series with the spine, that the cranial cavity is continuous with the spinal canal, and that the cranial vault is formed in the wall of the embryonic cerebro-spinal canal. The skull also, like the spine, is transversely segmented, but whether we regard these segments as vertebrae or not wiD depend upon the conception we entertain of the meaning of the term vertebra. If with Owen we define a vertebra to be “one of those segments of the endo-skeleton which constitute the axis of the body and the protective canals of the nervous and vascular trunks,” then we may support the vertebral nature of the cranial segments on the following grounds :—lsi, The presence of a series of bones extending forwards from the foramen magnum along the basis cranii, in series with the bodies of the spinal vertebrae,— e.g., the basi-occipital, basi-sphenoid, pre-sphenoid, mes-ethmoid (Fig. 7); 2d, The presence of a series of neural arches which enclose and complete the wall of the cranial cavity, and lie in series with the neural arches of the spinal vertebrae,— e.g., the ex-and supra-occi-pitals, which form the neural arches of the basi-occipital segment; the ali-sphenoids and parietals, which form the neural arches of the basi-sphenoid segment; the orbito-sphenoids and frontal, which form the neural arches of the pre-sphenoid segment; 3d, The presence of a series of visceral arches of which the mandibular and hyoidean enclose the alimentary and vascular canals, just as the ribs enclose them in the thorax; and 4th, The presence of foramina between the cranial segments like the inter-vertebral foramina between the spinal vertebrae for the transmission of nerves,— e.g., the sphenoidal fissure and the jugular foramen.

But if we are to regard a vertebra as a segment of the axial skeleton, which in course of its formation passes through a definite series of developmental changes, then the cranial segments cannot be regarded as vertebrae in the same sense as the spinal segments; for, lsi, The chorda dorsalis is not co-equal in length with the basis cranii, as with the bodies of the spinal vertebrae, so that if the basi-occipital and basi-sphenoid segments, the bodies of which are developed around it, were to be regarded as cranial vertebrae, the pre-sphenoidal and ethmoido-nasal would not be morphologically the same, as they are formed in front of the anterior end of the chorda. 2d, Proto-vertebrae are formed in the spine, but not in the basis cranii, ⅛d, The spine is transversely segmented in its cartilaginous stage of development, but the skull is not. Atlι, The transverse segmentation of the skull only appears when the bones are formed, but the individuality of the segments becomes again concealed by the fusion of the pre-and basi-sphenoids and the basi-occipital into a continuous bar of bone, a condition which is not found in the spine except in the sacro-coccygeal region, ζ>th, The neural arches in the spine are, like the bodies, ossified in cartilage, but in the cranium they are for the most part ossified in membrane. These differences in the mode of development of the spine and basis cranii may be summarised as below :—

Spine. 1st Stage, 2d Stage, ⅛d Stage, 4th Stage,

Unsegmented Proto-vertebrae. Segmented Segmented chorda. cartilage. bones.

Basis Cranii. 1st Stage, 2d Stage, 3d Stage, 4th Stage,

Unsegmented Unsegmented Segmented Unsegmented chorda in part. cartilage. bones. bones.

It is evident, therefore, that, although both skull and spine are developed in the walls of the cerebro-spinal groove, yet, to quote the words of Huxley, “though they are identical in general plan of construction, the two begin to diverge as soon as the one puts on the special character of a skull and the other that of a vertebral column; the skull is no more a modified vertebral column than the vertebral column is a modified skull.”

The limbs, at their first appearance, sprout like little buds or lappets from the sides of the trunk; cartilage forms within them, which assumes the shape of the future bones, and as the limbs grow outwards, manifestations of joints appear, and the subdivision of each limb into its several segments takes place. The clavicle, which ossifies before any of the other bones, begins, to form, however, in fibrous membrane; and at a much later period the ends of the bone, which are formed in cartilage, unite with the intermediate shaft. The scapula ossifies from one centre for its expanded plate and spine, two small centres each for the acromion and vertebral border, and one for the coracoid. In many vertebrates, more especially birds and reptiles, the coracoid is a distinct bone from the scapula, but they articulate with each other to form the glenoitf fossa. Each of the three rod-like bones of which the innomina⅛ bone is composed, ossifies from one centre for the shaft of the bone, and one for each extremity; in the ilium these terminal centres an situated at the crest and acetabulum; in the ischium, at the tubeτ and acetabulum; and in the pubis, at the symphysis and acetabulum. Each of the long bones of the shafts of the limbs ossifies from a single centre for the shaft, and one or more centres for each articular extremity. Each carpal and tarsal bone ossifies from a single centre, except the os calcis, which possesses an independent centre for its posterior' surface. The metacarpal and metatarsal bones and the phalanges ossify each from two centres, one for the shaft and one for one of the extremities. In the metacarpal bones of the fingers and the four outer metatarsals, the distal end is that which ossifies independently; in the metacarpal of the thumb, in the metatarsal of the great toe, and in all the phalanges, the proximal end is that which ossifies independently. As the method of ossification oî the first metacarpal and first metatarsal corresponds with that of the phalanges, some anatomists hold that these bones are really the first phalanges of their respective digits, and that the bone which is absent in these digits, when compared with the other digits, is not a phalanx, but a meta-carpal or tarsal bone. When the extremity of a bone ossifies from a centre distinct from the centre from which the shaft arises, it 13 called an epiphysis. The epiphysis is united to the shaft of the growing bone by an intermediate plate of cartilage, and so long as any of this cartilage remains unossified the bone can continue to grow in length. The ossification is not completed in the different bones until from the twentieth to the twenty-fifth year. In the case of the long bones, the epiphysis situated at the end of the bone, towards which the canal in the shaft which transmits the nutrient artery is directed, ossifies to the shaft before the epiphysis at the other end. In the humerus, tibia, and fibula, where the canal is directed downwards, the epiphyses at the lower ends of the bones first unite with the shaft; whilst in the femur, radius, and ulna, where the canal is directed upwards, the ossification first takes place between the upper epiphysis and the shaft.

All anatomists hold that the bones of the shaft and distal part of a limb belong to the appendicular part of the skeleton, but there is a difference of opinion as to the place in the skeleton to which the bones of the shoulder girdle and haunch are to be referred. Owen considers that the scapular and pelvic arches belong to the axial skeleton, and are homologous with the ribs; the scapula and coracoid as the visceral or rib-arch of the occipital vertebra, the clavicle of the atlas, and the innominate bone of the upper sacral vertebra. Goodsir objected to this conclusion of Owen’s on the ground that the shoulder girdle was not in series with the visceral arches, but was developed outside the visceral wall, at the junction of the cervical and thoracic regions, from which region the upper limb receives its nerves, ana not from the occipito-atlantal region, whence they would have proceeded had it been an appendage of the rib-arches of those segments. Owen’s chief argument for regarding the scapula and coracoid as the costal arch of the occipital vertebra is because in fish the scapula is attached to the occipital bone by a bone which Cuvier called the supra-scapula, and which he believed to be homologous with the supra-scapular cartilage of many other vertebrates. Parker, however, has recently pointed out that the so-called supra-scapula of a fish is not homologous with the supra-scapula of a reptile or mammal, that it is not a cartilage bone, but is a splint or scale-like bone, developed as a part of the dermo-skeleton. Between the scapula and coracoid and the innominate bone, anatomists have long recognised homologies to exist; the scapula is generally admitted to be the homotype of the ilium and the coracoid of the ischium, so that if these elements of the shoulder girdle be not a costal arch, neither can those of the pelvic girdle. The clavicle has by some been regarded as the homotype of the pubis; but in all probability the pubis is homologous with the procoracoid bone which is found in the amphibia and some reptiles, but is absent in crocodiles, birds, and mammals; whilst the clavicle is represented in the pelvic girdle, not by a bone, but by a fibrous band called Poupart’s ligament. Between the bones of the shafts of the limbs homologies exist: the humerus is the homotype of the femur, the radius of the tibia, the ulna of the fibula; whilst the patella has no representative in the human upper limb. The scaphoid and semilunar bones in the carpus are homotypes of the astragalus in the tarsus, the cuneiform is the homotype of the os calcis, the cuboid of the unciform; the trapezium of the ento-cuneiform, the trapezoid of the meso-, and the os magnum of the ecto-cuneiform. The tarsal scaphoid is not, as a rule, represented in the human carpus, but its homotype is the os intermedium, found in many mammals. The carpal pisciform is a sesamoid bone developed in the tendon of a muscle. The metacarpal bones and phalanges are homologous with the metatarsal bones and phalanges; the thumb with the great toe, and the fingers with the four outer toes. During the growth of the limbs outward, and their change from the simple lappet-like form to their elongated condition, a rotation of the proximal segment of the shaft takes place—that of the upper limb a quarter of a circle backward, that of the lower limb a quarter of a circle forward—to produce in the former case a supine position of the fore-arm and hand, with the thumb as the outermost digit; in the latter case, a prone condition of the· leg and foot, with the great toe as the innermost digit. The range of movement at the radio-ulnar joints enables us, however, to pronate the hand and fore-arm by throwing the radius across the ulna, so as to make the thumb the innermost digit. In many quadrupeds the fore-leg is fixed in this position, so that these animals walk on the soles of both the fore and hind feet.

General Observations on the Articulatory and Muscular Systems.

A Joint or Articulation is the junction or union of any two adjacent parts of the body. Most usually the term is employed to signify the connection established between contiguous bones. It is by the joints that the various bones are knit together to form the skeleton. Joints may be either immovable or movable.

The immovable joints are divided into the synchondroses and the sutures. A synchondrosis is the junction of two bones by the interposition of an intermediate plate of cartilage, the fibrous membrane or periosteum which invests the bones being prolonged from one bone to the other over the surface of the cartilage. A suture is the connection of two bones by the interlocking of adjacent toothed margins; the periosteal fibrous membrane is prolonged from one bone to the other, and is also interposed between their adjacent margins. In a young skull the basi-occipital and basi-sphenoid are united by synchondrosis, but junction by sutures is the mode of union which prevails in the bones of the head. In old persons the sutures become obliterated by the ossification of the intermediate fibrous membrane, and the bones are permanently fused together. The cranial sutures may conveniently be arranged in three groups: a, Median longitudinal, consisting of the frontal suture, which connects the two halves of the frontal bone, and the sagittal suture, between the two parietal bones; ⅛, Lateral longitudinal, consisting on each side of the head of the fronto-nasal, fronto-maxillary, fronto-lachrymal, frontoethmoidal, fronto-malar, fronto-sphenoidal, parieto-sphe-noidal, parieto-squamous, parieto-mastoid sutures; ^Vertical transverse, consisting of the coronal or fronto-parietal, the lambdoidal or parieto-occipital, the sphenoido-malar, sphenoido-squamous and occipito-mastoid sutures. As the skull grows by ossification of the cartilage of the base and the membranous vault, the direction of growth is perpendicular to the margins of the bones and the sutures and synchondroses which connect them together. The growth of the skull in length is perpendicular, therefore, to the basi-cranial synchondrosis and the vertical transverse group of sutures; its growth, in breadth, to the median longitudinal group, and in height to the lateral longitudinal group. So long as any of the cartilage or membrane between the margins of the bones remains unossified, bone may continue to form, and the skull may increase in size. It sometimes happens that the cartilage or membrane is prematurely ossified in a particular locality, and the further growth of the skull put a stop to in that region; if the brain is still growing, the skull must increase in other directions to permit of the expansion of the cranial cavity, and deformities of the skull are thereby occasioned. One of the most usual of these deformities is due to premature closure of the sagittal suture, causing «stoppage of the growth of the skull in breadth, and, byway of compensation, great increase in its length, so as to produce a very elongated and somewhat boat-shaped cranium.

The movable joints are divided into the amphiarthrodial and the diarthrodial joints. An amphiarthrosis or halfjoint has only a feeble range of movement. It consists of two bones, each of which has its articular surface covered by a plate of cartilage, and which plates are firmly connected together by an intermediate disc of fibro-cartilage. The centre of this disc is soft, or may even be hollowed out into a cavity, lined by a smooth synovial membrane, and containing a little fluid.

Ligamentous bands, continuous with the periosteum investing the bones, invest the fibro-cartilage, and assist in binding the bones together. The best examples of amphiarthrodial joints are furnished by the articulations between the bodies of the true vertebrae.

A diarthrosis admits of more or less perfect movement. In it the two articular surfaces are each covered by a plate of encrusting cartilage, the free surface of which is smooth and polished; between these surfaces is a cavity containing a glairy fluid, the synovia, for lubricating the smooth surfaces of the cartilage and facilitating the movements of the joint. This cavity is enclosed by ligaments, which are attached to the bones, and the inner surface of these ligaments is lined by a synovial membrane which secretes the synovia. Sometimes a plate or meniscus of fibro-cartilage is interposed between, without, however, being attached to the encrusting cartilages of a diarthrodial joint, so as more or less perfectly to subdivide the cavity enclosed by the ligaments into two spaces. The articular surfaces of diarthrodial joints are retained in apposition with each other, sometimes by investing Hgaments, at others by surrounding muscles and tendons; at others by atmospheric pressure, aided by the adhesive character of the interposed synovia. The form of the articular or movable surfaces varies very materially in different examples of these joints, and the modifications in form determine the direction of the movements of the joints. In some, as the carpal and tarsal joints, the surfaces are almost flat, so that they glide on each other; the movement is comparatively slight, and about an axis perpendicular to the moving surfaces: these are called planesurfaced joints. In other joints the articular surfaces may be regarded as produced by the rotation of a straight or curved line about an axis lying in the same plane; these are called rotation joints, and they present various modifications according to the direction and relation of the rotating line to the axis. One form of a rotation joint is the pivot joint, in which the movement takes place about the axis of one of the bones, which is the axis of rotation of the joint; examples of this joint are found in the joint between the atlas and odontoid process of the axis and the radio-ulnar joint. Another form is the ginglymus oι hinge joint, in which the axis of rotation of the joint is perpendicular to the axis of the two bones; the mover ments of the hinge are called flexion when the angle between the two bones is diminished, and extension when the angle is increased. An important modification of the ginglymus is the screwed-surfaced joint, examples of which are found in the elbow and ankle; here the plane of flexion is not perpendicular, but oblique to the axis of the joint. The saddle-shaped and oblong joints are also modified hinges, but allow motion about two axes; in the oblong both axes are on the same side of the joint; but in the saddle-shaped there is an axis of rotation on each side of the joint. The best example of the saddle-shaped is found between the metacarpal bone of the thumb and the trapeziam; of the oblong between the fore-arm and the carpus. In the ball-andrsocket joint a spheroidal head fits into a cup, and rotation takes place about any diameter of the sphere; the joint therefore is multi-axial; the hip and shoulder joints are the best examples. Some joints, in which the forms of the articular surfaces are more complex, are called composite; in them the movements of a hinge and of a ball-and-socket joint may be combined; the knee may be cited as an example of this form of articulation. In a large number of movable joints only portions of the opposite articular surfaces are in contact with each other at a given time; but, as the joint describes its path of movement, different parts of the surfaces come into contact with each other successively, and it is not unusual to find the articular surface both of the cartilage and the subjacent bone mapped out into distinct areas or facets, which are adapted to corresponding facets on the opposite articular surface in particular positions of the joint. When the corresponding facets on opposite articular surfaces break contact with each other, the space between becomes occupied by synovia, or in some joints, more especially the knee, by folds of synovial membrane enclosing clumps of fat, which have been called synovial pads. In the simple hinge, in that with screwed surfaces, in the oblong and composite joints, the principal ligaments are situated at the sides of the joint, and are called lateral; they not only prevent lateral displacement of the bones, but, by a tightening of their fibres, check excessive movement forwards or backwards during flexion and extension. In the saddle-shaped and ball-and-socket joints, the joint is included within a bag-like ligament called capsular. In the pivot joints the cavity in which the pivot fits is completed by a transverse or a ring-shaped ligament.

The Muscles are the organs which, by their contraction or shortening, move the bones on each other at the joints. The muscles constitute the flesh of the body. They are so arranged as to be capable not only of moving the various bones on each other, but the entire body from place to place. Hence the muscles are organs both of motion and locomotion. As they can be brought into action at the will of the individual, they are called voluntary muscles. Some of the muscles are engaged in the movement of other structures than the bones, such as the eye-ball, tongue, cartilages of the larynx, &c. About 400 muscles are usually enumerated, and the names applied to them express either their position, or relative size, or shape, or direction, or attachments, or mode of action. The word muscle is itself derived from the Latin musculus, a little mouse, from a fancied resemblance between that animal and some of the most simply formed muscles. It is customary to distinguish in a muscle a central part, or belly, and two extremities, one of which is the head or the origin, the other the insertion. The belly is the fleshy part of the muscle, and possesses a deep-red characteristic colour; it is the active contractile structure, the source of motor power. The two extremities are called the tendons of the muscle, or sinews; the tendons are bluish-white in colour, possess no power of contractility, and are merely, as it were, the ropes by which the belly of the muscle is attached to the bone or other structure which is moved by its contraction. The term tendon of origin, applied to one extremity of the muscle, signifies the fixed end of the muscle, that to which it draws during its contraction; as a rule this is the end nearest the trunk, the proximal end. The term tendon of insertion is applied to the end which is moved by the contraction; as a rule this is the end most removed from the trunk, the distal end. Entering the substance of each muscle is at least one artery, which conveys blood for its nutrition; this artery ends in a network of capillary blood-vessels, from which a vein arises and conveys the blood out of the muscle again; another small vessel, called a lymphatic, also arises within the muscle, and conveys the fluid lymph out of the muscle. Each muscle also is penetrated by a nerve, through which it is brought into connection with the brain, so as to be subject to the influence of the will. The will is the natural stimulus for exciting muscular action, which action is in many cases so rapid that scarcely an appreciable interval of time intervenes between willing and doing the action.

The bones form a series of rod-like levers, and, in studying the mode of action of the muscles, the place of insertion of the muscle into the bone—that is to say, the point of application of the power which causes the movement— and its relations to the joint, or fulcrum, or centre of motion, and to the weight or resistance which is to be overcome, have to be kept in view. The relative positions of fulcrum, point of application of power, and resistance, are not the same in all the bony levers. As a rule, the muscles are inserted into bones between the fulcrum and the moveable point of resistance, and nearer the fulcrum than the movable point, as may be seen in the muscles which bend the fore-arm at the elbow-joint. Although from the weightarm of the lever being in these cases much longer than the power-arm, the muscles, as regards the application of the power, act at a disadvantage, yet the movement gains in velocity. Sometimes the muscle is inserted, as is the case in the great muscle which straightens or extends the fore-arm, at one end of the lever, and the fulcrum or joint is placed between it and the movable point. At other times, as in the case of the chief depressor muscle of the lower jaw, whilst the muscle is attached to one end of the lever, the fulcrum is at the opposite end. When a muscle is so placed that its tendon of insertion is perpendicular to the bone to which it is attached, it acts to great advantage; when placed obliquely or nearly parallel, a loss of power occurs. Many muscles at the commencement of contraction lie obliquely to the bones which they move, but as contraction goes on they become more nearly perpendicular, so that they act with more advantage near the close than at the commencement of contraction. If a muscle passes over only one joint, it acts on that joint only: but if it passes over two or more joints, it acts on them in succession, beginning with the joint next the point of insertion. A given movement may be performed by the contraction of a single muscle, but as a rule two or more muscles are associated together, and they are not unfrequently so arranged that one muscle initiates the movement, which is then kept up and completed by the rest. Muscles producing movement in one direction have opposed to them muscles which by their contraction effect the opposite movement; w r hen both groups act simultaneously and with equal force, they antagonise each other, and no motion is produced; when a muscle is paralysed or divided, its antagonistic muscle draws and permanently retains the part to its own side. The rapidity of action of a muscle is proportioned to the length of its fasciculi, its power of contraction to their number.

Each muscle is invested by a sheath formed of connective tissue. In the limbs and in the neck not only has each muscle a sheath, but a strong fibrous membrane envelopes the whole of the muscles, and assists materially in giving form and compactness to the region. This membrane is called generally a fascia or aponeurosis, but special descriptive names are given to it in the different regions— e.g., cervical fascia, brachial aponeurosis, fascia lata, or fascia of the thigh. In some localities muscles arise from the fascia, and in others they are inserted into it. The fascia is separated from the skin by a layer of subcutaneous fatty tissue, and in this layer muscles are in some localities developed. In the fat of the inner border of the palm of the hand a small muscle, the palmaris brevis, is found, which is inserted into the skin covering the ball of the little finger; at each side of the neck, also, lies a thin muscle called platysma myoides, and the muscles on the face and scalp which move the skin of the face and head belong to the same category. These muscles form the group of subcutaneous or dermal muscles which, except in the localities above referred to, are not represented in the human body, but are well known in the bodies of the mammalia generally as the panniculus carnosus.

In arranging the muscles for descriptive purposes, either a morphological, a topographical, or a physiological method may be pursued. The morphological arrangement is to be preferred when the object is to compare the muscular system in man with that in different animals, and the basis of the arrangement should be into muscles of the axial, the appendicular, and the axi-appendicular skeletons, and sub-cutaneous muscles; a topographical arrangement is most suitable for the purposes of the practical surgeon; a physiological arrangement, when the object is to study the action of the muscles in connection with the movements of the joints. In Plates XV. and XVI., a front and back view of the voluntary muscles of the body is given.

Joints and Muscles of the Axial Skeleton.

The Intervertebral Joints are complex in construction. The bodies of the true vertebrae are connected together by an amphiarthrodial joint: the fibro-cartilaginous plate or intervertebral disc is tough and fibrous in its peripheral part, but soft and pulpy within. (Fig. 15.) Remains of the chorda dorsalis are said to occur in the soft pulp, and sometimes a distinct cavity, lined by a synovial membrane, is found in the centre of the disc, which in the finner whales is expanded into a large central cavity containing many ounces of synovia. A diarthrodial joint connects the superior and inferior articular processes of adjacent vertebrae on each side. Elastic yellow ligaments, the ligamenta subflava, pass between their laminae. Inter- and supra-spinous ligaments connect -adjacent spinous processes, and in the neck the supra-spinous ligament forms a broad band, the ligamentum, nuchoe. In those mammals which possess big heads or heavy horns, this ligament of the back of the neck forms a powerful elastic band for the support of the head. The joints between the atlas and axis, and the atlas and occiput, are specially modified in connection with the movements of the head on the top of the spine. The intervertebral discs are absent, and the range of movement either from before backward, as in nodding the head, or from side to side, as in looking over the shoulder, are more extensive than between any of the other true vertebrae. The head rotates along with the atlas around the odontoid or pivot process of the axis, which is lodged between the anterior part of the atlas and a strong transverse ligament which lies behind the odontoid. Too great movement to one side or the other'is prevented by the check ligaments, which pass from the top of the odontoid to the occipital bone, in front of the foramen magnum. The nodding movements take place between the occiput and atlas, and are permitted by the size and shape of the occipital condyles and hollow upper articular surfaces of the atlas. These joints are all diarthrodial. The spine is flexible and elastic; except in the joints above referred to, the range of movement between any two true vertebrae is very small, but the sum of the movement in the entire spine, owing to the number of bones, is considerable. The elasticity of the spine is partly due to the numerous diarthrodial joints between its articular processes, but more especially to the discs of fibro-cartilage interposed between the bodies of the vertebrae, which act like elastic pads or buffers to prevent shock. The spine and trunk may be bent either forwards or backwards, or to the right and left side; or without being bent, the spine may be screwed to the right or to the left, the screwing movement being permitted by the oblique direction of the articular processes.

The muscles which move the vertebrae on each other are principally situated on the back of the trunk. In the hollow on each side of the vertebral spines lies the great erector spin» muscle, the fibres of which pass longitudinally upwards. When both muscles act together, the entire spine is bent back; but when the muscle of one side only contracts, then the spine is bent to that side. These muscles also act in raising the spine from the bent to the erect position, and they are assisted by small inter-spinal muscles, situated between the spines in the cervical and lumbar regions. The spine is bent forward by the psoae and longi colli muscles; and the straight muscles of the abdomen, inserted into the lower true ribs, assist in this movement. The screwing movements of the spine are effected by a series of muscles, the fibres of which pass obliquely between the laminae and spines of adjacent vertebrae, and are known as the semispinales, multifidi, and rotatores spinae muscles.

The head is balanced on the summit of the spine, and is maintained in a quiescent position without any appreciable muscular action, but it can be moved in various directions by the muscles inserted into its bones. The nodding movements of the head on the atlas are due to the posterior recti, the two superior obliques, the two splenii, and the two complexus muscles, which draw it backwards, and the anterior recti and stemo-cleido-mastoid muscles, which draw it forwards. When the right splenius and greater posterior rectus and inferior oblique act along with the left complexus and sterno-mastoid, the head is rotated to the right shoulder; the opposite rotation being due to the action of the corresponding muscles on the other side of the body.

In the formation of the walls of the abdomen proper, bones and joints play but a small part. The lumbar vertebrae behind, the expanded wings of the iliac bones below, and the false ribs above, are the only bones to be considered. Three pairs of greatly expanded muscles— the external oblique, internal oblique, and transverse—lie at the sides and in front, and two pairs of muscles tho recti and pyramidales—are situated wholly in front. The internal oblique and the transverse muscles are attached above to the ribs, behind to the lumbar spine, below to the iliac crest and to a strong band, Poupart’s ligament, extending from the crest of the ilium to the pubic spine; the external oblique has similar connections above and below, but is not attached behind to the lumbar spine. The muscles all terminate in front in strong expanded tendons, called the anterior abdominal aponeuroses, which blend together in the middle line anteriorly to form the band called linea alba, which stretches longitudinally from the xiphi-ternum to the pubic symphysis. These expanded tendons enclose the recti muscles, which pass from the pubis upwards to the cartilages of the lower true ribs, and the pyramidal muscles, which pass from the pubis to be inserted into the hnea alba. The entire arrangement is admirably adapted for completing the walls of the great abdominal chamber, and for enabling the muscles to compress the abdominal viscera, an action which takes place when the contents of the bowels and bladder are being expelled during defecation and micturition.

Bones and joints play a more important part in the formation of the walls of the thoracic than of the abdominal cavity. Not only are there thoracic vertebrae behind, and the sternum in front, but on each side the twelve ribs arch more or less completely forward from the spine; each rib is articulated behind to one or two vertebrae, and the seven upper ribs, through their costal cartilages, articulate with the sternum.

The Costo-vertébral Joints are situated between the head of the rib and the vertebral body; also, except in the floating ribs, between the tubercle of the rib and the transverse process of the vertebra, the joints being diarthrodial, and completed in the usual manner by ligaments and synovial membrane. The Costo-stèmal Joints are also diarthrodial (except the first costal cartilage, which is directly united to the prae-sternum), a capsular ligament, lined by a synovial membrane, connecting the cartilages of the true ribs to the sternum. The cartilages from the sixth to the ninth ribs are also united by ligamentous fibres.

The movements of the ribs and sternum at the costovertebral and costo-sternal joints are of the utmost importance in the process of breathing. . Breathing or respiration consists of two acts—breathing in, or inspiration, and breathing out, or expiration. During inspiration, the air rushes through the nose or mouth down the windpipe, and dilates the air-cells of the lungs; together with the expansion of the lungs the walls of the chest rise, so that the capacity both of lungs and chest at the end of a full inspiration is nearly doubled. During inspiration the following changes occur in the walls of the chest: the ribs are elevated and rotated, the lower borders of their shafts are everted, while their surfaces are at the same time rendered more oblique, and the width of the intercostal spaces is thereby increased; the elevation and rotation of the ribs throw the sternum upwards and forwards, and make the thoracic part of the spinal column straighter; the diaphragm is depressed, and the antero-lateral walls of the abdomen are thrown forward. The muscles which cause these movements are as follows:—In each of the spaces between the different ribs a pair of intercostal muscles is situated; these elevate and rotate the ribs, and the movements are assisted by the levatores costarum, and, in the case of the upper and lower ribs, by the scaleni and serrati postici muscles; and by these agents the transverse and antero- 'posterior diameter of the chest is increased. The increase in its vertical diameter is due to the action of the diaphragm or midriff, the great muscle which, arising by its circumference from the xiphi-sternum, six lower ribs, and bodies of the lumbar vertebrae, forms the floor of the thoracic and the roof of the abdominal cavity. It constitutes a great arch, with its convexity directed to the cavity of the chest. By the contraction of its fibres the arch is rendered less convex, and the floor of the chest is thereby depressed. Under circumstances which require more powerful efforts of inspiration, the muscles which pass from the walls of the chest to the upper limbs may, by taking their fixed points at the limbs, act as elevators of the ribs. During expiration the ribs are depressed, their lower borders inverted, the width of the intercostal spaces diminished, the sternum depressed, the spine more curved, and the diaphragm more convex. These movements are principally due to the recoil of the elastic tissue of the lungs previously rendered tense by the inflation of the air-cells, and to the untwisting of the ribs when the inspiratory muscles cease to elevate and rotate them. Muscular action plays but a small part in quiet expiration, but the expulsion of the air from the lungs may be facilitated by contracting the abdominal muscles, which, pressing the abdominal viscera against the under surface of the diaphragm, force that muscle upwards.

The Temporo-maxillary Joints are the only diarthrodial articulations in the head. The condyle of the lower jaw on each side is received into the glenoid fossa of the temporal bone; each joint is enclosed by a capsular ligament, and between the articular surfaces is a meniscus, which sub-dirides the interior of the joint into two cavities, each lined by a synovial membrane. The movements of the lower jaw. take place simultaneously at both its articulations during mastication and speech, through the action of the several muscles which are inserted into it. This bone is elevated by the temporal muscles, inserted into the coronoid processes; and by‘the masseterics, inserted into the outer surface, and the internal pterygoids, into the inner surface of each angle. It is depressed partly by its own weight and partly by the action of the digastrics and genio-hyoids, inserted close to the symphysis; by the platysma, inserted into the outer surface of each horizontal ramus; and the mylo-hyoids, into their inner surfaces. The elevators of the jaw are much more powerful than the depressors, for they not only have to overcome the weight of the bone, but during mastication have to exercise force sufficient to cut or break down the food between the teeth. In carnivorous animals, more especially those which, like the tiger or hyaeua, crack the bones of their prey, these muscles attain a great size. The lower jaw can be projected in front of the upper by the external pterygoid muscles, inserted into the neck of the bone on each side; but excessive movement forward is checked by the action of the stylo-maxillary ligaments, which pass from the styloid processes to the angles of the bone; when projected forward, the jaw is drawn back by the posterior fibres of the temporal muscles. When the elevator, depressor, protractor, and retractor muscles are successively brought into action, the lateral or grinding movements of the bone, so important in mastication, are produced.

Along with the movements of the lower jaw those of the hyoid bone and larynx must be considered, for the digastrics, the genio-and mylo-hyoids, which depress the lower jaw, act, when their action is reversed, along with the stylohyoid muscles in elevating the hyoid bone and larynx, which structures can be depressed or drawn downwards by the action of the sterno-hyoids, sterno-thyroids, thyro-hyoids, and omo-hyoids; the elevation of the hyoid, when drawn down by its depressor muscles, is effected by the elastic stylo-hyoid ligaments attached to its small cornua, which, by their recoil when the depressor muscles have ceased to contract, draw the bone up to its former position.

Numerous muscles are situated immediately beneath the skin of the scalp and face. They are not of so deep red a colour as the muscles of the trunk and limbs, and whilst they arise from one or other of the bones of the head, they are inserted into the deep surface of the skin itself. Hence when they contract they move the skin of the scalp and face, and as they are the instruments through which the various passions and emotions are expressed, they are grouped together as the Muscles of Expression (Plate XV., figs. 2 and 3). The occipito-frontalis, or great muscle of the scalp, passes from the occipital bone over the vertex to the forehead; when it contracts, the skin of the forehead is wrinkled transversely, the eyebrows are elevated, and an expression of amazement or surprise is produced. Some persons have a greater power over this muscle than others, and by the alternate contraction of its occipital and frontal portions can move the hairy scalp to and fro with great rapidity. A pair of muscles, the corrugatores supercilii, arises from the supra-ciliary ridges, on the frontal bone, to be inserted into the eyebrows: they draw the eyebrows downwards and inwards, wrinkle the skin of the forehead longitudinally, and contract with great rigour in the act of frowning. The auricle of the external ear has three small muscles inserted into it, one behind, the posterior, one above, the superior, one in front, the anterior auricular muscle: in man, as a rule, these muscles are feeble, and have little action; but in many mammals they are large, and by them the animal pricks its ears to detect the faintest sound of danger. The eyelids are drawn together, so as to close the eye as in the act of sleep, by the orbicularis palpebrarum, the fibres of which lie in the eyelids and on the borders of the orbit, and surround the fissure between the eyolids. This muscle is a characteristic specimen of the group of sphincter muscles, i.e., muscles which surround orifices, and by their contraction close them. When the upper fibres of the muscle alone contract, the uoρer eyelid is depressed,— a movement which takes place almost involuntarily and with great frequency during our waking hours, so as to wash the tears over ehe exposed part of the eyeball and keep it moist. In expressing a “knowing wink,” the lower fibres alone of the orbicularis contract, and the lower lid is elevated. The elevation of the upper lid, as in opening the eye, is due to the levator palpebrae superioris, Which, arising within the orbit, is inserted into the upper eyelid. Muscles are inserted into the framework of the nostrils so as to increase or diminish the size of their orifices, and thus to promote or impede the passage of air into the nose. The size of the orifice is increased by two elevator muscles inserted into the ala, or side of the nostril; and when violent exercise is being performed, or respiration is from any cause impeded, the nostrils are always widely dilated. One of these elevator muscles, which also sends a slip down to the upper lip, and is consequently called the common elevator, is the muscle by the contraction of which a sneer is expressed. A partial closure of the nostril can be effected by small muscles which depress and compress the alae of the nose: in man these muscles are rudimentary as compared with the seal and other aquatic mammals, in which a powerful sphincter muscle closes the nostrils in the act of diving. The lips can be elevated or depressed so as to close or open the mouth; they can be protruded or retracted, or the corners of the mouth can be drawn to one side or the other, by the action of various muscles which are inserted into these movable folds of the integument. The orbicularis oris is a sphincter muscle, the fibres of which lie both in the upper and lower lips; by its contraction the mouth is closed and the lips pressed against the teeth, as when a firm resolution is intended to be expressed. The mouth is opened by the elevator muscles of the upper and the depressors of the lower lip; it is transversely elongated by the zygomatic and risorius muscles, which pass to its corners, and which are brought into action in the acts of smiling and laughing. But the muscles of the lips also play an important part in connection with the reception of food into the mouth, and with the act of articulation.

The cavity of the mouth forms the commencement of the alimentary canal, and is lined by a soft mucous membrane. In it the teeth and tongue are situated, and into it the secretion called saliva is poured. It opens behind into the pharynx. The side walls of the mouth are called the cheeks, and into the formation of each cheek a flattened quadrilateral muscle, the buccinator, enters. This muscle is attached above and below to the upper and lower jawbones, behind to a fibrous band, to which the upper constrictor muscle is also connected, so that the walls of the mouth and pharynx are continuous with each other, whilst in front the buccinator blends with the structures in the lips. It compresses the cheeks, and drives the air out of the cavity of the mouth as in playing a wind instrument; hence the name, “trumpeter’s muscle.”

The aperture of communication between the mouth and pharynx is named the isthmus of the fauces. It is bounded below by the root of the tongue, on each side by the tonsils, and above by the soft palate. The soft palate is a structure which hangs pendulous from the posterior edge of the hard bony palate. From its centre depends an elongated body, the uvula, and from each of its sides two folds extend, one downwards and forwards to the tongue, the other downwards and backwards to the pharynx. These folds are called the anterior and posterior pillars of the fauces or palate. Between the anterior and posterior pillar, on each side, the tonsil is seated. The soft palate and its pillars are invested by the mucous lining of the mouth and pharynx, and contain small but important muscles. The muscles of the soft palate and uvula, termed the elevators and tensors, raise and make them tense during the process of deglutition. The muscles of the posterior pillars, or palato-pharyngei, by their contraction, approximate the walls of the pharynx to the soft palate and uvula, whilst the muscles of the anterior pillars, or palato-glossi, diminish the size of the fauces.

The pharynx is a tube with muscular walls, lined by a mucous membrane, which communicates above and in front with the cavities of the nose, mouth, and larynx, whilst below it is continuous with the oesophagus or gullet. It serves as the chamber or passage down which the food goes from the mouth to the oesophagus in the act of swallowing, and through which the air is transmitted from the nose or mouth to the larynx in the act of breathing. It lies immediately behind the nose, mouth, and larynx, and in front of the five upper cervical vertebrae. Its length is from 4∣ to 5∣ inches; its widest part is opposite the back of the mouth. The principal muscles in its walls are called the constrictors, and are named, from above downwards, superior, middle, and inferior. They are arranged in pairs, and arise from the cartilages of the larynx, from the hyoid bone,lower jaw, and internal pterygoid process of the sphenoid; whilst the superior also springs from the fibrous band to which the buccinator is attached; their fasciculi curve backwards to the middle line of the posterior wall of the pharynx, to be inserted into a tendinous band which extends longitudinally along this wall of the tube.

The action of the muscles of the mouth, palate, and pharynx may now be considered in connection with the process of deglutition or swallowing. When the food is received into the mouth, it is moistened by the secretion of the salivary and other buccal glands, and is broken down by the grinding action of thé molar teeth. The buccinator muscles press it from between the gums and the cheek, and, along with the movements of the tongue, aid in collecting it into a bolus on the surface of that organ. During the process of mastication the palato-glossi contract so as to close the fauces. When the bolus is sufficiently triturated and moistened, the palato-glossi relax, the tip of the tongue is pressed against the roof of the mouth, and by a heave backward of that organ the bolus is pressed through the posterior orifice of the mouth into the pharynx, where it is grasped by the superior constrictor muscles, and forced downwards by them and the other constrictor muscles into the oesophagus, and thence into the stomach. As both the nose and larynx open into the pharynx, the one immediately above, the other immediately below the orifice of the mouth, it is of great importance that none of the food should enter into these chambers, and obstruct the respiratory passages. To guard against any accident of this kind, two valvular structures are provided,—viz., the soft palate and the epiglottis,—which, whilst leaving the orifices into their respective chambers open during breathing, may effectually close them when deglutition is being performed. As the bolus is being projected through the fauces into the pharynx, the soft palate and uvula are elevated and made tense, and at the same time the wall of the pharynx is brought in contact with it by the contraction of the palatopharyngei; the part of the pharynx into which the nose opens is thus temporarily shut off from that into which the mouth opens. If laughter, however, be excited at this time, the tension of the soft palate is destroyed, and part of the food may find its way upwards into the nose. The closure of the larynx by the epiglottis is due partly to the depression of that valve and partly to the elevation of the larynx. The backward heave of the tongue relaxes the ligaments which connect the front of the epiglottis to that organ, and enables the small epiglottidean muscles to depress the valve. The elevation of the hyoid and larynx is due to the action of the mylo-hyoid, digastric, and genio-hyoid muscles, which pass from the lower jaw to the hyoid, and of the thyro-hyoid, which pass from the hyoid to the thyroid cartilage of the larynx; preliminary to their action, the lower jaw must be fixed, which is done by the closure of the mouth prior to the act of swallowing. The aperture of the larynx is thus brought into contact with the depressed epiglottis, which is adapted more exactly to the opening by a change in its form due to the projection of a cushion-Hke pad from its posterior surface. By these ingenious arrangements the adaptation of a single chamber to the very different functions of breathing and swallowing is effectually provided for.

Joints and Muscles of the Upper Limb.

The upper limb is jointed to the trunk at the sternoclavicular articulation. This is a diarthrodial joint: the bones are retained together by investing ligaments; a meniscus is interposed between the articular surfaces, so that the joint possesses two synovial membranes. A strong ligament, which check&Aoo great upward movement, connects the clavicle and first rib. The two bones of the shoulder girdle articulate with each other at the diarthrodial acromio-clavicular joint; but, in addition, a strong ligament, which checks too great displacement of the bones, passes between the clavicle and coracoid. The movements of the upper limb on the trunk take place at the sterno clavicular joint, and consist in the elevation, depression, and forward and backward movement of the shoulder. The movements at the acromio-clavicular joint occur when the scapula is rotated on the clavicle in the act of elevating the arm above the head. The muscles which cause these movements are inserted into the bones of the shoulder girdle; the trapezius into the clavicle, acromion, and spine of the scapula; the rhomboid, levator anguli scapulae, and serratus magnus into the vertebral border of the scapula; the pectoralis minor into the coracoid; and the subclavius into the clavicle. Elevation of the entire shoulder, as in shrugging the shoulders, is due to the contraction of the trapezius, levator scapulae, and rhomboideus; depression partly to the weight of the limb and partly to the action of the subclavius and pectoralis minor; movement forward to the serratus and pectoralis; and backward to the trapezius and rhomboid. In rotation of the scapula on the clavicle, the inferior angle of the scapula is drawn forward by the serratus and lower fibres of trapezius, and backward by the levator scapulae, rhomboid, and lesser pectoral.

The Shoulder Joint is a ball-and-socket joint, the ball being the head of the humerus, the socket the glenoid fossa of the scapula. A large capsular ligament, which is pierced by the long tendon of the biceps muscle, and lined by a synovial membrane, encloses the articular ends of the two bones, and is so loose as to permit a range of movement greater than takes place in any other joint in the body. The muscles which cause these movements are inserted into the humerus; the supra-spinatus, infraspinatus, and teres minor into the great tuberosity; the sub-scapularis into the small tuberosity; the latissimus dorsi and teres major into the bottom of the bicipital groove; the pectoralis major into its anterior border; the coraco-brachialis into the inner aspect, and the deltoid, which forms the fleshy prominence of the shoulder, into the outer aspect of the shaft. Abduction and elevation or extension of the arm outwards at the shoulder joint are due to the supra-spinatus and deltoid; adduction or depression, to the coraco-brachialis, latissimus, and teres major, assisted by the weight of the limb; movement forwards and elevation, to the anterior fibres of the deltoid, pectoralis, and subscapularis; backward movement to the latissimus and teres; rotation outwards to the infra-spinatus and teres minor; rotation inwards to the subscapularis, pectoralis, latissimus, and teres. A combination of abduction, movement forwards, adduction, and movement backwards, produces the movement of circumduction. Certain movements of the upper limb, however, take place not only at the shoulder joint, but between the two bones of the shoulder girdle; for in elevating the arm, whilst the supra-spinatus and deltoid initiate the movement at the shoulder joint, the farther elevation, as in raising the arm above the head, takes place by the trapezius and serratus, which rotate the scapula and draw its inferior angle forward. The free range of movement of the human shoulder is one of its most striking characters, so that the arm can be moved in every direction through space, and its efficiency as an instrument of prehension is thus greatly increased. The movement of abduction, or extension, which elèvates the arm in line with the axis of the scapula, is characteristically human, and a distinct articular area is provided on the head of the humerus for this movement.

The Elbow Joint is the articulation between the humerus, radius, and ulna: the great sigmoid cavity of the ulna is adapted to the trochlea of the humerus, and the cup of the radius to the capitellum. The joint is enclosed by a capsular ligament lined by a synovial membrane, which is subdivided into anterior, posterior, internal, and external bands of fibres. Flexion and extension are the two movements of the joint, and the range of movement is limited by the locking at the end of flexion of the coronoid process into the coronoid fossa of the humerus, and at the end of extension of the olecranon process into the olecranoid fossa. The elbow joint is a hinge with screwed surfaces; the path described by the hand and fore-arm is a spiral, so that during flexion they are thrown forwards and inwards. The muscles which cause the movements are inserted into the bones of the fore-arm. The flexors are the brachialis anticus, inserted into the coronoid of the ulna; the biceps, which forms the fleshy mass on the front of the upper arm, into the tuberosity of the radius; the supinator longus into the styloid process of the radius. The only extensor is the triceps-anconeus, which forms the fleshy mass on the back of the upper arm, and is inserted into the olecranon.

The Radio-ulnar Joints are found between the two bones of the fore-arm. · The head of the radius rolls in the lesser sigmoid cavity of the ulna, and is retained in position by a ring-like ligament which surrounds it; the shafts of the two bones are connected together by the interosseous membrane, their lower ends by a capsular ligament and a triangular fibro-cartilage or meniscus. The radius rotates round an axis drawn through the centre of its head and the styloid process of the ulna; rotation of the fore-arm and hand forward is called pronation,—rotation backwards, supination. The supinator and pronator muscles are all inserted into the radius: the supinators are the longus and brevis and the biceps; the pronators are the teres and quadratus. Where delicate manipulation is required the fore-arm is semi-flexed on the upper arm, for the cupshaped head of the radius is then brought into contact with the capitellum of the humerus, and the rotatory movements of the bone can be performed with greater precision.

The Wrist or Radio-carpal Joint is formed above by the lower end of the radius and the triangular meniscus, below by the upper articular surfaces of the scaphoid, semi-lunar, and cuneiform bones. An investing ligament, lined by a synovial membrane, and subdivided into anterior, posterior, internal, and external bands of fibres, encloses the joint. It is the oblong form of hinge-joint, and possesses two axes, a long and a short; around the long axis movements occur which bend the hand forwards, or bring it in line with the fore-arm, or bend it backwards; arouud the short axis the hand may be moved towards the radial or ulnar margins of the fore-arm. The flexors forward are the palmaris longus, inserted into the palmar fascia; the flexor carpi radialis into the metacarpal bone of the index; the flexor carpi ulnaris into the pisiform bone; the extensors and flexors backwards are the longer and shorter radial extensors inserted into the metacarpal bones of the index and middle fingers, and the ulnar extensor into the metacarpal bone of the little finger; the flexors and extensors of the fingers have also a secondary action on the wrist joint. The ulnar flexor and ulnar extensor of the wrist draw the hand to the ulnar side, and the radial flexor and extensor, together with the extensors of the thumb, draw the hand towards the radial border of the fore-arm.

The Carpal and Carpo-metacarpal Joints are constructed thus :—The articular surfaces are retained in contact by certain ligaments passing between the dorsal surfaces of adjacent bones, by others between their palmar surfaces, and by interosseous ligaments between the semi-lunar and cuneiform, semi-lunar and scaphoid, os magnum and unciform, os magnum and trapezoid; lateral ligaments also attach the scaphoid to the trapezium, and the cuneiform to the unciform. Similarly, the trapezoid, os magnum, and unciform are connected to the metacarpal bones of the fingers by dorsal, palmar, and interosseous ligaments, and the metacarpal bones of the fingers have a like mode of union at their carpal ends; further, a transverse ligament extends between the distal ends of the metacarpal bones of the fingers, and checks too great lateral displacement. The range of movement at any one of these carpal joints is very slight, but the multiplicity of joints in this locality contributes to the mobility of the wrist, and makes the junction, between the hand and fore-arm less rigid in its nature. The metacarpal bone of the thumb is not jointed to the index, and has a distinct saddle-shaped articulation with, the trapezium, invested by a capsular ligament, so that its range of movement is extensive.

The Metacarpophalangeal and Interphalangea! Joints are connected by lateral ligaments passing between the bones, and by an arrangement of fibres on their dorsal and palmar surfaces.

In studying the muscles which move the digits, it will be advisable, on account of the freedom and importance of the movements of the thumb, to examine its muscles independently. These muscles either pass from the fore-arm to the thumb, or are grouped together at the outer part of the palm, and form the elevation known as the ball of the thumb; they are inserted either into the metacarpal bone or the phalanges. The thumb is extended and abducted, i.e., drawn away from the index, by three extensormuscles descending from the fore-arm, and inserted one into each of its three bones, and a small muscle, specially named abductor pollicis, inserted into the outer side of the first phalanx: its bones are bent on each other by a long and short flexor muscle; it is drawn back to the index by an adductor muscle; and the entire thumb is thrown across the surface of the palm by the opponens pollicis, which is inserted into the shaft of the metacarpal bone.

The four fingers can be either bent, or extended, or drawn asunder, ⅛.e., abducted; or drawn together, i.e., adducted. The ungual phalanges can be bent by the

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action of the deep flexor muscle, the four tendons of which are inserted into them; the second phalanges by the superficial flexor, also inserted by four tendons, one into each phalanx; these muscles descend from the front of the forearm into the palm in front of the wrist, where they are enclosed in a canal by a strong band, the anterior annular ligament, and their surfaces are invested by a synovial membrane, which facilitates their movements to and fro beneath that ligament; as they pass downwards in front of the fingers they are enclosed in a strong fibrous sheath lined by a synovial membrane, and the tendon of the superficial flexor is pierced by the deep flexor, so that the latter may reach the third phalanx into which it is inserted. Four rounded muscles, the lumbricales, arise in the palm from the deep flexor tendons, turn round the radial borders of the first phalanges, and are inserted one into the extensor tendon on the dorsum of each finger; these muscles bend the first phalanges on the metacarpal bones, but from their insertion into the extensor tendons they also extend the second and third phalanges on the first; as they are much used in playing stringed instruments, they have been called “fiddlers’ muscles.” The fingers are extended or straightened by muscles inserted into the back of the second and third phalanges; the extensor muscles descend from the back of the fore-arm,—one, the common extensor, subdivides into four tendons, one for each finger, but in addition the index and little have each a separate extensor muscle, the tendon of which joins that of the common extensor. The index finger possesses more independent movement than the other digits—hence its more frequent use as a “pointer;” the extensor tendons of the little and ring fingers are usually united together, so that these digits are associated in their movements. Abduction and adduction of the fingers are caused by seven small muscles situated in the intervals between the metacarpal bones,—hence called interossei; four of these lie on the back of the hand, three on its palmar surface; they are inserted into the sides of the first phalanges, and e^ιer pull the fingers away from a line drawn through the middle finger or approximate them to that line. Too great abduction is checked by the transverse metacarpal ligament. The human hand is a perfect instrument of prehension; not only can the individual fingers be bent into hooks, but the thumb can be thrown across the front of the palm, so that it can be opposed to the several fingers, and objects can therefore be grasped between it and them; but further, this power of opposing the thumb permits objects to be held in the palm of the hand, which may be hollowed into a cup or made to grasp a sphere. The movements of the joints are indicated on the surface of the palm by tegumentary folds,—an oblique fold for the thumb, and two oblique folds for the metacarpo-phalangeal joints of the fingers; the joints of the second and third phalanges are also marked on the surface by folds.

Joints and Muscles of the Lower Limb.

The innominate bones are connected to the spinal column by the sacro-iliac joints and the sacro-sciatic ligaments. The Sacro-iliac Joint is between the side of the sacrum and the internal surface of the ilium, the articular surfaces of which bones are covered by cartilage, and connected together by short, strong ligaments. The sacro-sciatic ligaments stretch from the side of the sacrum and coccyx to the spine and tuberosity of the ischium. The two innominate bones are also connected together at the pubic symphysis, which is an amphiarthrodial joint. The sacroiliac joints and pubic symphysis permit only slight movement; that at the former is around an imaginary axis, drawn transversely through the second sacral vertebra, which 'allows the base of the sacrum to be thrown forward and its 'apex backward in the stooping position of the body; but 'too great movement backward of the apex is checked by the sacro-sciatic ligaments. As the weight of the trunk, or of what may be carried in the arms or on the back, is transmitted through the haαnch-bones to the lower limbs, the sacro-iliac ligaments require to be of great strength, because the sacrum, and with it the entire trunk, are suspended by them on the two innominate bones.

The Hip Joint is a ball-and-socket joint; the ball is the head of the femur, and the socket the cup-shaped acetabulum in the haunch bone, the depth of the cup being in-< creased by a ligament which is attached around the brim. A large capsular ligament, which is especially strong in front, encloses the articular surfaces. The ligament is lined by a synovial membrane, which also invests the neck of the thigh bone. Within the joint is the round or suspensory ligament attached to the head of the thigh bone and to the sides of the depression at the bottom of the acetabulum. Whilst the hip joint possesses considerable mobility, it has much more stability than the shoulder, owing to the acetabulum being deeper than the glenoid fossa, and the greater strength and tension of the fibres of its capsular ligament. The muscles which move the thigh at the hip joint are situated either behind the joint, where they form the fleshy mass of the buttock, or at the front and the inner side of the thigh. They are inserted either into the femur or fascia lata, and the great and small trochanters serve as their principal surfaces of attachment. The thigh can be bent on the abdomen by the action of the psoas, iliacus, and pectineus, which lie in front of the joint; it can be extended or drawn into line with the trunk by the glutaeus-maximus and medius; it can be abducted or drawn away from the opposite thigh by the glutaeus maximus, medius, and minimus, which muscles are of large size, and form the fleshy mass of the buttocks. It can be adducted or drawn to touch its fellow, or, if slightly bent, drawn in front of its fellow, by the adductor longus, brevis, and magnus, which muscles are inserted into the linea aspera, and form the fleshy mass on the inner side of the thigh; and by the pectineus and quadratus femoris. It can be rotated outwards by the obturator and gemelli muscles, the glutaeus maximus, pyriformis, and quadratus femoris; and rotated inwards by the glutaeus medius, minimus, and tensor fasciae femoris. In standing erect the hip joints are fully extended, and the mechanical arrangements in and around these articulations are such as to enable them to be retained in the extended position with but a small expenditure of muscular power. As the weight of the body in the erect attitude falls behind the joints, the strong anterior fibres of their capsular ligaments are made tense, and the extended position of the joints is preserved. So long as the centre of gravity falls within the basis of support of the body, i.e., the space between the two feet when standing on both legs, the body will not fall. If the body is made to lean forward, then the capsular ligament is no longer tense, and the glutaeal muscles are put in action to re-extend the trunk on the thigh, and prevent it from falling forward; if the body is made to lean to one side or the other, the round ligament is made tense, or the strong ilio-tibial band of the fascia lata of the thigh, which stretches from the ilium to the tibia, is put on the stretch, and falling sideways is prevented. When, in standing erect either on one or both feet, the balance of the body is disturbed, then various muscles both of the trunk and lower limb are brought into action to assist in preserving the erect position. In the erect position the weight of the trunk is transmitted through the acetabula to the heads of the thigh-bones, but the position and connections of the round ligament enable it to suspend that portion of the trunk the weight of which is thrown upon it, and to distribute the weight over the head of the femur. . , i

The Knee is the largest and most complicated joint in the body. It consists of the femur, tibia, and patella; The patella moves up and down the trochlear surface of the femur, whilst the condyles of the femur roll upon the semilunar cartilages and articular surfaces of the tibia. A powerful investing ligament encloses the articular surfaces. This ligament is subdivided into bands, one on each side of the joint—the internal and external lateral ligaments—a posterior and an anterior. The anterior extends from the patella to the anterior tubercle of the tibia, and serves both as a ligament and as the tendon of insertion of the extensor muscles of the leg. Within the investing ligament two interarticular or crucial ligaments pass from the inter-condyloid fossa to the upper surface of the tibia; and interposed between the tibia and femoral condyles are two menisci, which from their shape are called the semilunar cartilages. The synovial membrane not only lines the investing ligaments, biιt covers the front of the femur for some distance above the trochlea, and forms folds or pads within the joint itself, which in certain movements are interposed between the articular surfaces of the bones. The movements at this joint are those of flexion and extension. The flexors are the three great muscles on the back of the thigh, called the ham-strings; they all arise from the ischial tuberosity, and are inserted—the biceps into the head of the fibula, the semi-tendinosus and semi-mem-branosus into the upper end of the tibia. The extensors form the fleshy mass on the front and outer side of the thigh ; one muscle, the rectus, arises from the ilium—the others, the vasti, from the shaft of the femur; and they are all inserted by a powerful tendon into the patella, and through the anterior ligament of the knee into the tibia. The patella is indeed a sesamoid bone, developed in the tendon of these muscles (Fig. 18). The knee can be bent so that the calf can touch the back of the thigh, and in this position the patella is drawn down in front of the joint, as in kneeling. The articular surface of the patella is divided into seven areas or facets, and in passing from the bent to the extended position of the joint, these facets come successively into contact with the articular surface of the femur, until, when the leg is fully extended on the thigh, the whole of the patella is raised above the femoral trochlea, except the lowest pair of narrow facets. It is in order to provide a smooth surface for the patella in this position that the synovial membrane of the joint covers the front of the lower end of the femur. At the commencement of flexion a slight rotation inwards of the leg and foot takes place through the action of the sartorius, gracilis, and semi-tendinosus, which are inserted close together into the tibia; whilst the extensor muscles cause, at the completion of extension, a slight rotation outwards of the leg and foot. The movements of flexion and extension are not simply in the antero-posterior plane, but along oblique paths which are determined by the screwed configuration of the femoral condyles. In complete extension of the leg the joint is “screwed home;” and as this position is necessary for the preservation of the erect attitude, the lateral, the posterior, and the anterior crucial ligaments are then all tense, to prevent displacement of the bones. The muscles which rotate the leg and foot inwards initiate the act of flexion by unlocking the joint.

The Tιbio-fibular Joints are found between the upper and lower ends of the bones, and in addition a strong interosseous membrane fills up the interval between their shafts. The movement between the two bones is almost inappreciable.

The Ankle Joint is formed by the convex upper and the lateral surfaces of the astragalus fitting into the concavity formed by the lower end of the tibia and the two malleoli. An investing Jigament, lined by synovial membrane, encloses the joint; the lateral portions of this ligament form distinct bands, and are much stronger than the anterior and posterior fibres. A diarthrodial joint also exists between the astragalus and os calcis, between which bones a powerful interosseous ligament passes. Between the astragalus and scaphoid, and the os calcis and cuboid, important diarthrodial joints are found, which are enclosed by ligamentous bands. The remaining tarsal bones are connected together usually by dorsal, plantar, and interosseous ligaments, and a similar mode of union is found between the distal row of tarsal bones and the metatarsals, except between the great toe and ento-cuneiform, where there is no interosseous ligament. The four outer metatarsals are also connected at their proximal ends by distal, plantar, and interosseous ligaments; and further, a transverse metatarsal ligament passes between the distal ends of all the metatarsal bones. The metatarsal bones articulate with the phalanges, and the phalanges with each other, in a similar manner to that described in the corresponding bones of the hand.

At the ankle joint movements of flexion and extension take place. The dorsum of the foot is bent towards the front of the leg by the direct action of the muscles on the front of the leg, more especially the tibialis anticus, inserted into the ento-cuneiform and metatarsal of great toe, and the peroneus tertius, inserted into the metatarsal of little toe; the opposite movement, the so-called extension of the foot, is due to the action of the gastrocnemius and soleus, the great muscles of the calf of the leg, which are inserted by the Tendo Achillis into the posterior prominence of the os calcis or heel. This movement is made at every step in walking or running, and the great size of the calf-muscles is in relation to their use in the act of progression. The foot cannot, however, be drawn so far back as to be brought into direct line with the leg. In standing erect the foot is at right angles to the axis of the leg, the astragalus is locked in between the two malleoli, and the fibres of the lateral ligaments are tense, so as to check movement forwards or backwards, and prevent displacement.

Between the several bones of the tarsus a certain amount of gliding is permitted, more especially between the os calcis and cuboid and the astragalus and scaphoid, so that it is possible to invert or evert the foot, i.e., to raise its inner or outer borders from the ground. The inversion is performed by the tibialis anticus and by the tibialis posticus, which latter is inserted into the scaphoid bone; the eversion by the peroneus longus and brevis muscles, situated on the outer side of the leg, the tendons of which pass behind the outer malleolus,—the brevis to be inserted into the metatarsal bone of the little toe, the longus into the plantar surface of the metatarsal bone of the great toe. The individual toes are bent on the sole by the action of the flexor muscles inserted into the plantar surface of the phalanges, and they are straightened by the extensor muscles inserted into their dorsal surfaces; the toes also can be drawn asunder or abducted, and drawn together or adducted, chiefly by the action of the interossei muscles. The hallux or great toe is the most important digit; a line prolonged backwards through it to the heel forms the proper axis of the foot, and the sole chiefly rests upon the pads of integument situated beneath its metatarso-phalangeal joint and the heel. The hallux is much more restricted in its movements than the thumb: the configuration of its tarso-metatarsal joint and the attachment of the transverse metatarsal ligament prevent the great toe from being thrown across the surface of the sole as the thumb is thrown across the palm in the movement of opposition; an object can, however, be grasped between the hallux and second toe by the action of its adductor muscles, and persons can be trained to write with a pen or pencil held in this position.

The act of walking consists in the movement forwards of the trunk by the alternate advancement of the lower limbs. Suppose a person to be standing erect, with one leg a little in advance of the other; the body, being inclined slightly forwards, is pushed in advance by the extension of the hindmost limb, so that the weight falls more and more upon the advanced leg, which at the same time is shortened by bending the knee and ankle. The heel of the hindmost limb being then raised by the action of the muscles of the calf, the toes press against the ground so as to push the trunk so far in front of the advanced limb as to be no longer safely supported by it; the hindmost limb is then raised from the ground by muscular action, and allowed to swing forward by its own weight, but guided by the muscles, until the toes touch the ground in front of the opposite limb. A step has now been made, and the limbs are in a corresponding but opposite position from that in which they were when the step commenced: a repetition of the act constitutes another step, and so. the alternate action continues. At one moment in each step both feet touch the ground at the same time, Z.e., when the hind foot presses against the earth. The act of running consists in a repetition of the movements of walking performed with so much greater rapidity that the feet never touch the ground at the same moment; the heels also are never brought to the ground. The propulsive action is also greatly increased by the extension of the hip and knee joints, so that a succession of small leaps on to alternate feet takes place. In leaping from the standing position the joints of both lower limbs, previously flexed, are suddenly and simultaneously extended, and the body is projected forwards with a rapid impulse.

Development and Homologies of the Voluntary Muscular System.

The voluntary muscles, like the bones and joints with which they are so intimately associated, are developed out of the middle of the three layers—the mcso-blast —into which the germinal area or blastoderm of the young embryo is divided. The muscles of the axial skeleton are capable of subdivision into a group situated outside the endo-skeleton, i.e., between it and the integument— which muscles have recently been called epi-skeletal— and a group lying on the ventral surface of the vertebral bodies and within the rib arches, which have been termed the haemal or hypo-skeletal muscles. The epi-skeletal muscles, like the vertebrae themselves, are developed within the proto-vertebrae, but it is not known if the hypo-skeletal group have the same origin. In fishes the epi-skeletal muscles preserve their fundamental arrangement with but little modification. They are disposed in transverse segments or myotomes, which equal in number the vertebrae. These myotomes are separated from each other by bands of fibrous tissue, the intermuscular septa. In man and the higher vertebrates the simple transversely segmented arrangement is to a large extent lost. Traces are preserved, however, in the interspinales and intertransversales muscles, situated in the intervals between the spines and transverse processes of some of the vertebral segments; in the external intercostale and in the recti abdominis muscles, in the last-named of which tendinous bands subdivide the muscle into several transverse segments. More usually, the intermuscular septa either are not formed or disappear, and adjacent myotomes become blended into a continuous mass of muscle. In some instances the fibres of this muscle run longitudinally, and the entire mass subdivides longitudinally into separate and distinct parallel muscles, as is seen in the subdivision of the great erector spinae into the sacro-lumbalis, musculus accessorius, cervicalis ascendens, longissimus dorsi, transversalis cervicis, trachelo-mastoid, and spinalis dorsi muscles. In other instances the muscles run obliquely; some on the back of the body pass obliquely from below upwards and outwards, as the splenius and obliquus inferior; others obliquely from below, upwards and inwards, as the complexus, obliquus superior, semispinalis, multifidus and rotatores spinae; others again, as the external and internal oblique muscles of the abdomen, extend obliquely from behind forwards to the ventral mesial line.

Of the hypo-skeletal group of muscles, the internal intercostals display the transverse segmentation. As a rule, however, the muscles of this group extend longitudinally, andform the prae-vertebra± group, named anterior recti, longi colli, and psoae; though the diaphragm, triangulares sterni, transversi abdominis, and levatores ani, which lie in relation to the inner surfaces of the ribs and visceral cavities, are not longitudinal, but are specially modified in arrangement for functional reasons. The plane of demarcation between the hypo-and epi-skeletal groups of muscles, where they form together the walls of the great visceral cnambers,—the thorax and abdomen,— is marked ofi' by the position and course of the intercostal series of spinal nerves.

The muscles of the appendicular skeleton are either limited to the limbs (purely appendicular, therefore), or pass from the axial part of the body to the limb (axi-appendicular). The axi-appen-dicular group are undoubtedly prolongations of the axial system of muscles. They are in the upper limb derived from the epi-skeletal subdivision, and form the trapezius, rhomboid, levator anguli scapulae, latissimus dorsi, serratus magnus, greater and smaller pectorals, and subclavius muscles of each superior extremity. In the lower limb they are in part derived from the hypo-skeletal subdivision, and form the psoas and pyriformis; and in part, as the glutaeus maximus, from the epi-skeletal subdivision. It is not improbable that the purely appendicular muscles are also prolongations of the axial system, and that as the limbs, in their development from their fundamental bud-like lappets, undergo both a transverse and a longitudinal segmentation, so the muscular mass, prolonged into them, differentiates both transversely and longitudinally into a motor apparatus, fitted for the performance of the special functions of each extremity.

Anatomy of the Textures or Tissues. Introductory.

Before proceeding to the description of the other organic systems of which the human body is built up, it may be well to enter into the consideration of the minute or microscopic structure of its constituent parts. These parts may primarily be divided into fluids and solids. The fluids are the blood, the lymph, the chyle, the secretions of the various glands, and of the serous and synovial membranes. The solids form the framework of the several organic systems, and assume different appearances in different localities. Sometimes they are arranged in compact solid masses, as in cartilage; at others they are elongated into fine threads or fibres, as in muscle, tendon, nerve; at others they are expanded into thin membranes, as in the fasciae or aponeuroses, the serous, synovial, and mucous membranes; at others they are hollowed out into distinct tubes for the conveyance of fluids, as in the blood-vessels, the lymph and chyle vessels, and the ducts of glands. To the solids of the body, whatever their form may be, the general name of Tissues or Textures is applied. Each organic system may be regarded as in the main composed of a tissue or texture peculiar to and characteristic of itself. Thus, the bones are essentially composed of the osseous tissue, the muscles of the muscular tissue, the nervous system of the nervous tissue, fibrous membranes of the fibrous or connective tissue, &c. But though the essential constituent of each organic system is a tissue peculiar to that system, yet in most localities certain other tissues are mingled with that which is to be regarded as the characteristic texture of the part. In a muscle, for example, not only is the muscular tissue present, but mingled with it are connective tissue, nerve tissue, bloodvessels, and lymph-vessels. A gland also not only consists of its proper tissue, the secreting cells, but of more or less connective tissue, nerves, blood and lymph vessels, and gland ducts. Indeed, there are few localities in which, along with the proper tissue of the part, blood and lymph vessels, nerves and connective tissue, are not found; and to a part built up of two or more tissues the name of an Organ is applied. Thus the muscular system consists of the series of organs which we call the muscles, the glandular system of the several organs called glands, and so on. Each tissue and each organ, into the construction of which that tissue enters as the characteristic· texture, possesses not only distinctive structural, but also distinctive functional properties. Thus the muscular tissue is characterised by the property of contractility, and the muscles, of which it forms the essential texture, are organs of motion oι locomotion; the osseous tissue is characterised by its hardness and strength, and the bones, of which it forms the essential texture, are organs of protection and support.

But the study of the textures embraces an inquiry not only into the special, structural, and functional properties of each tissue, and organ—into the special part which each plays in the animal economy—but the consideration of their properties as living structures. It would be out of place in this article to enter into a discussion of the meaning of the term Life, or Living, or to attempt an analysis of the various definitions of the term which have been suggested from time to time by different philosophers, which will naturally find a place in the article Physiology. It will suflice for our present purpose to adopt the old Aristotelian definition, and to speak of Life as the faculties of self-nourishment, self-growth, and self-decay. All the tissues, over and above the special properties which they possess, have the power of growing and of maintaining themselves in full structural perfection and functional activity for a given period of time. After a time they begin to exhibit signs of diminished perfection and activity, they degenerate or decay; ultimately they die, and the entire organism of which they form the constituent parts is resolved by the υutrefactive υrocess into more simple forms of matter.

General Considerations on Cells.

The simplest form of organic matter capable of exhibiting the phenomena of life is called Cyto-blastema or Protoplasm. It possesses a viscous or jelly-like consistency. Under the highest powers of the microscope it seems to be homogeneous, or dimly granulated, like a sheet of ground glass. Not only can it assimilate nutriment and increase in size, but it possesses the power of spontaneous movement and contractility. It enters in a very important manner into the structure of the bodies of the lower animals. The elongated processes, or pseudopodia, to which Dujardin applied the name of sarcode, which the Rhizopoda can project from their surface into the surrounding medium, and again withdraw into their substance, consist of protoplasm, and may be cited as furnishing excellent examples of its motive and contractile power. From the recent researches of Haeckel it would appear that protoplasm is capable of forming, without the superaddition of any other structure, independent organisms, which stand at the lowest grade of organisation, and from their extreme simplicity are named by him Monera. To the group Monera belong the genera Protamoeba, Protogenes, and Bathybius. Of these, Bathybius is that which has attracted most attention. It has been regarded as a layer of soft slimy undifferentiated protoplasm covering the bottom of the deep sea, and capable of exhibiting the phenomena of contractility, growth, assimilation of food, and reproduction, Doubts, however, have been expressed regarding the nature of this Bathybius, so that it cannot now be cited as so definite an organism as the freely-swimming Protamoeba and Protogenes. Haeckel has referred these simple organisms to a subkingdom of Protistae, which he considers to lie on the confines of both the animal and vegetable kingdoms. To a mass of protoplasm, whether it forms, as in one of these Protistae, an independent organism, or is merely a portion of the substance of the body of a higher organism, he has given the general name of a Cytode. Sometimes a cytode is a naked clump of soft protoplasm, without a trace of differentiation either on its surface or in its substance, as in the freely-moving Monera; at others the peripheral part of the cytode hardens, and differentiates into a more or less perfect envelope, as in the genera Protomonas and Protomyxa. So far back as 1861, Lionel Beale had described, under the name of germinal matter (Bioplasm), minute living particles of vegetable protoplasm, and in 1863 he demonstrated the presence of extremely minute particles of living matter in the blood. More recently Stricker has also called attention, in the bodies of the higher animals, to minute detached clumps of protoplasm which exhibited the phenomena of life.

As a rule, however, in both vegetable and animal organisms the specks or clumps of protoplasm assume definite shapes, and show evidence of an internal differentiation. In the midst of a minute clump of this substance a sharply-defined body called a nucleus is found, which differs from the surrounding protoplasm in not being contractile; and sometimes a minute speck, or nucleolus, exists within the nucleus. When a definite clump of protoplasm contains a nucleus in its interior, whether a nucleolus be present or not, it is called a Nucleated Cell. Cells are definite anatomical and physiological units, and exhibit all the phenomena of life. Some of the lowest organisms consist merely of a single cell, others of two or more cells united together, and these are called uni-or multi-cellular organisms. Cells also enter in the most material manner into the constitution of the textures of all the higher forms of plants and animals. Not unfrequently the peripheral part of the protoplasm of the cell differentiates into a distinct investing envelope, technically named a cell uaU or cell membrane.

In the earlier periods of investigation into the minute structure of cells it was believed that a cell wall was constantly present, and that each cell was a minute microscopic vesicle or bladder, which in its typical shape was globular or ovoid, but capable of undergoing various modifications both in form and chemical composition. The material enclosed by the cell wall was termed the cell con tents, and either in the midst of these contents or in contact with the cell wall was the nucleus, which might or might not contain a nucleolus. Schwann believed that the cell wall was the most active constituent of the cell, i.e., possessed the power not only of producing chemical and physical changes in its own substance and in the cell contents, but of separating materials from the surrounding media,—of secreting them, as it were, into the interior of the cell. In this manner he accounted for the formation in some cells of fat, in others of pigment, in others of the characteristic secretion of glands, and so on.

It was then maintained by John Goodsir that the nucleus was the part of a cell which in all probability was concerned in separating and preparing its characteristic cell contents, and in its nutrition. Martin Barry and Goodsir also contended that the reproduction and multiplication of cells were due to self-division of the nucleus, which was thus the source of successive broods of young cells. They gave to the nucleus, therefore, an importance in the economy of the cell greater than had previously been assigned to it.

As the investigations into cell structure became more extended, it was ascertained that a cell wall was by no means always present; that in many of the cells in which it had been supposed to exist it could not satisfactorily be demonstrated, and that in others, more especially in young actively-growing cells, no trace of an investing envelope could be observed. Hence the importance of the cell wall as an essential compc nent of a cell was still further diminished; and Leydig then defined a cell to be a little mass composed of a soft substance enclosing a central nucleus.

But a most important advance in our conceptions of the essential structure of a cell was made when Brücke pointed out that the contents of cells not unfrequently possessed the property of spontaneous movement and contractility, and when Max Schultze determined that the contractile substance termed sarcode, which forms so large a part of the bodies of the lower animals, was analogous and apparently homologous with the contents of young actively-growing animal and vegetable cells, before a differentiation of these contents into special secretions or other materials had taken place. As the term “protoplasm” had been introduced by Von Mohl to express the contents of the vegetable cell, which undergoes changes in the process of growth, it was adopted by the animal histologist; and Max Schultze suggested that a cell should be defined to be a nucleated mass of protoplasm,—a definition which is adopted in this article. Now, as protoplasm, whether it occurs along with a nucleus in the form of a cell, or in independent clumps or cytodes, exhibits not merely the property of contractility, but the power of growing and maintaining itself, it is regarded as the functionally active constituent of the cell. And thus our conceptions as to the part of the cell in which its functional activity resides have passed through three phases. In the first, the cell wall; in the second, the nucleus; in the third, the protoplasm cell contents, or cell substance, has been regarded as the active constituent, not only as regards its nutrition, but the reproduction of young cells. But though the protoplasm can of itself perform these offices, yet there can be no doubt, as Barry and Goodsir were the first to show, that the nucleus of the cell plays a part not unfrequently in the multiplication of cells by self-division.

One of the most characteristic cells is the mammalian ovum. In it a cell wall exists, known as the 20na pellucida or vitelline membrane; within this envelope is the granular yelk or cell contents, in the midst of which is imbedded the nucleus or germinal vesicle, which in its turn contains the nucleolus or germinal spot. The granules of the yelk are a special metamorphosis of the protoplasm cell substance.

Schwann made the important generalisation that the tissues of the animal body are composed of cells, or of materials derived from cells, “that there is one universal principle of development for the elementary part of organisms, however different, and that this principle is the formation of cells.” The ovum is the primordial or fundamental cell, or germ-cell, from which, after being fertilised by the male sperm, the tissues and organs of the animal body are derived. Within the fertilised ovum multiplication of cells takes place with great rapidity. It is as yet an unsettled question how far the original nucleus of the ovum participates in this process of multiplication; but there can be no doubt that the protoplasm cell contents divide, first into two, then four, then eight, then sixteen segments, and so on. Each of these segments of protoplasm contains a nucleus—is, in short, a nucleated cell, and the protoplasm of these cells exhibits the property of contractility. The ovum or germ-cell is therefore the immediate parent of all the new cells which are formed within it, and mediately it is the parent of all the cells which, in the subsequent processes of development and growth, are descended from those produced by tne segmentation of the yelk. The process of development of young cells within a parent cell, whether it occurs in the ovum or in a cell derived by descent from the ovum, is called the endogenous reproduction of cells. But cells may multiply by a process of fission—i.e., a constriction, gradually deepening, may take place in a cell until it is subdivided into two; the -nucleus at the same time participating in the constriction and subdivision. A third mode of multiplication of cells is by budding: little clumps of protoplasm bud out from the protoplasm of the parent cell, become detached, and assume an independent vitality. If a nucleus differentiates in the interior of such a clump, it becomes a cell; if it remains as a mere clump of protoplasm, it is a cytode.

These various methods of multiplication are all confirmatory of Schwann’s generalisation of the descent or derivation of cells from pre-existing cells. But as the nucleated cell, either with or without a cell wall, is not, in the present state of science, regarded as the simplest and most elementary unit capable of exhibiting vital phenomena, and as these phenomena can be displayed by individual clumps of protoplasm, without the presence of a nucleus, some modification of the doctrine, as regards the formation of the tissues from nucleated cells, seems to be necessary. For, although there can be no doubt that all the tissues are mediately derived from the ovum or fundamental cell, and that most of the tissues are derived directly from nucleated cells, yet there is reason to think that a differentiation of a cytode clump of protoplasm into tissue may take place, so that the direct formation of such a tissue would be, not from a nucleated cell, but from the more simple cytode. Hence a more comprehensive generalisation, to which observers have gradually been led from the consideration of numerous facts, has now been arrived at,—that the tissues and organs of the body, whatever may be their form and composition, are formed of protoplasm, or produced by its differentiation; and that the protoplasm itself is derived by descent from the protoplasm substance of the primordial germ-cell. Some, indeed, have contended that protoplasm, cells, and their derivatives can arise by a process of precipitation or aggregation of minute particles or molecules in an organic infusion, and that living matter may be thus spontaneously generated. But the evidence which has been advanced in support of this hypothesis is by no means satisfactory or conclusive, whilst the correctness of the theory of the direct descent of protoplasm from pre-existing living protoplasm is supported by thousands of observations made by the most competent inquirers.

In the process of conversion of protoplasm into the several tissues, there takes place a differentiation of form and structure (i.e., a morphological differentiation), and of composition (i.e., a chemical differentiation), as the result of which a physiological differentiation is occasioned, whereby tissues and organs are adapted to the performance of special functions. Hence arise the several forms of tissue which occur in the human body and in the higher animals. Many of the tissues consist exclusively of cells which present in different parts of the body characteristic modifications in external configuration, in composition, and in properties, as may be seen in the fatty tissue, pigmentary tissue, and epithelium. Other tissues, again, consist partly of cells, and partly of an intermediate material which separates the constituent cells from each other. Here also the cells present various modifications; and the intermediate material, termed the matrix or intercellular substance, varies in structure, in composition, and in properties in the different textures, as is seen in the connective, cartilaginous, osseous and muscular tissues It is not an easy matter to devise a classification of the tissues, based on their structural characters, which shall be in all respects logically perfect; but a convenient basis of arrangement for descriptive purposes may be found by dividing them into those which consist—1st, of cells suspended in'fluids; 2d, of cells placed cn free surfaces; 3d, of cells imbedded in solid tissues.

lsi Group.—Cells Suspended in Fluids.

The fluids of the body which have cells or other minute solid particles suspended in them are the blood, the lymph, and the chyle. Sometimes cells are found floating in the secretions of glands.

The Blood. —The blood is the well-known red fluid which circulates throughout the blood-vascular system. As its composition and general properties will be described in the article Physiology, the solid particles only, which are suspended in the liquor sanguinis, will be considered here. If a drop of human blood be examined under the microscope, crowds of minute bodies, the blood corpuscles, or blood globules, may be seen in it. These present two different appearances, and are distinguished by the names of red and white blood corpuscles.

The red corpuscles, which are by far the more numerous, are minute circular discs, slightly concave on both surfaces. Their average diameter is about π3 ⅛ σ th of an inch, and their thickness about ∣th of that measurement; hence they are not spheres, as the old name blood globules would imply. They are non-nucleated. Single corpuscles have a faint fawn-coloured hue, but collectively they give to the blood its characteristic red colour. This colour is due to the presence in the corpuscles of the substance termed haemoglobin. It has beèn estimated by Vierordt and Weicker that 5,000,000 red corpuscles are present in every cubic millimetre of healthy human ¾lood, The red corpuscles in the blood of all mammals, except the tribe of camels, are circular bi-concave discs; but in these exceptional mammals they have an elliptical outline. In all mammals the red corpuscles are non-nucleated, though appearances of nucleation have been seen in exceptional individual cases; for Rolleston saw a nucleated appearance in a small proportion of the dried red blood corpuscles of a two-toed sloth; and Turner observed in a proportion of the red blood discs of a Hoffmann’s sloth an appearance of a central nucleus.

In all birds, reptiles, and amphibia the red corpuscles are oval or elliptical, and in each corpuscle an oval or elliptical nucleus is situated. In all fishes they are nucleated and also elliptical in form, except in some of the Cyclostomata, which possess circular discs. In the elliptical nucleated corpuscles the surfaces are not biconcave, but have central projections, which correspond in position to the nucleus (2, 4, 5, Fig. 28). The red corpuscles vary materially in size in different vertebrata, and these variations have been especially studied by Gulliver. He has found them to vary in mammals from an average diameter of τ 5 - 7 ⅛ κ th of an inch in the elephant, and r r 7Wh in Orycteropus capensis, to τ ⅛ κ th in Tragulus javanicus, and he concludes that the smallest blood discs occur in the small species of an order or family, the largest in the large species. In birds they are larger than in mammals, and vary in length from an average of τ ⅛∙g∙ inch in Casuarius javanicus to ^ i l r ^th in Linaria minor. In reptiles they are still larger, and vary in length from an average of τ τ ∖∙ s ∙th in Anguis fragilis to τ5⅛5th in Lacerta, viridis. In amphibia the largest corpuscles, according to Gulliver, are about 1 ⅜- 5 - inch in length in Proteus and Siren, though Riddell states that in Amphiuma, tridactylum they are ⅛d larger; whilst the smallest, as in the common frog, average in length ττ⅛κ inch. In cartilaginous fish the corpuscles are larger than in osseous. In Lamna cor-nubica Gulliver found their long diameter to be v ⅜ 7 inch; while in the Salmonidae, which have the largest blood discs among osseous fish, the long diameter in the salmon and common trout is only about τκ ⅛ 7 inch.

The white or colourless corpuscles are comparatively few in number in the healthy human blood. Weicker has estimated the normal relative number as one white to 335 red; in pregnant and menstruating women the proportion is increased to about 1 to 280. In some forms of disease the proportion is so very materially increased that they appear to be almost as numerous as the red. They are rounded in form, finely granulated or mulberry-like in appearance, and nucleated—the nucleus becoming more distinct after the addition of acetic acid; moreover, they are larger than the red corpuscles, their average diameter being from ^⅛∙ σ th to ^∙ σ ⅛∙jth of an inch. Corpuscles of a similar form are found in the blood of all vertebrata. They do not vary so much in size in different animals as do the red corpuscles. In Triton, according to Gulliver, their average diameter is ∙ 2 - δ x 0 - 5 -th, whilst in Herpestes griseus they are not more than inch. The white blood corpuscles are minute nucleated clumps of protoplasm; they are therefore minute cells. It is very doubtful if they possess a cell wall, the evidence being against rather than in favour of its presence.

The red blood corpuscles in all vertebrata, except the mammalia, are nucleated clumps of protoplasm; they are therefore minute cells. In mammals, owing to the absence of a nucleus, they do not accord with the definition of a cell adopted in this article, and they are not therefore morphologically identical with the red corpuscles in other vertebrates. What their precise homology may be is somewhat difficult to say, owing to the obscurity which prevails as to their exact origin. If they are merely clumps of specially modified protoplasm, budded off from the white corpuscles, then they are cytodes. If, as some have supposed, they are the nuclei of the white corpuscles, specially modified in composition, then they are free nuclei. If, again, they are the white corpuscles, the cell substance of which has undergone a special differentiation, and the nucleus has disappeared, then they are potentially cells, though no nucleus is visible. Whatever may be their exact homology, there can be no doubt that the non-nucleated mammalian red corpuscle, and that part of the nucleated red corpuscle which lies outside the nucleus, are functionally identical with each other; the protoplasm having undergone a special chemical differentiation into haemo-globin, a proximate principle characterised by containing iron as its essential constituent. The action of water, spirit, acids, alkalies, various gases, heat, cold, and electrical currents, on the red corpuscles has been studied by several observers, and the conclusion has been reached that the corpuscles consist of a “stroma,” with which the colouring matter is blended, but from which it may be separated without the stroma affording any evidence of the presence of an investing envelope or membrane. When blood is drawn from the vessels the red corpuscles, in about half a minute, run together into piles, like rouleaux of coins (Fig. 29), which arrange themselves into irregular meshes. In inflammatory diseases, and in the blood of pregnant women, the piles of corpuscles form more readily, and at the same time sink rapidly below the surface of the fluid, so as to cause the “buffy coat” seen in the blood coagulum. In the healthy blood of horses a bufly coat is formed as a natural condition of the coagulation.

One of the most curious properties possessed by the living white blood corpuscle is that of protruding delicate processes from its circumference, which processes may change their shape, or be again withdrawn into the substance of the corpuscle, which then resumes its former circular outline. These processes resemble the sarcode prolongations which Amoeba and other Rhizopods can project from various parts of their circumference; and as a white blood corpuscle, like an Amoeba, can by the movements of the processes change its position, the term “amoeboid movements” has been applied to the phenomena in question. Like an Amoeba, also, a white corpuscle can by these movements include within its substance minute particles of solid matter which it may come in contact with in its path. Thirty years ago W. Addison stated that the white blood corpuscles could pass through the walls of the blood-vessels into the surrounding tissue, where they formed mucus corpuscles, and, under certain pathological conditions, the corpuscles of pus or inflammatory lymph. The passage of white blood corpuscles through the wall of the capillaries was seen in 1846 by A. Waller; and though for many years his observations were ignored, yet the more recent inquiries of Cohnheim and others into the subject have anew directed attention to them. It is now generally admitted that the migration of these corpuscles from the blood through the wall of the capillaries into the tissues does take place, and that they may then “wander” to and fro, owing to the mobility of their contractile protoplasm. These migrated corpuscles are also believed to play an important part in many physiological and pathological processes.

But the blood contains, in addition to the red and white corpuscles, still more minute particles, which are, however, inconstant in number. Minute globules have been described by Beale and Max Schultze, which are probably detached fragments of protoplasm budded off from the white corpuscles; and Zimmermann has described, as elementary corpuscles, minute particles, which are apparently derived from broken-up red corpuscles.

In the very young embryo the blood corpuscles, like the capillary blood-vessels themselves, are formed by special differentiation of certain of the cells of the embryo, and these young corpuscles seem to have the power of multiplying by fission. At first they are colourless, but afterwards assume a red colour. Even in mammals the earliest red blood corpuscles are nucleated and larger than the future red discs, but as development goes on, non-nucleated red corpuscles appear, and as their number increases, both absolutely and relatively with the progress of the foetus, in course of time all the nucleated red corpuscles have disappeared, and are replaced by the non-nucleated discs. In adults the red corpuscles are believed to be derived from the white corpuscles, though the exact process of metamorphosis has not been satisfactorily ascertained. It is also believed that red corpuscles may be new-formed in the spleen, and Neumann has recently stated that the red marrow of the bones may serve as a centre of origin for the red blood corpuscles. In the foetus the liver apparently serves as a centre of origin for the white corpuscles, but its blood corpuscle forming function ceases at the time of birth. Throughout extra-uterine life the spleen and the lymphatic glands are without doubt organs of formation of the colourless corpuscles,— those produced in the lymphatic glands, under the name of lymph corpuscles, being mingled with, the blood-stream where the fluid lymph flows into the venous system. When mixed with the blood, the lymph corpuscles become the white blood corpuscles.

Corpuscles are also found in the blood of the Invertebrata. They are as a rule colourless, but R. Wagner pointed out that in the Cephalopods they are coloured. They are sometimes round, at others oval or fusiform, and in worms and insects have even branched processes. They are always nucleated.

The Lymph and Chyle. —The lymph is the fluid found in a subdivision of the vascular system named the lymph vascular system. It is transparent and colourless, and contains numerous corpuscles floating in it, which correspond, in appearance, structure, and the possession of the property of amoeboid movements, to the white corpuscles of the blood. The lymph corpuscles are formed in the glands situated in the course of the lymph vessels, and are carried away from the glands by the stream of lymph which flows through them.

The chyle is a milky fluid found during the period of digestion in the delicate lacteal vessels which pass from the walls of the intestine. The lacteals join the lymphatics at the back of the abdomen to form the thoracic duct in which the lymph and chyle become mingled together. The chyle contains corpuscles similar to the lymph corpuscles, which are apparently derived from the lymph glands in the mesentery, through which the chyle flows on its way to the thoracic duct. The fluid of the lymph, the chyle, and the blood, in which the corpuscles are suspended, is sometimes described as a fluid intercellular substance. Corpuscles possessing the type of structure of the lymph corpuscles, are named lymphoid cells or leucocytes.

Cells are also met with floating free in the secretions formed in the interior of some of the glands. They are more particularly found in the secretion of mucus from the mucous glands, and of saliva from the salivary glands. They are round, colourless, nucleated corpuscles, not unlike the white corpuscles of the blood, and have been detached from their original position in the gland follicles.

2<7 Group.—Cells placed on Free Surfaces.

By the term free surface is meant a surface which is not blended with or attached to adjacent structures, but is free or separable from them without dissection. Every free surface is covered by one or more layers of cells. Sometimes these cells are named an Epithelium, at others an Endothelium. By the term Epithelium is meant the cells situated on free surfaces which are exposed either directly or indirectly to the air. By the term Endothelium is meant the cells situated on free surfaces which are not exposed either directly or indirectly to the air.

Epithelium. —The free surfaces covered by an epithelium are the skin and the membranes, named, from the character of their secretion, mucous membranes. The Mucous Membranes line internal passages and canals, and are continuous at certain orifices with the skin,—e.y., the mucous membrane of the alimentary canal opens on the surface at the mouth and anus; the respiratory mucous membrane opens on the surface at the nostrils, and is continuous in the pharynx with the alimentary mucous membrane—it is also prolonged through the Eustachian tube into the tympanum, and is continuous through the nasal duct with the conjunctiva; the genito-urinary mucous membrane opens on the surface at the orifice of the urethra and vagina. Mucous membranes also line the ducts of the various glands which open on the surface either of the skin or the several mucous membranes. The epithelial cells are as a rule arranged in layers or strata, and the shape of the cells is by no means uniform in the different layers. The cells of the deeper strata are usually smaller, softer, more rounded, and more recently formed than those of the superficial strata, though sometimes, as in the bladder, conjunctiva, and some other mucous surfaces, they may be irregular in form and size, or even elongated into short columns. The cells next the free surface have a tendency to be shed, and their place is then taken by the cells of the deeper layers, which become modified in form as they approach the surface. The form of the cells of the superficial layer varies in different localities, which has led to a division of epithelium into groups bearing appropriate names. Epithelium is distinguished further by being devoid of bloodvessels, i.e., it is non-vascular; and also, with some exceptions, devoid of nerves, i.e., non-sensitive.

The epithelial cells, whether arranged in one or several strata, rest upon a subjacent tissue, which, from its relation to the cells, may be called sub-epithelial. The sub-epithelial tissue is a delicate modification of the fibrous form of connective tissue, to be subsequently described, and in it the nerves and the blood and lymph vessels of the skin and mucous membranes ramify; hence it is sometimes described as a fibro-vascular tissue or corium. It was for a long time believed that between the deeper surface of the epithelium and the corium a homogeneous continuous membrane, named by Bowman a basement membrane, intervened. Bowman, however, himself admitted that in some of the localities where this membrane was theoretically supposed to exist it could not satisfactorily be demonstrated; and the general opinion of anatomists now is, that a distinct separable membrane does not intervene between the epithelium and the fibro-vascular corium, but that the cells of the former rest directly upon the surface of the latter. The corium is also the seat of the numerous glands, with the blood and lymph vessels and the nerves belonging to them, found in connection with both the skin and the mucous membranes; and the epithelial lining of the g 1 ands is continuous at their orifices with the epithelial investment of the corium. The surface both of the skin and mucous membranes is usually more or less undulated—sometimes it is thrown into strong folds or rugae, at others it is elevated into minute, frequently conical, processes, named in some localities papillae, in others villi; but in all these cases the epithelium is prolonged as a continuous covering over the undulating free surface. The free surface of all mucous membranes is kept moist by the secretion or mucus which lubricates it.

Tessellated, pavement, scaly, or squamous epithelium is situated on the free surface of the mucous lining of the mouth, pharynx, oesophagus, vestibular entrance to the nose, ocular conjunctiva, and entrance to the urethra and vagina. It forms, under the special name of the horny layer of the cuticle or epidermis, the superficial investment of the skin. Its cells are nucleated flattened scales, varying in diameter from κ J ιy th to τ0⅛-5-th inch. Those in the same layer, being in contact by their edges, form a tessellated, pavement-like arrangement, whilst the cells in adjacent layers have their flattened surfaces in contact with each other. Sometimes the cells have jagged, serrated edges, or fluted surfaces, and usually they contain scattered granular particles. In the formation of this epithelium a morphological differentiation of the protoplasm of the rounded cells of the deeper strata into flattened scales, and at the same time a chemical differentiation of their soft contents into a horny material, have occurred.

Columnar or cylindrical epithelium is situated on the free surface of the mucous lining of the alimentary canal from the oesophageal orifice of the stomach to the anus, it is prolonged into the ducts of various glands which open on the alimentary mucous membrane; it covers the mucous lining of the urethra and the mucous membrane of the gall bladder. Its cells are elongated, cylindrical columns, about τ ⅜- 5 ∙th inch long, placed side by side like a row of palisades, and with their long axes perpendicular to the surface on which the cells rest. Sometimes the cells are uniformly cylindrical; at other times they are compressed at the sides; at others they vary in circumference,—the broader end, lying next the surface, being rounded or polygonal; the deeper extremity being narrower and more pointed. The nuclei are distinct, and the cell contents are finely granular. Usually this epithelium forms only a single layer of cells. The columnar cells which cover the intes r tinal villi have a clear space at their broad free ends, which is often streaked with fine parallel lines. Intermingled with the cells of the columnar epithelium of the alimentary canal are small goblet-shaped cells.

Ciliated epithelium is situated on the free surface of the nasal mucous membrane, which extends into the air-sinuses within the cranial bones, into the nasal duct and lachrymal sac, into the Eustachian tube and tympanum; on the free surface of the mucous membrane of the windpipe as far as the terminal branches of the bronchial tubes; on the mucous surface of the uterus and Fallopian tubes; on the mucous lining of the commencement of the vas deferens, and on the lining membrane of the ventricles of the brain and central canal of the spinal cord. It generally consists of columnar cells, which have at their free ends extremely slender, soft, pellucid, hair-like processes, or cilia. These cilia are specially differentiated at the free ends of the epithelium cells from which they project. Beale states that the soft bioplasm (protoplasm) of the body of the cell is prolonged along the axis of each cilium, whilst the periphery possesses the firmer consistence of formed or differentiated material.

During life these processes move rapidly to and fro in the fluid which moistens the surface of the membrane on which this form of epithelium is situated. In the human body the cilia are not more than from ^5⅛∙<5th to j 75 ⅛ 7 th inch in length; but in various marine Invertebrata they are both longer and stronger. Sometimes, as in the lining membrane of the cerebral ventricles and central canal of the spinal cord, the cells carrying the cilia are either spheroidal or cylindrical; but as the cavities lined by these cells are shut off from the air, the cells ought rather to be referred to the endothelial than the epithelial series of structures. Cilia occasion currents in the fluid in which they move, and play an important part in the economy of many animals; in some of the Invertebrata they serve as organs of locomotion, in others they propel currents over respiratory surfaces, and in others aid in bringing food within the animal’s reach.

Spheroidal or glandular epithelium is situated on the free surface of the follicles or ultimate secreting apparatus of glands, and the commencement of gland ducts. The cells are often spheroidal in form, though not unfrequently they are polyhedral. Their contents are specially differentiated into the secretion of the particular gland in which they are situated.

The epithelial cells of a Secreting Gland rest upon a sub-epithelial tissue. Not unfrequently this tissue has the appearance of a membrane; it represents, indeed, the basement membrane of Bowman, and is called membrana propria. Deeper than this apparent membrane is a delicate connective tissue in which the blood and lymph vessels and the nerves of the gland ramify. The anatomical structures necessary for secretion are cells, blood-vessels, and nerves-The blood-vessels convey the blood from which the secretion has to be derived; the cells, as Goodsir showed by a variety of proofs, are the active agents in separating the secretion from the blood; the nerves regulate the size of the blood-vessels, and therefore the amount of blood which circulates through the gland, and perhaps also exercise some direct influence on the activity of the cells. The connective tissue and the membrana propria are merely supporting structures for the cells, vessels, and nerves. All secreting glands have the same general type of structure, though they differ from each other, as will be pointed out when the individual glands are described, in the degree of complexity in which their constituent parts are arranged.

Transitional epithelium is the name applied to epithelial cells, situated on some free surfaces, which possess transitional forms either between the columnar and tessellated epithelia, or the columnar and spheroidal The epithelium of the mucous lining of the bladder is transitional between the columnar and scaly varieties; and in many glands the continuity of the epithelial layer from the spheroidal epithelium of the gland follicles to the columnar epithelium of the ducts is preserved by the interposition of intermediate transitional forms of cells.

The epithelial surfaces of the upper part of the mucous lining of the nose and of the back of the tongue arc specially modified in connection with the senses of smell and taste localised in those regions, as will afterwards be considered when their anatomy is described.

Endothelium. —The free surfaces covered by an endothelium are the serous membranes, the inner surface of the walls of the lymph and blood vessels and of the heart, the synovial membranes of the joints and of synovial bursae, the free surface of the osseous and membranous labyrinth of the internal ear, and the free surface of the ventricular cavities of the brain and central canal of the spinal cord. The tubes, canals, and cavities lined by an endothelium are shut off from all communication with the external atmosphere. The cells of the endothelium are arranged so as to give perfect smoothness to the surface which they cover. In the blood and lymph vessels this smoothness of surface is in order to facilitate the flow of the blood and lymph in the course of .the circulation. The serous and synovial membranes are found covering the surfaces oι parts which move on eacn other, and thθ smoothness of their respective surfaces, by permitting freedom of movement, diminishes the friction.

Each Serous Membrane consists of a portion which invests the viscus or organ, named the visceral layer, and a portion which lines the walls of the cavity in which the organ is situated, named the parietal layer. Between these two layers is the so-called serous cavity, the wall of which is formed by the smooth surfaces of both the parietal and the visceral layers. The serous membranes are as follows :—The two pleurae situated in the cavity of the chest, one investing each lung, and lining the interior of that part of the thoracic cavity in which the lung is situated; the pericar dium, which invests the heart, and lines the bag in which the heart is contained; the peritoneum, which invests the abdominal viscera, and lines the. abdominal cavity; and the arachnoid membrane, which invests the brain and. spinal cord, and is regarded by many as lining the dura mater, which encloses these important organs. The smooth free surfaces of the serous membranes are moistened by a limpid fluid, or serum, which facilitates their movement on each other, just as the free smooth surfaces of the synovial membranes are lubricated by the viscid synovia which they secrete.

Endothelial cells form usually only a single layer, and are, as a rule, flattened scale-like cells, arranged after the manner of a tessellated epithelium. Endothelium, like epithelium, is non-vascular, and, so far as is known, non-nervous.

The endothelial cells rest upon a sub-endothelial tissue, consisting of a delicate modification of the fibrous form of connective tissue. Here, as in the surfaces covered by epithelium, a basement membrane was at one time supposed to intervene between the cells and the connective tissue; but it is now believed that the cells are in direct contact, by their deeper surface, with the connective tissue itself. In the serous membranes and in the coats of the larger blood-vessels elastic fibres are present in considerable numbers in the sub-endothelial tissue, which serves as the framework of support for the blood and lymph vessels and the nerves of the part. In the serous membranes the lymphvessels are very abundant in the sub-epithelial tissue, where they form a layer parallel to the free surface of the membrane, from which short vessels pass vertically to open by minute orifices into the serous cavity. The serous membranes are attached by the sub-endothelial connective tissue to the organs which they invest.

The endothelium of the Serous Membranes consists of irregular and squamous cells, the edges of which may be smooth or slightly serrated. The cells are closely adapted to each other by their edges, so as to form a continuous smooth layer, which forms the free surface of the serous membrane. Scattered irregularly over this surface are the minute orifices, or stomata, which open into lymphatic vessels. The cells which surround the stomata differ in form and appearance from the ordinary endothelium; they are smaller, and are polyhedral, their contents are granular, and the nucleus is more distinct.

The endothelium lining the Lymphatic Vessels consists of flattened cells, which, instead of having an irregular shape, are elongated spindles, slightly sinuous in outline. The endothelium of the lymphatics is continuous with that of the serous membranes through the stomata, so that the cavities of the serous membranes are now regarded as great lymph-sacs.

The endothelial lining of the Blood Vessels corresponds in general characters with that of the lymphatics. In the small blood capillaries the cells are fusiform; in those of larger size, more irregular: in the veins they are broader, more irregular, and less distinctly fusiform than in the arteries. The endothelial covering of the endocardial lining of the heart consists of a layer of flattened cells with irregular outlines. The endothelial lining of the blood-vascular system is continuous with that of the lymph-vascular system, where the thoracic duct and other large lymph-vessels open into the great veins, and thus a continuity of surface is established between the serous membranes and the lining membrane of the blood-vascular system through the lymphatics.

( The endothelium of the Synovial Membranes is formed of roundish, or polygonal, or tessellated cells, arranged after the manner of a stratified epithelium. Not unfrequently processes of the sub-endothelial vascular connective tissue covered by the endothelium project into the cavities of joints and synovial bursae. They have been called synovial fringes, and contribute to the formation of the synovia which lubricates the surfaces of a synovial membrane.

The endothelium of the Cerebral Ventricles and Central Canal of the spinal cord is, as already stated, formed of spheroidal or cylindrical cells, possessing cilia on the free surface. The endothelial lining of the osseous labyrinth consists of flattened scales, whilst the membranous laby rinth possesses a layer of polygonal cells.

3J Group.—Cells imbedded in Solid Tissues.

The cells which are imbedded in the solid tissues are either grouped together in considerable masses, or, as not unfrequently happens, are more or less separated by an intermediate matrix or intercellular substance. The matrix substance varies in its character in different tissues, and sometimes is so abundant as to obscure the cells. The textures which are constructed on this plan are of great importance, and constitute by far the larger proportion of the tissues not only of the human body, but of the bodies of animals generally. Sometimes these tissues are elongated into delicate threads or fibres, at other times they are expanded into thin membranes, at others they form solid masses of considerable thickness.

Connective Tissue. —By the term connective tissue is meant a group of tissues which, though the members of the group differ in various respects from each other, both in naked eye and microscopic characters, yet agree in the property of binding or connecting together other tissues or parts of the body, and in serving as a supporting framework for more delicate tissues. This group of tissues is the most extensively diffused of all the textures, for there is no organ in the body which does not contain one or other of its forms. The following varieties, based on modifications in their appearance and structure, may be recognised.

a. Neuroglia. This name, which means nerve glue, has been applied by Virchow to the delicate tissue in the central organs of the nervous system, and of the retina, which supports the nerve cells, nerve fibres, and blood-vessels of those parts. Microscopically it consists of small round or ovoid corpuscles, imbedded in a soft undifferentiated protoplasm. Λ form of tumour, named Glioma, is sometimes produced by the excessive growth in the brain or retina of this variety of connective tissue.

b. Retiform connective tissue constitutes the stroma or supporting framework of the lymphatic and other glands which possess the adenoid type of tissue. It also forms the middle subdivision of the enamel organ of the teeth. It consists of stellate branching cells, the branches of which blend with each other, and form a delicate anastomosing network or reticulum. In the lymph glands, the colourless lymph corpuscles are set in the meshes of this network. In the solitary and Peyer’s glands of the alimentary canal, in the tonsils, the back of the tongue, the posterior wall of the nasal part of the pharynx, the palpebral conjunctiva, the thymus gland, the pulp and Malpighian bodies of the spleen, colourless lymph-like corpuscles are also included in the meshes of a reticulum. The name adenoid or lymphoid tissue is sometimes employed in describing this type of structure, and in some forms of disease the tissue increases in certain localities so largely in quantity as to form well-defined lymphoid tumours.

c. Gelatinous or mucous connective tissue (fi>cldeimgewebe), forms the connective tissue of the embryo, the vitreous humour of the eye-ball, and the jelly of Wharton, which invests the blood-vessels of the umbilical cord. It is soft and jelly-like in consistency. Microscopically it consists of rounded, or spindlelike, or stellate cells, imbedded in a soft gelatinous intercellular substance. Sometimes the intercellular substance is in part differentiated into short delicate fibres. Under some pathological conditions, this form of tissue increases largely in quantity in some parts of the body, and forms a kind of tumour named Myxoma.

d. Fibrous connective tissue presents four modifications in appearance. It may be soft and delicate, with the fibres short and but faintly marked, as in the sub-epithelial tissue of the skin and mucous membranes. It may be loose, flocculent, and filamentous, and may contain small spaces or areolae (when it is called areolar tissue), which is well seen in the subcutaneous tissue of the adult, and in the omenta. It may be expanded in the form of a fibrous membrane, as in the fasciae or aponeuroses, and the threads or fibres, strong and well marked, sometimes ran parallel, sometimes cross each other at various angles. It may be collected into rounded or flattened bands, as in tendons and ligaments, where it forms the tendinous and ligamentous tissues. Here also the threads or fibres may be distinctly recognised and seen to run in parallel bundles, so as to ^connect together the two structures between which the tendon or ligament passes.

In the fibrous form of connective tissue, both cells and intercellular substance, the latter of which is differentiated into fibres, may be recognised. The cells are, as a rule, either elongated, or fusiform, or caudate, or stellate branched cells, and are familiarly known as the connective tissue corpuscles. In these cells the nucleus is round or oval, and usually well marked. It is surrounded by granular protoplasm, but it is very doubtful if the protoplasm is invested by a cell wall. Not unfrequently, more especially where the cells are stellate, the delicate branched protoplasm processes of adjacent cells appear to blend at their extremities with each other, and form an anastomosing network. In tendons the cells are arranged in linear rows, which lie parallel to the long axis of the tendon itself. In adults these cells are flattened, but in younger tendons they are more polygonal in form. There seems reason to think, indeed, as Thin has shown, that the bundles of connective tissue are invested by a layer of flattened cells. The wide diffusion of the connective tissue throughout the body, and the great importance of its cellular elements, have been especially dwelt on by Virchow as sources of origin of the new cell forms which arise in various pathological processes.

The intercellular substance consists of fibres, which are not uniform in shape, and are divided into the two groups of white and yellow fibres.

The white fibres of connective tissue constitute the most common form, and make up the great bulk of most ligaments, tendons, and fibrous membranes. They consist of excessively delicate filaments, varying from ^ κ ⅜ 7π th to κ < τ ⅛ 7 )th inch in thickness, which are united together in bundles or fasciculi of variable size. The bundles, as well as the filaments of which they are composed, have a wavy course, and the filaments in each bundle lie almost parallel to each other. The bundles also in some cases are parallel, though in others they cross at various angles. Not only the filaments in each bundle, but the bundles themselves, are cemented together; the firmness of the adhesion varies in the different modifications of the fibrous connective tissue, being much more decided in the tendons, ligaments, and fascise, than in the lax areolar tissue.

The yellow fibres of connective tissue, named elastic fibres, from their elasticity, make up the mass of the ligamentum nuchae, the ligamenta sub-flava, and the yellow .elastic coat of the arteries. They are also found, mingled with the white fibres, in the fibrous membranes, the skin, mucous and serous membranes, the areolar tissue, in tendons, and some ligaments. In the ligamenta sub-flava and nuchae the yellow fibres are arranged in bundles, the individual fibres of which are comparatively broad, with a distinct dark outline. They branch, and their branches readily break across, and the broken end then curls upon itself. Their diameter is about ΓδW i h In the coats of the arteries the elastic fibres form an anastomosing network. When mingled with the white fibres they are much finer, and sometimes do not exceed 5 y⅜- ff7 th inch in diameter. They possess, however, a distinct and definite outline; they branch and occasionally anastomose; and the individual fibres, possessing a ring-like, spiral, or twisted course, are wound around the bundles of the white fibres. The white fibres yield gelatine on boiling, but the elastic fibres do not. The white fibres swell up and become so transparent under the action of acetic acid as to be no longer recognisable. The yellow fibres, again are not affected by that reagent. Quekett pointed out that the elastic fibres of the ligamentum nuchae of the giraffe were marked by transverse striae, and Μ. Watson has seen a similar appearance in the elastic pericardiac ligament of the elephant. These transverse striae are apparently cracks in the fibre; and, as Beale has shown, are not unfrequently seen in the elastic fibres in beef and mutton which have passed through the alimentary canal.

Bearing on the mode of nutrition of the tendons, and other fibrous forms of connective tissues, the existence of plasma, or juice, canals has been described, along which, not blood, but the liquor sanguinis is supposed to flow. Virchow conceived that the connective tissue corpuscles formed an anastomosing network for this purpose. Brücke believed that delicate channels or lacunae existed between the bundles of connective tissue, whilst Recklinghausen maintained that serous canaliculi were situated in the homogeneous substance which connects the fibrous fasciculi and lamellae of the connective tissue with each other. These lacunae or canaliculi are, in all probability, the rootlets of origin of the lymphatic system of vessels. There can indeed be no doubt, as the recent injections of Ludwig and Schweigger-Seidel have shown, that tendons and fasciae are well provided with lymph vessels, for they have injected in them a minute network, consisting in part of polygonal meshes, and in part of vessels running longitudinally and parallel to the connective tissue bundles, and the walls of these vessels were formed of endothelial cells. Recklinghausen and others have recently described corpuscles in the connective tissue which resemble in size and appearance the white corpuscles of the blood and lymph. These corpuscles are believed to move about in the juice canals already referred to, and it is possible that they may have migrated into the tissue through the walls of its nutrient blood-vessels.

The vascularity of the connective tissue varies in different localities. The periosteum and perichondrium are very vascular; but their numerous vessels are concerned in the nutrition not merely of these fibrous membranes, but of the bone and cartilage which they invest. The sheath of connective tissue which invests a tendon is more vascular than the substance of the tendon itself. As a rule, it may be stated that the fibrous connective tissues are not highly vascular, and that the nutritive changes which take place in them after their growth is completed are not very active.

The mode of development of the connective tissue has been much discussed by anatomists, and various views have been advanced as to the changes which lead to its production. It is now, however, generally admitted that it arises from the embryonic cells by a special morphological and chemical differentiation of their protoplasm, but the degree to which this differentiation may proceed varies with the particular form of the texture. In the neuroglia the. tissue is apparently a simple nucleated protoplasm. In the retiform connective tissue the cells have assumed a stellate shape, and their branches anastomose. In the gelatinous and fibrous forms an intercellular matrix is extensively produced, and exhibits a differentiation into fibres. In these' last-named forms, which are the most characteristic varie-' ties of the tissue, the cells of the embryo change their form, and assume a fusiform, caudate, or stellate shape; and, subsequently a delicate fibrillated structure appears between them, which assumes the characters of the bundles of white fibrous tissue, and by separating the cells from each other forms the fibrous intercellular matrix. It has been much disputed whether these white fibres take their rise immediately from the peripheral portion of the cells by a direct differentiation of their protoplasm, or whether this protoplasm is not in the first instance converted into a homogeneous matrix in which the fibrous differentiation then occurs. There can be no doubt that the fibres are formed by a metamorphosis of the protoplasm of the cells; whether the metamorphosis takes place directly, or through the intermediate stage of a homogeneous matrix, is a secondary question, and in all probability both modes of conversion take place at different times and in different localities. As the differentiation into fibres progresses, the tissue becomes firmer and tougher, and the proportion of the cellular to the fibrous element diminishes. Hence, say in a young tendon, the rows of connective tissue cells are not only closer together, but are much more readily seen than in an adult tendon, in which the increased production of fibres obscures the cellular element.

The mode of origin of the yellow elastic fibres has also been much discussed. At one time it was believed that they were derived from nuclei, and on this supposition they were named nuclear fibres. But from more recent observations there is reason to believe that they are produced, like the white fibres, by a special differentiation of the protoplasm of the embryonic cells, or of a homogeneous matrix derived from that protoplasm. In sαch localities as the ligamentum nuchae, where the fibres are both large and numerous, the whole of the cell protoplasm appears to become converted into elastic tissue. In tendons, and those parts where these fibres are slender and scanty, and coil round the bundles of white fibrous tissue, they apparently arise from a differentiation of the protoplasm on the surface only of the formative embryonic ceÜs.

Adipose Tissue. —The adipose or fatty tissue varies in its amount in different individuals. It is especially found in the marrow of the bones; as a layer beneath the skin, differing in thickness in different individuals; and collected in the cavity of the abdomen in the folds of peritoneum, known as the mesentery and omenta, in which, and indeed in the other localities where it occurs, it is intimately associated with the connective tissue. It consists of cells, which vary in size from 5 ⅜- 5 -th to ^⅜- 5 -th inch, usually ovoid or spherical in form, though when collected into masses they may be laterally compressed. These cells are sometimes isolated, though most usually arranged in rows or clusters to form lobules of fat. The number of cells in a given lobule varies with the size of the lobule. The distinctive contents of these cells is a minute drop of oil, which, when examined by transmitted light, presents a bright appearance; but when seen by reflected light, looks, as Monro primus described it long ago, like.

a cluster of pearls. Each fat cell possesses a distinct wall, as can be readily demonstrated by digesting these cells in ether, when the oil is dissolved out and the membranous wall remains. The nucleus of the fat cell is more difficult to demonstrate, and when seen is found attached to the inner surface of the cell wall. In the fat of old persons, and in specimens of this tissue which have been removed from the body for a length of time, a stellate group of acicular crystals is not unfrequently to be seen in the interior of the cell, which consists either of margarin or margaric acid, one of the constituents of human fat. The lobules of fat cells are included between bundles of the areolar variety of connective tissue, which form their supporting framework. But in addition, they are more or less perfectly surrounded by a network of capillary vessels, which not only serves to convey to them blood for their nutrition, but aids in retaining them in position.

The close anatomical relation between the adipose and the connective tissue points to a genetic relationship between them. It has now been ascertained that the first stage in the formation of a fat cell consists in the appearance of extremely minute drops of oil in the protoplasm of the connective tissue corpuscles of the part; as these run together larger drops are produced, a cell wall at the same timo differentiates from the peripheral part of the protoplasm, and as the cell becomes distended with oil, by the conversion into fat of its substance, it swells out into a spherical or ovoid cell. Klein has recently shown that the fatty tissue of the omentum and mesentery is formed by the production of oil drops within the branched cells, which form the reticular tissue that supports the lymphoid cells found so abundantly between these folds of peritoneum.

Pigmentary Tissue. —In some parts of the body a yellow, brown, or black pigment is found in the interior of cells, which gives to the tissue and organ a characteristic colour. In the coloured races of mankind, and in certain parts of the body of the white races, pigment is produced in the cells of the cuticle or epidermis, more especially in the cells of the deeper strata or rete Malpighi. In the connective tissue corpuscles, also, more especially in the dermis of fish, amphibia, and reptiles, pigment is found in considerable abundance. The choroid coat of the eyeball owes its dark brown or black colour to the presence of pigment in the interior of the cells. The pigment cells of the choroid are usually polyhedrons, 5 or 6-sided, and are arranged to form a mosaic pattern. In the centre is a nucleus, and the cell substance is occupied by numbers of minute brown granules. In the connective tissue on the outer surface of the choroid, the pigment is contained in stellate cells. In the skin of fishes and amphibia, the stellate pigment cells branch and subdivide so as to form highly complex patterns, and the cells are crowded with brown or yellow granules. The production of pigment, either in the interior of epidermal cells, in the polyhedral cells of the choroid, or in the stellate connective tissue corpuscles, is owing to a special metamorphosis or differentiation of the protoplasm substance of these cells.

Cartilaginous Tissue. —By the term cartilage, or cartilaginous tissue, is meant a group of tissues which, though usually found in the form of plates or bars, yet differ in various aspects from each other, both in naked eye and microscopic characters. They agree, however, in forming solid textures, opaque when seen in mass, but, in thin slices, translucent, pearly, or bluish white, firm in consistence, but easily cut with a knife, endowed with considerable elasticity, and yielding chondrine on boiling. Cartilage is of greater importance in the foetus, and in the immature condition of the body than in the adult, for in early life the bones are in a great measure formed of it. As development and growth proceed, a considerable proportion of the cartilage becomes converted into bone, and is called, therefore, temporary cartilage, whilst the remaining portion continues as cartilage throughout life, and is termed permanent. The following varieties of cartilage, based on modifications in structure and appearance, may be recognised

Celluιar cartilag Cells with matrix substance.

Matrix homogeneous. Matrix fibrous.

(Hyaline cartilage.) (Fibro-cartilage.)

White fibro-cartilage. Yellow fibro-cartilage.

The Cellular or Parenchymatous Cartilage does not exist in the adult human body. It occurs, however, in the human embryo, in the embryos of all the vertebrata, and in the larval stage of development of the tunicata, as the slender rod named chorda dorsalis or notochord. In all the higher vertebrata the chorda dorsalis disappears as development advances, but in the lower vertebrates it persists throughout life as a more or less perfect structure. In the lamprey and myxine it forms a continuous rod in the vertebral region. In fish generally, but more especially in the cartilaginous group, it forms a jelly-like mass, occupying the concavities between the bodies of the vertebrae. The cells lie in contact with each other. They are comparatively large in size, are sometimes rounded, but more usually compressed late rally. The nucleus is often very distinct, though at other times more difficult to detect, and the cell wall is well marked. Sometimes a little intercellular substance is found. By some anatomists the chorda dorsalis is regarded as a variety of connective tissue, and not of cartilage

The cartilaginous framework of the ear of some small mammals—as the mouse, the bat, and the rat—is formed of cellular cartilage, the cells of which are smaller in size than those of the chorda dorsalis, irregularly polygonal, and closely packed together so as to form a solid tissue.

The Hyaline Cartilage consists of cells imbedded in a pellucid or hyaline matrix, which, under some conditions, however, may assume a dimly granulated appearance. The xiphoid and costal cartilages, the encrusting cartilages at the articular ends of the bones, the cartilages of the nose, those of the windpipe, except the epiglottis and cornicula laryngis, belong to this variety, as also the temporary cartilages. In hyaline cartilage the cells are ovoid or polygonal, or even fusiform, and sometimes flattened, the flattened form of cell being found next the surface of the cartilage. They lie singly, or in groups of two, or three, or four; sometimes they are arranged in linear series, at other times they are irregularly grouped together. The cell contents are dimly granular, with a well-defined nucleus containing a nucleolus. Not unfrequently two or more nuclei are present in a cell; and in old cartilage the contents are often coarsely granular, or even infiltrated with drops of oil. Heidenhain has shown that powerful induction shocks cause contraction of the protoplasm of the cells towards the central nucleus. The cells lie in cavities in the matrix substance, and the part of the matrix which forms the immediate wall of the hollow is named the capsule of the celL Two or more cells may sometimes lie in the same hollow.

The matrix of hyaline cartilage is usually homogeneous. In some animals the matrix appears to have a concentric arrangement around the cells; and Rollett has stated that by the use of dilute sulphuric acid or chromic acid the matrix may be made to split up into concentric layers. Sometimes the matrix appears granulated, a change which is very apt to occur in sections of cartilage which have been removed for some time from the body. In the costal cartilages of old persons the matrix becomes fibrous; and it is by no means uncommon to find in advanced age these bars of cartilage converted into bone.

In the articular or encrusting cartilages the arrangement of the cells is quite distinctive. If a vertical section be made through a plate of this cartilage, the cells next the bone are seen to be arranged in parallel rows perpendicular to the surface of the bone on which the cartilage rests; the cells are smaller than those of the costal cartilage, oblong in form, and the adjacent rows are separated by intermediate hyaline matrix. Near the free surface of the cartilage the cells are flattened, placed parallel to the plane of the surface, and so closely packed together that the proportion of matrix is much reduced. In the intermediate parts of the cartilage the cells lie irregularly in the matrix, and are rounded in form. It was from the study of the changes which take place in articular cartilage in disease that Goodsir was enabled to establish the production of new cells by the multiplication of the normal pre-existing cells of the cartilage,—an observation which formed the starting-point of the modern doctrine of cellular pathology.

Fibro-cartilages are divided into white and yellow. White fibro-cartilage may form the connecting medium between the articular surfaces of an amphiarthrodial joint, as in the intervertebral discs; or it may form plates in the interior of joints, as in the semilunar cartilages of the knee and other menisci in diarthrodial joints; or it may extend around the margin of the socket of a joint, as in the cotyloid ligament of the hip; or it may invest the surfaces of bones over which tendons have to play, as where the tendons of the peronei muscles play in the groove on the back of the external malleolus. In the intervertebral discs, which give the best illustrations of the structure of white fibro-cartilage, the cells are ovoid in form and distinctly nucleated. Sometimes two or three are grouped together, but not unfrequently they occur singly. They are separ rated from each other by short fibres. In these discs the fibrous matrix is always stronger and more distinct in the peripheral than in the central part. The other forms of white fibro-cartilage are transitional between the true cartilage and connective tissue, i.e., the cells possess the characters of cartilage cells, whilst the fibrous matrix approximates to the matrix of the connective tissue.

The yellow elastic fibro-cartilages are the epiglottis, the cornicula laryngis, the cartilaginous framework of the auricle of the human ear, and the ears of mammalia generally, and the cartilaginous wall of the Eustachian tube. The cells are rounded or ovoid, distinctly nucleated, and usually arranged singly or in pairs. The matrix is distinctly fibrous; the fibres, which form a close intersecting network, branch and sometimes anastomose. They resist the action of acetic acid like the yellow fibres of connective tissue; and Donders has described a continuity between them and the elastic fibres of the connective tissue, which forms the investing perichondrium of this form of cartilage. The yellow fibro-cartilage has no tendency to ossify.

The bars and plates of cartilage,—except the encrusting hyaline cartilages, and the interarticular, marginal, and investing white fibro-cartilages,—are surrounded by a fibrous membrane or perichondrium,. In the adult human body cartilage is not penetrated by blood-vessels, but is nourished by the vessels which ramify in its investing perichondrium. In the foetus, however, and in the large masses of cartilage which are found in the skeletons of the cetacea and of the cartilaginous fishes, the cartilage is permeated by canals in which blood-vessels ramify. In the encrusting cartilages, the cartilage is nourished by the blood-vessels of the synovial membrane of the joint, which, in the case of the articular cartilage, form a vascular ring around its margin; and, both in it and in the forms of white fibro-cartilage that do not possess a perichondrium, by the vessels of the bone, to which these cartilages are as a rule attached. In the movable joints, after the child has begun to use its limbs, the synovial membrane is not continued over the free surface of the articular cartilage, but stops at its margin along the line of the vascular ring. In the foetus, however, it has been stated that both blood-vessels and synovial membrane are prolonged over the free surface of the articular cartilage.

- In the development of hyaline cartilage the contents of the embryonic cells of the part, where the cartilage is to be produced, become clear, and a cell wall differentiates around the exterior of the cell. The nuclei in the cells divide and subdivide, so that a multiplication of the cells by endogenous reproduction takes place. Hyaline matrix substance then appears between the cells, and is concentrically arranged around them; it is believed to be formed by a special conversion of successive layers of the cell protoplasm into a substance which yields chondrine on boiling. The fibro-cartilages, both white and yellow, but especially the latter, yield but little chondrine on boiling, for the fibrous matrix of the white fibro-cartilage is a gelatineyielding substance, like the white fibres of connective tissue, whilst the fibres of the yellow fibro-cartilage partake of the nature of elastic tissue. The fibro-cartilages, therefore, form a group which links together the connective and cartilaginous tissues.

f. Osseous Tissue. —The osseous tissue, or bone, is that .which constitutes the hard framework of the skeleton. i Each bone consists of a hard, more or less dense, tough, and but slightly elastic material. The elasticity of the bones is more marked in young than in adult and aged persons. From differences in their external configuration, bones are divided into long or cylindrical, e.g., femur; short, e.g., carpal or tarsal bones; flat or plate-like, e.g., scapula; irregular bones, e.g., vertebrae. These variations in shape do not, however, involve differences either in composition or minute structure. Bone consists chemically of an earthy and an animal substance intimately blended together. The earthy matter forms about two-thirds of it, and consists chiefly of phosphate of lime, which, from its abundance in bone, is frequently called “bone earth.” Carbonate of lime and a small proportion of soda and magnesia salts are also present. The hardness of bone is due to the presence of the earthy matter. The animal matter forms the remaining third, and yields gelatine on boiling; it imparts elasticity and toughness to the bone, and binds together the particles of earthy matter.

Bone presents two different structural characters to the naked eye. The outer part of a bone is its hardest part, and forms a dense external shell, technically called the compact tissue. The interior of a bone is much less firm, and is made up of thin delicate plates or bars, or trabecles, which intersect each other at various angles, and form a lattice-like arrangement, technically called the spongy or cancellated tissue. The plates and bars of the spongy tissue are continuous with the inner surface of the compact tissue. In the long bones the interior of the shaft is hollowed into a canal, named the medullary canal, the walls of which are formed by the compact tissue, and the cancellated tissue is found only at the articular ends of these bones; the thickness of the compact tissue in a long bone is always greater at the centre of the shaft than at or near the articular ends. (

If the outer surface of the compact tissue of a long bone and the wall of the medullary canal be examined with a pocket lens, they will be seen to be riddled by multitudes of minute orifices, which are the mouths of minute tubular passages or canals that traverse the compact tissue. These passages are named Haversian canals, and their arrangement may be studied by making thin sections through the compact tissue, and submitting these to microscopic examination, when they will be seen to pass longitudinally or very obliquely through its substance, so as to terminate by rounded orifices either on its outer surface, or on the inner surface, which forms the wall of the medullary canal. These canals are connected together at intervals by short transverse or oblique canals. Owing to these communications the dense osseous tissue is permeated by an anastomosing network of canals, which, as they contain bloodvessels, may be named vascular canals. These canals are circular in section, and vary in diameter from about τ ⅜ π th to χδ⅛τyth inch. They not unfrequently are dilated at the inner end, where they open into the spaces of the cancellated tissue. The compact tissue of all bones possesses a system of canals similar to those found in the long bones, but when bone occurs in the form of very thin plates the canals may be absent. In addition to the Haversian canals, irregular spaces, named Haversian spaces by Tomes and De Morgan, may also be seen in sections through the compact tissue. They are met with not only in young but in adult bones, and are regarded as produced by absorption of the bone in those particular localities. In thin sections through bone, more especially when the Haversian canals are transversely divided, the dense tissue or matrix of the bone which surrounds the canals is seen to be arranged in concentric rings, as if it were built up of a series of lamellce superimposed on each other. These lamellae do not at all times form complete circles, and the number which surround a canal may vary from two or three to half a dozen; they are sometimes called the Haversian lamellae. Other lamellae lie in relation to the periosteal surface of the bone, and are called peripheral lamellae; whilst others again are, as it were, intercalated between adjacent Haversian systems of lamellae, and are named intermediate or interstitial. It has been pointed out by Sharpey that a bone lamella, after the earthy matter has been dissolved out by the action of an acid, is made up of multitudes of fine transparent fibres, which intersect each other and form a network. But he has further shown that the lamellae are perforated by fibres, or bundles of fibres, which pass through them either perpendicularly or obliquely, so as to bolt adjacent lamellae together. With a little care, the perforating fibres of Sharpey may be drawn out of the holes or sockets in which they are lodged.

When thin sections through a macerated and dried bone are examined under the higher powers of the microscope, the lamellated matrix is seen to exhibit a very peculiar appearance, which is characteristic of the osseous tissue. Between the surfaces of adjacent lamellae irregularly elongated spaces, called lacunae, are to be seen in considerable numbers; these lacunae, like the lamellae between which they are situated, have a concentric arrangement around the Haversian canals. The lacunae, the lamellae, and the Haversian canal which they surround, are sometimes named a Haversian system. From the ends and sides of any one of these lacunae very minute branching canals, termed canaliculi, proceed,which penetrate the lamellae and anastomose with the canaliculi proceeding from adjacent lacunae, whilst the canaliculi, springing from the sides of those lacunae which lie nearest to the Haversian canal, open on the wall of the canal itself. The lacunae average in length inch, and their transverse diameter is about 3κ ⅛- 5 ∙th inch; the canaliculi vary from - r2 ~¼- δ ∙th to ∙^⅜j∙ 3 ∙th inch in diameter. When examined in a dried bone by transmitted light, the lacunae look like solid, black bodies, and the canaliculi seem to be processes branching off from them, hence they were erroneously called by the earlier observers bone-corpuscles. But if a little turpentine be added to the section, the fluid displaces the air which the lacunae and canaliculi contain in the dried bone, renders the part more transparent, and affords a satisfactory demonstration that they are, in a macerated and dry bone, not solid bodies, but a minute system of spaces and anastomosing little canals; and that all those which lie in the same Haversian system not only freely communicate with each other, but, either directly or indirectly, with the Haversian canal which they surround.

But a macerated and dried bone, such as one sees in museums and in articulated skeletons, and the structure of which has just been described, is a bone which has been deprived of several soft tissues by the process of putrefaction, which tissues are of the utmost importance in the economy of the bone in the living animal. A living bone is a complex organ, and a macerated bone is only the skeleton of a living bone. It is essential, therefore, in studying the structure of bone, that the attention should not be limited to the appearances presented by the macerated bone, but that the arrangement and structure of its soft tissues should be considered. The soft tissues of a bone are the periosteum and its prolongations, the marrow, the minute masses of nucleated protoplasm which occupy the lacunae of the bone, the blood and lymph vessels, and the nerves.

The Periosteum is a strong fibrous membrane which invests all the exterior of a bone, except where the encrusting cartilage is continuous with its articular end. It is subdivided into two layers: a, a firm external fibrous layer, consisting of bundles of connective tissue, which decussate with each other in various directions, and amidst which a network of small blood-vessels is freely distributed prior to their passage into the Haversian canals; b, a softer internal layer, which is especially well marked in young growing bones. This soft layer partly consists of very delicate connective tissue, in which rounded or oval cells are found, which give off slender processes at various points of their periphery, and partly of larger granular cells, which lie next the bone itself. Processes of the soft inner layer are prolonged into the Haversian canals, in which, as Goodsir pointed out, a layer of cellular substance lies between the wall of the canal and its contained blood-vessel, so that these canals are not, as in macerated bones, empty passages, but are filled up by the blood-vessels and the cellular layer.

The Marrow occupies the medullary canal of a long bone and the spaces in the cancellated tissue of bones generally. It occurs in two forms, red and yellow marrow. Red marrow is found in the bones of the foetus generally, and in the cancelli of the plate-like, short, and irregular bones at a more advanced period. It consists principally of large many-nucleated masses of protoplasm, the myeloid cells of KöUiker and Robin, lying in a very delicate areolar tissue, and supplied by a network of capillary blood-vessels. It contains little or no fat Yellow marrow, again, is composed of fat cells lying in a delicate areolar tissue with accompanying blood-vessels. The areolar tissue, which supports the marrow cells, lines the medullary canal and cancelli, and is named the medullary membrane, or the endosteum.

In the fresh bone the lacunae are not empty spaces as in the macerated bone. They are filled up by nucleated clumps of protoplasm, and are therefore, as Goodsir was the first to show, the seats of h’ttle masses of nucleated cells, which cells are the true bone-corpuscles. The protoplasm of these cells is apparently prolonged into the canaliculi. Hence the hard part of the osseous texture has within it a system of nucleated cells, some of which occupy the lacunae and canaliculi, while others form a lining to the Haversian canals.

The blood-vessels of a bone are abundant. It receives its arteries partly from the small arteries which ramify in the periosteum, the fine branches of which enter the Haversian canals, and form within them an anastomosing network of capillaries; partly through a special artery which enters the nutrient canal in the bone, to be distributed chiefly to the marrow; partly through small arteries which enter openings in the compact tissue near the articular extremities. The veins of bones are also abundant. In the cancellated tissue they are large, and leave the interior of the bone partly through foramina situated near the articular ends, and partly by a vein which accompanies the artery that traverses the nutrient canal. In the plate-like bones of the skull the veins lie in distinct channels in the diploe, and in the bodies of the vertebrae the veins pass out through large holes in the posterior surface. Bones possess lymph-vessels, but their exact mode of arrangement has not yet been ascertained. Fine nerves have been traced into bones accompanying the arteries which enter the nutrient and Haversian canals.

It is clear, therefore, that a bone, hard and dense though its texture seems to be, is yet hollowed out by spaces, passages, and canals which, under the several names of medullary canal, cancellated spaces, nutrient canal, Haversian canals, Haversian spaces, lacunae, and canaliculi, are occupied by blood-vessels or other soft tissues. By the penetration of blood-vessels into the bone, blood is conveyed not only to the medulla, but into the very substance even of the compact tissue; and there can be no doubt that the nucleated masses of protoplasm which occupy the lacunae and canaliculi, and line the Haversian canals, are, as Goodsir long ago pointed out, centres concerned in the nutrition of the matrix substance of the bone in their immediate neighbourhood. These cells.

together with the periosteum, the medulla, and their bloodvessels, are active agents in the development, growth, and nutrition of the osseous tissue.

In the description of the development of the skeleton, it was stated that the bones are formed by ossification in cartilage and fibrous membrane, so that bones are produced by secondary changes in a pre-existing material. The mode of production of the osseous tissue in the cartilaginous and fibrous tissues will now be considered, and it should be clearly understood at the outset that, in normal ossification, bone is not formed by a mere calcification of the matrix of the pre-existing tissue, and a conversion of the cartilage or connective tissue corpuscles into bono corpuscles; but, as the researches of Sharpey, Bruch, H. Müller, Lovén, and Gegenbaur have made known, is due to a development of new corpuscles, which Gegenbaur has named osteo-blasts, accompanied by an abundant formation of blood-vessels.

When the process of ossification in temporary cartilage begins, a change takes place in the arrangement of its cells at the centre, or point, or nucleus of ossification. The cells, instead of preserving their irregularly scattered arrangement in the matrix, are now collected into longitudinal parallel rows, not unlike what was described in a previous section, in the deeper cells of encrusting cartilage. In each row the cells lie with their long axes transverse, and apparently multiply by a process of fission. The cells at the end of the rows which lie nearest the centre of ossifie change swell out and become more rounded. Calcification of the matrix substance, which separates not only the parallel rows of cells, but also the cells in the same row, from each other, then takes place, which calcification includes also the capsules of the cartilage cells. A general opacity of the cartilage is the result of this calcification, and the further progress of ossification is rendered obscure. It is necessary, therefore, to dissolve out by an acid the calcareous matter, in order to follow the steps of the process, i Spaces or canals now form in the ossifying cartilage, into which blood-vessels, continuous with the vessels of the perichondrium, are prolonged. These spaces are lined by concentric layers of small rounded cells, not unlike lymphoid cells in size and appearance, and form the medullary spaces of foetal cartilage, whilst the cells and blood-vessels form the medulla. Respecting the source of origin of the cells of this medulla, there have been difficulties in arriving at a correct conclusion. Some have believed them to be descended from the cartilage cells, though no demonstration of their derivation from this source has ever been obtained. Henke conceived that they might be blood corpuscles migrated from the blood-vessels within the spaces. But the recent observations of Stieda seem satisfactorily to show that the layers of medulla cells are continuous with similar layers beneath the perichondrium, which layers are prolonged along with the blood-vessels into the medullary spaces as they form in the ossifying cartilage. But, whatever be their derivation, there can be no doubt that these cells undergo certain modifications which are of the utmost importance in the further stages of the ossifie process. A few become elongated into fusiform or stellate corpuscles, like those of connective tissue; others have oil drops forming in their interior, and become the cells of yellow marrow; others become the corpuscles of red marrow; others, again, which form the osteo-blasts, properly so-called, are the direct agents in the production of the osseous tissue itself.

The formation of the medullary spaces in cartilage is owing to an absorption of the calcified cartilaginous tissue. Kölliker points out that the absorption is effected through the agency of colossal, many-nucleated cells (myeloplaxes ∖ which he believes to be derived from the osteo-blastic cells of the medulla already described, so that a destruction of the calcified cartilage precedes the formation of the proper osseous tissue. As the absorption of the cartilage goes on, an irregular series of medullary spaces communicating more or less freely with each other is produced. But along with the destructive changes in the cartilage the production of the new osseous tissue takes place. Certain of the cells of the medulla are arranged in layers around the walls of the medullary spaces, and undergo an important change both in composition and shape. They become granular, their protoplasm hardens from the periphery towards the nucleated centre of the cell, so as to give origin to the dense matrix substance of a bone lamella; but the nucleus, and the protoplasm immediately investing it, do not harden, —they form the soft contents of the lacunae and canaliculi. A second layer of osteo-blastic medulla cells then passes through a similar metamorphosis, and a second lamella is formed. By a repetition of this process around the walls of the several medullary spaces, the lamellae of the bone are produced. Hence it would appear that the dense solid matrix of the osseous tissue is produced by a special hardening of the protoplasm of the osteo-blastic cells in the medullary spaces, and as layer after layer of these cells is ossified successive lamellae are produced. The persistence, however, of the nucleus of each osteo-blast, and of a small portion of its investing protoplasm, preserves within the hard matrix a certain amount of soft material, which being destroyed when a bone is macerated, leaves the lacunary and canalicular system already described. The formation of successive lamellae diminishes the size of the medullary spaces, which then form the Haversian canals. The vascular and cellular contents of these canals are therefore the remains of the contents of the medullary spaces of the foetal cartilage, and are continuous with the deeper layer of the periosteum.

So long as any cartilage remains in a foetal or young bone the process of replacement of the cartilaginous tissue by the proper osseous tissue goes on, until none of the cartilage is left, except the thin layer of encrusting cartilage at each articular extremity. Bones grow in length by an ossification in cartilage; and a provision for their longitudinal increase is furnished up to, and even beyond the age of puberty, by the plate of cartilage which senar-rates the epiphysis from the shaft of a bone. The ossification of this plate of cartilage marks the period when growth ceases in the long axis of the bone. But bones also grow in thickness, and this addition to their girth takes place by an ossification of material situated at their circumference. It has already been pointed out that a bone is invested by a fibrous membrane, the periosteum, which fulfils for it the same purpose as does the perichondrium for the cartilage. On the deeper surface of the periosteum, i.e., next the bone itself, are osteo-blastic cells, similar to those which lie in the medullary spaces of the foetal cartilage. These cells pass through a similar series of changes, and produce successive layers of new bone at the periphery. The importance of the periosteum as a centre of origin of new bone has, indeed, long been recognised by both surgeons and pathologists. The parts of this membrane in which the special bone-producing power resides is the deep layer of osteo-blastic cells, whilst the blood-vessels furnish the pabulum for their nutrition. If 6trips of periosteum be removed, along with the cells of the deeper osteo-blastic layer, from a bone, and transplanted to other parts of the living body, bone will continue to be produced by their agency.

The intra-membranous ossification of bone was first recognised by Nesbitt, and has been worked out in most of its details by Sharpey, Kölliker, and Gegenbaur. The tabular bones of the skull offer the best illustration of this mode of ossification. Sharpey has pointed out that a network of minute spicula of bone forms in the membrane, and extends in radiating lines from the centre of ossification towards the circumference of the bone. The ossifying tissue consists of fibres, of multitudes of granular corpuscles or osteo-blasts, and of blood-vessels. The osteo-blasts invest the fibres, but as the investing osteo-blastic cells calcify, from the periphery towards the nucleus, they assume a stellate configuration, and pass through a series of changes similar to those described in the intra-cartilaginous mode of ossification. The fibres, which are in the first instance soft, also calcify, and contribute to the formation of the bone. Here, however, as in the intra-cartilaginous ossification, the active agents in the ossifie process are the osteoblastic cells. The lamellated structure is due to ossification of successive layers of these cells, and the formation of the lacunae and canaliculi is owing to the persistence of their nuclei with a small proportion of unossified investing protoplasm. The increase in thickness of a membrane bone, like that of a cartilage bone, takes place through ossification in a deep periosteal layer of osteo-blasts. Hence it follows that, though the tissue which precedes the appearance of bone in the skeleton is not uniformly the same, in some cases being membrane, in others cartilage, there is an identity in the ossifie process in the two forms of pre-existing tissue, in both of which the osteo-blastic cells are the active agents in ossification. The chemical differentiation which takes place in the protoplasm of the Osteo-blasts during bone-formation is not merely a calcification, but a coincident production of a gelatine-yielding substance, within which the minute calcareous particles are deposited.

Stress has been laid by some anatomists, in discussing the homologies of the several bones of the skeleton, on the differences met with in the place of their formation. Thus, it has been supposed that a bone originally developed in cartilage cannot be homologous with one originally developed in membrane, and that a fundamental morphological distinction should be drawn between cartilage bones and membrane bones. But when it is considered that, though the place of formation may vary, the method of formation is the same in all localities, it does not appear that so much importance should be attached to the distinction between cartilage and membrane bones as it has sometimes received. Moreover, the differences between these two varieties of bones are, during the growth of the bone, still further diminished, for in both cases increase in thickness takes place in the same kind of pre-existing tissue, and in the same way, viz., by ossification of the deep periosteal layer of osteo-blasts.

In the description of the development of bone in the foetus and young person, the formation of medullary spaces was referred to. But the production of spaces in bone is by no means limited to its early stages of growth. The medullary canal in a long bone can scarcely be said to exist in the bones of an infant’s limbs. The hollowing out of the shaft of a long bone into a large canal, and the enlargement of the spaces of the cancellated tissue, goes on not only up to the period of adult life, but even to advanced years; so that in an old person the relative size of this canal is greater than in the prime of life. The Haversian spaces also, as Tomes and De Morgan pointed out, are produced by the absorption of the lamellae of the osseous tissue surrounding the Haversian canals, and the production of these spaces is constantly going on during the life of the bone. The air-sinuses in the cranial bones are also formed by the absorption of the diploe, and consequent separation of the two tables of the skulL Bones, therefore, are organs which are continually undergoing change. During growth additions are being made to their length and thickness, and additional lamellae are being formed in the walls of the Haversian canals. At the same time a hollowing out of spaces in their interior is going on, so that an increase in weight commensurate with their growth does not take place. The interstitial absorptive changes, whether occurring during growth or after growth is completed, are due, as Kölliker has shown, to the action of many-nucleated colossal cells which line the walls of the spaces where absorption is going on, which cells he has named osteo-klasts. The development and configuration of a bone is therefore, as has been well expressed by Kölliker, the product of the formation of osseous tissue by the agency of the osteo-blasts, and of its absorption or destruction by the action of the osteo-klasts.

From the fact that osseous tissue may be produced either in the cartilaginous or in the fibrous tissues, and that all three contribute to the formation of the skeleton, it is evident that these tissues are closely allied. To express this alliance they have all been grouped together under the common term connective substances. ι

Muscular Tissue. —The muscular tissue is that which is actively concerned either in the movement of parts of the body on each other, or in the movement of the entire body from place to place; it is the active agent, therefore, both in motion and locomotion. It forms a large proportion of the general mass of the body, is the essential constituent of the muscles or flesh, and enters into the formation of the walls of the hollow viscera. It consists structurally of threads or fibres, some of which are distinguished by being marked with transverse stripes or striae; others have no such markings. Hence it is customary to divide the fibres of the muscular tissue into transversely striped fibres and non-striped fibres. As a rule, the striped fibres are collected together to form those muscles which are under the influence of the will, so that both the muscles and the fibres of which they are composed are called voluntary. One important exception to this rule is, however, met with, for the muscular fibres of the heart, though transversely striped, are involuntary; the will exercises no control over the action of the heart. The non-striped fibres, and the muscles into the construction of which they enter, are in no instance, however, subject to the influence of the will; so that, without exception, they may be named involuntary.

The Non-striped or Involuntary fibre, sometimes called pale or smooth muscular fibre, enters into the formation of the walls of the hollow viscera— e.g., stomach, intestines, bladder, uterus—of the walls of the air-tubes, gland-ducts, blood and lymph vessels, of the skin, and various mucous membranes. The fibres are usually collected into bundles or fasciculi, which are not aggregated together into such compact red masses as in the voluntary muscles, but are of a paler red colour, and are set farther apart, and often cross and interlace with each other in the walls of the tubes and hollow viscera, in which this form of muscle is found. The fasciculi are separated from each other by a delicate, areolar connective tissue, or perimysium. The size of the fasciculi varies in different localities; in the hollow viscera they are so large that their arrangement can be observed with the naked eye; but in the skin, the walls of glandducts, &c., they can only be seen with the aid of the microscope. If a fasciculus be carefully torn up with needles it can be resolved into its constituent fibres, and the number of the fibres varies with the size of the fasciculus. The non-striped fibres are pale and almost colourless, with soft, ill-defined outlines, from ιy ¼th to τ⅛jyth inch in diameter; they are rounded in form or laterally compressed, and are so easily flattened by artificial pressure, that they have erroneously been regarded as flat or ribbon-shaped fibres. When digested for a few hours in dilute nitric or hydrochloric acid, and sometimes even without any reagent, the fibres may be resolved into elongatedfusiform cells—the contractile fibro-cells of Kölliker —which vary in length from s ⅜- y th to κ ⅜ τ th inch, and which taper off usually into attenuated ends. In the middle of each cell is a characteristically elongated, rodshaped nucleus, and sometimes the substance of the cell is finely granular, or even faintly longitudinally striped. No tell wall or sarcolemma can be distinguished. In some localities, as was pointed out by Lister in the minute arteries in the web of the frog’s foot, isolated contractile fibro-cells are wound spirally around the wall of the vessel.

The Transversely Striped fibre is the characteristic tissue of the voluntary muscular system, and is found wherever energetic movements are to be performed. In these muscles the fibres are collected together in fasciculi, which bundles usually lie parallel to each other, and extend from the tendon of origin to the tendon of insertion. Each muscle is invested by a membranous sheath formed of connective tissue, the perimysium externum, which sheath gives off processes that dip into the substance of the muscle, so as to form delicate partitions between the fasciculi, and from these partitions still more slender prolongations of connective tissue, named perimysium internum, pasβ between the fibres. The number and size of the fasciculi vary with the size and texture of the muscle; in some, as the deltoid and glutaeus maximus, the fasciculi are large and coarse; whilst in others, as the gracilis and omo-hyoid, they are much finer. The number of fibres in a fasciculus varies with its length and thickness, and the fibres which are adjacent to each other in a fasciculus lie parallel. The striped fibres are cylindrical or laterally compressed; they usually taper off at their extremities, and apparently do not, even in muscles with long fasciculi, exceed 1∣ inch in length. The transverse diameter of the striped fibres varies, in different localities in the human body, from i ⅜- c ∙th to 3y4⅛ σ th inch, according to the measurements of Kölliker. Much wider differences in diameter are found in the animal series, in insects the fibres being of extreme minuteness, whilst in cold-blooded animals they are much larger than in man and mammals.

If a fibre be carefully separated from a fasciculus, and examined microscopically by transmitted light, transverse stripes may be readily seen to extend across it from side to side. These transverse striae are not mere surface marks, but, as Bowman pointed out, pass through its entire thickness, and lie parallel to each other. The striation is due to the structure of the fibre, which consists of dark and light bands or discs, alternately dark and light. The discs differ in optical properties, for, as Brücke’s observations show, the light discs refract light singly—are isotropic; whilst the dark discs refract light doubly, and consist of an anisotropic substance. Busk and Huxley described in 1853 a dark line passing across the light disc, so as to subdivide it into two halves; and this appearance has also been figured by Sharpey, Krause, and others. It is believed to be due to the presence of a strongly refracting stripe in the middle of the feebly refracting disc. More recently Hensen has directed attention to a slender, feebly refracting stripe passing transversely across the strongly refracting disc, so as to subdivide it also into two halves. In addition to the transverse striae, the fibres not unfrequently show markings which extend longitudinally, but these are irregular in position, do not correspond to the whole length of the fibre, or necessarily pass through its entire thickness.

The transverse and longitudinal markings indicate that a muscular fibre has a disposition to split up transversely or longitudinally into smaller particles. The transverse subdivision of the fibre is promoted by digesting a piece of muscle for some hours in dilute hydrochloric acid. If the fibres be then examined, gaps or fissures will be seen to extend transversely into the substance of the fibre; and, if the digestion has been sufficiently prolonged, the fissures have extended completely across the fibre, and have subdivided it into a multitude of plate or disc-shaped bodies—the muscular-fibre discs. These discs are the strongly and feebly refracting discs already described, and the transverse diameter of each disc corresponds to that of the fibre from which it has been derived. The longitudinal marks in the fibre are best seen by digesting a piece of muscle in strong spirit of wine, or in a solution of chromic» acid. If a fibre so treated be teased out with needles, and the thin covering glass be smartly tapped, the fibre will split up longitudinally into multitudes of minute, elongated threads—the muscular-fibre fibrilloe. A fibrilla may be regarded as equalling in length the fibre of which it formed a part, and like the fibre is transversely striped; but its breadth is not definite, and depends upon the minuteness with which the fibre has been split up in the longitudinal direction. If in the same fibre the processes of transverse and longitudinal splitting were to go on simultaneously, then the fibre would be resolved into an immense multitude of rectangular particles—the sarcous elements of Bowman. If these particles be regarded as the ultimate subdivisions of the fibre, then the discs may be conceived to be built up of a number of these particles, possessing similar optical properties, arranged side by side, so as to occupy the entire diameter of the fibre in any transverse plane: whilst the fibrillae are built up of the particles arranged end to end, so as to correspond to the entire length of the fibre; but in this longitudinal arrangement, particles with different optical properties, the one singly refracting, the other doubly refracting, alternate with each other with the utmost regularity.

Another view of the structure of muscular fibre has just been advanced by E. A. Schäfer. He describes the dark, or, as seen in a living fibre, the “dim discs,” as traversed by multitudes of excessively fine, dark, rod-shaped particles parallel in their direction to the fibre itself, which extend into the contiguous bright discs, near the middle of which each muscle rod ends in a knob-like extremity, and the series of knobs form a line of minute dark dots, passing transversely across each bright disc. The muscle rods are imbedded in a “ground substance,” that forms the alternating dim and bright discs, which substance he believes to be anisotropous, whilst the muscle rods are isotropous. He regards the ground tabs tance as the true contractile part of the fibre.

Each transversely striped fibre is invested by a homogeneous membrane, the sarcolemma or myolemma, which is so transparent as to allow the characteristic transverse striae to be distinctly seen through it. The sarcolemma is so closely incorporated with the periphery of the fibre, that its isolation and demonstration as a distinct membrane are attended with some difficulty, but when water is added to a living fibre it is absorbed, and elevates the sarcolemma from the sarcous contractile particles. If acetic acid be added to a muscular fibre the transverse striae become less distinct, and a number of oval bodies come into view. These are especially to be seen next the periphery of the fibre in relation to the inner surface of the sarcolemma, though some apparently lie deeper in the substance of the fibre. These bodies have long been known as the nuclei of the striped fibre. More recent investigations have, however, shown that each nucleus lies in a little finely-dotted protoplasm, which often extends in a fusiform manner beyond the ends of the nucleus. These nuclei, with their investing protoplasm, have the anatomical characters of nucleated cells, and are called the muscle corpuscles.

Some peculiar modifications of ’the striped muscular fibre are met with in certain localities. As a rule, this form of fibre does not branch; but in the muscles of the tongue and lip, and other muscles of the face, these fibres usually branch prior to their insertion, and the branches taper off to finely attenuated ends. In the heart also the fibres branch; and the branches of adjacent fibres anastomose, so that the muscular wall of this organ consists of a compact network of fibres. The individual fibres are smaller than those of the voluntary muscles, the transverse striation is much less distinct, and it is doubtful if an investing sarcolemma be present.

Some difficulty has been experienced in determining the exact mode of connection of the fibres of the belly of a muscle with those of its terminal tendons. By some it has been supposed that the fibres of the one are directly continued into those of the other; whilst Weismann has described the muscular fibre as terminating in a sharply-defined, rounded, or pointed extremity, to which the fibres of the tendons are closely apposed.

Both the striped and non-striped forms of muscle are well provided with blood-vessels, which ramify in the substance of the muscle lying in the areolar connective tissue that separates the fasciculi and fibres from each other. The capillaries form an elongated network, the principal strands of which ' lie parallel to the muscular fibres, but never penetrate the sarcolemma. Hence, though the belly of a muscle is a highly vascular organ, its individual fibres are extra-vascular. The vascularity of the fleshy belly is much greater than that of the terminal tendons of attachment, and the nutritive changes are much more active in it than in them.

The contractile fibro-cells of the non-striped muscular fibre are formed by the gradual elongation of the rounded cells of the middle germinal layer of the embryo into spindle-shaped cells, the oval nuclei at the same time becoming elongated, so as to assume a rod-shaped form. Usually the spindle cells which lie in the same linear series become cemented together into the smooth fibres of this form of muscle.

The mode of development of the striped fibre is more difficult to follow out, and various statements have been made as to the successive stages of its formation.' Schwann believed that a fibre was built uo of the embryonic cells of the part, which arranged themselves in linear series, coalescing with each other at their surfaces of contact; that the contents of the cells then became transversely striated, and that the cell walls formed the sarcolemma. Savory and Lockhart Clarke maintained that a formation of blastema took place around free nuclei, and that this blastema gradually assumed the striated character. Remak, Kölliker, Wilson Fox, and Frey have, however, by studying the earliest stages of development in the very young embryo, established the fact that the striped fibres are developed from the cells of the embryo, though not in the manner described by Schwann. The process, briefly stated, is as follows: The embryonic cells elongate, the nucleus may remain single, but more usually it divides and subdivides, so that many nuclei appear in the interior of the elongated cell. The nuclei lie in linear series, and may either be separated from each other, or two or more may be in contact, and they may lie either near the periphery of the elongated cell, or in its axis. With this multiplication of the nuclei, the cell increases in length and assumes the form of a fibre. The cell protoplasm, both in the single and many-nucleated fibres, then differentiates into the sarcous particles of the transverse striae, and as this progresses the fibre assumes its characteristic striped appearance. The whole amount of the protoplasm does not, However, assume the transversely striped appearance, for a small quantity remains around each nucleus and forms with it a muscle corpuscle. The differentiation of the protoplasm occasions an anatomical and chemico-physical change in the fibre, and confers on it the property of energetic contractility. W. Engelmann has endeavoured to show that the opaque anisotropic discs of the fibre are those in which the power of contractility resides, and that the clear isotropic discs possess only elastic properties. The mode of development of the sarcolemma is still somewhat obscure. By some it is regarded as the wall of the embryonic cell, which has become metamorphosed into a muscular fibre; by others it is regarded as a special differentiation of the protoplasm at the periphery of the fibre taking place at the time when the transverse striae are being formed; whilst by others it is considered to be a special modification of connective tissue formed around the fibre. In the development of the muscular fibres of the heart, the cells of the embryo heart branch and anastomose, and the nuclei multiply. By the transverse striation of the protoplasm of these cells the branched muscular fibres of the heart are produced.

In the growth of a muscle the individual fibres increase in size, so that they are bigger in the adult than at the time of birth. The observations of Budge, Weismann, and Beale show that new fibres may also form in a muscle. Weismann believes that this increase may be due to a longitudinal splitting of a pre-existing fibre; but Beale maintains that the new fibres are produced in the muscle in the same manner as the original fibres of the part.

Nebvous System.

The Nervous System consists of a number of organs which are named respectively Nerve Centres, Nerves, and Peripheral End-organs. The largest and most important Nerve Centres are the brain and spinal cord, which together constitute the cerebro-spinal nervous axis, and are lodged in the cranial cavity and spinal canal. But, in addition, numerous small bodies, usually oval in form, technically called ganglia, are situated in the axial part of the body, and form smaller nerve centres. The Nerves are white cords which traverse the different regions of the body, both axial and appendicular, for a greater or less distance, for the purpose of connecting together the other sub-divisions of the nervous system. Tire Peripheral End-organs are minute structures connected with the peripheral extremities of the nerves. These end-organs are situated in the skin and other organs of sense, in the glands, blood-vessels, and muscles. The nerves establish communications and conduct nervous impulses, either between different nerve centres, or between nerve centres and peripheral end-organs, so as to associate together in their action parts of the nervous system often widely separated from each other. Nerves, therefore, are internuncial structures. When a nerve connects two nerve centres together it is intercentral. When a nerve connects a nerve centre with a peripheral endorgan, and conducts impulses from the centre to the end-organ, it is a centro-peripheral or centrifugal nerve. When a nerve connects a peripheral end-organ with a centre, and conducts impulses from the end-organ to the centre, it is a periphero-central or centripetal nerve. Owing to the different directions in which impulses are conducted by nerves, the varying nature of their endorgans, and the functional differentiation of the nerve centres, or portions of the nerve centres in which their central extremities terminate, nerves vary so in. their functions, that a classification of the nerves, based upon their functional properties, has been proposed. Of the centro-peripheral nerves, those which end in the muscles are motor nerves; those which end in the muscular coat of the blood-vessels are vaso-motor nerves; whilst some physiologists have named nerves which they believe to terminate in connection with the secreting cells of a gland, secretory nerves; and others, which they believe to terminate in the tissues and to be concerned in the regulation of their nutrition, trophic nerves. It should be stated, however, that it is not yet absolutely determined that the secreting cells of glands and the cell elements of the tissues have special nerves terminating in connection with them for the purpose of exercising a direct influence over secretion and nutrition. Should these special nerves be non-existent, then the secretory and nutritive functions would be influenced solely by the vaso-motor nerves, which regulate the size of the blood-vessels and the amount of blood which flows through a part in a given time. Of the periphero-central nerves, those which arise in the end-organs in the skin, terminate in a nerve centre, and excite in it the sense of touch, are nerves of common sensation; those which arise in the end-organs in the eye, ear, nose, and tongue, and excite in their appropriate nerve centres the sensations of sight, sound, smell, and taste, are nerves of special sense; whilst nerves which conduct impulses from peripheral end-organs to a nerve centre, and, instead of exciting in the latter a sensation, have the impulses reflected to motor nerves, are reflex or excito-motory nerves.

The nerve centres, nerves, and peripheral end-organs are arranged in two groups or systems—a Cerebro-spinal and a Sympathetic. The Cerebro-spinal nervous system consists of the brain and spinal cord, the nerves which arise from or terminate in these large centres, the small ganglia connected with these nerves, and the end-organs at their peripheral terminations. The Sympathetic nervous system consists of the sympathetic ganglia, with their nerves and end-organs.

Nervous Tissue. —The several parts of the nervous system are not uniform in colour, some being white, others grey. The nerves, at least those of the cerebrospinal system, are invariably white, and white masses, variable in size, are met with in the brain and spinal cord; they constitute the white matter of the nervous system. In the nerve centres, both of the cerebro-spinal and sympathetic systems, grey matter is found, sometimes in considerable quantities. This grey colour is so characteristic, that it may be regarded as marking the position of a nerve centre.

The nervous system possesses a characteristic form of tissue—the nervous tissue—which in part consists of fibres (Nerve Fibres), and in part of cells (Nerve Cells). The nerve cells are found in the grey matter—that is, in the nerve centres—and sometimes also in the peripheral end-organs. The nerve fibres constitute the nerves, enter into the nerve centres, and pass into the peripheral end-organs; they form the white matter. But in addition to the characteristic nervous tissue, the nervous system also contains a considerable quantity of connective tissue, numerous bloodvessels, and some lymph vessels.

Nerve Fibres.— Nerve fibres are of two kinds: α, the white, medullated, or dark-bordered fibres, which are the characteristic fibres of the cerebro-spinal nervous system, though they do also sparingly occur in the sympathetic system; b, the pale, non-medullated, or gelatinous nerve fibres, which are the characteristic fibres of the sympathetic nervous system.

Medullated Nerve Fibres.— To examine the structure of these fibres, a portion of a cerebro-spinal nerve may be selected. In the first place, it will be seen to be invested by a sheath of connective tissue, the j9eππewr⅛jn, which gives off processes that pass into the nerve, and subdivide it into fasciculi or funiculi. Each fasciculus is in its turn composed of nerve fibres, which are separated from each other by bundles of delicate connective tissue, prolonged from the perineurium, in which the nutrient bloodvessels of the nerve ramify. The size of a nerve is in relation to the number and size of its fasciculi, and the size of a fasciculus is in relation to the number of its fibres. The fibres and the fasciculi lie parallel to each other in the same nerve; but as nerves branch at intervals, the more external of the fasciculi diverge from the main stem to form the branches. In the white matter of the brain and spinal cord the nerve fibres are not arranged in such definite fasciculi as in a distributory nerve, and the connective tissue between the fibres is the soft, delicate form called neuroglia.

A medullated nerve fibre is an elongated cylinder, which, when examined in the body of a living animal, or immediately after removal from the living body, consists apparently of a soft, homogeneous, or glassy-looking substance enclosed within a limiting membrane. When examined some time after death, or after the addition of reagents, such as water, spirit, ether, collodion, acetic acid, &c., it loses its homogeneous aspect, and the following structures can be distinguished in it: A (Fig. 56), a delicate transparent investing membrane, — the so-called tubular or primitive membrane, or neurilemma; C, a delicate thread, extending along the axis of the fibre,—the axial cylinder or central band of Remak; B, a substance which lies between the primitive membrane and the axial cylinder,—the white substance of Schwann, or the medullary sheath. Within the external outline of the fibre, formed by the investing membrane, is a second line, not quite parallel to the first, and the presence of these two lines gives to the fibre a characteristic double-contoured appearance. The investing membrane is a perfectly pellucid, homogeneous structure, with nuclei arranged at intervals in it. It is believed to be absent from the nerve fibres in the brain and spinal cord, as well as at the peripheral terminations of many nerves. The medullary sheath is a fatty and albuminous substance, which refracts the light strongly. Not unfrequently it collects into little ball-like masses, and sometimes causes irregular bulgings on the fibre, and produces a knotted, varicose appearance; at other times it becomes granular, and makes the fibre opaque. By gentle pressure it can be squeezed out of the broken end of a fibre. The axial cylinder is a pale, grey, cylindriform band, usually about one-third or one-fourth the diameter of the fibre, which possesses more tenacity than the medullary sheath, and not unfrequently, as in Fig. 56, 2, projects for some distance beyond the broken end of a fibre. Max Schultze showed that it is not homogeneous, but exhibits a very delicate longitudinal fibrillation, and at the ends of the nerves these primitive fibrillae may separate from each other.

Although from its great delicacy the axial cylinder cannot be seen in the living fibre of a cerebro-spinal nerve, yet there are many reasons for regarding it as a structure existing in the living nerve, and not the product of a post mortem, change. It is the part of a fibre which first appears in the course of development, the medullary sheath and primitive membrane being secondary investing structures, superadded as development proceeds. It forms not unfrequently the only constituent of a nerve fibre at its central and peripheral terminations, and is therefore the part of the fibre which is anatomically continuous with the nerve cell, or with the peripheral end-organ. As it is the sole constituent of many nerve fibres at their terminations, and of all nerve fibres in the earlier stage of development, and as it forms the medium of connection between them and the structures in which they terminate, it is obviously of primary importance, both anatomically and physiologically, and is believed to be the part of the fibre directly concerned in the conduction of impulses; whilst the investing structures serve the purpose of insulating materials. Lister and Turner pointed out, in 1859, that essential differences in chemical composition existed between the axial cylinder and the medullary sheath; the former being unaffected by chromic acid, though the latter is rendered opaque and brown, and concentrically striated under its influence; while, on the other hand, the axial cylinder is stained red by an ammoniacal solution of carmine with great facility, although the medullary sheath is unaffected by it. They further showed that these differences in the mode of action of chromic acid and carmine might advantageously be employed in the demonstration of the structure of nerve fibres. Ranke has subsequently stated that the axial cylinder possesses an acid, and the medullary sheath an alkaline reaction.

Medullated nerve fibres vary materially in diameter in different parts of the nervous system. In the brain, for instance, they are sometimes as fine as the τ2 -⅜∙ δ -∙ 5 th inch; whilst, in the distributory nerves, fibres of - r √⅛ 7 th of an inch in diameter may be seen; though it should be stated that, even in the nerves of distribution, fibres of great minuteness are often placed in the same bundle with those of the largest size. Nerve fibres do not branch in their course, but only at their central or peripheral terminations, and much more frequently at the latter than the former.

Non-medullated Nerve Fibres.— These fibres, which are characterised by the absence of a medullary sheath, are chiefly found in the sympathetic nervous system, but they occur also in the cerebro-spinal system. The fibres of the olfactory nerve are non-medullated, so also are the peripheral terminations of the cerebro-spinal nerves, and indeed all nerve fibres in the first stage of their development. In Petromyzon it has been stated that all the nerve fibres are distinguished by the absence of a medullary sheath.

This form of nerve fibre consists of pale grey, translucent, flattened bands, the o l o th to κ - δ ⅛- c ∙th inch in diameter. They usually appear as if homogeneous or faintly granular; but Schultze showed that, when carefully examined, they present a delicate fibrillated appearance, like that seen in the axial cylinder of a medullated nerve; so that, like that cylinder, they are supposed to be composed of multitudes of extremely delicate primitive fibrillae imbedded in a finely granulated material. Sometimes these fibres consist solely of this fibrillated material, at other times they are invested by a sheath similar to the primitive membrane of a medullated fibre. Nuclei are also found both in the substance of the fibre and in relation with the primitive membrane. The presence of multitudes of fibres in the sympathetic nervous system, formed either entirely, or almost entirely, of a material precisely similar in structure to the axial cylinder of a medullated fibre, and by which the proper function of the fibre can alone, therefore, be exercised, is, of course, an additional argument to those previously advanced, in favour of the existence of the axial cylinder as a normal constituent of the fibre, and of its functional importance.

Nerve Cells. —Nerve cells constitute an important division of the nervous tissue. They are the characteristic structures in the nerve centres, are susceptible to impressions, or nervous impulses, and are the texture in which the molecular changes occur that produce or disengage the special form of energy named nerve energy, the evolution of which is the distinctive mark of a nerve centre. The central extremities of the nerve fibres lie in relation to, and are often directly con-tinous with, the nerve cells. It was at one time thought that nerve cells were globular in form; but it is now generally understood that, though the body of the cell is not unfrequently globular, two or more processes or poles project from it, and are continuous with its substance. Nerve cells are distinctly nucleated; the nuclei are usually large, and contain one, and often two nucleoli. The cell substance is granular, and not unfrequently brown or yellow pigment is co∏ected around the nucleus. A cell wall is sometimes apparently present, though at others it cannot be demonstrated. The nerve cells in the grey matter of the brain and spinal cord are imbedded in the neuroglia. In the smaller nerve centres, as the sympathetic ganglia and the ganglia on the posterior roots of the spinal nerves, the nerve cells are surrounded by a capsule of connective tissue. Fräntzel, Kölliker, and others, have described this capsule as lined by an endothelium formed of flattened cells; and it should be stated that Ranvier has described a similar endothelium in relation to the connective tissue investment of the cerebro-spinal nerves. It is not improbable that these endothelial cells form the walls of delicate capillary rootlets of the lymphatic vascular system.

Nerve cells from which two poles or processes proceed are called bipolar. Characteristic specimens of these cells, as was first pointed out by Robin and R. Wagner, may be recognised without difficulty in the ganglia on the posterior roots of the spinal nerves of fishes, and it is probable that similar cells exist in the corresponding centres in other vertebrates. These cells usually possess a globular body, though sometimes it may be elongated; and from opposite points of the surface of the body a strong process is given off, which is directly continued into a nerve fibre. The axial cylinder of the fibre is continuous with the cell substance, and Schultze has shown that both exhibit a delicate fibril-lated structure. The medullary sheath and the primitive membrane are also usually continued from the fibre over the nerve cell Hence these bipolar cells seem to be, as Schultze expressed it, nucleated enlargements of the axial cylinder.

A remarkable modification of the bipolar nerve cell, carefully studied and described by Lionel Beale, is found in the sympathetic ganglia of the frog. The cells are pear-shaped, and from the narrow end of the pear two nerve fibres arise, one of which, called the straight fibre, forms, as it were, the stalk of the pear; whilst the other, or spiral fibre, winds spirally round the straight fibre, and then passes away from the cell in the opposite direction. Both fibres are nucleated, and at their origin consist, apparently, of axial cylinder substance only; but in their course they may acquire both a medullary sheath and a primitive membrane. The straight fibre passes into the interior of the cell substance, and Arnold and Courvoisier believe that they have traced it into the nucleus; but the spiral fibre apparently arises nearer the periphery of the cell. The pyriform cells are invested by a distinct capsule of connective tissue. The nerve fibres of these pyriform cells, although they both arise close together from one end of the cell, represent its poles. Should one of the poles, either in this, or in the bipolar form of nerve cell described in the preceding paragraph, be from any cause removed or not developed, then the cell would be unipolar; and if both poles were absent it would be apolar.

In other localities, as in the sympathetic ganglia of man and many other vertebrates, and in the several subdivisions of the cerebro-spinal nervous axis, the nerve cells have more than two poles or processes projecting from them. Cells of this kind are called multipolar, and in many localities they present characteristic forms. In the grey matter of the spinal cord, more especially in its anterior horn, they give rise to numerous processes, and have a stellate or radiate form. In the grey matter on the surface of the convolutions of the cerebrum they are pyramidal in shape. The apex is directed to the surface of the convolution, the base towards the white matter. The processes arise from the base, apex, and sides of the pyramid. In the grey matter on the surface of the cerebellum the body of the cell is almost globular. From that aspect of the cell which is directed towards the white matter a slender central process arises; from the opposite or peripheral aspect of the cell two strong, many-branched processes extend for a considerable distance. In the human sympathetic ganglia, again, the stellate form of cell prevails, and the existence of a capsule of connective tissue around the individual cells can be recognised. The processes which arise from a multipolar nerve cell, as a rule, divide and subdivide as they pass away from the body of the cell, until at last they give rise to branches of extreme tenuity. These branching processes apparently consist exclusively of cell protoplasm, and have been called protoplasm, processes. Gerlach has described the protoplasm processes of the multi-polar nerve cells of the brain and spinal cord as forming an excessively minute network, from which minute medullated nerve fibres arise; and F.

Boll conceives that a similar arrangement occurs in the cells of the cerebellum. One, at least, of the processes of a multipolar nerve cell does not branch, but becomes directly continuous with a nerve fibre, and has been named the axial - cylinder process. This process was first recognised by Deiters in the cells of the spinal cord; but Hadlich and Kosch-ennikoff have since described the central process of the cells of the cerebellum as continuous with a medullated nerve fibre; and the latter observer has pointed out, that from the base of a pyramidal nerve cell in a cerebral convolution a process may be traced directly into a nerve fibre. Hence it would appear that the multipolar nerve cells may have two modes of union with nerve fibres—one directly through the passage of the non-branched axial-cylinder process into a fibre, the other through the origin of fibres from the minute network in which the branched protoplasm processes terminate. The branched processes of adjacent nerve cells may also blend with each other, so as to form an anastomosing cell network, though these anastomoses are, in all probability, not so frequent as was at one time supposed. Schultze has pointed out that not only the protoplasm substance of the body of a multipolar nerve cell, but both the non-branched and branched processes, possess a fibrillated structure similar to that described by him in the axial cylinder of the nerve fibres.

Peripheral Endrθrgans or End Bodies.— Nerve fibres at their peripheral extremities terminate in connection with peculiar structures, named end-bodies, terminal bodies, or peripheral end-organs, which are situated in the several organs of the body. The motor nerves end in the voluntary and involuntary muscles; the vaso-motor nerves end in the muscular coat of the blood-vessels; the sensory nerves end in the skin, mucous membranes, and organs of special sense; and it has been stated that secretory nerves terminate in connection with the ultimate cell elements of the secreting glands. These end-organs possess certain structural peculiarities, which are by no means uniform in the different parts, so that the end-body connected with the peripheral termination of a nerve is distinctive of the organ in which it is situated. It will be a matter of convenience to defer the consideration of the peripheral end-bodies in the skin, organs of special sense, coats of the blood-vessels, and the several glands, until these parts are described. In this place the mode of termination of the motor nerves in the voluntary and involuntary muscles, of the sensory nerves in the mucous membranes, and of the ending of the nerves in the remarkable bodies named Pacinian corpuscles, will alone be examined.

After a nerve has entered a voluntary muscle it ramifies in the connective tissue, which lies between the fasciculi, and at the same time divides and subdivides into smaller branches. These branches interlace with each other and form plexuses, from which slender nervous twigs, often consisting of only a single medullated nerve fibre, proceed, which ramify in the connective tissue, separating the individual muscular fibres from each other. The single nerve fibres in their turn branch, accompanied by a splitting of the axial cylinder, and these branches usually lose the nιedullated character. The mode of termination of these very delicate branches has been a subject of much dispute. Beale described them as forming a minute network, situated on the exterior of the sarcolemma, but in contact with it, and the fibres of this nervous network were distinctly nucleated. Other observers have, however, described peculiar bodies, called motorial end-plates, at the extremity of these nerves. These end-plates consist of a clump of richly nucleated protoplasm, somewhat oval or perhaps irregular in form, into which the axial cylinder of the nerve fibre penetrates. The exact position of these end-plates in relation to the muscular fibres is difficult to determine. Krause holds that they lie outside the sarcolemma, but adherent to it; whilst Kühne, Margo, and Rouget maintain that the end-plate lies within the sarcolemma, and that the nerve fibre has to pierce that membrane before it can enter the end-plate. After the axial cylinder has entered the end-plate it subdivides into very minute branches. Each muscular fibre has apparently only a single end-plate, and consequently only a single axial cylinder in connection with it.

Tn flιe non-striped muscles the nerves are distributed in the connective tissue which separates the fasciculi from each other. Here they form plexuses, which in some localities, as in the myenteric plexus of Auerbach in the muscular coat of the intestines, have collections of nerve cells, forming microscopic ganglia lying in them. From these plexuses fibres arise which subdivide into delicate non-medullated fibres possessing nuclei. These delicate fibres form still finer plexuses, which in their turn give origin to minute fibres, which pass between the muscιdar fibre cells to form a still more minute intra-muscular network. Frankenhäuser maintains that the delicate nerve fibrils which arise from this terminal network penetrate the muscular fibre cells, enter the nucleus, and terminate in the nucleolus; but Arnold considers that, after having entered the nucleus, the fibril again gives off a filament, which passes out of the cell to join the intra-muscular plexus; the ending of the nerve, therefore, within the nucleus is only apparent, and is rather to be regarded as the nodal point of a fine intra-nuclear plexus.

The termination of the sensory nerves in the mucous membranes has been especially studied in the conjunctiva, the mucous membrane of the soft palate, and the glans of the penis and clitoris. In these parts Krause discovered oval or globular end-bodies, which consisted of a soft, homogeneous substance invested by a nucleated capsule of connective tissue. A nerve fibre pierces the capsule and terminates in the interior of the end-body, which forms a bulbous enlargement at the end of the nerve, and is called the end-bulb. After the nerve has entered the end-bulb, it may consist only of the axial cylinder and terminate in a pointed extremity, or it may twist upon itself and form a coil within the end-bulb. When the structure of the skin is described, it will be seen that the ending of the nerves in the cutaneous papillae bears a general resemblance to their termination in the end-bulbs of a mucous membrane.

But in certain of the mucous membranes delicate nerves have been traced into the layer of epithelium, situated on the free surface of the membrane. Petermöller described nerve fibres continuous with the nerves of the cornea passing into the layer of conjunctival epithelium on the front of the cornea. Klein recognised an intra-epithelial nervous network in the same locality. Chrschtschonovitsch traced non - medullated nerve fibres proceeding from a sub-epithelial network into the deeper epithelial layers of the vaginal mucous membrane, and similar nerve fibres have been seen by Elin to end in the epithelial investment of the mucous membrane of the mouth.

Connected with the sensory nerves in some localities are the remarkable bodies named the Corpuscles of Pacini, which were the first terminal organs discovered in connection with the peripheral distribution of the nerves. These corpuscles have been found attached to the nerves which pass to the skin of the fingers and toes, to the nerves which supply the skin of the neck and arm, to the intercostal nerves, to the nerves of the joints, to the nerves of the periosteum, to the nerves of the genital organs, and to the mesenteric nerves. In cats they are often extremely abundant both in the mesentery and omenta. A Pacinian corpuscle can be seen by the naked eye, and looks like a minute grain from - 1 1 5 -th t θ ^ nc ^ 1 l° n e>∙ It is elliptical in form, and may either be sessile or attached to the nerve stem by a slender stalk. Examined microscopically, it is seen to consist of numerous layers of connective tissue concentrically arranged, which form its capsule, and surround a central core. Numerous connective tissue cor puscles may be seen in the concentric layers, and Hoyer has recently shown that an endothelial-like appearance exists on the inner surface of the corpuscle. Entering one pole of the corpuscle is a nerve fibre which extends along the axial core for a considerable distance, and usually terminates in a slight bulbous enlargement. The nerve fibre parts with its perineurial sheath after it enters the Pacinian corpuscle; and as it lies in the core it loses its medullary substance, so that its terminal part consists only of the axial cylinder. Sometimes the nerve fibre divides into two branches within the corpuscle. Capillary blood-vessels are distributed to the concentric layers of the Pacinian corpuscle.

The mode of origin of the nervous tissue in the course of development of the embryo is still involved in some obscurity. It is, however, believed that the nerve cells are derived from the embryo cells, which multiply, and tho young cells then grow and assume characteristically granular and finely fibrillated contents. Processes or poles then appear at the periphery of the cells, which, according to the observations of Beale, connect adjacent cells together. As the growth of the part goes on, the cells are more widely separated from each other, and the anastomosing processes in consequence become considerably elongated, and form the axial cylinder of the nerve fibre. In the course of time the medullary sheath and the primitive membrane may form around this axial cylinder so as to insulate it. The exact mode of formation of the medullary sheath is not properly understood; but it is believed that the primitive membrane, and the perineurial connective tissue, arc derived from those surrounding embryonic cells which differentiate into connective tissue. Of the two originally contiguous cells from which the nerve fibre is, as it were, spun out, one, as Hensen conceived, may form a cell in a nerve centre, the other may differentiate into a peripheral end-organ. In the tail of the tadpole the formation and growth of nerve fibres have been studied by Kölliker and others, and it has been seen that the terminal part of a fibre may have fusiform or tri-radiate cells connected with it, the processes of which cells gradually differentiate into nerve fibres. The young cerebro-spinal nerve fibres are distinctly nucleated, and correspond in appearance and structural characters to the non-medullated nerve fibres of the adult. If in a young or adult person a nerve be cut across, its conducting power is destroyed; but after a time it reunites, and its function is restored. The part of the nerve which lies between the place of section and its peripheral extremity, undergoes, as Waller pointed out, degenerative changes. To how great an extent the degeneration affects the various constituents of each fibre, it is difficult to determine; for whilst some experiments would seem to show that only the medullary sheath broke up into granular particles and was absorbed, in others both it and the axial cylinder disappeared. In process of time, however, these parts may be reproduced, and the nerve then recovers its functional activity.

Descriptive Anatomy of the Cerebro-spinal Nervous System.

In this section the anatomy of the Brain and Spinal Cord, and of the numerous distributory Nerves which arise from them, will be described. The brain and spinal cord are the largest and most important of all the nerve centres. They occupy the cranial cavity and spinal canal, and are continuous with each other through the foramen magnum in the occipital bone. As the arrangement of the structures which compose the brain and spinal cord is extremely complex, and as the names applied to the several parts are numerous and often very arbitrary, it may be well, before commencing a detailed description, to make a few general observations on their mode of development.

Development of the Cerebro- Spinal Nervous Axis.— The brain and spinal cord are developed in the cranio-spinal groove of the embryo, and appear originally as a thin band extending along the whole length of this groove. About the time when the walls of the groove meet posteriorly to complete the cranio-spinal cavity, the margins of this band become elevated, bend backwards, and meet, so that the originally simple band becomes converted into a cylindri-form cerebro-spinal tube. In the walls of this tube the nervous structures of the brain and spinal cord are formed, whilst the axis of the tube forms a central canaL In the part which becomes the Spinal Cord the central canal persists as the central canal of the spinal cord, and around it a layer of ciliated cylindrical endothelium is developed. Outside this layer a mass of grey matter containing nerve cells is formed, which is subsequently divided into two lateral crescent-shaped masses. Outside the grey matter white matter is produced, which ultimately becomes aιτanged in the form of longitudinal columns of nerve fibres. With the formation and growth of these columns and of the internal grey matter, a longitudinal mesial fissure appears on the anterior and another on the posterior surface of the cord, which gradually increase in depth until the cord is almost completely divided into two lateral halves. At the bottom of tho anterior median fissure the nerve fibres of the anterior commissure are developed, and at the bottom of the posterior median fissure those of the posterior commissure. These commissures unite the two halves of the cord together.

The upper or cerebral end of the cerebro-spinal tube becomes the Encephalon, or Brain. At first the cerebral part of the tube is uniform in appearance with the spinal part, but it soon expands into three vesicular dilatations — the primary cerebral vesicles. These vesicles, named (from before backwards) anterior, middle, and posterior, are separated from each other by constrictions, and as the development progresses the vesicles bend on each other and on the upper end of the spinal cord. As each vesicle is an expansion of the cerebro-spinal tube, it is necessarily hollow, and the space in its interior is continuous with the central canal of the spinal cord. In the walls of the vesicles the nervous structures are produced, which form the several subdivisions of the encephalon.

The posterior cerebral vesicle bends first forwards from the upjier end of the spinal cord, and then backwards; the part which bends forward becomes the medulla oblongata; that which bends backward the cerebellum, whilst the pons is developed at the angle where these two parts are continuous with each other; the central hollow forms the central canal of the medulla oblongata and the dilated space called the fourth ventricle. In the medulla oblongata shallow anterior and posterior median furrows then appear continuous with those in the cord, and each lateral half differentiates into grey matter and into a longitudinal arrangement of nerve fibres continuous with the corresponding structures in the cord. A large proportion of these fibres are continued upwards through the pons as its longitudinal fibres. The cerebellum consists at first of a central lobe, and in the lower vertebrates its development does not proceed beyond this stage; but in mammals, including man, a lateral lobe or hemisphere is superadded on each side, and with the growth of these lateral lobes numerous transverse fibres, which connect the two hemispheres together, are developed in the pons. The cerebellum is also connected below with the medulla oblongata by the pair of restifoπn bodies, or inferior peduncles, and above with the corpora quadrigemina by the pair of superior peduncles.

The middle cerebral vesicle bends forwards from the posterior vesicle. In its roof the optic lobes are formed; in its floor the crura cerebri; whilst the central hollow becomes the aqueduct of Sylvius. At first the optic lobes form a single structure, but about the sixth month of embryo life a median furrow divides this structure into two lateral halves (the corpora bigemina), and in the lower vertebrates the development does not proceed beyond this stage; but in the seventh month of embryo life of the human foetus each lateral half is subdivided into two by a transverse fissure, so that four bodies (the corpora quadrigemina) are produced. The crura cerebri form the two cerebral peduncles, which, diverging from each other, pass upwards to the hemisphere of the cerebrum. They consist almost entirely of nerve fibres continuous with the longitudinal fibres of the pons, a few of which go to the corpora quadrigemina, but the greater number ascend to the cerebrum.

The anterior cerebral vesicle bends downwards from the middle vesicle. The posterior part of this vesicle forms at first a simple hollow sac, but subsequently divides into the two optic thalami, one on each side of the central hollow, which hollow becomes the third ventricle. This ventricle is prolonged downwards into a funnel-shaped process, the infundibulum, which is connected with the pituitary body, or hypophysis cerebri, lodged in the pituitary fossa in the sphenoid bone, whilst posteriorly it is continuous with the aqueduct of Sylvius. In its upper and posterior wall the 2n'τιmZ body, or epiphysis cerebri, is developed, and from this body two white peduncles run forwards on the sides of the optic thalami. Immediately below these peduncles the transverse fibres of the posterior commissure are developed, which pass between the two optic thalami. The anterior wall of this ventricle is closed in by the lamina cinerea or lamina terminalis, and behind it are formed the transverse nerve fibres of the anterior commissure, and the vertical fibres of the anterior pillars of the fornix. These fornix fibres pass to the base of the brain, and form the corpora albicantia, prior to entering the optic thalami. The posterior part of the anterior vesicle gives off from each side a flask-shaped prolongation., the primary optic vesicle. The stem of the prolongation, at first hollow, becomes solid, and forms the optic nerve and tract, whilst the expanded distal end forms the nervous elements of the retina.

The antero-lateral part of the anterior cerebral vesicle is prolonged forward as two hollow processes, the hemisphere vesicles, which become the cerebral hemispheres, and are separated from each other by a median longitudinal fissure; whilst the hollow in the interior of each forms the lateral ventricle. In the floor of each hemisphere vesicle is developed a large grey mass, striated with bundles of nerve fibres, the corpus striatum, which lies immediately in front and to the outer side of the optic thalamus; a curved band, the taenia semicircularis, is formed along the junction of the thalamus with the corpus striatum, and at the inner and anterior end of this band, immediately behind the anterior pillars of the fornix, the two lateral ventricles become continuous with each other and with the third ventricle through the foramen of Monro. The roof and side walls of each hemisphere vesicle form a grey expansion or mantle, which is at first smooth, but subsequently becomes divided into lobes and convolutions, separated from each other by fissures. A deep gap or fissure now appears on the inner wall of each hemisphere vesicle, and is bounded above by a longitudinal band of fibres, which, continuous anteriorly with the anterior pillar of the fornix, joins its fellow in the middle line to form the body of the fornix, and then again diverging from its fellow passes backwards, downwards, and forwards as the posterior pillar of the fornix or the taenia hippocampi. A transverse arrangement of fibres then forms in each hemisphere vesicle, above the plane of the fornix, which, reaching the mesial plane, joins its fellow, connects the two hemispheres together, and forms the corpus callosum. In the hinder part this corpus rests upon the upper surface of the fornix, but more anteriorly it lies some distance above the fornix, and then bends down in front of it. Hence there is enclosed between the fornix and the antero-inferior part of the corpus callosum two thin layers of grey matter, one belonging to the inner surface of each hemisphere vesicle, and called the septum lucidum. Between these two layers is a narrow space, the fifth ventricle, which, unlike the other ventricles, is not derived from the cerebro-spiual tube, but is merely a portion of the longitudinal median fissure shut in by the development of the corpus callosum and fornix. Each hemisphere vesicle also gives off from its anterior part a hollow process, which expands in front into a bulbous dilatation, named the olfactory bulb, from which the nerves of smell arise, whilst the stalk of the bulb solidifies and forms the olfactory peduncle. '->-m

Owing to the great development of the mantle of the hemisphere vesicles in the human brain, and the size and complexity of the convolutions, these parts of the hemispheres grow forward so as to overlap the olfactory bulbs and peduncles, and backward, so as to conceal not only the corpora striata and optic thalami, but also the corpora quadrigemina, crura cerebri, cerebellum, pons, and medulla oblongata, so that when the human brain is looked at from above, none of these structures can be seen. It is only when the brain is turned over and its base exposed that the medulla, pons, cerebellum, and crura are visible; and before the corpora quadrigemina, optic thalami, and corpora striata can be exposed, portions of the hemisphere substance must be removed. The great growth of the hemisphere vesicle leads also to a great expansion of the central hollow or lateral ventricle, which is prolonged forwards, backwards, and downwards as the anterior, posterior, and descending cornua. In tho descending cornu 13 a projection, the hippocamus major, along which the taenia hippocampi of the fornix runs; in the posterior cornu is a smaller eminence, the hippocampus minor; and at the junction of these two cornua is a third elevation, the eminentia collateralis.

Immediately investing the spinal cord and encephalon a vascular membrane, the pia mater, is developed, processes from which dip into the fissures between the two halves of the cord and between the cerebral convolutions. A broad band, the velum interpositum, which possesses two marginal fringes, the choroid plexuses, is admitted into the lateral ventricle through the gap or fissure in the inner wall of each hemisphere vesicle. This fissure is bounded above by the arch-shaped fornix, with its taenia hippocampi. When the two hemispheres are in situ, and the two halves of the fornix are joined together to form the body of that structure, the fissure, with its contained velum interpositum, passes across the mesial plane from one hemisphere to the other, having the fornix and taeniae for its. roof, and the optic thalami and corpora quadrigemina for its floor; it is known as the great transverse fissure of the cerebrum. t cλt , Membranes of Brain and Spinal Cord. —These nerve centres are invested by three membranes or meninges, which lie betλveen them and the bones that form the walls of the cranial cavity and spinal canal. The membranes are named dura mater, arachnoid mater, and pia mater.

Dura mater.— The most external membrane, named dura from its firmness, consists of a cranial and a spinal subdivision. The cranial part is in contact with the inner table of the cranial bones, and is adherent along the lines of the sutures and to the margins of the foramina, which transmit the nerves, more especially to the foramen magnum. It forms, therefore, for these bones an internal periosteum, and the meningeal arteries which ramify in it are the nutrient arteries of the inner table. As the growth of bone is more active in infancy and youth than in the adult, the adhesion between the dura mater and the cranial bones is greater in early life than at maturity. From the inner surface of the dura mater strong bands pass into the cranial cavity, and form partitions between certain of the subdivisions of the brain. A vertical longitudinal mesial band, named, from its sickle shape, falx cerebri, dips between the two hemispheres of the cerebrum. A smaller sickleshaped vertical mesial band, the falx cerebelli, attached to the internal occipital crest, passes between the two hemispheres of the cerebellum. A large band arches forward in the horizontal plane of the cavity, from the transverse groove in the occipital bone to the clinoid processes of the sphenoid, and is attached laterally to the upper border of the petrous part of each temporal bone. It separates the cerebrum from the cerebellum, and, as it forms a tentlike covering for the latter, is named tentorium, cerebelli. Along certain lines the cranial dura mater splits into two layers, to form tubular passages for the transmission of venous blood. These passages are named the venous blood sinuses of the dura mater, and they are lodged in the grooves on the inner surface of the skull referred to in the description of the cranial bones. Opening into these sinuses are numerous veins, which convey from the brain the blood that has been circulating through it; and two of these sinuses, called cavernous, which lie at the sides of the body of the sphenoid bone, receive the ophthalmic veins from the eyeballs situated in the orbital cavities. These blood sinuses pass usually from before backwards: a superior longitudinal along the upper border of the falx cerebri as far as the internal occipital protuberance; an inferior longitudinal along its lower border as far as the tentorium, where it joins the straight sinus, which passes back as far as the same protuberance. One or two small occipital sinuses, which lie in the falx cerebelli, also pass to join tho straight and longitudinal sinuses opposite this protuberance; several currents of blood meet, therefore, at this spot, and as Herophilus supposed that a sort of whirlpool was formed in the blood, the name torcular Herophili has been used to express the meeting of these sinuses. From the torcular the blood is drained away by two largo sinuses, named lateral, which curve forwards and downwards to the jugular foramina to terminate in the internal jugular veins. In its course each lateral sinus receives two petrosal sinuses, which pass from the cavernous sinus backwards along the upper and lower borders of the petrous part of the temporal bone.

The spinal part of the dura mater hangs loosely in the spinal canal. It does not form a periosteum for the vertebrae, but is separated from their bony rings by loose fat and a plexus of veins. It gives off no bands from its inner surface, and it does not split into two layers for the lodgment of venous blood sinuses. The spinal dura mater forms a tubular envelope for the spinal cord and the origins of the spinal nerves. It extends from the foramen magnum, where it is continuous with the cranial dura mater, to the lower end of the sacral canal, ends below in a funnel-shaped prolongation, and is pierced laterally by the roots of the several spinal nerves in their passage outwards to the intervertebral foramina.

Both the cranial and the spinal parts of the dura mater consist of a tough, fibrous membrane; somewhat flocculent externally, but smooth, glistening, and free on its inner surface. The inner surface has the appearance of a serous membrane, and when examined microscopically is seen to consist of a layer of squamous endothelial cells, similar to those drawn in fig. 34. Hence the dura mater, is sometimes called a fibro-serous membrane. The dura mater is well provided with lymph vessels, which in all probability open by stomata on the free inner surface. Between the dura mater and the subjacent arachnoid membrane is a fine space containing a minute quantity of limpid serum, which moistens the smooth inner surface of the dura and the corresponding smooth outer surface of the arachnoid. It is regarded as equivalent to the cavity of a serous membrane, and is named the arachnoid cavity, or, more appropriately, the sub-dural space. Arachnoid mater.— The arachnoid is a membrane of great delicacy and transparency, which loosely envelopes both the brain and spinal cord. It is separated from these organs by the pia mater; but between it and the latter membrane is a distinct space, called sub-arachnoid. The sub-arachnoid space is more distinctly marked beneath the spinal than beneath the cerebral parts of the membrane, which forms a looser investment for the cord than for the brain. At the base of the brain, and opposite the fissures between the convolutions of the cerebrum, the interval between the arachnoid and the pia matter can, however, always be seen, for the arachnoid does not, like the pia mater, clothe the sides of the fissures, but passes directly across between the summits of adjacent convolutions. The sub-arachnoid space is subdivided into numerous freely-communicating loculi by bundles of delicate areolar tissue, which bundles are invested, as Key and Retzius have shown, by a layer of squamous endothelium. The space contains a limpid cerebro-spinal fluid, which varies in quantity from 2 drachms to 2 ounces. The fluid is alkaline, of sp. gr. l ’ 005, contains a little albumen, and a substance which, as Turner pointed out, reduces blue oxide of copper to the state of yellow sub-oxide. The arachnoid membrane is made up of delicate connective tissue. The free surface next the sub-dural space is smooth, like a serous membrane, and covered by a layer of squamous endothelium. This layer is reflected on to the roots of the spinal and cranial nerves, and, when they pierce the dura mater, it becomes continuous with the endothelial lining of that membrane. As the arrangement and structure so closely correspond with what is seen in the serous membranes, many anatomists regard the arachnoid as the visceral layer of a serous membrane, and the endothelial lining of the dura mater as the parietal layer, whilst the sub-dural space is the intermediate cavity.

When the skull cap is removed, clusters of granular bodies are usually to be seen imbedded in the dura mater on each side of the superior longitudinal sinus; these are named the Pacchionian bodies. When traced through the dura mater they are found to spring from the visceral or proper cerebral arachnoid. The observations of Luschka and Cleland have proved that villous processes invariably grow from the free surface of that membrane, and that when these villi greatly increase in size they form the bodies in question. Sometimes the Pacchionian bodies greatly hypertrophy, occasion absorption of the bones of the cranial vault, and depressions on the upper surface of the brain.

Pia mater.— This membrane closely invests the whole outer surface of the brain. It dips into the fissures between the convolutions, and a wide prolongation, named velum interpositum, lies in the interior of the cerebrum. With a little care it can be stripped off the brain without causing injury to its substance. The pia mater invests the spinal cord, and is more intimately attached to it than to the brain, for not only does it send prolongations into the anterior and posterior fissures of the cord, but slender bands pass repeatedly from its inner surface into the columns of the cord. Hence it cannot be stripped off the cord without causing injury to its substance. The pia matter is prolonged on to the roots both of the cranial and spinal nerves, and on to the filum terminale. This membrane consists of a delicate connective tissue, in which the arteries of the brain and spinal cord ramify and subdivide into small branches before they penetrate the nervous substance, and in which the veins conveying the blood from the nerve centres lie before they open into the blood sinuses of the cranial dura mater and the extradural venous plexus of the spinal canal. The arteries which pass from the pia mater into the brain and spinal cord are invested by a loose sheath, which has been described as forming , the wall of a peri-vascular lymphatic vessel; but Key and Retzius have shown that the space between the blood-vessel and the sheath opens into the subarachnoid space, and contains cerebro-spinal fluid. A network of lymph vessels ramifies freely in the pia mater. It is also well provided with nerves, which arise from the posterior roots of the spinal nerves, from some of the cranial nerves, and from the carotid and vertebral plexuses of the sympathetic. The epi-cerebral and epi-spinal spaces described by His as existing?between this membrane and the brain and spinal cord are in all probability artificial productions.

In the spinal canal a slender fibrous band projects from the pia mater covering the side of the cord, and, pushing the arachnoid membrane in front of it, is attached by about twenty-two pairs of denticulated processes to the inner surface of the dura mater. It is named ligamentum denticulatum, and its teeth alternate with the successive pairs of spinal nerves.

Spinal Cord. —The Medulla Spinalis, or Spinal Cord, occupies the spinal canal, and extends from the foramen magnum to opposite the body of the first lumbar vertebra. In the early foetus it equals in length the canal itself; but as the spinal column grows at a greater proportional rate than the cord, the latter, when growth has ceased, is several inches shorter than the column. The cord is continuous above with the medulla oblongata, whilst it tapers off below into a slender thread, the filum terminale, which lies in the axis of the sacral canal, and is attached below to the back of the coccyx, or to the fibrous membrane which closes in below the sacral canal. The length of the cord is from 15 to 18 inches. It approaches a cylinder in shape, but is flattened on its anterior and posterior surfaces, and presents two enlargements which have a greater girth than the rest of the cord. The upper, called the cervical or brachial enlargement, extends from opposite the third cervical to the first dorsal vertebra, and from it arise the nerves which supply the upper limbs; the lower, called the crural or lumbar enlargement, is opposite the last dorsal vertebra, and supplies with nerves the lower limbs. The cord is almost completely divided into right and left lateral halves by two fissures, named respectively anterior and posterior median fissures, which do not quite reach the centre of the cord, for at the bottom of the anterior fissure are the transverse fibres of the anterior white commissure, and at the bottom of the posterior fissure the fibres of the posterior grey commissure. By these commissures the two halves of the cord are united together. The fibres of the posterior commissure surround a canal, called the central canal, which extends along the whole length of the cord, and even passes into the upper end of the filum terminale. This canal is lined by a ciliated columnar endothelium, and expands superiorly into the cavity of the fourth ventricle. Each lateral half of the cord is subdivided into three columns by two depressions, which mark the points of emergence of the roots of the spinal nerves. The anterior nerve roots pass through the antero-lateral depression or fissure, and between it and the antero-median fissure is the anterior column of the cord. The posterior nerve roots pass through the postero-lateral fissure, and between it and the postero-median fissure is the posterior column, whilst between the anterior and posterior nerve roots lies the lateral column. In the cervical region, the part of the posterior column which lies next tue postero-median fissure is marked off by a fissure into a small internal or postero-median column. The subdivision of each lateral half of the cord into the columns, and the arrangement of its nervous tissues, are well seen in transverse sections through its substance. The cord is composed of white and grey matter. The white matter is external, and forms the columns of the cord.

The grey matter is surrounded by the white, and has in each lateral half of the cord a crescentic shape. The horns of the crescent are directed towards the fissures of emergence of the nerve roots; the anterior horn is rounded; the posterior long and narrow. The proportion of grey matter to the white varies in different parts of the cord. At the commencement of the filum terminale there is scarcely any white matter; but the white matter increases in amount from below upwards, so that its absolute quantity is greatest in the cervical part of the cord. The grey crescents are thicker in the upper and lower enlargements than in the intermediate part.

The cord contains both nerve fibres and nerve cells. The external, columnar, white part of the cord consists of nerve fibres, with a supporting reticular framework of connective tissue and blood-vessels derived from the pia mater. Well-formed stellate connective tissue corpuscles lie in this supporting framework. The nerve fibres of the various columns extend longitudinally, and lie parallel to each other, so that in transverse sections through the columns the fibres are transversely divided. The individual fibres vary much in diameter, but in all the axial cylinder and medullary sheath can be distinctly seen. Wherever the nerve roots enter into the cord, the fibres of these roots pass transversely or obliquely in their course inwards to the grey matter. Horizontal fibres are also found in the white anterior commissure, and a similar appearance can be seen in the posterior commissure. Horizontal fibres have also been traced from the lateral columns into the adjacent part of the grey matter. .

The grey crescentic portion of the cord contains connective tissue, blood-vessels, nerve fibres, and nerve cells. The nerve fibres in the grey matter are numerous; and whilst some possess a medullary sheath, others consist only of the axial cylinder; they divide and subdivide, and, as Gerlach has shown, form a narrow - meshed network of extremely minute fibres. The nerve cells are multipolar, and are chiefly collected in the anterior and posterior horns of each crescent. The cells of the anterior cornu are large, distinct, and stellate, and form a well-defined group of nerve cells. Those of the posterior cornu are smaller in size, more elongated in shape, but with stellate branched processes. They are not so distinct as in the anterior horn, owing to the connective tissue with its corpuscles being so abundant. This tissue is best marked at the tip of the posterior horn, where it forms the substantia gelatinosa of Rolando. Lockhart Clarke has described an intermedio-lateral group of nerve cells situated at the outer side of the grey matter, about midway between the anterior and posterior horns, in the upper part of the cervical portion of the cord, and in the thoracic part between the brachial and crural enlargements.

The course of the fibres in the cord and their relations to the nerve cells should now be considered. There can be no doubt that of the longitudinal fibres some ascend from below upwards, and conduct either excito-motory impulses to the regions of the spinal cord itself, or sensory impulses to the brain. Other longitudinal fibres again descend from the brain and higher regions of the cord to the lower, and conduct motor and vaso-motor impulses from above downwards. The horizontal and oblique fibres of an anterior or motor nerve root enter the grey matter of the anterior cornu, and seem to have the following arrangement: some become directly continuous with the axial cylindrical processes of the nerve cells; others pass into the anterior commissure; others extend as far as the grey matter of the posterior horn. The nerve cells of the anterior cornu give origin, therefore, directly to nerve fibres by their unbranched processes. Gerlach’s observations show that the branched processes of these cells become continuous with the network of extremely minute fibres already described in the grey matter; from this network medullated fibres appear to arise which leave the grey matter; some enter the lateral column, and ascend as the fibres of this structure; others pass as fibres of the anterior commissure to the opposite side of the cord, and ascend as the anterior column of that side. The anterior and lateral columns, therefore, are constantly receiving accessions of fibres from the enclosed grey matter.

The fibres of a posterior or sensory nerve root on entering the cord subdivide into two bundles; one does not enter the grey matter, but applies itself to the posterior column, of which it forms some of the vertical fibres. These fibres may ascend to the brain, or they may at some higher point in the cord enter the grey matter of the posterior horn. The other bundle of posterior root fibres at once enters the posterior horn of grey matter. The connections and ultimate arrangement of these fibres in the grey matter have not been satisfactorily made out Gerlach states that, as they frequently subdivide on entering the grey matter if is possible they may form the fine nerve fibre plexus of the grey substance; but a direct continuity between them and the axial-cylinder processes of the cells of the posterior horn does not seem to have been observed. From the plexus, formed by the much subdivided processes of these cells, fibres arise, which, forming the fibres of the posterior commissure, pass both in front of and behind jι the central canal to the opposite side, where they ascend towards the brain, “partly in the vertical fasciculi of the posterior cornua and partly in the posterior columns.”

The structure of the spinal cord shows it to be both a nerve centre and a conductor of nervous impulses. The nerve cells in its grey matter give rise either directly, or through the delicate plexus formed by their branching processes, to nerve fibres, which may either pass out of the cord as the anterior and posterior roots of the spinal nerves, or may ascend to the brain as the columns of the cord. Hence the cord is anatomically continuous, on the one hand, through the nerves which arise from it, with the peripheral end-organs in the skin, and muscular system in which those nerves terminate; and, on the other hand, it is continuous with the brain. It serves, therefore, to conduct the impulses of touch-sensation from the skin upwards to the brain, and the motor impulses from the brain downwards to the muscles. But further, the cord is the great nerve centre concerned in reflex excito-motory actions. It must, also, be remembered that the two halves of the cord are anatomically continuous with each other through the nerve fibres of the commissures, so that it acts as a single organ, and not as two organs. Experiments have shown that sensory impulses are conducted upwards through the cord, not by that half from which the nerves arise that have been excited, but by the opposite half of the cord, which is obviously due to the crossing of the fibres of the posterior commissure. Motor impressions are, however, conducted downwards by that half of the cord from which the nerves arise that pass to supply the muscles to be moved.

The spinal cord is well supplied with blood by numerous arteries, which terminate in a diffused capillary network. The capillaries are much more numerous in the grey matter of the cord than in the white columns.

Origin, Arrangement, and Distribution of the Spinal Nerves. —The spinal cord gives origin to thirty-one pairs of Spinal nerves, which pass out of the spinal canal through the intervertebral foramina. These nerves are arranged in groups, according to the region of the spine through the foramina in which they proceed. There are eight pairs of cervical nerves; the first or sub-occipital emerges between the occipital bone and the atlas, the eighth between the seventh cervical and first dorsal vertebrae. Twelve dorsal or thoracic nerves pass out on each side in relation to the dorsal vertebrae: five pairs of lumbar nerves in the region of the loins; five pairs of sacral nerves through the sacral foramina; and one pair of coccygeal nerves through the lowest openings in the spinal canaL Each spinal nerve arises by two roots, an anterior and a posterior, from the side of the cord. These roots are distinguished from each other both anatomically and physiologically. The posterior root has a swelling or ganglion on it, whilst no ganglion exists on the anterior root. The posterior root consists of sensory nerve fibres, t.e., of fibres which conduct impulses from the periphery into the nerve centre; whilst the anterior root is composed of motor nerve fibres, i,e., of fibres which conduct impulses from the centre to the periphery. The ganglion is situated on the posterior root, as a rule, in the intervertebral foramen; but the lower sacral nerves have the ganglia. on their posterior roots in the spinal canaL These ganglia contain bipolar nerve cells, and the nerve fibres, as they pass through each ganglion, are apparently connected with the poles of the cells. The roots of the spinal nerves vary in direction and length. Those of the cervical nerves are short, and run almost horizontally outwards to their respective intervertebral foramina; those of the dorsal are longer and more oblique; whilst the roots of the lumbar and sacral nerves, owing to the cord ending much above the foramina through which the nerves proceed, are very long, and form a leash of nerves in the lower part of the spinal canal, which surrounds the filum terminale, and, from its general resemblance in arrangement to the hairs of a horse's tail, has been named cauda equina.

The anterior nerve root joins the posterior immediately outside the ganglion, and by their junction a spinal nerve is formed. This nerve contains a mixture of both motor and sensory fibres, and is compound therefore in function. Almost immediately after its formation the nerve separates into two divisions, an anterior and a posterior, and each division, like the nerve itself, contains both motor and sensory fibres.

The Posterior Primary Divisions of the spinal nerves, smaller than the anterior, are distributed both to the muscles and skin on the back of the axial part of the body. Their general arrangement is as follows: each division, with some three or four exceptions, subdivides into an internal and an external branch. In the back of the neck and the back of the upper part of the chest, the external branches of these nerves supply the deep muscles; the internal branches pierce the muscles close to the spines of the vertebrae, and end in the skin; the internal branch of the second nerve, called great occipital, and that of the third cervical, pass to the skin over the occipital bone. In the back of the lower part of the chest and of the loins, the internal branches supply the deep muscles, the external branches pass to the skin, those of some of the lumbar nerves extending as far as the skin of the buttock.

The Anterior Primary Divisions are not so uniform either in arrangement or distribution as are the posterior. They supply the front and sides of the axial part of the neck and trunk, and the extremities. The anterior divisions of the twelve thoracic nerves have the most simple arrangement. Each nerve, called from its position an intercostal nerve, runs outwards, immediately below the lower border of a rib, and gives origin to three series of branches, named communicating, muscular, and cutaneous. By the G ommunicating branch each intercostal nerve is connected with an adjacent ganglion on the thoracic portion of the sympathetic system. By the Muscular or motor branches these nerves supply the intercostal muscles, the levatores costarum, and the triangularis sterni, whilst the lower intercostal nerves run forwards and downwards into the wall of the abdomen, and supply the two oblique, the transverse, rectus, and pyramidalis muscles. The skin of the sides of the thorax and abdomen receives its nervous supply from the Lateral Cutaneous branches, whilst the skin on the front of the trunk is supplied by the Anterior Cutaneous terminations of these nerves. The lateral cutaneous branches of the second and third intercostal nerves are comparatively large in size, and assist in the supply of the skin of the inner side of the upper arm; hence they are called intercosto-humeral nerves.

In the regions of the neck, loins, and pelvis, the anterior divisions of the spinal nerves do not pass simply outwards to their distribution. In each region adjacent nerves interlace with each other, and form what is technically called a nervous plexus. When a branch arises from a thoracic nerve, it contains fibres derived from that nerve only; but when a branch arises from a plexus, it may contain, fibres, not of one only, but of two or more of the nerves which, by their interlacement, form the plexus. Hence the parts which are supplied by these branches are brought into connection with a greater number of nerves, and consequently with a greater extent of the spinal cord or nerve centre, than are the parts which receive branches from a single nerve only. These plexuses are especially found in connection with the nerves which supply the extremities, where, owing to the complexity of the muscular movements, the co-ordination of these movements through the nervous system is rendered necessary.

The anterior divisions of the eight cervical nerves are arranged in two plexuses, named cervical and brachial.

The Cervical plexus (Pl. XVII.) is formed of the four upper cervical nerves, which make, by interlacement with each other, a series of loops in front of the transverse processes of the cervical vertebrae. Arising either directly from these nerves, or from the plexus which they form, are communicating, muscular, and cutaneous branches. The Communicating branches connect these nerves with the large superior cervical ganglion of the sympathetic system, also with the vagus, accessory, and hypoglossal cranial nerves, and with the descending branch of the hypoglossal. The Muscular branches supply the anterior recti muscles of the neck, the levator scapulae, the posterior scalenus, the diaphragm, and in part the sterno-mastoid and trapezius. The branch to the diaphragm, or the phrenic nerve, is the most important (Pl. XVII. φ) ; it springs from the third, fourth, and fifth cervical, and passes down the lower part of the neck, and through the thorax, to supply its own half of the diaphragm. The Cutaneous branches are as follows:—the occipitalis minor, to the skin of the occiput; the auriculo-parotidean, to the skin over the parotid gland and the adjacent part of the auricle; the transversalis colli, to the skin of the front of the side of the neck; the supra-clavicular nerves, to the skin of the lower part of the side of the neck, and upper part of the chest.

The Brachial plexus (PL XVII. 1, 2, 3, 4) is formed of the four lower cervical nerves, and of the larger portion of the first intercostal, called also first dorsal nerve. It is of large size, and is principally for the supply of the upper limb. Its exact mode of arrangement presents many variations, but the following is not unfrequently found :—The fifth and sixth nerves join to form a large nerve, which, after a short course, is joined by the seventh; in this manner the upper cord of the plexus is formed. The eighth cervical and the first dorsal then join, to form the lower cord of the plexus. These cords then pass behind the clavicle and subclavius muscle into the axilla, where they become modified in arrangement. From each a large branch arises, and these two branches then join to form a third cord. These three cords have special relations to the axillary artery: the one which lies to its outer side is named the outer cord; that to the inner, the inner cord; that behind, the posterior cord. These nerves and the cords formed by them give origin to communicating, muscular, cutaneous, and mixed branches. The Communicating branches join the middle and inferior cervical and first thoracic ganglia of the sympathetic system. The Muscular branches supply the scaleni, longus colli, rhomboid, and subclavius muscles; the supra and infra-spinatus muscles, through a branch called suprascapular; the serratus magnus, through the posterior thoracic branch; the greater and lesser pectorals, through the two anterior thoracic branches; and the subscapularis, teres major, and latissimus dorsi, through the three subscapular branches. The Cutaneous branches arise from the inner cord, and are the lesser internal cutaneous, which ends in the skin of the inner side of the upper arm, and joins the intercosto-humeral; and the internal cutaneous, which not only sends branches to the skin of the upper arm, but supplies the skin of the inner side of the forearm, both on its anterior and posterior surfaces. The Mixed branches are large and very important:— a, The Circumflex, from the posterior cord, supplies the deltoid and teres minor muscles, the skin over the deltoid, and the shoulder joint, b, The Musculo-Spiral, also from the posterior cord, supplies the triceps and anconeus, the supinator longus and extensor carpi radialis longior muscles; and by its external cutaneous branch, the skin of the outer side of the back of the forearm. It then divides into the radial and posterior interosseous branches. The radial passes through the forearm to the hand, and supplies the skin on the back of the thumb, index and middle digits, and radial side of the ring digit. The posterior interosseous supplies the muscles on the back of the forearm and the articulations of the carpal joints, c, The Musculo-Cutane-ous branch of the outer cord of the plexus supplies the biceps, brachialis anticus, and coraco-brachialis muscles, and ends in an external cutaneous branch, which supplies the skin of the outer side of the forearm, both in front and behind, d, The Ulnar nerve arises from the inner cord, passes through the upper arm, and enters the forearm between the inner condyle and olecranon, where it supplies the elbow joint. Here it may easily be compressed, when, a pricking sensation is experienced in the course of its distribution. In this spot it is popularly called the “funny bone.” In the forearm the ulnar nerve supplies the flexor carpi ulnaris and inner part of the flexor profundus digitorum muscles. In the hand it supplies the muscles of the ball of the little finger, the two inner lumbricales, the interossei muscles, and the adductor and deep part of the short flexor of the thumb. It also supplies a dorsal cutaneous branch to the back of the hand, and the back of the little and of the ulnar side of the ring digits. Palmar cutaneous branches are also given to the palm and the palmar aspects of the same digits, e, The Median nerve arises by two roots, one from the inner, the other from the outer cord of the plexus. It enters the forearm in front of the elbow joint, supplies, either directly or through its anterior interosseous branch, all the flexors and pronators, except those supplied by the ulnar; is continued to the hand, where it supplies the abductor, opponens, superficial part of the short flexor of the thumb, and two outer lumbrical muscles. It also supplies a palmer branch to the skin of the palm, and gives digital cutaneous branches to the thumb, index and middle digits, and radial side of the ring digit.

The Lumbar plexus, of large size, is situated at the back of the abdominal cavity in the region of the loins, and is formed by the four upper lumbar nerves, which form a series of loop-like interlacements in front of the transverse processes of the lumbar vertebrae. It gives origin to communicating, muscular, cutaneous, and mixed branches. The Communicating branches join the four upper lumbar ganglia of the sympathetic system. The Muscular branches supply the quadratus lumborum muscle, and give branches to the psoas. The Cutaneous branches are named— a, Ilio-hypogastric, which gives an iliac branch to the skin of the buttock, and a hypogastric branch to the skin of the abdomen above the pubic symphysis; δ z Ilio-inguinal, which supplies the skin of the groin; this nerve is by some said to send a branch to the internal oblique muscle; c, External Cutaneous, which supplies the skin on the outer aspect of the thigh. The Mixed branches are as follows :— a, Genito-crural, which supplies the cremaster muscle, and a cutaneous branch to the skin of the groin, δ, Anterior Crural, a large nerve which enters the thigh by passing behind Poupart’s ligament, and supplies the great extensor muscles of the knee-joint, and also the sartorius, the psoas-iliacus and the pectineus, which act as flexors of the hip-joint; it gives off the following cutaneous branches :—An internal cutaneous to the skin of the inner side, a middle cutaneous to the skin of the middle of the front of the thigh, and the long saphenous nerve, which supplies the skin of the inner side of the knee-joint, the inner side of the leg and the foot, c, Obturator k&cnq, which leaves the pelvis through the obturator foramen, and supplies the obturator externus and adductor muscles of the thigh, and sends a branch to the pectineus; it also supplies the hip and knee joints, and not unfrequently gives a branch to the skin of the lower part of the inner side of the thigh, d, An Accessory Obturator nerve is sometimes present, which goes to the pectineus, to the hip-joint, and also joins the obturator nerve.

The Lumbo-sacral Cord is formed of the fifth lumbar nerve and of a branch from the fourth lumbar. It joins the sacral plexus. Before the junction it gives origin to a communicating and a muscular branch. The Communicating joins the fifth lumbar ganglion of the sympathetic. The Muscular, branch, named the superior glut<xal nerve, supplies the glutaeus medius and minimus and the tensor fasciae femoris muscle.

The Sacral plexus is situated in the cavity of the pelvis, and is the largest of all the plexuses. It is formed by the junction of the lumbo-sacral cord, the first, second, third, and partof the fourth sacral nerves, and appears as aflattened mass in front of the sacrum. It gives origin to communicating, muscular, and mixed branches. The Communicating branches join the upper sacral ganglia of the sympathetic system. The Muscular branches supply the upper fibres of the glutaeus maximus, the pyriformis, gemelli, quadratus femoris, and obturator internus muscles. The Mixed nerves are as follows :— a, Pudic f which supplies the muscles and skin of the external organs of generation, b, Small Sciatic, which supplies not only the lower fibres of the glutaeus maximus muscle, but the skin of the buttock, the back of the thigh, of the popliteal space, and of the leg; it also gives a long pudendal branch to the skin of the perineum, c, Great Sciatic; this is the largest nerve in the body. It leaves the pelvis through the great sciatic foramen, and passee down the back of the thigh, when it divides into external and internal popliteal branches. Before dividing it supplies the hamstring muscles, and gives a branch to the adductor magnus. The external popliteal branch gives offsets to the knee-joint, passes down the outer side of the leg, supplies the peronei longus and brevis, gives off the communicans peronei branch to the skin of the outer side of the back of the leg, and ends as the external cutaneous nerve for the dorsum of the foot and the dorsal surfaces of all the toes, except the outer side of the little and the adjacent sides of the great and second toes. The internal popliteal branch, gives offsets to the knee-joint, and supplies the communicans tibialis nerve, which joins the communicans peronei, and forms with it the external saphenous nerve that passes to the outer side of the foot and little toe. The internal popliteal also supplies the muscles of the calf and the popliteus muscle, and is prolonged downwards as the posterior tibial nerve. The anterior tibial passes to the front of the leg, supplies the tibialis anticus, peroneus tertius, and extensor muscles of the toes, and terminates as the cutaneous digital nerve for the adjacent sides of the great and second toes. The posterior tibial nerve passes down the back of the leg, supplies the tibialis posticus and long flexors of the toes, gives off a cutaneous branch to the skin of the heel, and terminates by dividing into the internal and external plantar nerves. The internal plantar nerve supplies the skin of the sole and sends digital branches to the skin of the great, second, third, and tibial side of the fourth toes; it also supplies the abductor pollicis, flexor brevis digitorum, flexor brevis pollicis, and two inner lumbrical muscles. The external plantar nerve supplies digital branches to the skin of the little and fibular sides of the fourth toes, and branches to all the muscles of the sole of the foot which are not supplied by the internal plantar nerve.

The Sacro-Coccygeal is the smallest plexus belonging to the anterior divisions of the spinal nerves. It is formed by a part of the fourth sacral, the fifth sacral, and the coccygeal nerves. It lies in front of the last sacral and the first coccygeal vertebrae, and gives origin to communicating, visceral, muscular, and cutaneous branches. The Communicating branches join the lower sacral and the coccygeal ganglia of the sympathetic system; the Visceral pass to the pelvic plexus of the sympathetic, and through it to the bladder and rectum; the Muscular to the levator ani, coccygeus, and sphincter ani externus muscles; the Cutaneous to the skin about the anus and tip of the coccyx.

The Brain. —By the term Brain or Encephalon is meant all that part of the central nervous axis which is contained within the cavity of the skull. It is divided into several parts, named medulla oblongata, pons, cerebellum, and cerebrum. The medulla oblongata is directly continuous with the spinal cord through the foramen magnum. The cerebellum lies above, and immediately behind the medulla oblongata, with which it is directly continuous. The pons lies above and in front of the medulla, with which it is directly continuous. The cerebrum is the highest division, and lies above both pons and cerebellum, with both of which it is directly continuous. Several figures of the brain are given in Plate XVIII.

The Medulla Oblongata rests upon the basi-occipital. It is somewhat pyramidal in form, about 1∣ inch long, and 1 inch broad in its widest’part. It is a bilateral organ, and is divided into a right and a left half by shallow anterior and posterior median fissures, continuous with the corresponding fissures in the spinal cord; the posterior fissure ends above in the 4th ventricle. Each half is subdivided into elongated tracts of nervous matter. Next to, and parallel with the anterior fissure is the anterior pyramid (Pl. XVIII. figs. 1 and 2, P). This pyramid is continuous below with the cord, and the place of continuity is marked by the passage across the fissure of three or four bundles of nerve fibres, from each half of the cord to the opposite anterior pyramid; this crossing is called the decussation of the pyramids. To the side of the pyramid, and separated from it by a faint fissure, is the olivary fasciculus, which at its upper end is elevated into the projecting oval-shaped olivary body (Pl. XVIII. figs. 1 and 2,0). Behind the olive, and separated from it by a faint groove, is the strong tract named restiform body; as it ascends from the cord it diverges from its fellow in the opposite half of the medulla oblongata. By this divergence the central part of the medulla is opened up, and the lower half of the cavity of the 4th ventricle is formed. Internal to the restiform body is the posterior pyramid, which is continuous with the postero-median column, and bounds the postero-median fissure. Where the restifoπn bodies diverge from each other, there also the posterior pyramids diverge outwards from the sides of the postero-median fissure. At the upper part of the floor of the 4th ventricle a longitudinal tract of nerve fibres, the fasciculus teres, ascends on each side of its median furrow (Fig. 68, 7). Slender tracts of nerve fibres, the arciform fibres, arch across the side of the medulla immediately below the olive; and white slender tracts emerge from the median furrow of the 4th ventricle, pass outwards across its floor, and form the strice medullares or acousticoe, the rootsof origin of the auditory nerve(Fig. 68,8).

The medulla oblongata, like the spinal cord, with which it is continuous, consists both of grey and white matter. But the exterior of the medulla is not so exclusively formed of white matter as is the outer part of the cord, for the divergence from each other of the restiform bodies and posterior pyramids of opposite sides opens out the central part of the medulla, and allows the grey matter to become superficial on the floor of the 4th ventricle. The nerve fibres which enter into the formation of the pyramids and the other tracts just described, are partly continuous below with the columns of the spinal cord, and are prolonged upwards either to the pons and cerebrum, or to the cerebellum, or they partly take their rise in the medulla oblongata itself from the cells of its grey matter. As the medulla is a bilateral organ, its two halves are united together by commissural fibres, which cross obliquely its mesial plane from one side to the other, and as they decussate in that plane, they form a well-marked mesial band or raphé. Further, the medulla is a centre of origin for several pairs of the more posterior encephalic nerves, and for the vaso-motor nerves. In the passage upwards through the medulla of the columns of the cord, a re-arrangement of their fibres takes place; just as in a great central railway station, the rails, which enter it in one direction, intersect and are rearranged before they emerge from it in the opposite direction. The fibres of the posterior median column of the cord are prolonged upwards as the posterior pyramid. The fibres of the posterior column of the cord are for the most part prolonged upwards into the restiform body, though some fibres pass to the front of the medulla to participate in the decussation of the anterior pyramids. The lateral column of the cord divides into three parts: a, the greater number of its fibres pass inwards across the anterior median fissure, to assist in forming the anterior pyramid of the opposite side, so as to produce the decussation already referred to; b, others join the restiform body; c, ethers form the fasciculus teres situated on the floor of the 4th ventricle. The anterior column of the cord also divides into three parts: a, some fibres form the arcifoπn fibres and join the restiform body; b, others assist in the formation of the olivary fasciculus; c, others are prolonged upwards in the anterior pyramid of the same side (Fig. 67).

The anterior pyramid consists partly of fibres of the anterior column of the cord of the same side, partly of decussating fibres of the anterior commissure, partly of decussating fibres from the posterior columns and posterior cornu of grey matter, but principally of the decussating fibres of the lateral column of the opposite side of the cord. The fibres of the anterior pyramid are prolonged through the pons to the cerebrum. Owing to the decussation of the lateral columns of the cord in the formation of the pyramids, the motor nerve fibres from one-half of the brain are transmitted to the opposite side of the cord, so that injuries affecting one side of the brain occasion paralysis of the motor nerves arising from the opposite half of the cord. The olivary fasciculus is formed partly of fibres of the anterior column of the same side, and partly of fibres arising from the grey matter of the olive. It is continued upwards through the pons to the cerebrum. The restiform body is formed principally of fibres of the posterior column of the same side, but partly of fibres of the lateral column, and also of the arciform fibres from the anterior column, and from the grey matter of the superior and inferior olives. As the restiform body is continued upwards to the cerebellum, and forms its inferior peduncles, the arciform fibres have been called by Solly the superficial cerebellar fibres of the medulla. Through the restiform body the cerebellum is connected with the posterior, lateral, and anterior columns of the cord as well as with the olivary nuclei in the grey matter of the medulla oblongata. The posterior pyramid consists of the posterior median column of the cord, and is prolonged through the pons to the cerebrum. The fasciculus teres is formed of a small part of the lateral column of the cord, and is also prolonged through the pons to the cerebrum.

The grey matter of the medulla oblongata, which contains numerous multipolar nerve cells, is in part continuous with the grey matter of the Spinal cord, and in part consists of independent masses. As the grey matter of the cord enters the med∏11⅞ it loses its crescentic arrangement. The posterior cornua are thrown outwards towards the surface, lose their pointed form, and dilate into rounded masses named the grey tubercles of Rolando, whilst portions are prolonged into both the posterior pyramid and the restiform body. The grey matter of the anterior cornua and of the intermedio-lateral tracts loses its continuity, and becomes subdivided into numerous small masses, owing to being traversed by bundles of nerve fibres, which give rise to a network termed formatio reticularis, in the meshes of which the groups of nerve cells are contained. In the lower part of the medulla a central canal continuous with that of the cord exists, but when the restiform bodies and posterior pyramids on the opposite sides of the medulla diverge from each other, the central canal loses its posterior boundary, and dilates into the cavity of the 4th ventricle. The grey matter in the interior of the medulla appears, therefore, on the floor of the ventricle; that which corresponds to the anterior cornua being situated immediately on each side of the median furrow, whilst that which is continuous with the grey tubercles of Rolando and the posterior cornua is some distance external to it. This grey matter forms collections of nerve cells, which are the centres of origin of several important encephalic nerves.

Of the independent masses of grey matter of the medulla, that which forms the corpus dentatum within the olivary body is the most important, and constitutes the nucleus of the inferior olive. It is folded on itself in a zig-zag or denticulated manner, and forms a sort of capsule open on the inner aspect, through which openings a bundle of nerve fibres from the interior of the capsule proceeds. These fibres aid in the formation of the olivary fasciculus, and as Deiters and Meynert have pointed out, in part arch across the mesial plane and join the restiform body on the opposite side, whilst some apparently join the posterior pyramid. The nerve cells of the olive are multipolar and flask-shaped, and in all probability give origin to the nerve fibres proceeding from the interior of the capsule. Separated from the inner part of the olive by a layer of reticular substance is a smaller grey mass, called by Stilling nucleus olivaris accessorius. Crossing the anterior surface of the medulla oblongata, immediately below the pons, in the majority of mammals is a transverse arrangement of fibres forming the trapezium, which contains a grey nucleus, named by Van der Kolk the superior olive. In the human brain the trapezium is concealed by the lower transverse fibres of the pons, but when sections are made through it, as L. Clarke pointed out, the grey matter of the superior olive can be seen. Meynert states that its nerve cells give origin to some fibres, which run straight backwards to the restiform body of the same side, and to others which pass across the mesial plane to the opposite corpus restiforme.

The Pons Varolii or Bridge (PL XVIII. figs. 1,2, 3,N) is cuboidal in form: its anterior surface rests upon the dorsum sellae of the sphenoid, and is marked by a median longitudinal groove; its inferior surface receives the pyramidal and olivary tracts of the medulla oblongata; at its superior surface are the two crura cerebri; each lateral surface is in relation to a hemisphere of the cerebellum, and a peduncle passes from the pons into the interior of each hemisphere; the posterior surface forms in part the upper portion of the floor of the 4th ventricle, and in part is in contact with the corpora quadrigemina.

The pons consists of white and grey matter: the nerve fibres of the white matter pass through the substance of the pons, either in a transverse or a longitudinal direction. The transverse fibres go from one hemisphere of the cerebellum to that of the opposite side; some are situated on the anterior surface of the pons, and form its superficial transverse fibres, whilst others pass through its substance and form the deep transverse fibres. The transverse fibres of the pons constitute, therefore, the commissural or connecting arrangement by which the two hemispheres of the -cerebellum become anatomically continuous with each other. The longitudinal fibres of the pons ascend or pass vertically upwards from the medulla oblongata, and consist of the fibres of the anterior pyramids, olivary fasciculi, fasciculi teretes, and posterior pyramids. They leave the pons.by emerging from its upper surface as fibres of the two crura cerebri. The pons possesses a median raphe continuous with that of the medulla oblongata, and formed like it by a decussation of fibres in the mesial plane.

The grey matter of the pons is scattered irregularly through its substance, and appears on its posterior surface; but not on the anterior surface, which is composed exclusively of the superficial transverse fibres. It is traversed both by the longitudinal and deep transverse fibres, which form a well-defined formatio reticularis. To a portion of grey matter, containing nerve cells charged with dark pigment, the name of locus caeruleus is apphed. The locus lies on the floor of the 4th ventricle, close to the entrance to the aqueduct of Sylvius, and serves as the origin of the sensory root of the 5 th, and perhaps of the posterior root of the 4th cranial nerve. The nerve cells of the pons are multipolar and stellate. The pons acts as a conductor of impressions through its nerve fibres, and as a centre of origin of nerve fibres from nerve cells. Meynert states that some of the fibres of the crura cerebri end in the nerve cells of the pons, which cells again give origin to fibres that pass outwards to the cerebellum.

The Cerebellum, Little Brain, or After Brain (Pl. XVIII. fig. 2, c), occupies the inferior pair of occipital fossae, and, along with the pons and medulla oblongata, lies below the plane of the tentorium cerebelli. It consists of two hemispheres or lateral lobes, and of a median or central lobe, which in human anatomy is called the vermiform process. It is connected below with the medulla oblongata by the two restiform bodies which form its inferior peduncles, and above to the corpora quadrigemina of the cerebrum by two bands, which form its superior peduncles; whilst the two hemispheres are connected together by the transverse fibres of the pons, which form the middle peduncles of the cerebellum. On the superior or tentorial surface of the cerebellum the median or vermiform lobe is a mere elevation, but on its inferior or occipital surface this lobe forms a well-defined inferior vermiform process, which lies at the bottom of a deep fossa or vallecula; this fossa is prolonged to the posterior border of the cerebellum, and forms there a deep notch which separates the two hemispheres from each other; in this notch the falx cerebelli is lodged. Extending horizontally backwards from the middle cerebral peduncle, along the outer border of each hemisphere is the great horizontal fissure, which divides the hemisphere into its tentorial and occipital surfaces. Each of these surfaces is again subdivided by fissures into smaller lobes, of which the most important are the amygdala or tonsil, which forms the lateral boundary of the anterior part of the vallecula, and the flocculus, which is situated immediately behind the middle peduncle of the cerebellum. The inferior vermiform process is subdivided into a posterior part or pyramid; an elevation or uvula, situated between the two tonsils; and an anterior pointed process or nodule. Stretching between the two flocculi, and attached midway to the sides of the nodule, is a thin, white, semilunar-shaped plate of nervous matter, called the posterior medullary velum.

The whole outer surface of the cerebellum possesses a characteristic foliated or laminated appearance, due to its subdivision into multitudes of thin plates or lamellae by numerous fissures. The cerebellum consists both of grey and white matter. The grey matter forms the exterior or cortex of the lamellae, and passes from one to the other across the bottoms of the several fissures. The white matter lies in the interior of the organ, and extends into the core of each lamella. When a vertical section is made through the organ, the prolongations of white matter branching off into the interior of the several lamellae give to the section an arborescent appearance, known by the fanciful name of arbor vitoe (Pl. XVIII. fig. 3, c). Independent masses of grey matter are, however, found in the interior of the cerebellum. If the hemisphere be cut through a little to the outer side of the median lobe, a zig-zag arrangement of grey matter, similar in appearance and structure to the nucleus of the olivary body in the medulla oblongata, and known as the corpus dentatum of the cerebellum, is seen; it Ees in the midst of the white core of the hemisphere, and encloses white fibres, which leave the interior of the corpus at its inner and lower side. Stilling has described, in connection with the anterior end of the inferior vermiform process, which projects forwards into the valve of Vieussens, and aids in the formation of the roof of the 4th ventricle, two grey masses, named roof nuclei. They possess flask-shaped nerve cells Eke those of the corpus dentatum. The white matter is more abundant in the hemispheres than in the median lobe, and is for the most part directly continuous with the fibres of the peduncles of the cerebellum. Thus the restiform or inferior peduncles pass from below upwards through the white core, to end in the grey matter of the tentorial surface of the cerebellum, more especially in that of the central lobe; on their way they are connected both with the grey matter of the corpus dentatum and of the roof nuclei. The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, reach the grey cortical matter, more especially an the inferior surface of the cerebellum, though they also form connections with the corpus dentatum. The middle peduncles form a large proportion of the white core, and their fibres terminate in the grey matter of the foliated cortex of the hemispheres. But, in addition to these peduncular fibres, which connect the cerebellum to other subdivisions of the encephalon, its white matter contains fibres proper to the cerebellum itself. The fibroe propriae have been especially described by Stilling; some, which he has termed the median fasciculi, lie near the mesial plane, and connect the grey matter on the tentorial aspect of the middle lobe with that of the inferior vermiform process, whilst others cross directly the mesial plane to unite opposite and symmetrical regions of the hemispheres. Further, the auditory nerve was said by Foville to derive some of its fibres of origin from the cerebellum; the connection of this nerve with the cerebellum has been strongly insisted on by Meynert, and this anatomist has also ascribed a cerebellar origin to a portion of the sensory root of the 5th cranial nerve.

The grey matter of the cortex is divided into two well-defined layers, an external grey, and an inner rust coloured layer of about equal thickness. The rust coloured layer is distinguished by containing multitudes of so called “granules,” the well-defined nucleus in which, as described by Strachan, is invested by a small quantity of branched protoplasm. These “granules” are, therefore, minute stellate cells. Where the rust coloured layer joins the grey layer the characteristic nerve cells of the cerebellum, named the corpuscles of Purkinje, are situated. A slender central process arising from each cell enters the rust coloured layer, and, as the observations of Hadlich and Koschennikoff show, becomes continuous with the axial cylinder of a medullated nerve fibre; for the nerve fibres of the white core enter this layer, divide into minute fibres, and ramify amidst the granules. From the opposite aspect of each cell two peripheral processes arise, and ramify in an antler-like manner in the external grey layer. Obersteiner and Hadlich maintain that the finer branches of these processes curve back towards the rust coloured layer, where, according to Boll, they form a network of extreme minuteness, from which it is believed that nerve fibres may arise. The substratum of the grey layer, in which the branched processes of the cells of Purkinje lie, consists of a very delicate neuroglia, in which scattered corpuscles are imbedded; but, in the outer part of this layer, delicate supporting connective tissue-like fibres are also met with.

The Fourth, Ventricle is the dilated upper end of the central canal of the medulla oblongata. Its shapo is like an heraldic lozenge. Its floor is formed by the gιey matter of the posterior surfaces of the medulla oblongata and pons; its roof partly by the inferior vermiform process of the cerebellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or anterior medullary velum ; its lower lateral boundaries, by the divergent restiform bodies and posterior pyramids; its upper lateral boundaries, by the superior peduncles of the cerebellum; the reflection of the arachnoid membrane from the back of the medulla to the inferior vermiform process closes it in below, but allows of a communication between its cavity and the sub-arachnoid space; above, it communicates with the aqueduct of Sylvius, which is tunnelled through the substance of the corpora quadrigemina. Along the centre of the floor is the median furrow, which terminates below in a pen-shaped form, the so-called calamus scriptorius. Situated on its floor are the fasciculi teretes, striae acous-ticae, and deposits of grey matter described in connection with the medulla oblongata. Its endothelial lining is continuous with that of the central canal.

The Cerebrum or Great Brain lies above the plane of the tentorium, and forms much the largest division of the encephalon. It is customary in human anatomy to include under the name of cerebrum, not only the convolutions, the corpora striata, and the optic thalami, developed in the anterior cerebral vesicle, but also the corpora quadrigemina and crura cerebri developed in the middle cerebral vesicle. The cerebrum is ovoid in shape, and presents superiorly, anteriorly, and posteriorly a deep median longitvr dînai fissure, which subdivides it into two hemispheres. Inferiorly there is a continuity of structure between the two hemispheres across the mesial plane, and if the two hemispheres be drawn asunder by opening out the longitudinal fissure, a broad white band, the corpus callosum, τaay be seen at the bottom of the fissure passing across the mesial plane from one hemisphere to the other. The outer surface of each hemisphere is convex, and adapted in shape to the concavity of the inner table of the cranial bones; its inner surface, which bounds the longitudinal fissure, is flat and is separated from the opposite hemisphere by the falx cerebri; its under surface, where it rests on the tentorium, is concave, and is separated by that membrane from the cerebellum and pons. From the front of the pons two strong white bands, the crura cerebri or cerebral peduncles, pass forwards and upwards to enter the optic thalami in their respective hemispheres. Winding round the outer side of each crus is a flat white band, the optic tract. These tracts converge in front, and join to form the optic commissure, from which the two optic nerves arise. The crura cerebri, optic tracts, and optic commissure enclose a lozenge shaped space, which includes— a, a grey layer, called pons Tarini, which, from being perforated by several small arteries, is often called locus perforatus posticus; b, two white mammillae, the corpora albicantia ; c, a grey nodule, the tuber dnereum, from which, d, the infundibulum projects to join the pituitary body. Immediately in front of the optic commissure is a grey layer, the lamina cinerea or lamina terminalis of the 3d ventricle; and between the optic commissure and the inner end of each Sylvian fissure is a grey spot perforated by small arteries, the locus perforatus anticus.

The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as the convolutions or gyri of the cerebrum. These convolutions are separated from each other by fissures or sulci, some of which are considered to subdivide the hemisphere into lobes, whilst others separate the convolutions in each lobe from each other. In each hemisphere of the human brain five lobes are recognised: the temporo-sphe-noidal, frontal, parietal, occipital, and the central lobe or insula. Passing obliquely on the outer face of the hemisphere from before, upwards and backwards, is the well-marked Sylvian fissure, which is the first to appear in the development of the hemisphere. Below it lies the tem-poro-sphenoidal lobe, and above and in front of it, the parietal and frontal lobes. The frontal lobe is separated from the parietal by the fissure of Rolando, which extends on the outer face of the hemisphere from the longitudinal fissure obliquely downwards and forwards towards the Sylvian fissure. About two inches from the hinder end of the hemisphere is the pariet o-occipital fissure, which, . commencing at the longitudinal fissure, passes down the inner surface of the hemisphere, and transversely outwards for a short distance on the outer surface of the hemisphere; it separates the parietal and occipital lobes from each other.

The Temporo-Sphenoidal Lobe presents on the outer surface of the hemisphere three convolutions, arranged in parallel tiersivoτa above downwards, and n&vaedsuperior,middle,and inferior temporo-sphenoidal convolutions. The fissure which separates the superior and middle of these convolutions is called the parallel fissure. The Occipital Lobe also consists from above downwards of three parallel convolutions, named superior, middle, and inferior occipital. The Frontal Lobe is more complex; immediately in front of the fissure of Rolando, and forming indeed its anterior boundary, is a convolution named ascending frontal, which ascends obliquely backwards and upwards from the Sylvian to the longitudinal fissure. Springing from the front of this con volution, and passing forwards to the anterior end of the cerebrum, are three convolutions, arranged in parallel tiers from above downwards, and named superior, middle, and inferior frontal convolutions, which are also prolonged on to the orbital face of the frontal lobe. The Parietal Lobe is also complex; its most anterior convolution, named ascending parietal, ascends parallel to and immediately behind the fissure of Rolando. Springing from the upper end of the back of this convolution is the postero-parietdl convolution, which, forming the boundary of the longitudinal fissure, extends as far back as the parieto-occipital fissure; springing from the lower end of the back of this convolution is the supra - marginal convolution, which forms the upper boundary of the hinder part of the Sylvian fissure; as this gyrus occupies the hollow in the parietal bone, which corresponds to the eminence, it may appropriately be named the convolution of the parietal eminence. Continuous with the convolution of the parietal eminence is the angular convolution, which bends round the posterior extremity of the Sylvian fissure. Lying in the parietal lobe is the intra-parietal fissure, which separates the convolution of the parietal eminence from the postero-parietal convolution. The occipital is connected with the parietal lobe by two annectent or bridging gyri, which bridge across the transverse external part of the parieto-occipital fissure; the depth and extent of this fissure vary in different brains in proportion to the size of these bridging convolutions. The superior annectent gyrus passes between the postero-parietal and the superior occipital convolutions, whilst the second annectent gyrus connects the middle occipital with the angular gyrus. Two annectent gyri also pass from the inferior occipital convolution to the lower convolutions of the temporo-sphenoidal lobe. These lobes of the cerebrum, though named after the bones which form the vault of the skull, are not exactly co-terminous with them. The frontal lobe not only lies under cover of the frontal bone, but extends backwards under the anterior part of the parietal; for the fissure of Rolando, which form3 its posterior boundary, lies from 1J to 2 inches behind the coronal suture. The occipital lobe is not limited to the upper tabular part of the occipital bone, but extends forwards under cover of the posterior part of the parietal, for the parieto-occipital fissure lies about I inch in front of the apex of the lambdoidal fissure. The temporo-sphenoidal lobe not only lies under the squamous-temporal and great wing of the sphenoid, but passes upwards under cover of the lower part of the parietal, for the Sylvian fissure passes from below obliquely upwards and backwards across the line of the squamous suture near its middle. The area covered by the parietal bone so far, then, from being co-terminous with the parietal lobe of the cerebrum, is trenched on anteriorly by the frontal, posteriorly by the occipital, and inferiorly by the temporo-sphenoidal lobe. The convolutions of the parietal lobe itself are grouped around the parietal eminence, and in the interval between it and the sagittal suture. The inner table of the cranial bones is an almost exact mould of the convolutions of these lobes; but this is not so with the exterior of the skull, the configuration of which is modified by the formation of ridges and processes for the attachment of muscles, by variations in the thickness of the diploe, and by the development of the frontal and mastoid airsinuses. Hence the outer surface of the skull does not correspond in shape to the outside of the brain.

The Central Lobe of the hemisphere, more usually called the insula or island of Heil, does not come to the surface of the hemisphere, but lies deeply within the Sylvian fissure, the convolutions forming the margin of which conceal it. It consists of four or five short convolutions, which radiate from the locus perforatus anticus, situated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus, which insulates it from the adjacent convolutions. It lies opposite the upper part of the ali-sphenoid, where it articulates with the parietal and squamous-temporal.

Convolutions also exist on the inner surface of the hemisphere, and on the under surface which rests on the tentorium, but these have no relation to the bones of the cranial vault. They may be studied in connection with the corpus callosum or great transverse commissure, which connects the two hemispheres, and with certain fissures situated on these surfaces of the hemisphere. The small convolutions which lie behind the internal part of the parieto-occipital fissure form the inner convolutions of the occipital lobe, or the occipital lobule (Fig. 73). Those which lie immediately in front of the same fissure belong to the inner face of the parietal lobe, and form the quadrilateral lobule. It is customary, however, to name the convolution which extends forwards from that fissure along the margin of the longitudinal fissure to the anterior end of the hemisphere, and which then turns back to the locus perforatus anticus as the marginal convolution. This is separated by a fissure called callosomarginal, from the callosal convolution or gyrus fornicatus, which, commencing at the locus perforatus anticus, turns round the anterior end of the corpus callosum, extends parallel to its upper surface, and then turns round its posterior end. It is separated from the corpus callosum by the callosal fissure, at the bottom of which the grey matter of the gyrus fornicatus termintes in a well-defined edge.

1 The above view of the brain in situ shows the relations of the surface convolutions to the regions of the skull. R, fissure of Rolando, which separates the frontal from the parietal lobe. ΓO, parieto-occipital fissure between the parietal and occipital lobes. SS, fissure of Sylvius, which separates the temporo-sphenoidal from the frontal and parietal lobes. SF, MF, IF, the supero-, mid-, and infcro-frontal subdivisions of the frontal area of the skull; the letters are placed on the superior, middle, and inferior frontal convolutions; the inferior frontal region is separated from the middle frontal by the frontal part of the curved line of the temporal ridge; the mid-from the supero-frontal by an antero-posterior line through the frontal eminence. SAP, the supero-antero-parietal area of the skull; S is placed on the ascending parietal convolution, AP on the ascending frontal convolution. IAP, the infero-antero-parietal area of the skull; I is placed on the ascending parietal, AP on the ascending frontal convolution. SPP, the supero-postero-parietal area of the skull; the letters are placed on the angular convolution. IPP, the infero-postero-parietal area of the skull; the letters are placed on the mid-tβmporo-sphenoidal convolution; the temporal ridge separates the supero-and infero-parietal regions from each other; a vertical line drawn through the parietal eminence separates the antero-and postero-parietal regions. X, the convolution of the parietal eminence, or supramarginal gyrus. 0, the occipital area of the skull; the letter iβ placed on the mid-occipital convolution. Sq, tho squamoso-temporal region of the skull; the letters are placed on the mid-temporo-sphenoidal convolution. AS, the ali·sphenoid region of the skull; the letters are placed on the tip of the supero-temporo-sphenoidal convolution. The black lines mark the boundaries of different cranial regions.

The callosal convolution encloses the corpus callosum Within the concavity of its arch, and from its direction is appropriately called fornicatus (arch-shaped). The posterior end of the callosal convolution curves downwards and then forwards, under the name of gyrus hippocampi, to the tip of the inner surface of the temporo-sphenoidal lobe. This gyrus is separated anteriorly by a narrow curved fissure called hippocampal fissure, from a white band, the taenia hippocampi, which band possesses a free curved border, round which the pia mater and choroidal artery enter the lateral ventricle through the great transverse fissure of the cerebrum. The hippocampal fissure is continuous round the posterior end of the corpus callosum with the callosal fissure, and at the bottom of the hippocampal fissure the grey matter of the gyrus hippocampi terminates in a well-defined dentated border {fascia dentata). The hippocampal fissure on this surface of the hemisphere marks the position of an eminence in the descending cornu of the ventricle called hippocampus major. The gyrus hippocampi is separated posteriorly from the adjacent temporo-sphenoidal convolution by a fissure, named collateral, which marks the position on this surface of the hemisphere of the collateral eminence in the interior of the ventricle. From the lower end of the parieto-occipital fissure an offshoot, called the calcarine fissure, passes almost horizontally backwards in the occipital lobe, which fissure marks on this surface of the hemisphere the eminence named calcar avis, or hippocampus minor, in the posterior cornu of the ventricle.

If a horizontal slice be removed from the upper, part of each hemisphere, the peripheral grey matter of the convolutions will be seen to follow their various windings, whilst the core of each convolution consists of white matter continuous with a mass of white matter in the interior of the hemisphere. If a deeper slice be now made down to the plane of the corpus callosum, the white matter of that structure will be seen to be continuous with the white centre of each hemisphere. The corpus callosum does not equal the hemispheres in length, but approaches nearer to their anterior than their posterior ends (Pl. XVIII. fig. 3, B.) It terminates behind in a free rounded end, whilst in front it forms a knee-shaped bend, and passes downwards • and backwards as far as the lamina cinerea. If the dissection be performed on a brain which has been hardened in spirit, the corpus callosum is seen to consist almost entirely of bundles of nerve fibres, passing transversely across the mesial plane between the two hemispheres; these fibres may be traced into the white cores and grey matter of the convolutions, and apparently connect the corresponding convolutions in the opposite hemispheres. Hence the corpus callosum is a connecting or commissural structure, which brings the convolutions of the two hemispheres into anatomical and physiological relation with each other. On the surface of the corpus callosum a few fibres, the strias longitudinales, run in the antero-posterior or longitudinal direction. If the corpus callosum be now cut through on each side of its mesial line, the large cavity or lateral ventricle in each hemisphere will be opened into.

The lateral ventricle is subdivided into a central space or body, and three bent prolongations or cornua; the anterior cornu extends forwards and outwards into the frontal lobe; the posterior cornu curves backwards, outwards, and inwards into the occipital lobe; the descending cornu curves backwards, outwards, downwards, forwards, and inwards, behind and below the optic thalamus into the temporo-sphenoidal lobe. On the floor of the central space may be seen from before backwards the grey upper surface of the pear-shaped corpus striatum, and to its inner and posterior part a small portion of the optic thalamus, whilst between the two is the curved flat band, the taenia semicircularis. Resting on the upper surface of the thalamus is the vascular fringe of the velum interpositum, named choroid plexus, and immediately internal to this fringe is the free edge of the white posterior pillar oj the fornix. The anterior cornu has the anterior end of the corpus striatum projecting into it. The posterior cornu has an elevation on its floor, the hippocampus minor, and between this cornu and the descending cornu is the elevation called eminentia collateralis.

Extending down the descending cornu and following its curvature is the hippocampus major, which terminates below in a nodular end, the pes hippocampi; on its inner border is the white taenia hippocampi, continuous above with the posterior pillar of the fornix. If the taenia be drawn on one side the hippocampal fissure is exposed, at the bottom of which the grey matter of the gyrus hippocampi may be seen to form a well-defined dentated border (the so-called fascia dentata). The choroid plexus of the pia mater turns round the gyrus hippocampi, and enters the descending cornu through the great transverse fissure between the taenia hippocampi and optic thalamus. The lateral ventricle is lined by a cylindrical endothelium, which is in many parts ciliated, and which rests on a layer of neuroglia. This lining is continuous through the foramen of Monro with that of the third ventricle, which again is continuous with the lining of the fourth ventricle through the aqueduct of Sylvius. A little fluid is contained in the cerebral ventricles, which, under some pathological conditions, may increase greatly in quantity, so as to occasion considerable dilatation of the ventricular cavities.

If the corpus callosum be now divided about its middle by a transverse incision, and the posterior half of this structure be turned back, the body of the fornix on which the corpus callosum rests is exposed. If the anterior half of the corpus callosum be now turned forward, the grey partition, or septum lucidum, between the two lateral ventricles is exposed. This septum fits into the interval between the under surface of the corpus callosum and the upper surface of the anterior part of the fornix. It consists of two layers of grey matter, between which is a narrow vertical mesial space, the fifth ventricle. If the septum be now removed, the anterior part of the fornix is brought into view.

The fornix or arch is an arch-shaped band of nerve fibres extending in the antero-posterior direction. Its anterior end forms the anterior piers or pillars of the arch, its posterior end ∖ he posterior piers or pillars, whilst the intermediate body of the fornix forms the summit or crown of the arch. It consists of two lateral halves, one belonging to each hemisphere. At the summit of the arch the two lateral halves are conjoined to form the body; but in front of the body the two halves separate from each other, and form two anterior pillars, which descend in front of the third ventricle to the base of the cerebrum, where they form the corpora albicantia, and then enter the substance of the optic thalamus. Behind the body the two halves diverge much more from each other, and form the posterior pillars; each of which curves downwards and outwards into the descending cornu of the ventricle, and, under the name of taenia hippocampi, forms the free border of the hippocampus major. If the body of the fornix be now divided by a transverse incision, its anterior part thrown forwards, and its posterior part backwards, the great transverse fissure of the cerebrum is opened into, and the velum interpositum lying in that fissure is exposed.

The velum interpositum is an expanded fold of pia mater, which passes into the interior of the hemispheres through the great transverse fissure. It is triangular in shape; its base is in a line with the posterior end of the corpus callosum, where it is continuous with the external pia mater; its lateral margins are fringed by the choroid plexuses, which are seen in the bodies and descending cornua of the lateral ventricles, where they are invested by the endothelial lining of those cavities. Its apex, where the two choroid plexuses blend with each other, lies just behind the anterior pillars of the fornix. The interval between the apex and these pillars is the aperture of communication between the two lateral ventricles and the third, already referred to as the foramen of Monro. The choroid plexuses contain the small choroidal arteries, which supply the corpora striata, optic thalami, and corpora quadrigemina; and the blood from these bodies is returned by small veins, which join to form the veins of Galen (Fig. 75). These veins pass along the centre of the velum, and, as is shown in Fig. 63, open into the straight sinus. If the velum interpositum be now carefully raised from before backwards, the optic thalami, third ventricle, pineal gland, and corpora quadrigemina are exposed.

The optic thalamus is a large, somewhat ovoid body situated behind the corpus striatum, and abovp the crus cerebri. Its upper surface is partly seen in the floor of the body of the lateral ventricle, but is for the most part covered by the fornix and velum interpositum. Its postero-inferior surface forms the roof of the descending cornu of the ventricle, whilst its inner surface forms the side wall of the third ventricle. At its outer and posterior part are two slight elevations, placed one on each side of the optic tract, and named respectively corpus geniculatum internum and externum.

The third ventricle is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of fornix; its floor, by the pons Tarini, corpora albicantia, tuber cinereum, infundibulum, and optic commissure; its anterior boundary, by the anterior pillars of the fornix, anterior commissure, and lamina cinerea; its posterior boundary, by the corpora quadrigemina and posterior commissure. The cavity of this ventricle is of small size in the living head, for the inner surfaces of the two thalami are connected together by intermediate grey matter, named the middle or soft commissure; but in taking the brain out of the cranial cavity this commissure is usually more or less torn through, and the cavity is consequently enlarged. Immediately in front of the corpora quadrigemina, the white fibres of the posterior commissure pass across between the two optic thalami. If the anterior pillars of the fornix be separated from each other, the white fibres of the anterior commissure τa⅛γ be seen entering the two corpora striata.

The pineal body is a reddish cone-shaped body, enveloped by the velum interpositum, and situated upon the more anterior pair of the corpora quadrigemina. From its broad anterior end two white bands, the peduncles of the pineal body, pass forwards, one on the inner side of each optio thalamus. Each peduncle joins, along with the taenia semicircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of a vascular stroma of connective tissue, in the meshes of which lymphoid cells are contained. Branched corpuscles are also found not unlike nerve cells. Amylaceous and gritty calcareous particles, constituting the brain sand, are also found in it. Usually it is hollowed out into two or more small cavities. The function of the pineal body is not understood, but both it and the pituitary body, which possess a certain structural correspondence, are usually referred to the type of the ductless glands.

The corpora quadrigemina or optic lobes are situated behind and between the two optic thalami, and rest upon the posterior surface of the crura cerebri. The division into two lateral halves is marked by a shallow longitudinal fissure, and the subdivision of each half into an anterior and a posterior eminence, by a shallow transverse fissure. The anterior pair of eminences are called nates; the posterior, testes. From each testis a strong white band, the superior peduncle of the cerebellum, passes backwards to the cerebellum, and stretching between the pair of peduncles is the valve of Vieussens or anterior medullary velum. The corpora quadrigemina are tunnelled in the antero-posterior direction by the aqueduct of Sylvius, which opens anteriorly into the third ventricle immediately below the posterior commissure, and posteriorly into the fourth ventricle under cover of the valve of Vieussens. It is lined by a cylindrical ciliated endothelium.

Internal Structure of the Cerebrum. —The cerebrum is composed both of grey and white matter, the general relations of these two forms of nerve matter to each other may be seen by making sections through the cerebrum. The determination, however, of their minute structure, and of the relations and connections of the nerve fibres to the nerve cells is, owing to the delicacy of the organ, one of the most difficult departments of anatomical study. Several anatomists have endeavoured to trace out the course of the nerve fibres in the organ, and though our knowledge is by no means complete, yet many important facts have undoubtedly been ascertained. These facts have been summarised, and numerous valuable additions made to them by Meynert in a recent elaborate memoir, which has been frequently consulted and made use of in writing the following description.

The Grey Matter of the cerebrum is disposed in three great groups: a, The grey matter of the cortex of the hemispheres; b, the grey matter of the great ganglia of the base of the cerebrum ; c, the central grey matter which forms the wall of the cerebral end of the cerebro-spinal tube.

a, The grey matter of the cortex of the hemisphere forms the superficial part of the convolutions, and is known as the great hemispherical ganglion, but in some localities, as at the loci perforati antici and the septum lucidum, it has received distinctive names. When a convolution is divided vertically the grey matter is seen to be confined to the surface and to enclose a white core. The grey matter presents a laminated appearance, and as a rule consists of five or six layers, which are composed of the characteristic pyramidal nerve cells of the cortex of the cerebrum, of nerve fibres, of matrix or neuroglia, and of blood-vessels. The most superficial layer consists of neuroglia, in which nerve fibres extend parallel to the surface of the convolutions. In the deeper layers are found the pyramidal nerve cells, which lie with their long axes vertical to the surface of the convolutions, and which contain angular nuclei. From the observations of Lockhart Clarke, Arndt, Cleland, and Meynert, there can be no doubt that the pyramidal nerve cells vary in relative size and in numbers in the different layers of the grey cortex, and that the largest sized pyramidal cells lie in the third and fourth layers. L. Clarke stated that the cells of all the layers of the posterior or occipital lobe were small and of nearly uniform size, whilst in the convolutions anterior to it numerous cells of a much larger kind were found; but though it is undoubtedly true that large pyramidal cells are found in the frontal lobe in considerable numbers, and that the greater number of the cells of the occipital lobe are small and nearly uniform in size, there is no difficulty in recognising in the occipital lobe a small proportion of cells, quite equal in magnitude to the largest cells of the frontal lobe, interspersed amongst the smaller pyramidal cells. The nerve fibres which ascend into the grey matter from the white core of the convolution radiate into its several layers, and are apparently continuous with the basal axis-cylinder processes of the nerve cells. According to Cleland, the elongated apices of the cells, which are directed to the surface of the convolution, are continuous with the nerve fibres situated in the superficial layer of horizontal fibres. Immediately subjacent to the large pyramidal cells numerous small, irregularly shaped nerve corpuscles, like those of the internal granule layer of the retina, form the so-called granule layer of the grey matter. Fusiform cells, which give off lateral processes, are found in the deepest layer of the grey matter, and form the claustral layer of Meynert. Gerlach has described here, as in the spinal cord, a network of extremely minute nerve fibres, with which the branched lateral processes of the nerve cells are apparently continuous. The neuroglia contains , multitudes of small rounded corpuscles. In it also are found small stellate cells, provided with minute branched processes, which cells, as Meynert states, are so pellucid, that in the healthy brain they seem to be only free nuclei; it is difficult to say whether these cells belong to the neuroglia, or are nerve cell elements. The grey cortex of the cerebrum is much more vascular than the white matter. The arteries derived from the pia mater pass vertically into it, and end in a close polygonal network of capillaries; but it is also traversed by the arteries, which terminate in the capillary network of supply for the white matter.

In the grey matter of the cortex of the occipital lobe eight layers have been described by Clarke and Meynert. The increase in number is due to the intercalation of two additional granule layers, which coalesce and form a distinct white band in the grey matter, owing, as Meynert states, to the absence of pigment in the cells of the granule layers.

The grey matter of the cortex of the island of Beil and of the convolutions bounding the Sylvian fissure contains a very large proportion of fusiform cells. They form the chief constituent of the grey claustrum, situated deeper than the grey matter of the island, and separated from the outer part of the corpus striatum by a thin layer of white matter. Fusiform cells also occur abundantly in the nucleus amygdala, a grey mass situated below the corpus striatum, which in some sections seems as if isolated, but in reality is continuous with the grey matter of the inferior temporo-sphenoidal convolution.

The grey matter of the cortex of the gyrus hippocampi and of the hippocampus major is apparently destitute of both the granule and claustral layers of cells. Its superficial layer has been named the nuclear lamina, and contains small and scattered nerve corpuscles. Next this lamina lies the striatum reticulare, in which the apices of the numerous pyramidal cells of the third layer branch and again unite to form a delicate network. Deeper than the pyramidal cells is a thick layer of so-called “granules,” which A. B. Stirling recognised some years ago as like the granules of the rust coloured layer of the cerebellum; like them they consist of a well-defined nucleus invested by delicate branched protoplasm. The grey matter of the two layers of the septum lucidum, though included between the corpus callosum and fornix, is yet in the same plane as the grey matter of the cortex of the inner surface of the hemispheres, but is cut off from it by the development of the transverse fibres of the corpus callosum. The grey matter of the locus perforatus anticus contains dusters of minute granules and a compact arrangement of small nerve cells.

δ, The great ganglia of the base of the cerebrum are the corpora striata, the optic thalami, the corpora geniculata, the corpora quadrigemina, and the locus niger in each crus cerebri.

The corpus’striatum cerebri consists of two masses of grey matter separated from each other by numerous striae of white fibres, which ascend from below upwards through its substance. The upper mass of grey matter projects into the lateral ventricle, and is called the intra-ventricular portion or nucleus caudatus. The lower extra-ventricular portion or nucleus lenticularis forms the outer and lower part of the corpus striatum, and is separated by the claustrum from the island of Reil. Multipolar nerve cells are found in both the caudate and lenticular masses, and in the latter cells of large size have been seen. The optic thalamus forms an almost continuous mass of grey matter traversed by nerve fibres, which are not, however, collected into definite striae. The nerve cells in the grey matter are both multipolar and fusiform. The external corpus geniculatum consists of alternate layers of grey and white matter, due to the zig-zag folding of the grey matter; the nerve cells are multipolar, and contain pigment. In the internal corpus geniculatum the cells are smaller in size and fusiform. The grey matter of the corpora quadrigemina consists of two distinct masses. One, the zonular ι layer, lies near the surface, and contains small multipolar nerve cells; the other, the Sylvian or central layer, lies at the sides of the Sylvian fissure and belongs to the grey matter of the wall of the cerebro-spinal tube, and serves as a centre of origin for the roots of both the 3d and 4th cranial nerves. The grey matter of the crus cerebri occupies the centre of the cerebral peduncle. Its cells are multipolar, and contain dark brown or black pigment, so that the name locus niger is applied to this collection of nerve cells.

c, The central grey matter of the cerebrum is in series with the grey matter of the floor of the 4th ventricle and the grey matter of the spinal cord. It is situated around the Sylvian aqueduct, and at the sides and floor of the third ventricle, which form the cerebral portion of the cerebrospinal tube. That which is situated in relation with the aqueduct of Sylvius forms the Sylvian or central layer just described in the corpora quadrigemina. That which lies in relation to the third ventricle forms the middle or soft commissure, and the well-defined grey layer which covers the inner wall of each optic thalamus; also the grey masses situated at the base of the brain between and in front of the crura cerebri, viz., the pons Tarini, tuber cinereum, lamina cinerea, infundibulum, and the grey matter of the pituitary body. By some anatomists the grey matter of the pineal body is referred to the same category, but Arnold has pointed out that it is separated by its peduncle from the soft commissure; and Meynert is disposed to regard it as a ganglion of origin of the tegmentum. Both the pituitary and pineal bodies contain, besides the nervous matter, structures of the type of the glands without ducts.

The White Matter of the cerebrum consists of tracts or fasciculi of nerve fibres, of which— a, some connect the cerebrum with the lower divisions of the encephalon; δ, others connect the two hemispheres together; c, others connect different structures in the same hemisphere; d, others serve as roots of origin for the more anterior encephalic nerves.

a, The tracts of fibres which connect the cerebrum with the lower divisions of the encephalon are called peduncular fibres. The largest of these peduncles are the two crura cerebri or cerebral peduncles. Continuous below with the longitudinal fibres of the pons they ascend into the optic thalami and corpora striata, and their fibres are named the peduncular fibres. From the corpora striata and optic thalami fibres radiate into the convolutions of the lobes of the hemisphere and form the corona radiata. To some extent the fibres of the corona are directly continuous with those of the cerebral peduncles, but there can be no doubt that a large portion of the peduncular fibres terminate in the grey matter of the ganglia of the base of the cerebrum, and that a still larger number arise from their nerve cells to aid in the formation of the corona radiata. The direct continuity, therefore, of many of the peduncular fibres with those of the corona is broken or interrupted by the interposition of the cerebral ganglia, which Meynert has named ganglia of interruption. The peduncular fibres and those of the corona constitute the cerebral portion of the projection system of fibres of Meynert, a term devised to express that they conduct upwards to the grey cortex of the hemispheres sensory impulses derived from the external world, the image of which is projected upon the cortex. But it should also not be forgotten that many of the fibres of this system conduct motor impulses downwards to be propagated along the'motor cranial and spinal nerves. The peduncular fibres of the crura cerebri are arranged in two groups, named respectively crusta and tegmentum, which are separated from each other by the nerve cells of the locus niger. The crusta forms the superficial or anterior part of the crus. Its fibres are in greater part continuous with the longitudinal fibres of the pons derived from the anterior pyramids of the medulla; but it receives additional fibres from the grey matter of the locus niger, and from the cells of the Sylvian layer in the corpora quadrigemina. Some of the fibres of the crusta pass directly upwards as radiating fibres to the grey cortex of the occipital and temporal lobes, but the larger number terminate in the nucleus caudatus and nucleus lenticularis of the corpus striatum. From these nuclei a great mass of fibres radiates into the cortex of the fronto-parietal lobes, more especially the frontal, but a few also, bearing the special name of stria cornea, pass to the grey matter of the apex of the temporal lobe; fibres also enter the convolutions of the insula. In addition to the radiating fibres, the grey matter of the corpus striatum gives origin to fibres of the middle root of the olfactory peduncle, and to connecting fibres with the grey matter of the septum lucidum. The tegmentum forms the posterior or deeper part of the crus cerebri. Its fibres are continuous with the longitudinal fibres of the pons derived from the olivary fasciculi, fasciculi teretes, and posterior pyramids of the medulla. A. few of the fibres of the tegmentum enter the corpora quadrigemina and corpora geniculata, but the great majority enter the optic thalami, in the grey matter of which many evidently terminate, though some may pass through into the cortex of the hemispheres as fibres of the corona radiata. But the grey matter of the thalamus gives origin to numerous radiating fibres: those which arise in its posterior part radiate into the occipital and temporal lobes, whilst those proceeding out of its anterior part radiate into the frontal, parietal, and temporal lobes, and the insula. In the optic thalamus the fornix arises. Its fibres emerge from the under surface of the thalamus, form the corpus albicans, and then pass backwards as the upper boundary of the great transverse fissure to end as the taenia hippocampi in the gyrus hippocampi; hence this convolution has a special connection with the optic thalamus through the fornix In the corpus albicans the fibres of the fornix are arranged in loops, in the concavities of which nerve cells are situated. The optic thalamus also gives origin to the middle root of the optic tract. Owing to the connections of the locus niger, nucleus caudatus, and nucleus lenticularis with the crusta, Meynert has named them the ganglia of the crusta ; whilst the optic thalami, corpora quadrigemina, and geniculata are the ganglia of the tegmentum. The comparison of the human brain with those of diffβreκi mammals has shown that the development of the hemispheres bears a direct relation to the size of the crusta and its ganglia, whilst the development of the hemispheres is in inverse relation to the size of the tegmentum and its ganglia.

The superior peduncles of the cerebellum connect that organ with the cerebrum. They arise in the grey matter of the vermiform process, ascend to the corpora quadrigemina, and some fibres are even prolonged apparently into the tegmentum, and through it doubtless into the optic thalamus.

b, The fibres which connect together the two hemispheres are called commissural fibres. The largest of these commissures is the corpus callosum, which, as has already been described, connects corresponding convolutions in the opposite hemispheres. As its fibres lie on a plane superior to those of the corona radiata, the two systems of fibres intersect with each other on their way to the convolutions. The anterior commissure, though often described as connecting the two corpora striata, yet, as Spurzheim pointed out half a century ago, passes through these bodies to the convolutions around the Sylvian fissure, and gives a root of origin to the olfactory nerve. The posterior commissure passes into the two optic thalami; some of its fibres are said to extend into the tegmentum, and others into the substance of the hemisphere.

c, The tracts which connect different convolutions in the same hemisphere are named arcuate fibres, orfibroe proprioe. The arcuate fibres are situated immediately beneath the inner surface of the cortex of the hemispheres, and connect together the grey matter of adjacent convolutions. In some localities they are strongly marked, and have ^received special names.

The fasciculus uncinatus passes across the Sylvian fissure, traverses the claustrum and amygdala, and connects the convolutions of the frontal with those of the temporo-sphenoidal lobe. The fillet of the gyrus fornicatus extends longitudinally in that convolution, immediately above the corpus callosum, from its anterior to its posterior ends, and connects two different parts of its grey matter together. The longitudinal fibres of the corpus callosum, or nerves of Lancisi, also connect the anterior and posterior ends of the callosal convolution. The longitudinal inferior fasciculus connects the convolutions of the occipital with those of the temporal lobe. Longitudinal fibres lie on the inner surface of the septum lucidum, and extend into the gyrus fornicatus.

The corpora quadrigemina are connected with the optic thalami by nervous tracts called brachia, and smaller tracts also connect the thalami with the corpora geniculata. The peduncles of the pineal gland connect that body with the fornix, and are probably continued into the optic thalamus. The taenia semicircularis is also at one end apparently connected with the optic thalamus, but its posterior termination is not well ascertained.

The great cerebral ganglia and the central masses of grey matter are centres of origin for sensori-motor nerves. The hemispherical ganglia, again, are the parts of the brain associated with the intellectual processes. The question has often been put, Are not the individual convolutions distinct organs, each endowed with special properties 1 and various arguments based on physiological, pathological, and anatomical grounds have been advanced in support of this proposition. In connection with the anatomical branch of the argument it may be stated that the convolutions possess, not only in man, but in all animals with convoluted brains, great regularity both in position and arrangement; but specialisation of form is not in itself a sufficient test of specialisation of function. A gain, though the convolutions have definite forms they are not disconnected from each other, for the grey matter forms a continuous layer over the whole surface of the hemisphere. Hence a group of cerebral convolutions differs from a group of muscles, each member of which is undoubtedly a distinct organ, for each muscle is isolated from those around it by a definite investing sheath. As regards internal structure, evidence has already been given that all the convolutions are not constructed on precisely the same plan, and it has also been pointed out that the convolutions are not all connected in the same way with the great cerebral ganglia. These structural modifications unquestionably point to functional differences in the several parts in which they are found. But further, special connections through the arcuate fibres are established between certain convolutions and not between others, and it is possible not only that particular combinations of convolutions through an interchange of internuncial fibres may condition a particular state of intellectual activity, but that these combinations associate various convolutions together in the performance of a given intellectual act, just as in the muscular system several muscles are as a rule associated together for the performance of a given movement. A clue to the special functions of the convolutions may perhaps be obtained by studying their connections, just as the action of the members of a group of muscles is ascertained by examining the direction of their fibres and the attachment of their terminal tendons.

Mass and Weight of the Brain. —The human brain is absolutely bigger and heavier than the brain of any animal, except the elephant and the larger whales. It is also heavier relatively to the bulk and weight of the body than are the brains of lower animals, except in some small birds and mammals. Considerable variations, however, exist in the size and weight of the human brain, not only in the different races of mankind, but in individuals of the same race and in the two sexes. The heaviest brains occur in the white races. The average weight of the adult European male brain is 49 to 50 oz., that of the adult female 44 to 45 oz.; so that the brain of a man is on the average fully 10 per cent, heavier than that of a woman. The greater weight of the brain in man as compared with woman is not in relation merely to his greater bulk, but is a fundamental sexual distinction; for, whilst there is a difference of 10 per cent, in the brain weight, the average stature of women is, as Thurnam has calculated, only 8 per cent, less than that of men. Dr Boyd states that the average weight of the brain in the newly born male infant is 11 *67 oz.; in the female only 10 oz. The exact age at which the brain reaches its maximum size has been variously placed at from the 3d to the 8th years by different authors; but it continues to increase in weight to 25 or 30, or even 40. After 60 the brain begins to diminish in weight; in aged males the average weight is about 45 oz., in females about 41 oz. In some cases the adult brain considerably exceeds the average weight. The brains of several men distinguished for their intellectual attainments have been weighed: the brain of Cuvier weighed 64£ oz.; of Dr Abercrombie, 63 oz.; of Professor Goodsir, 57⅜ oz.; of Spurzheim, 55 oz.; of Sir J. Y. Simpson, 54 oz.; of Agassiz, 53 ’ 4 oz.; and of Dr Chalmers, 53 oz. But high brain weights have also been found where there was no evidence of great intellectual capacity. Peacock weighed four male brains which ranged from 62 ’ 75 to 61 oz.; Boyd, a specimen of 60 ’ 75 oz.; and Turner has recorded one of a boy aged fifteen which weighed 60 oz. In the brains of the insane high brain weights have also been observed. Bucknill met with a brain in a male epileptic which weighed 64∣ oz.; Thurnam, one which weighed 62 oz.; and in the West Riding Asylum, out of 375 males examined, the weight of the brain in 30 cases was 55 oz. or upwards, and the highest weights were 61 oz. in a case of senile dementia, 60∣ oz. in a case of dementia, and 60 oz. in one of melancholia. No case has as yet been recorded of the weight of the brain in a woman possessing intellectual eminence; but Boyd met with a woman’s brain as high as 55 ’ 25 oz., and many instances of upwards of 50 oz. in women where there was no evidence of high mental endowment. Skae, in a female monomaniac, observed a brain which weighed 61∣ oz.; and of 300 females examined in the West Riding Asylum the weight of the brain in 26 cases was 50 oz. or upwards, the highest weights being 56 and 55 oz. in two cases of mania. The size and weight of the brain do not therefore, per se, give an exact method of estimating the intellectual power of the individual, and a high brain weight and great intellectual capacity are not necessarily correlated with each other. It seems certain, if the human brain, even amongst the most uncultivated peoples, falls below 30 oz., that this low weight is not merely incompatible with intellectual power and activity, but is invariably associated with idiocy or imbecility; so that the human brain has a minimum weight below which intellectual action is impossible. Amongst the more cultivated races the minimum weight-limit of intelligence is, however, in all probability higher than 30 oz. It has been placed by Broca at 32 oz. for the female, and 37 oz. for the male brain; and Thurnam’s numbers are almost the same. To how low a weight the brain in the microcephalous idiot may fall is well shown in a case recorded by Theile, where it weighed only 10 ’ 6 oz., in Gore’s case of 10 oz. 5 grs., and in Marshall’s case, 8 J oz. But instances are not wanting in which the brains of idiots have exceeded even 50 oz. Langdon Down observed the brain of a male idiot aged 22, which weighed 59J oz.; and J. B. Tuke has recently met with a brain of 60 oz. in a male idiot aged 37, the capacity of whose cranium was 110∣ cubic inches. In the West Riding Asylum tables the brain weights in 10 idiots were not less than 34 oz., and in 5 cases exceeded 40 oz. As yet the opportunities of weighing the brain in the coloured races of men have been but scanty. But from a very extensive series of observations made by Barnard Davis, not on the brains themselves, but on the cubic capacities of crania, from which an approximate estimate of the brain weight may be obtained with a fair measure of accuracy, the following facts are derived :—The average weight of the male brain in the African races is 45 - 6 oz.; of the female brain, 42 ’ 7 oz.: the average weight of the male brain in the Australian races is 42 ’ 8 oz.; of the female brain, 39 ’ 2 oz.: the average weight of the male brain in the Oceanic races, 46’5 oz.; of the female brain, 43 oz. The conclusions which may legitimately be drawn from an analysis of Barnard Davis’s observations are as follows :— 1st, That the average brain weight is considerably higher in the civilised European than in the savage races; 2d, That the range of variation is much greater in the former than in the latter; 3d, That there is an absence, almost complete, of specimens heavier than 54 oz. in the exotic races, so that the higher terms of the series are not represented; 4th, That though the male brains are heavier than the female, there is not the same amount of difference in the average brain weight between the two sexes in the uncultivated as in thè cultivated peoples.

No reliable determinations have as yet been made of the exact proportion, as regards bulk and weight, which the convolutions bear to the corpora striata, optic thalami, and corpora quadrigemina, but data are obtainable of the relative weight of the pons, cerebellum, and medulla to the entire encephalon. Between the ages of 20 and 70 the ratio of weight of the pons, cerebellum, and medulla, to the entire brain, is as 13 to 100, and this relative weight is virtually the same in both sexes.

Origin, Arrangement, and Distribution of the Encephalic Nerves. —Several pairs of nerves, called Cranial or Encepπalic, arise from the under surface or base of the encephalon, and pass outwards through foramina situated in the floor of the cranial cavity. Continental anatomists usually enumerate twelve pairs of cranial nerves; but because in one locality two of these nerves lie together and pass through the same foramen, and in another spot three of these nerves emerge together from the skull, British anatomists have restricted the number to nine pairs. These nerves are numbered from before backwards, in the order in which they are seen at the base of the brain. The names applied to the individual nerves, and their numerical designations, according to both the Continental and British methods, are given in the following table :— [table]

These nerves may be arranged in three groups according to the presence or absence of motor and sensory fibres.

First group.— Sensory nerves, or nerves of special sense: a, olfactory, the nerve of smell; b, optic, nerve of sight; c, auditory, nerve of hearing.

Second group.— Motor nerves: a, oculo-motor, the principal nerve of supply for the muscles of the eyeball; b i trochlear, the nerve for the superior oblique muscle; c, abducent, the nerve for the external rectus; d, portio dura, the nerve for the facial muscles of expression; e, spinal accessory, the nerve which gives a motor root to the pneumogastric, and supplies the sterno-mastoid and trapezius muscles; f, hypoglossal, the nerve for the muscles of the tongue.

Third group.— Mixed nerves: a, trifacial, distributed to the muscles of mastication, the skin of the face, various mucous membranes, and to the anterior and lateral surfaces of the tongue, where it may play the part of a nerve of the special sense of taste; b, glosso-pharyngeal, distributed to the mucous membrane of the pharynx, to certain palatopharyngeal muscles, and to the mucous membrane of the back of the tongue, where it acts as a nerve of the special sense of taste; c, the pneumogastric, conjoined with the internal division of the spinal accessory, is distributed to several muscles, mucous membranes, and internal organs.

The consideration of the 1st group of cranial nerves may appropriately be deferred until the organs of sense, in which they terminate, are described. The anatomy of the motor nerves is as follows :—

The Oculo-motor or third nerve springs out of the inner surface of the crus cerebri. When its fibres are traced into the crus, some are seen to pass to the nerve cells of the locus niger, whilst others sink into the corpora quadrigemina, and extend as far as the Sylvian group of large nerve cells. The nerve, after it has emerged from the crus, runs forwards in the outer wall of the cavernous sinus, and enters the orbit through the sphenoidal fissure. It supplies the levator palpebrae superioris, the superior, inferior, and internal recti muscles, and the inferior oblique. It contributes the short or motor root to the ciliary ganglion, and through it influences the iris and ciliary muscles within the eyeball. It also communicates with the cavernous· plexus of the sympathetic.

The Trochlearis or fourth, the smallest cranial nerve, lies at the outer side of the crus cerebri. When traced backwards to its origin it is seen to sink into the valve of Vieussens, where its fibres divide into three roots: one decussates across the valve with a root of the corresponding nerve on the opposite side; another passes backwards to the locus caeruleus; the third sinks into the corpora quadrigemina and reaches the Sylvian group of nerve cells, from which the third nerve also arises. The fourth nerve runs forward in the outer wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the superior oblique muscle. It also communicates with the cavernous plexus of the sympathetic.

The Abducent or sixth nerve springs out of the groove between the lower border of the pons and the anterior pyramid of the medulla oblongata. Its roots sink deeply into the pons, and arise from a nucleus of grey matter at the floor of the fourth ventricle, common to it and the portio dura. The sixth nerve runs forward in the inner wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the external rectus muscle. It communicates with the carotid plexus of the sympathetic.

The Portio dura or motor facial portion of the seventh nerve springs out of the groove between the lower border of the pons and the restiform body. Its roots sink deeply into the pons, and whilst some of its fibres arise from a grey nucleus, at the floor of the fourth ventricle, common to it and the sixth nerve, others ascend from a nucleus which, according to Meynert, lies just on the outer side of the superior olivary body, and others again decussate across the median raphe of the pons. An accessory portion, called portio intermedia, which is said to arise from the lateral columns of the cord, joins the portio dura. The portio dura enters the internal auditory meatus in the petrous-temporal bone along with the auditory nerve; but at the bottom of the meatus it leaves that nerve and enters the aqueduct of Fallopius, along which it is conducted through the bone to emerge at the stylo-mastoid foramen. When in the aqueduct it forms a hnee-shaped bend, and expands into a small ganglion, which is joined by the great, small, and external petrosal nerves, and through the external petrosal it communicates with the sympathetic. The portio dura gives off— a, a minute branch to the stapedius muscle; b, the chorda tympani, which, entering the tympanum, passes across that cavity, emerges through the Glaserian fissure, and joins the lingual branch of the fifth nerve, which it accompanies as far as the submaxillary ganglion; it gives a branch to the ganglion, and one to the lingualis muscle. After the portio dura has passed through the stylo-mastoid foramen it gives off— c, the posterior auricular branch to the occipital belly of the occipitofrontalis and to the retrahens aurem muscle, and d, the digastric branch to the posterior belly of the digastric and stylo-hyoid muscles; and then runs forwards through the parotid gland to the face, where it breaks up into numerous (e) facial branches to supply the facial muscles of expression and the buccinator muscle. The facial is also the secretory nerve for the salivary glands. Through the chorda tympani it influences the secretion of the submaxillary and sublingual glands, and through the connection between its lesser petrosal nerve and the auriculo-temporal in the otic ganglion it influences the parotid gland.

The Spinal Accessory is the lowest division of the eighth nerve. It springs out of the side of the medulla oblongata, and from the lateral column of the cervical part of the spinal cord as low as the fifth cervical nerve: its roots arise from the intermedio-lateral group of nerve cells in the cord, and from a nucleus of grey matter in the floor of the fourth ventricle. The spinal fibres of origin enter the skull through the foramen magum, join the fibres from the medulla, and leave the cranial cavity through the jugular foramen. This nerve, purely motor in function, is subdivided into two parts, an internal and an external. The external passes obliquely outwards across the side of the neck, pierces the sterno-mastoid, and ends in the trapezius, both of which muscles it supplies. The internal joins the pneumogastric nerve, of which it forms the motor or accessory root, and is distributed along with it.

The Hypoglossal or ninth nerve springs out of the groove between the anterior pyramid and olivary body of the medulla oblongata, in series with the anterior roots of the spinal nerves. Its roots pass through the medulla to the floor of the fourth ventricle, to arise from the nerve cells in two nuclei of grey matter situated close to the median furrow. This grey matter is in series with the anterior cornua in the spinal cord. The nerve passes out of the skull through the anterior condyloid foramen, and arches across the side of the neck to the tongue, to end in glossal branches for the supply of the intrinsic and extrinsic muscles of the tongue. It also gives off— a, the descendens noni branch, which, after been joined by the communicantes noni from the cervical plexus, supplies the omo-hyoid, sterno-hyoid, and sterno-thyroid muscles; b, the thyro-hyoid branch to the thyro-hyoid muscle; c, the genio-hyoid branch to the genio-hyoid muscle. It communicates in the neck with the sympathetic, vagus, lingual branch of the fifth, and cervical plexus.

The group of mixed nerves will now be considered.

The Trifacial or fifth is the largest cranial nerve. It springs by two distinct roots out of the side of the pons. The smaller or motor root arises from the nerve cells of a nucleus of grey matter situated in the back of the pons, near the floor of the upper part of the fourth ventricle. The larger or sensory root has, according to Meynert, a complex origin— a, from a nucleus of grey matter in the pons to the outer side of the origin of the motor root; b, by descending fibres which arise from nerve cells in the substance of the corpora quadrigemina, from the grey matter of the locus caeruleus, and from the longitudinal fibres of the pons; c, by ascending fibres which apparently arise from the grey tubercle of Rolando; d, probably by fibres which traverse and embrace the superior peduncle of the cerebellum. As the large sensory root of the fifth lies on the petrous bone it expands into the Gasserian ganglion, which resembles in structure the ganglion on the posterior root of a spinal nerve. From this ganglion three large branches arise, named respectively the 1st, 2d, and 3d divisions of the ganglion.

The lsi or Ophthalmic division is the upper sensory nerve of the face, and divides into three branches, which pass out of the cranial cavity through the sphenoidal fissure. By its lachrymal branch it supplies the lachrymal gland, and the outer part of the skin and conjunctiva of the upper eyelid; by its frontal branch, the inner part of the skin and conjunctiva of the upper lid, and the skin of the forehead; by its oculo-nascd branch, it gives long ciliary nerves to the eyeball, and a nasal nerve to the mucous membrane of the nose, and the skin of the side of the nose. From the oculo-nasal nerve arises the long or sensory root of the ciliary ganglion, which lies in the cavity of the orbit, and which receives also a motor root from the third nerve, and a root from the sympathetic. This ganglion gives origin to the short ciliary nerves for the eyeball.

The 2cZ or Superior Maxillary division is the sensory nerve for the middle part of the face. It leaves the skull by the foramen rotundum, passes across the spheno-maxillary fissure, then lies in the canal in the floor of the orbit, from which it emerges on the face through the infra-orbital foramen as the infra-orbital nerve. It gives off a small orbital branch to a small part of the skin of the temple.

and that ôvér the cheek bone ; dental branches to £he teeth in the upper jaw; palpebral branches to the skin and conjunctiva of the lower eye-lid; nasal branches to the skin and mucous membrane of the nose; labial branches to the skin and mucous membrane of the upper lip. It also gives off, when in the spheno-maxillary fossa, sphenopalatine branches, which form the sensory root of the spheno-palatine or Meckel's ganglion. This ganglion receives a motor root through the great petrosal nerve from the knee-shaped bend of the portio dura, and a sympathetic root from the carotid plexus, which runs along with the great petrosal, and forms with it the vidian nerve. The ganglion gives origin to—α,an orbital branch, which supplies a layer of non-striped muscular fibres, described by H. Müller and Turner as developed in connection with the periosteum of the orbit, where it covers the spheno-maxillary fissure; b, upper nasal and naso-palatine branches to the mucous membrane of the nose and hard palate; c, descending palatine branches to the mucous membrane of the hard and soft palate; d, pterygo-palatine to the mucous membrane of the upper part of the pharnyx.

The 3<Z or Inferior Maxillary division passes out of the skull through the foramen ovale, and as it does so is joined by the motor root of the 5th. By the junction a mixed nerve is formed, which is the sensory nerve for the lower part of the face, and the skin of the temple, and the motor nerve for the muscles of mastication. Immediately after passing through the foramen this nerve divides into a small and large division, in each of which motor and sensory fibres are found. The small division supplies motor masticatory branches to the masseter, temporal, external, and internal pterygoid muscles; but further it gives off a long buccal branch, which, though often described as the motor nerve for the buccinator muscle, is really a sensory nerve for the skin and mucous membrane of the cheek. The sensory nature of this nerve is proved, not only by physiological and pathological experiments, but by tracing its fibres through the buccinator muscle to the mucous membrane. Turner has also recorded two cases in which the long buccal nerve arose as a branch of the sensory superior maxillary nerve. The large division separates into three branches— a, auriculo-temporal, which ascends to supply the parotid gland, the skin of the auricle, external meatus, and temple, and the temporo-maxillary joint; b, inferior dental, which enters the dental canal in the lower jaw, and supplies the lower set of teeth and the skin and mucous membrane of the lower lip; it also gives off a mylo-hyoid branch to the mylo-hyoid and anterior belly of the digastric muscle; c, lingual or gustatory, which runs forward along the side of the tongue to end in the filiform and fungiform papillae of its mucous membrane. The lingual branches are sensory nerves of touch, though some physiologists believe that they are also nerves of taste. Connected with the branches of the inferior maxillary division are two small ganglia, which, like the ciliary and spheno-palatine ganglia, are of a greyish colour, contain nerve cells, and receive roots from motor, sensory, and sympathetic nerves. The submaxillary ganglion lies under cover of the mylo-hyoid muscle, and receives a root from the motor chorda tympani nerve, a root from the sensory lingual, and a sympathetic root. It gives branches to the sub-maxillary and sublingual salivary glands. The otic ganglion lies close to the Eustachian tube, and receives a root from the muscular nerve to the internal pterygoid, a root from the sensory auriculo-temporal, and a sympathetic root. It also receives the small petrosal nerve, by which it is connected to the knee-shaped bend of the portio dura and to the glosso-pharyngeal nerve. It supplies the tensor tympani and tensor palati muscles. The branches of the three divisions of the fifth cranial nerve, which Dass to the ekin of the temple, forehead, and face, freely communicate with the branches of the portio dura, which supply the muscles situated in those regions.

The Glosso-pharyngeal or uppermost division of the eighth nerve springs out of the side of the medulla oblongata between the olivary and restiform bodies; its roots arise from two small masses or nuclei of grey matter in the floor of the 4th ventricle. The nerve passes out of the skull through the jugular foramen, where it possesses two small ganglia, named jugular and petrous. It then passes across the side of the neck and gives off carotid branches, which run along the internal carotid artery; pharyngeal branches to the mucous membrane of the pharynx; tonsil-itic branches to the tonsil and soft palate; glossal branches to the base of the tongue and the circumvallate papillae, which branches are unquestionably nerves of the special sense of taste; muscular branches to the stylopharyngeus and perhaps the constrictor muscles. Through the jugular and petrous ganglia the nerve communicates with the vagus and sympathetic. The petrous ganglion gives off the tympanic branch or nerve of Jacobson, which enters the tympanic cavity, supplies its mucous membrane, and gives off three communicating branches—one to the sympathetic; a second to the great petrosal, and through it to the knee-shaped bend of the facial; a third to the small petrosal, and through it to the otic ganglion.

The Pneumogastric or Vagus is the middle subdivision of the eighth cranial nerve. It springs out of the side of the medulla oblongata, between the olivary and restiform bodies; its roots arise from a nucleus of grey matter in the floor of the 4th ventricle, which nucleus, along with those for the glosso-pharyngeal nerve, is in series with the posterior cornu of grey matter in the spinal cord. It goes through the jugular foramen, is joined by the inner division of the spinal accessory which is its motor root, then passes down the side of the neck, enters the thorax, reaches the outer wall of the oesophagus, accompanies that tube through the diaphragm, and terminates in the wall of the stomach. The left nerve lies on a plane anterior to the right: it crosses in front of the arch of the aorta, and is distributed to the anterior wall of the stomach, whilst the right nerve supplies the posterior wall. Each nerve possesses high in the neck two enlargements, named upper and lower ganglia. The branches of the vagus are numerous and important. The upper ganglion gives origin to the auricular branch, which traversing a small canal in the petrous temporal bone, is distributed to the skin of the back of the auricle. The lower ganglion gives origin to— a, the pharyngeal branch, which forms a plexus with the glosso-pharyngeal and sympathetic nerves, from which the muscles of the pharynx are supplied; b, the superior laryngeal, which divides into an external branch to supply the crico-thyroid muscle, and an internal, which pierces the thyro-hyoid membrane, and supplies the mucous lining of the larynx and the mucous covering of the epiglottis. The trunk of the nerve gives origin to— a, the recurrent laryngeal branch, which on the right side turns round the subclavian artery, and on the left round the arch of the aorta, and ascends to the larynx to supply its intrinsic muscles except the crico-thyroid; b, cardiac branches, which arise from the nerve partly in the neck and partly in the chest, and. join the great cardiac plexus for the heart; c, pulmonary branches, which arise in the chest, pass into the substance of the lungs, and form along with the sympathetic an anterior plexus in front of, and a posterior plexus behind the root of the lung; d, oesophageal branches, which supply the coats of the oesophagus; e, gastric branches, which supply the coats of the stomach, and give important offshoots to the great solar plexus of~the sympathetic situated at the pit of the stomach.

Descriptive Anatomy of the Sympathetic Nervous System.

The Sympathetic Nervous System consists of a pair of ∣ gangliated cords, situated one on each side of the spinal column; of three great gangliated prevertebral plexuses situated in the thoracic and abdominal cavities ; of numerous smaller ganglia lying more especially in relation with the thoracic and abdominal viscera; of multitudes of fine distributory nerves.

Each Gangliated Cord of the sympathetic extends along the side of the spine from the base of the skull to the coccyx. In the neck it lies in front of the transverse processes of the vertebrae; in the thorax, in front of the heads of the ribs; in the abdomen, on the sides of the vertebral bodies; and as it descends in front of the sacrum it approaches its fellow, so that in front of the coccyx the two are united in a single ganglion, the ganglion impar (Fig. 66, c). Each cord consists of a number of ganglia united into a continuous cord by intermediate nerves. As a rule, the ganglia equal in number the vertebrae of the region. Thus, in the sacral region there are five ganglia, in the lumbar five, and in the thorax twelve; but in the neck there are only three, named superior, middle, and inferior; of these the superior is very large, and represents without doubt several smaller ganglia. From the superior cervical ganglion the cord is prolonged upwards by an ascending or cranial offshoot through the carotid canal into the cranial cavity, and forms a plexus around the internal carotid artery, both in the carotid canal, named the carotid plexus, and in the inner wall of the cavernous sinus, named the cavernous plexus. Through branches derived either directly or indirectly from these plexuses the sympathetic roots for the ciliary and spheno-palatine ganglia, described in connection with the fifth nerve, are derived.

From the gangliated cord and its ascending or cranial prolongation a communicating and a distributory series of branches are derived.

By the Communicating branches this portion of the sympathetic is connected with most of the cranial and with the anterior divisions of all the spinal nerves, so as to bring the cerebro-spinal and sympathetic systems into close anatomical and physiological relation with each other. It is important also to observe that each communicating branch contains not only non-medullated nerve fibres from the sympathetic system to the cerebro-spinal nerves, but medullated fibres from the cerebro-spinal to the sympathetic, so that a double interchange takes place between the two systems. The cranial prolongation of the sympathetic and the superior cervical ganglion communicate with the 3d and 4th nerves, the Gasserian ganglion of the 5th, the 6th, the portio dura of the 7th, the glosso-pharyngeal and pneumogastric of the 8th, and the 9th.cranial nerves, and with the anterior divisions of the four upper cervical spinal nerves. The middle cervical ganglion communicates with the 5th and 6th cervical nerves, the inferior cervical ganglion with the 7th and 8th cervical nerves, the twelve thoracic ganglia with the series of intercostal nerves, the five lumbar ganglia with the series of lumbar spinal nerves, the sacral and coccygeal ganglia with the sacral nerves and the coccygeal nerve.

The Distributory branches of the gangliated cord are as follows :—α, Pharyngeal branchée from the superior cervical ganglion, which join the pharyngeal branches of the glossopharyngeal and pneumogastric nerves, to form the pharyngeal plexus, which supplies the muscles and mucous membrane of the pharynx, δ, Articular branches from the upper thoracic and the lumbar ganglia to the articulations between the adjacent vertebrae. c, Pulmonary branches from the 3d or 4th thoracic ganglia, which join the posterior pulmonary plexus, d, Vaso-motor branches or ner ↑ n molies, which supply the muscular coat of the arteries: those which arise from the cranial prolongation of the superior cervical ganglion supply the internal carotid artery and its branches to the brain and eyeball: those which arise from the superior cervical ganglia itself supply the external carotid artery and its branches; from the branch accompanying the facial artery the submaxilliary ganglion derives its sympathetic root; from that accompanying the middle meningeal artery the otic ganglion derives its sympathetic root: the vaso-motor nerves which arise from the middle cervical ganglion supply the inferior thyroid artery, and pass to the thyroid gland: the vaso-motor branches of the inferior cervical ganglion supply the vertebral and basilar arteries and their several branches, which pass to the spinal cord and the hinder par⅛ of the encephalon. Vaso-motor nerves also arise from the thoracic ganglia, which pass to the thoracic aorta, from the lumbar ganglia to the abdominal aorta, and from the sacral ganglia to the middle sacral artery; the ganglion impar gives branches to a peculiar vascular structure, named the coccygeal body, developed in connection with the end of the middle sacral artery; a body of similar structure, called intercarotic body, situated in the angle of bifurcation of the common carotid artery, receives branches from the superior cervical ganglion, e, Cardiac branches from the superior, middle, and inferior cervical and the 1st thoracic ganglia, which pass into the thorax to join the prevertebral cardiac plexus, f, Splanchnic branches as follows: great splanchnic nerve, by the union of branches from the thoracic ganglia, the 3d to the 10th inclusive; it pierces the crus of the diaphragm, and passes to the prevertebral solar plexus; small splanchnic nerve, also to the solar plexus from the 10th or 11th thoracic ganglia; smallest splanchnic nerve, from the 12th thoracic ganglion to the renal plexus. g, Hypogastric branches, from the lumbar and sacral ganglia to the prevertebral hypogastric plexus.

The Prevertebral Cardiac plexus (Pl. XVII. c) is situated at the base of the heart, and is divided into a superficial part, which lies in the concavity of the arch of the aorta, and a deep part between the aorta and trachea. It receives the cardiac branches of the pneumogastric and the cervical ganglia of the sympathetic. It contains collections of nerve cells and a dense plexiform arrangement of nerve fibres. It gives off branches to the heart, which wind around the surface of that organ and penetrate its muscular substance: on these branches minute ganglia are found which regulate its rhythmical movements. Through these branches and the cardiac plexus the heart is brought into connection with both the cerebro-spinal and sympathetic systems of nerves. The sympathetic apparently regulates its contraction, for when this nerve is stimulated the action of the heart is accelerated. The pneumogastric again exercises an inhibitory or restraining influence on the contractions of the organ, for when this nerve is irritated the activity of contraction is diminished, but when divided it is greatly increased. The cardiac plexus also sends offsets to the anterior and posterior pulmonary plexuses for the supply of nerves to the lungs.

The Prevertebral Solar or Epigastric plexus is situated at the pit of the stomach around the coeliac axis, a branch of the abdominal aorta. It receives the great and small splanchnic nerves from the thoracic ganglia of the sympathetic, and some of the terminal branches of the pneumogastric nerve. It contains large collections of nerve cells, which form the two semilunar ganglia, and a dense plexiform arrangement of nerve fibres. It gives origin, either directly or indirectly, to numerous plexiform branches, which accompany, and are named after, the abdominal aorta and its various branches given off to the walls and viscera of the abdomen proper. In this manner, not only do the arteries which supply the abdominal viscera receive their vaso-motor nerves, but the muscular and mucous coats of the stomach, intestines, gall bladder, bile ducts, ureters, and seminal ducts, and the glandular structures of the liver, pancreas, kidneys, spleen, and supra-renal capsules. It is important also to observe that these plexuses of distribution not unfrequently contain small ganglia, and the branches which supply the muscular coat of the stomach and intestines have minute microscopic ganglia, with stellate nerve cells lying amidst them. The distribution of the pneumogastric nerve to the stomach, and its connection with the solar plexus, enables that nerve to stimulate its peristaltic contraction, and, according to some experimenters, that of the small intestine also; but the precise action of the sympathetic on these organs is still a disputed question.

The Prevertebral Hypogastric plexus is situated in front of the last lumbar vertebra. It receives branches from the lumbar ganglia of the sympathetic, and from the plexus surrounding the abdominal aorta. It divides into two parts, which lie one on each side of the rectum, and are called the pelvic plexuses; these plexuses are joined by branches from the sacral ganglia of the sympathetic, and from the 3d and 4th sacral spinal nerves, and contain small gangliform collections of nerve cells. From the pelvic plexuses numerous plexiform nerves arise, which accompany the internal iliac artery and its branches to the walls and viscera of the pelvis, and are named after them. These nerves not only supply the vaso-motor nerves for these blood-vessels, but also the muscular coat and mucous membrane of the bladder, rectum, and urethra, besides the prostate gland in the male, and the uterus and vagina, and in part the ovary, in the female; in connection with their distribution to these viscera, minute ganglia are found lying amidst the nerves, the nerve cells in which act undoubtedly as centres of reinforcement for the origin of additional, nerve fibres.

From the distribution of the branches of the gangliated cord of the sympathetic, and of the gangliated prevertebral plexuses, it will be seen that this nerve is especially related to the blood-vessels and to the viscera. contained within the great cavities of the body. As the cerebro-spinal system is engaged in the supply of nerves to the voluntary muscles, the sympathetic is the medium of supply for the involuntary muscular apparatus, both in the coats of the vessels and in the walls of the hollow viscera. But though the vaso-motor nerves branch from the sympathetic ganglia, it must not be supposed that they have no connection with the cerebro-spinal system. The communicating branches between the sympathetic ganglia and the anterior divisions of the spinal nerves establish a connection between them and the cerebro-spinal nervous axis. By recent experiments, the tract of transmission of the vasomotor fibres has been traced along with the anterior roots of the spinal nerves, through the lateral columns of the cord to the medulla oblongata, in which the vaso-motor nerve centre lies a little to one side of the mesial plane, above the calamus scriptorius. In the distribution of the sympathetic to the glandular viscera, not only is it important to attend to their terminations in the muscular coat of the blood-vessels of the glands, but the termination of the nerves in connection with the secreting cells themselves must be taken into consideration. The communications between the cerebro-spinal and sympathetic systems, not only through the spinal nerves, but also through the pneumogastric, are to be kept in mind in connection with the effects produced by varying mental conditions on the secretions of the glands.

Organs of Sense.

The organs of sense are the organs through the interme diation of which the mind becomes cognisant of the appearance and properties of the various objects in the external world. These organs are severally named nose, eye, ear, tongue, and skin. For the excitation and perception of a sensation three sets of structures are necessary: a, a peripheral end-organ; δ, a sensory nerve; c, a central organ. The peripheral end-organ is the part of the apparatus to which the stimulus necessary for the production of the sensation is applied. This stimulus causes nervous impulses to be propagated from the end-organ along the fibres of the sensory nerve to the central organ, in which that nerve terminates at its central extremity. These nervous impulses occasion molecular changes in the nerve cells of the brain, and the mind becomes conscious of a sensation. The shape and construction of each organ of sense is adapted to the application of the stimulus required for the production of the particular sensation to which the organ is subservient. Each organ of sense possesses its own characteristic form of end-organ. The touch corpuscles of the skin, the end bulbs found in several mucous membranes, and the Pacinian corpuscles, are the end-organs occurring in their several localities; they have the peripheral ends of the sensory nerves terminating in their substance, and the axial cylinder of the nerve fibre ends in their interior. The rods and cones of the retina, the rods of Corti in the cochlea, the olfactory cells of the nose, and the gustatory bodies in the tongue, are the end-organs belonging to their several organs of sense; the sensory nerve fibres which terminate in relation with them have not yet, however, been traced into actual continuity with their substance. A stimulus, whatever may be its nature, applied to any organ of sense can excite only that kind of sensation for the production of which the organ is subservient. Thus a stimulus applied to the eye, whether it be the natural stimulus of the waves of light, the mechanical stimulus of a blow, or an electric stimulus, can only excite the sensation of light. Stimuli applied to the ear can only excite the sensation of sound, and in like manner with the other senses. In studying the anatomy of the organs of sense the arrangement of numerous accessory structures, which assist either in conducting stimuli or in modifiying their effects, the arrangement and structure of the peripheral end-organs, and the origin, course, and distribution of the sensory nerves, will have to be considered.

The Nose or organ of smell is a large cavity situated in the face, between the orbits, above the mouth, and below the cribriform plate of the ethmoid bone. It communicates by the anterior nares, or nostrils, with the external atmosphere, by the posterior nares with the pharynx, and through it with the larynx, trachea, and lungs. It is the proper entrance to the respiratory passage, is accessory to the production of the voice, aids in the sense of taste, and forms one of the most important features of the face. It is subdivided into a right and a left chamber by a vertical mesial partition, the septum nasi, so that the nose is a double organ in the same sense as the eyes or ears are double. The walls of the cavity of the nose are formed partly of bone and partly of cartilage. The osseous walls are referred to on page 826. The cartilages form the point, the alae, and a part of the septum nasi. The mesial or septal cartilage is triangular in shape, and fits into the interval between the vomer, the mesial plate of the ethmoid, and the nasal spine of the superior maxilla. Anteriorly and inferiorly its border is free, projects on to the face, and forms the columna of the nose. The lateral cartilages form the tip and alae. On each side is an upper lateral cartilage attached by its outer margin to the free edge of the nasal bone and superior maxilla, whilst by its inner it is continuous with the anterior border of the septal cartilage. The lower lateral cartilage curves inwards upon itself, touches its fellow in the mesial plane at the tip, and forms the anterior and lateral boundary of the orifice of the nostril It is connected by fibrous membrane above to the upper lateral cartilage, and behind to the anterior edge of the superior maxilla. In this membrane two to five small cartilaginous plates, called the epactal cartilages, are often found imbedded. The skin of the nose which covers the lower lateral cartilages contains numerous sebaceous follicles, which open by comparatively large orifices on the surface. It is closely connected to these cartilages, and to the muscles of the alae. The lower lateral cartilage forms the wall of the vestibule or entrance to the nasal chamber, and the vestibule is lined by a prolongation of the integument, which is studded with numerous short hairs or vibrissae. Each nasal chamber is lined by a mucous membrane called the pituitary or Schneiderian. This membrane is prolonged into the meatuses and the air sinuses which open into them; posteriorly it is continuous with the mucous lining of the pharynx, and anteriorly it blends with the cutaneous lining of the vestibule. The pituitary membrane is thick and soft, and diminishes the size of the meatuses and the openings of the air sinuses as seen in the skeleton. The mucous membrane is divided into a respiratory and an olfactory region. The respiratory region corresponds to the floor of the nose, to the inferior turbinated bone, and to the lower third of the nasal septum. It is covered by a ciliated columnar epithelium, and contains numerous racemose glande for the secretion of mucus or pituita. It is also vascular, and the veins which ramify in it have a plexiform arrangement. The mucous lining of the air sinuses is also ciliated, but almost devoid of glands, except in the antrum, in which region the glands sometimes dilate into cystic tumours.

The olfactory region is the seat of distribution of the olfactory nerve and of its peripheral end-organs. It corresponds to the roof of the nose, to the superior and middle turbinals, and the upper g rds of the septum. The mucous membrane is thick, soft, easily destroyed, of a yellowish brown colour, and blended with the periosteum. When vertical sections through this membrane are examined microscopically the tubular glands discovered by Bowman may be seen in its vascular connective tissue layer. These glands contain roundish secreting cells with yellowish-brown pigment-stained contents. The epithelium is cylindrical, but not usually ciliated, though patches of ciliated epithelium cells are said to occur in man. Long, slender, and even branched processes proceed from the deeper end of each cell towards or even into the sub-epithelial connective tissue. The colls usually contain pigment granules. Between the epithelium cells the characteristic olfactory cells of Schultze are situated. Each olfactory cell consists of a globular or fusiform body, from which two long processes arise: one, the peripheral process, passes vertically between the adjacent cylindrical epithelium cells to the free surface of the mucous membrane: in amphibia, reptiles, and birds it projects beyond the plane of the epithelium as a simple hair-like structure, or subdivided into several slender “olfactory hairs;” in fish and mammals, man inclusive, it ends, without forming a hair-like prolongation, on the general plane of the mucous surface. The second or central process of the olfactory cell extends towards the sub-epithelial connective tissue: it is finer than the peripheral process, and has not unfrequently a varicose appearance like a nerve fibre.

In the description of the developmentof the brain (p. 864), the origin of the olfactory bulb and peduncle from the hemisphere vesicle was referred to. In the adult brain the olfactory peduncle is in contact with the under surface of the frontal lobe. It is a white band, which divides in front of the locus perforatus anticus into three so-called roots of the olfactory nerve. The external or long root passes outwards across the Sylvian fissure to the gyrus hippocampi, and perhaps also to the insula: a few fibres are continuous with the anterior commissure; but in mammals, where the olfactory peduncle forms a good-sized lobe, it receives many fibres from the commissure. The middle or grey root contains white fibres which proceed from the corpus striatum. The internal or short root has been traced into the anterior end of the gyrus fornicatus; hence the inner and outer roots of the olfactory peduncle are connected with the anterior and posterior extremities of the arch-shaped gyrus. The olfactory bulbs rest on the upper surface of the cribriform plate of the ethmoid, one on each side of the crista galli. The bulb consists both of grey and white matter, and sometimes retains the central cavity lined by a ciliated epithelium. The grey matter contains fusiform and pyramidal nerve cells imbedded in neuroglia (the stratum gelatinosum of L. Clarke). Between it and the central cavity is the white matter formed of nerve fibres interspersed with “granules,” similar to those seen in the rust coloured layer of the cerebellum. Between the grey matter and the surface is the stratum glomerulosum ol Meynert, which apparently consists of coils of the olfactory nerve fibres with interspersed “granules. ,, The olfactory nerve fibres form the first pair of cranial nerves or nerves ol smell; they leave these glomeruli in from 15 to 25 bundles, and enter the roof of the nose through the holes in the cribriform plate (Pl. XIX. figs. 1, 2); they lie in grooves in the bones of the olfactory region, and form a network from which bundles of fine non-medullated fibres arise that enter the mucous membrane and run between the glands into the epithelial layer. These nerves have a varicose appearance, and though their terminations have not been precisely ascertained, it is believed that they are connected with the central processes of the olfactory cells, which cells are therefore regarded as the peripheral end-organs of the olfactory nerve fibres. The mucous membrane of the nose also receives branches from the 1st and 2d divisions of the 5th cranial nerve. Their mode of termination in that membrane is not known, but they are associated with the sense of touch, and not with the special sense of smell

The Eyeball, globe of the eye, or organ of vision, is a complex optical apparatus situated in the cavity of the orbit, imbedded to a large extent in loose fat, and with several muscles attached to it. Its form approximates to the spheroidal, but it actually consists of segments of two spheres, the posterior of which is the larger.

The eyeball consists of three coats or tunics, which enclose three translucent refracting media. The first or external coat consists of a posterior, white, opaque part, the sclerotic, which corresponds in its area with the posterior larger segment of the ball, and of an anterior, translucent part, the cornea, which corresponds in its area with the anterior smaller segment of the eyebalL Piercing the sclerotic coat is the optic nerve, which enters the globe about ∣th inch to the nasal or inner side of its anteroposterior axis. The second or middle coat, or tunica vasculosa, consists of a posterior part or choroid, the area of which corresponds almost exactly with the sclerotic; this coat possesses anteriorly numerous folds, the ciliary processes, which are continuous with the iris, a structure which lies behind the cornea. The third or internal or nervous coat is named the retina, and in it the optic nerve terminates. The enclosed refracting media occupy the axis of the globe, and are named from before backwards the aqueous humour, crystalline lens, and vitreous body.

The Sclerotic coat, called from its white appearance the white of the eye, is a firm, unyielding fibrous membrane, which forms the posterior fths of the outer coat of the eyeball. It is thicker behind than in front, and where pierced by the optic nerve it has a cribriform structure, as the bundles of nerve fibres do not pass through one large, but several small openings. The sclerotic consists of the white fibrous form of connective tissue, intermingled with a small proportion of elastic fibres. The bundles of white fibres lie in two directions; some pass in the meridian of the globe from the optic nerve towards the cornea, others lie parallel to its equator. The sclerotic is joined by accessory fibres behind, derived from the perineurium of the optic nerve, where the nerve pierces it; and in front from the tendons of the recti and obliqui muscles, which are inserted into it. In the cetacea the sclerotic possesses extraordinary thickness. In fish and amphibia it consists largely of cartilage, and in birds a ring of bone is developed around its anterior margin. It is the protecting coat of the eyeball.

The Cornea forms the translucent anterior ⅜th of the outer coat of the eyeball. It is almost circular in form, and is blended at its circumference with the anterior border of the sclerotic. Its anterior surface is convex, and covered by the conjunctival epithelium. The forward projection of the cornea is always greater in young than in aged persons. Its posterior surface is concave, and bounds the chamber in which the aqueous humour is contained: if the chamber be punctured, and the humour evacuated, the cornea loses its translucency, its tension, and its forward convexity, and becomes flaccid and opaque. It has considerable thickness, and can be readily split up into laminae. When antero-posterior sections are made through it and the epithelium on its anterior and posterior surfaces, four distinct series of structures may be seen, viz., the anterior epithelium, the proper tissue of the cornea, the posterior elastic lamina, and the posterior epi-(endo)-thelium.

The anterior epithelium of the cornea, often called the conjunctival epithelium, is stratified. The deepest layer, which lies next the cornea, is formed of elongated cells, which lie vertically to the plane of the surface of the cornea. The more superficial layers are squamous cells, often with fluted surfaces and serrated or spinous edges. The intermediate layers are irregular in shape, and often possess, as Cleland pointed out, long digitate processes, which interlock with those of the adjacent cells.

The proper tissue of the cornea is a modified form of connective tissue. When examined fresh it appears as if perfectly homogeneous, but after a time, and more especially if hardened in alcohol, chloride of gold, and other reagents, it is seen to consist of cells and an intercellular matrix. The cells consist of two kinds,—those which belong to the cornea, and those which have migrated into it. The proper cornea cells or cornea corpuscles were first seen by Toynbee, and have been carefully studied by numerous subsequent observers. They are large stellate, flattened cells, and lie with their surfaces parallel to the surfaces of the cornea; they possess many branching processes, and the processes of adjacent cells anastomose to form a cell network. They consist of nucleated masses of protoplasm, which Kühne showed to be contractile, and are apparently destitute of a cell wall. In vertical sections through the cornea the corpuscles seem as if shaped like elongated spindles. The migrating cells of the cornea were first seen by Von Recklinghausen. They resemble white blood corpuscles, and possess active amoeboid movements, so that they can wander through the corneal tissue. In a healthy cornea they have migrated out of the marginal blood-vessels; but in an inflamed cornea, where their number is greatly increased, they are in part white corpuscles derived from the blood, and in part produced by proliferation of the proper cornea corpuscles. The intercellular matrix of the cornea consists of a laminated substance, the lamellae being arranged parallel to the surfaces of the cornea. The lamellae consist of fasciculi of extremely delicate filaments; immediately under the anterior epithelium the fasciculi decussate with each other, and at the circumference of the cornea the fasciculi run into the connective tissue of the sclerotic. Bowman described a translucent structureless layer or anterior elastic lamina between the conjunctival epithelium and the cornea proper, but it is doubtful if this layer exists as a constant arrangement. Bowman and other observers have injected tubular spaces in the cornea which are apparently situated between the lamellae. The exact nature of these spaces is somewhat doubtful, but Thin believes them to be lymph-vessels traversing its sub stance, for he has seen an endothelial lining similar to the endothelial cells of the lymphatics. It is probable that these spaces serve as the channels for the migrating corpuscles to wander through. Thin also describes the proper cornea corpuscles as lying in lacunae, which communicate with each other and with the lymph-vessels. The posterior elastic lamina forms a distinct translucent, structureless layer adherent to the back of the proper tissue of the cornea, from which it may be stripped off without much difficulty. When torn across, the edges curl inwards towards the corneal tissue. It is from ∙ 5 √ δ - δ ∙ to th inch thick, and resists the action of various reagents. This lamina thins off at its circumference and splits into fibres, which become continuous with the pectinate ligament of the iris.

The posterior epithelium of the cornea, also called the endothelium of the aqueous humour, forms a single layer of polygonal cells on the back of the posterior elastic lamina. It is continuous with the endotheiιal covering of the pectinate ligament and of the anterior surface of the iris.

The cornea is not in the adult traversed by blood-vessels, though in the foetus a layer of capillaries lies near its anterior surface. In the adult, however, the margin of the cornea is penetrated by a zone of capillary loops derived from the arteries of the conjunctiva; these loops, according to Lightbody, are invested by perivascular lymph spaces. The venous canal of Schlemm runs round the circumference of the cornea, at the junction of its deeper layers with the sclerotic. Leber states that it is not a simple canal, but a plexiform arrangement of veins. The nerves of the cornea first seen by Schlemm have been carefully examined by recent observers. They arise from the ciliary nerves, and enter the margin of the cornea in from twenty to forty fasciculi, which run from the circumference to the centre and to the anterior surface of the cornea, and give off numerous branches. The nerve fibres soon lose their medullary sheath, and branch; adjacent branches then communicate, and form plexuses which possess nuclei at the points of intersection of the nerves. From these plexuses delicate branches again arise, some of which penetrate between the cells of the anterior epithelium, whilst others end in the proper tissue of the cornea. Kühne stated that the terminal fibres ended in the cornea corpuscles, but this statement has not been confirmed.

The Choroid coat forms the largest portion of the middle coat of the eyeball. It lies immediately internal to the sclerotic, and extends as far forward as the corpus ciliare, or annulus albidus, where it forms the ciliary processes; it is pierced posteriorly by the optic nerve. It has a deep black colour, from the numerous pigment cells it contains, and is abundantly provided with blood-vessels and nerves. The Corpus ciliare, or annulus albidus, is a greyish-white ring which surrounds the anterior border of the choroid close to the junction of the sclerotic and cornea. It consists of two portions—an external, the ciliary muscle, which lies next the sclerotic, and an internal, the ciliary processes (Plate XIX. fig. 4). These processes, about 80 in number, are folds, separated from each other by furrows which extend forwards in the meridional direction as far as the iris, and form collectively a zone-like plated frill around the circumference of the iris. On the one hand, they are continuous with the vasculo-pigmentary structures of the choroid; on the other, with the vasculo-pigmentary structures of the iris.

The Iris is a circular, flattened disc-shaped diaphragm, situated behind the cornea, in front of the crystalline lens, and bathed by the aqueous humour. By its circumference or ciliary border the iris is not only continuous with the ciliary processes, ∣ but is connected by fibres, termed ligamentum pectinatum, with the posterior elastic lamina of the cornea. The iris is the structure which gives the characteristic colour to the eye—blue, grey, brown, hazel, as the case may be. It is perforated at, or immediately to the inner side of, its centre by a circular aperture, the pupil, the size of which is regulated by the contraction or relaxation of the muscular tissue of the iris.

The structure of the several divisions of the middle coat will now be considered. The Choroid coat has its inner or anterior surface formed by a distinct pigmentary layer of hexagonal pigment cells (Fig. 43). In the eyes of Albinos, though the cells are present, they contain no pigment. In many mammals also, the pigment is absent from the inner surface, so that the choroid possesses a beautiful iridescent lustre, the tapetum luendum. In ruminant animals and in the horse the iridescence is due to the reflection of the light by the bundles of the connective tissue stroma, but in cats from polygonal nucleated cells, which Schultze states contain double refracting crystals. Next the inner pigmentary layer is the lamina vitrea, the elastic layer of Kölliker. It forms a translucent membrane, described by some as structureless, but by Kölliker as faintly fibrous, which is intimately connected with the stroma of the choroid. The stroma consists of a plexiform arrangement of bundles of connective tissue, in the intervals between which numerous stellate pigment cells are situated, which give to the entire thickness of the choroid its black appearance. This stroma connects the outer surface of the choroid with the inner surface of the sclerotic, and forms the lamina fusca. Ramifying in the stroma are the blood-vessels and nerves. The vessels of the choroid are arranged in two layers. Next the lamina vitrea is a plexiform capillary layer, the meshes of which are so minute, and the vessels so compacted together, as to give the appearance of avascular membrane, long known as the membrana Ruyschiana. The capillaries radiate like minute stars from the terminal twigs of the choroidal arteries and veins. The choroidal arteries and veins form a layer external to the capillaries, i,e., next the lamina fusca. The arteries are the short posterior ciliary branches of the ophthalmic artery, which pierce the sclerotic close to the entrance of the optic nerve, and, running forwards in a tor* tuous manner, divide dichotomously before ending in the capillaries. The veins of the choroid are arranged in a series of remarkable whorls, named the venae vorticosae, which receive the blood not only from the capillaries of the choroid proper, but from those of the iris and ciliary body; they discharge their blood by means of from 4 to 6 veins into the ophthalmic vein. The ciliary muscle is the greyish white structure which forms the outer part of the ciliary body. It was at one time called the ciliary ligament, but its muscular nature was discovered almost simultaneously by Bowman and Brücke. It consists of smooth involuntary muscle, the fibres of which are arranged in two layers. The outer and thicker part of the muscle consists of fasciculi, which arise close to the canal of Schlemm, i.e., opposite the junction of the sclerotic and cornea, and radiate from before backwards in the meridian of the eyeball, between the ciliary processes and the sclerotic. The inner part of the muscle forms a ring-like arrangement of fasciculi close to the circumference of the iris, and is often called the annular muscle of Müller. Iwanoff has shown that in long-sighted persons (hypermetropic) the annular muscle is strongly developed; whilst in short-sighted (myopic) eyes its fasciculi are very feeble. The Ciliary Processes have on their inner surface a black pigmentary layer of cells continuous with that of the choroid. The vitreous layer is .also present, but according to H. Müller is no longer smooth but reticulated. The stroma does not contain so large a proportion of stellate pigment cells as in the choroid. The arteries have been carefully studied by Leber; they are the long posterior ciliary branches of the ophthalmic, and the anterior ciliary branches of the muscular branches of the ophthalmic. They pierce the sclerotic, run forwards, and at the anterior border of the ciliary muscle form by their anastomoses the circulus arteriosus, which gives origin to. the arteries for the ciliary processes and the iris. The arteries for the ciliary processes are short, and divide into tortuous branches, which frequently anastomose, and form highly complex vascular plexuses, from which arise veins that join the vense vorticosae. Before the long ciliary arteries contribute to the formation of the arterial circle they send branches to the ciliary muscle, and recurrent branches to the anterior part of the proper choroid coat.

The iris has its anterior surface covered by a layer of cells continuous with the endothelium of the aqueous humour. This layer is continuous at the pupillary border with a thick layer of cells filled with black pigment granules, the uvea, which covers the posterior surface of the iris, and is continuous at its ciliary border with the pigmentary layer of the ciliary processes. The connective tissue stroma of the iris also contains stellate pigment cells. The variations in colour of the iris in different eyes depends upon the distribution and amount of the pigment in the uvea and the ßtellate cells: in dark-coloured eyes, both are filled with dark pigment granules; whilst in light-coloured eyes the εtellate cells of the stroma are either devoid of pigment or only faintly coloured. The iris contains numerous fasciculi of involuntary or non-striped muscular fibre arranged in two directions. Circularly arranged fibres surround the aperture of the pupil, and form the sphincter muscle, by the contraction of which the size of the pupil is diminished. Smooth muscular fibres also radiate from the pupillary to the ciliary border of the iris and form the dilatator muscle. The muscular nature of these fibres in the human iris was long disputed, but was satisfactorily demonstrated in 1852 by Lister. Jeropheef has also described circular fasciculi surrounding the ciliary border. In birds and reptiles the muscular tissue of the iris consists of transversely striped fibres. The arteries of the iris arise from the circulus arteriosus, and run radially forwards towards the pupil, where they anastomose and form the circulus iridis minor. They possess relatively thick external and muscular coats. The capillaries form a plexus not so compact as that of the choroid coat. The veins of the iris end in the venae vorticosae. In the foetus the pupil is closed in by a delicate membrane, membrana pupillaris, into which the bloodvessels of the iris are prolonged. This membrane disappears by absorption during the later months of embryo life. The nerves of the middle coat of the eyeball are the long ciliary branches of the ophthalmic division of the 5th and the short ciliary branches of the ciliary ganglion (Pl. XIX. fig. 7, ~). They pierce the sclerotic near the optic nerve, and run forward in the lamina fusca of the choroid. They give off branches to the choroid which form in it a plexus in which H. Muller found nerve cells. From this plexus delicate branches pass to the muscular coat of the choroidal arteries. The ciliary nerves then enter the ciliary muscle, and form plexuses with interspersed nerve cells, from which branches pass to the muscular fibres. Other branches of the ciliary nerves enter the iris, and form plexuses, from which branches proceed to the muscular tissue.

The Retina is the delicate nervous coat of the eyeball which lies immediately internal to the choroid, and extends from the place of entrance of the optic nerve as far forward as the ciliary processes, where it forms a jagged border, the ora serrata. In the living eye it is translucent and colourless, but shortly after death it becomes grey: it is soft and so easily torn that it is difficult to display it in a dissection without injury. Its inner or anterior surface, concave forwards, is moulded on the vitreous body, and presents the following appearances :—Almost exactly in the anteroposterior axis of the eyeball is a transversely oval yellow spot, about τ 2 ⅛th inch in its long diameter, which amongst mammals is found only in man and apes, though, as Knox and Hulke have shown, it exists in reptiles; in the centre of this spot is a depression, the fovea centralis; about ⅜th inch to the inner side of the yellow spot is a slight elevation, the papilla optica, which marks the disc-like entrance of the optic nerve into the retina; here the fibres of the nerve radiate outwards and forwards to the ora serrata, and branches of the arteria centralis retin oe accompany then.

The retina is highly complex in structure, and consists of nerve fibres and cells, of peripheral end-organs, of connective tissue, and of blood-vessels, arranged in several layers. Max Schultze, who is the chief authority on the subject, recognises ten layers, but includes among these the layer of hexagonal pigment cells just described as the inner pigmentary layer of the choroid. If this layer be omitted, nine layers may then be recognised, and, following Schultze, be named from before backwards as follows:—1. Membrana limitans interna; 2. Layer of optic nerve fibres; 3. Layer of ganglion cells; 4. Internal granulated (molecular) layer; 5. Internal granule layer; 6. External granulated layer; 7. External granule layer; 8. Membrana limitans externa; 9. Bacillary layer (Fig. 78).

The nervous elements of the retina will first be considered. The optic nerve fibres (2), where they pierce the sclerotic, as a rule lose the medullary sheath, and radiate outwards as non-medullated fibres from the optic disc to the ora serrata immediately behind and parallel to the membrana limitans interna. These fibres vary greatly in size, and are frequently varicose. When any of the optic nerve fibres retain the medullary sheath the retina is there rendered opaque. Immediately behind the nerve fibres is the layer of ganglionic nerve cells These cells are either bi polar or multipolar. In the living eye the cell substance is hyaline and the nucleus transparent, but after death the substance both of the body of the cell and the processes assumes a fibrillated appearance, like the axial cylinder of an optic nerve fibre. One process, the central process, extends into the layer of optic nerve fibres; and another, the peripheral, into the internal granulated layer. Tho internal granulated layer (4) contains the branching processes of the nerve cells, some of which apparently become continuous with an arrangement of excessively fine fibrils, probably nervous in their nature. These fibrils are intermingled with a delicate plexus of connective tissue. The internal granule layer (5) contains numerous fusiform nucleated enlargements, the so-called internal granules, arranged in superimposed strata; from each fusiform enlargement a fibre proceeds in two directions, one centrally into the internal granulated layer, and one peripherally into the external granulated layer. These fibres possess varicosities, and resemble the optic nerve fibres. The external granulated layer (6) is very thin, and consists of an expanded network of minute fibres, with nuclei situated at the points of intersection of the fibres. Krause has called it the membrana fenestrata. The external granule layer (7) contains numerous fusiform nucleated enlargements, the so-called external granules, arranged in superimposed strata: from each enlargement a fibre proceeds in two directions, one centrally into the external granulated layer, and one peripherally through the membrana limitans externa to the bacillary layer, where it becomes continuous with the anterior end of either a rod or a cone, as the case may be. Hence these fibres of the external granule layer are called by Schultze rod and cone fibres, and the external granules are nucleated enlargements of these fibres. These fibres possess varicosities like those of the internal granule layer.

The bacillary layer (9) or membrane of Jacob consists of multitudes of elongated bodies arranged side by side like rows of palisades, and vertically to the surfaces of the retina. Some of these bodies are cylindrical, and are named the rods of the retina; others flask-shaped, and named the cones of the retina: the rods equal in length the entire thickness of the bacillary layer; the cones are shorter than the rods, and are interspersed at regular intervals between them; the apex of each cone is directed towards, but does not reach, the plane of the posterior or choroidal surface of the retina. The posterior or outer end of each rod rests against the pigmentary layer of the choroid.

The anterior or inner ends of both rods and cones are continuous with the rod and cone fibres of the external granule layer, as already described. Each rod and cone is subdivided into an outer strongly refractile and an inner feebly refractile segment. By the action of various reagents the outer segments both of the rods and cones exhibit a transverse striation, and ultimately break up into discs. Hensen has described a longitudinal striation in the outer segments, and Ritter has stated that both in the outer and inner segments of the rods an axial fibre exists. Max Schultze has also seen the inner segments of both rods and cones longitudinally striped on the surface. Modifications in the relative numbers and appearances of the rods and cones have been seen in the eyes of various vertebrata. In birds, for example, the cones are much more numerous than the rods, whilst the reverse is the case in mammals generally. In the cartilaginous fishes the cones are entirely absent; so also, as Schultze has shown, in the bat, hedge-hog, and mole; whilst in reptiles the bacillary layer is exclusively composed of cones. In all the vertebrata, except the mammalia, the twin or double cones described by Hannover probably exist. In the amphibia, lens-shaped bodies have been described in the inner segments of the cones. The rods and cones are the peripheral end-organs in connection with the fibres of the optic nerve, and their apparent relation to these fibres is as follows :—The optic nerve fibres are continuous with the central processes of the ganglion cells of the retina, the peripheral branching processes of which pass into the internal granulated layer, where they may possibly become continuous with the central processes of the inner granular layer. The peripheral processes of the inner granular layer enter the external granulated layer, but it is difficult to say whether or not they become continuous with the central processes of that layer. There can, however, be no doubt that the peripheral processes of this layer are directly continuous with the rods and cones of the bacillary layer. The entire arrangement is sometimes called the radial nervous fibres of the retina.

In addition to the nervous structures just described, the retina contains a delicate supporting connective tissue like the neuroglia of the brain and spinal cord. Not only does it lie between the fibres, cells, and so-called granules in the several nervous layers, and form in them a radial arrangement of supporting fibres, but it constitutes the two limitary membranes of the retina. The membrana limitans externa (8) is excessively thin, and appears in vertical sections through the retina as a mere line between the bacillary and external granular layers, continuous on the one hand with the connective tissue which passes for a short distance between the rods and cones, and on the other with the connective tissue framework of the external granule layer.

The membrana limitans interna (1) covers the anterior surface of the retina, and lies next the vitreous body; its posterior surface blends with the radial arrangement of connective tissue between the optic nerve fibres, but its anterior or hyaloid surface, as J. C. Ewart has recently shown, possesses a mosaic appearance, like that of a layer of squamous endothelium.

The yellow spot exhibits some structural differences from the rest of the retina. It owes its colour to the presence of yellow pigment deposited in the more anterior layers of the retina. Except at its central depression, the fovea centralis, it is thicker than the surrounding parts of the retina; but it is much softer, a condition which is due to the almost complete absence of the layer of optic nerve fibres, and a diminution in the amount of the supporting connective tissue; the membrana limitans interna is, however, relatively stronger. In the fovea centralis itself the rods of the bacillary layer have entirely disappeared, and are replaced by cones which are distinguished by their close arrangement, and the more slender form and increased length, especially of their outer segments. The external granule layer is well marked, and the central fibres belonging to it, instead of passing vertically forwards, incline very obliquely or almost horizontally outwards to the internal granule layer, which, together with the layers anterior to it, is so thin as almost to have disappeared. In the yellow spot surrounding the fovea the bacillary layer is also composed of cones which are not, however, so slender or so long as at the fovea itself. The layer of nerve cells and the inner part of the external granule layer are thicker than in the rest of the retina. The yellow spot is the part of the retina most sensitive to light.

At the ora serrata or anterior border of the retina the nervous layers, including the rods and cones, cease to exist. The radial connective tissue and internal limiting membrana are present; from the radial tissue a layer of cells is prolonged forward in contact with the deep surface of the ciliary processes as the pars ciliaris retinae. ∣

The retina is supplied with blood by the arteria centralis, which, traversing the axis of the optic nerve, reaches the retina at the optic disc. In the retina it branches dichotomously in the nerve fibre layer, avoiding however the yellow spot, and its terminal twigs reach the ora serrata. The capillaries form in the more anterior layers of the retina a distinct network, which does not enter the external granule and bacillary layers, but penetrates the yellow spot, though not the fovea centralis. The blood is conveyed from the retina by the central vein which accompanies the artery in the optic nerve, and opens either into the ophthalmic vein or directly into the cavernous sinus. The veins and capillaries of the retina have been described by His as completely invested by perivascular lymphatic sheaths, whilst the arteries only possess such sheaths for a limited part of their course.

The Optic Nerve itself passes from the orbit through the optic foramen into the cranial cavity, where it arises from the optic commissure. This commissure is a flattened band formed by the junction of the two optic tracts. Each tract winds backwards around the tuber cinereum and crus cerebri to arise from the optic thalamus, corpora quadrigemina, and geniculata; and some observers also state that it derives fibres from the tuber cinereum and lamina cinerea. In the commissure an interchange takes place between the fibres of opposite nerves and tracts, so that not only does an optic nerve contain fibres derived from the tract on its own side, but from the opposite tract, and it has even been stated that fibres pass across the commissure from one optic nerve to the other, and from one optic tract to the other.

The Aqueous Humour is a limpid watery fluid, containing a little common salt in solution, which occupies the space between the cornea and the front of the crystalline lens. In this space the iris lies, and imperfectly divides it into two chambers, an anterior and a posterior, which communicate with each other through the pupil. The anterior chamber, of some size, is situated between the iris and cornea; but as the iris is in contact with the front of the lens, the posterior chamber is reduced to a mere chink between the circumference of the iris and that of the lens.

The Crystalline Lens is situated behind the iris and pupil, and in front of the vitreous body. It is a transparent bi-convex lens, with its antero-posterior diameter ⅜d less than the transverse, its posterior surface more convex than the anterior, and with its circumference rounded. It consists of a capsule and the body of the lens enclosed by the capsule. The lens capsule is a transparent, smooth, structureless, and very elastic membrane, about twice as thick on the anterior as on the posterior surface of the lens. It is non-vascular in the adult, though in the foetus a branch of the central arterv of the retina, which traverses the vitreous humour, ramifies in its posterior portion. A single layer of polygonal cells lies between the body of the lens and the anterior portion of the capsule. The lens body is softer at its periphery than in its centre. It is built up of concentric layers, and on both the anterior and posterior surfaces lines are to be seen radiating from the central pole of each surface towards the circumference of the body. The radiated pattern varies in different animals. In the human foetus there are usually three lines, but in the adult they are more numerous. The lines on one surface do not lie immediately opposite those on the other, but are intermediate. By the action of strong spirit and other reagents the body of the lens can be split up from the periphery towards the centre in the direction of these lines, so that they mark the edges of apposition of its concentric laminse. Each lamina consists of numerous hexagonal fibres about rδ ⅛∙ ff th inch wide, which extend from one surface to the other over the circumference of the lens, so that a fibre which begins at the polar end of a radius on the one surface terminates at the circumferential end of a radius on the opposite. The edges of the fibres are sinuous in man, but denticulated in many animals, especially fishes, so that the fibres, not only in the same, but in superimposed layers, are closely interlocked. The lens fibres are nucleated, a structural fact which gives a clue to their true nature, and they are now regarded as peculiarly modified elongated cells. Babuchin states that he can trace the transition from the cells of the layer between the lens-body and capsule to the proper lens fibres. The lens-body is non-vascular and non-nervous. The surfaces of the lens become more flattened in old age, and its substance hardens and is less transparent.

The Vitreous Body is much the largest of the refracting media, and occupies the largest part of the space enclosed by the tunics. Anteriorly it Î3 hollowed out to receive the posterior convexity of the lens, but posteriorly it is convex, and the retina is moulded on it. It is as translucent as glass, jelly-like in consistency, and when punctured a watery fluid drains out. Its minute structure is difficult to ascertain, but as it, like the subcutaneous tissue of the embryo, contains rounded, stellate, and fusiform cells, it is customary to refer it to the gelatinous form of connective tissue; concentric lamellae, and even a radiated arrangement of fibres, have also been described. It has been customary also to consider it as invested by a delicate structureless membrane, the hyaloid membrane; but this is now regarded as belonging to the retina, where it is known as the membrana limitans interna. Almost opposite the ora serrata a membrane springs from the vitreous body, passes forwards for some distance in relation to the deep surface of the ciliary processes, but separated from them by the pars ciliaris retinae, and then inclines inwards to become attached to the anterior surface of the capsule of the lens close to its circumference. It is so closely connected at its origin with the membrana limitans that it is difficult to recognise it as a distinct membrane. It is named the suspensory ligament of the lens, or zonule of Zinn, and contains fibres, which run in the meridional direction. Where it leaves the vitreous body a narrow space is enclosed between it and that body, which space surrounds the circumference of the lens, and is called the canal of Petit. From the relation of the suspensory ligament to the ciliary processes it has a plicated surface, and when these processes are torn' away from it a portion of the pigment of the processes is often left behind, so that the zonule is sometimes named the ciliary processes of the vitreous body.

The Eyeball is an optical instrument, constructed on the plan of the camera obscura. The sclerotic forms the wall of the chamber. The choroid represents the black lining for absorbing the surplus rays of light. The cornea, aqueous humour, lens, and vitreous body are the translucent media which, like the glass lens of the camera obscura, bring the rays of light to a focus. The retina is the sensitive plate on which the optical picture is thrown. In considering the relation of the retina to the visual rays, it must be kept in mind that the place of entrance of the optic nerve is insensible to light, and that the most sensitive part of the retina is the yellow spot, with its fovea centralis, where the optic nerve fibres are absent, but where the bacillary layer reaches its maximum size. It is clear, therefore, that the rods and cones of this layer, and not the optic nerve fibres, are the structures in the retina which are stimulated by the light; and it is probable, as was suggested many years ago by Goodsir, that these rods and cones are impressed by the light, not as it enters the eye directly, but as it is reflected backwards from the choroid along their axes. The iris is the diaphragm which, by opening or closing the pupil, admits or cuts off the rays of light. The ciliary muscle represents the adjusting screw of the camera; through its attachment to the ciliary processes and their relation to the suspensory ligament of the lens, it is able to act upon the lens and modify the curvature of its anterior surface; for when the eye is to be accommodated to the vision of near objects the anterior surface of the lens becomes more convex than when distant objects are being examined.

It has already been stated on p. 864 that the retina is formed in the primary optic vesicle, which grows forwards to the integument. By the involution and growth of the skin at this spot a hollow is produced at the front of the vesicle, which gradually deepening forms a pouch, the secondary optic vesicle, in which the involuted part of the skin is lodged. From the included sub-epidermal tissue the vitreous body is derived; from the included epidermis, the lens; whilst the cornea sclerotic and iris are produced by the subcutaneous connective tissue. The optic nerve and retina are formed from the primary optic vesicle and its peduncle, and it is probable that the bacillary layer is a special development of its internal epithelial lining. The choroid coat again is derived from the pia mater. Hence the eyeball is compounded of structures derived partly from the integument and partly from the embryo brain.

Accessory Parts to the Eyeball. —In relation to the eyeball several accessory parts are found.

The Eye-Brows are projections of the integument, from which short, stiff hairs grow.

The Eye-Lids, or palpebroe, are two movable curtains, an upper and a lower, which protect the front of the globe. Between each pair of lids is a horizontal fissure, the palpebral fissure. From the free margins of the two lids project short hairs, the eye-lashes or cilia; the upper set curve downwards and forwards, the lower set upwards and forwards; they also protect the front of the globe. Each eye-Hd consists of skin; of the fibres of the orbicular sphincter muscle; of a thin plate of fibro-cartilage, the tarsal cartilage, to the inner end of which a fibrous band, the tendo palpebrarum, is attached, this tendon springing from the ascending process of the superior maxilla; and of the conjunctiva. Between the conjunctiva and the tarsal cartilage is a layer of glands, the Meibomian glands; each gland consists of a short duct, which expands at its sides into small sacculi. The sacculi contain short columnar cells; these secrete a sebaceous material, which escapes through the orifice of the duct at the border of the eye-lid.

The Conjunctiva is a mucous membrane, which forms the posterior layer of the eye-lid, and is reflected on to the anterior part of the sclerotic. At the inner angle of junction of the eye-lids is a soft reddish elevation of the conjunctiva, the caruncula lachrymalis, and immediately external to it is a vertical fold, the plica semilunaris, the rudiment of the third eye-lid, or membrana nictitans, so well developed in birds. The palpebral conjunctiva has small papillae scattered over its surface; its epithelium is stratified, with scaly cells on the free surface and elongated cells in the deepest layer. In the sub-epithelial tissue are small branched mucous glands, which are numerous in the caruncula. Little masses of adenoid tissue (p. 849) with lymphatic vessels are also found in it, and the conjunctiva of the front of the eyeball is thinner than the palpebral part. It is not glandular, and its nerves terminate in end-bulbs (p. 862). The palpebral conjunctiva, and in part that of the eyeball, receive their blood-vessels from those of the eye-lids, but the portion of the conjunctiva next the cornea is supplied by the arteries of the sclerotic coat.

The Lachrymal Apparatus is engaged in the secretion of the tears, and in conveying them away from the front of the globe. The lachrymal gland occupies a depression in the outer part of the roof of the orbit. It is smaller than an almond, is sub-divided into lobules, and belongs to the group of compound racemose glands. It consists of the ramifications of short ducts, which terminate in small sacculi The wall of each sacculus consists of a delicate membrana propria, and the cavity contains the polyhedral secreting cells. Outside the membrana propria is a capillary network derived from the lachrymal artery, but Giannuzzi and Boll have recently described a space between this network and the membrana propria which they believe to be continuous with the lymphatic system. Pflüger has described nerves as terminating in connection with the secreting celle. The excretory ducts of the gland are from six to eight, and open on the back of the upper eye-lid. The tears are washed over the surface of the globe by the involuntary winking of this lid. When the secretion is increased in quantity, in the act of crying, the tears flow over the cheek, but in ordinary circumstances they are conveyed away by two slender tubes, the lachrymal canals, which open by minute orifices, the puncta lachry-malia, one at the inner end of the free border of each eye - lid. These tubes open at their opposite ends into a small reservoir, the lachrymal sac, situated in a hollow in the lachrymal bone. From this sac a duct, the nasal or lachrymal duct, proceeds which opens into the inferior meatus of the nose, and here the tears mingle with the mucous secretion of that cavity.

Muscles of the Eyeball. —The sclerotic coat of the eyeball has six muscles inserted into it. Four of the muscles are called recti, and are situated, one superior, one inferior, one external to, another internal to, the globe. They all arise from the rim of bone which bounds the optic foramen; the external and internal muscles are inserted vertically into the sides of the sclerotic, but the superior and inferior recti have oblique insertions into its upper and lower aspects. The other two muscles are called obliqui. The superior oblique arises along with the recti, passes to the inner end of the upper border of the orbit, where its tendon goes through a pulley, and is directed back to be inserted obliquely into the upper and outer part of the sclerotic. The inferior oblique arises from the lower border of the orbit, passes outwards and upwards to be inserted obliquely into the sclerotic. These muscles roll the eyeball in the orbit, and, without entering into a minute analysis of their actions, their office may be stated generally as follows :— The internal rectus rolls it inwards, the external outwards, about its vertical axis; the superior rectus rolls it upwards, the inferior downwards, about its transverse horizontal axis, though from the obliquity of their insertions they give it at the same time a slight inward or outward movement as the case may be; the superior and inferior oblique roll the globe around its antero-posterior or sagittal axis, the superior upwards and outwards, the inferior downwards and outwards.

Periosteal Muscle of the Orbit.— The periosteum of the orbit contains, as H. Müller and Turner have described, a layer of non-striped muscular fibre in the part which covers over the spheno-maxillary fissure. In man it is rudimentary, but in the sheep, deer, elephant, <fcc., where the osseous wall of the orbit is deficient, this muscle forms a well-defined structure. It has been suggested that it acts as a protractor muscle of the globe.

The Ear, or organ of hearing, is a complex acoustic apparatus, situated in connection with the temporal bone. It is divided into three parts, named external, middle, and internal ear.

The External Ear consists of the pinna or auricle and the external auditory meatus. The auricle is the oblong convoluted body situated at the side of the head. Its incurved outer border is named the helix. Within this lies a curved ridge, the anti-helix, in front of which is a deep hollow, the concha, which leads into the external meatus. The concha is bounded in front by a prominence, the tragus, and behind by a smaller prominence, the anti-tragus; below the anti-tragus is the lobule, which forms the most depending part of the auricle. The framework of the auricle is formed of yellow elastic fibro-cartilage invested by integument, except the lobule, which consists merely of a fold of integument containing fat. Attached to the cartilage are not only the three auricular muscles referred to on page 836, but also certain smaller muscles called the proper muscles of the pinna. Thus the greater muscle of the helix is placed on its anterior border; the lesser muscle of the helix ia situated where it arises out of the concha; the muscle of the tragus lies on the front of that prominence; the muscle of the anti-tragus is placed on the back of that prominence; the transverse muscle on the posterior or cranial surface of the auricle.

The External Meatus leads from the bottom of the concha into the temporal bone, and is separated from the tympanum or middle ear by the membrana tympani. It is a crooked passage one and quarter inch long, inclined at first forwards and upwards, then downwards and inwards. The wall of the outer end of the passage is formed of fibro-cartilage continuous with the cartilage of the auricle, whilst that of the deeper end is formed of the plate-like tympanic part of the temporal bone. The passage is lined with integument continuous with the skin of the auricle, in which are situated numerous hairs, together with ceruminous glands which secrete the well-known yellow “wax.”

The Tympanum, or Drum, or Middle Ear, is a chamber irregularly cuboidal in form, situated in the temporal bone between the bottom of the meatus and the internal ear. The outer wall is formed of the membrani tympani, which inclines obliquely downwards and inwards at the bottom of the external meatus, at an angle of 55° to the axis of the meatus, whilst the membranes in the two ears form with each other an obtuse angle of 130° to 135°. The tympanic membrane is attached to a groove at the bottom of the meatus, and is concave on its outer, convex on its inner surface. It consists of three layers: an external tegumen-tary, continuous with the skin of the meatus, which contains no hairs or glands; an internal mucous, continuous with the mucous lining of the tympanum; and an intermediate membrana propria, which consists of unyielding fibres arranged both radially and circularly. The radial fibres radiate from the point of attachment of the handle of the malleus. The membrana propria is usually said to be destitute both of nerves and vessels, but Kessel states that nerves, blood, and lymph vessels exist in it as well as in the mucous and tegumentary layers. Immediately in front of the membrana tympani is the Glaserian fissure. The inner wall separates the tympanum from the labyrinth, and presents the following appearances: a rounded elevation or promontory caused by the first turn of the cochlea, on the surface of which promontory are grooves for the lodgment of the tympanic plexus of nerves; above the promontory is an oval opening closed in by a membrane, the fenestra ovalis, which corresponds with the vestibule; behind and below the promontory is a round opening closed in by a membrane, the fenestra rotunda, which corresponds with the tympanic passage in the cochlea. The floor of the tympanum is a narrow chink between the inner and outer walls; and the roof is formed by the anterior surface of the petrous-temporal bone. At its anterior wall the tympanum opens into the Eustachian tube, a canal which communicates with the nasal compartment of the pharynx immediately behind the inferior turbinal. The wall of the tympanic end of this tube is formed ςf bone, that of the pharyngeal end of a curved plate of hyaline cartilage, which is connected to the bone by fibro-cartilage; its pharyngeal orifice is dilated into a trumpet-shaped mouth; through this tube the ciliated mucous membrane of the nasal part of the pharynx is prolonged into the tympanum. The cartilaginous wall of the tube does not completely surround it, but is completed by a fibrous membrane, and a layer of voluntary muscle, named by Rüdinger the dilatator tubae. Above the tympanic orifice of the Eustachian tube is a fine canal, through which the tensor tympani muscle enters the tympanum. At its posterior wall the tympanum communicates with the air-sinuses in the mastoid temporal; here also is found a small hollow eminence, the pyramid, through a hole at the apex of which the tendon of the stapedius muscle passes; and a foramen which transmits the chorda tympani nerve.

The tympanic cavity contains three small bones, named malleus, incus, and stapes, arranged so as to form an irregular chain, stretching across the cavity from the outer to the inner wall.

The Malleus or hammer is the most external bone. In it may be recognised a head separated by a constricted neck from an elongated handle. Close to the junction of the neck and handle a long slender process projects downwards and forwards to be inserted into the Glaserian fissure, and near the root of the long process a short process projects outwards. By its handle the malleus is intimately connected with the centre of the membrana tympani; by its head it articulates with the incus; whilst ligamentous fibres pass from it upwards, forwards, outwards, and backwards to the tympanic walls.

The Incus, or anvil-shaped bone, possesses a body and two processes; on the anterior surface of the body is a saddle-shaped hollow in which the head of the malleus fits; the short process projects almost horizontally backwards, and is attached by a ligament to the posterior wall of the tympanum; the long process extends at first downwards and then inwards, to end in a rounded projection, named os orbiculare, through which it articulates with the stapes.

The Stapes, or stirrup-shaped bone, possesses a head and neck, a base and two crura; the head articulates with the os orbiculare of the incus; from the constricted neck the two crura curve inwards to the base, which is attached to the fenestra ovalis. The joint between the malleus and incus is diarthrodial and saddle-shaped, and the articular surfaces are enclosed by a capsular ligament. The joint between the incus and stapes is also diarthrodial, and possesses an investing capsular ligament. Toynbee and Rüdinger have described the base of the stapes and the margin of the fenestra ovalis as each invested by hyaline cartilage. Between these plates elastic fibres extend in a plexiform manner, and the intervals between them are occupied by fluid; the joint seems, therefore, a modified amphiarthrosis. The bones are moved on each other at these joints by small muscles. The tensor tympani arises from the apex of the petrous temporal, and the cartilage of the Eustachian tube enters the tympanum at its anterior wall, and is inserted into the malleus near the root. The laxator tympani muscle arises from the spine of the sphenoid, and the cartilage of the Eustachian tube enters the tympanum through the Glaserian fissure, and is inserted into the neck of the malleus. The stapedius arises within the pyramid, enters the tympanum through the hole at its apex, and is inserted into the neck of the stapes. The tympanum is lined by a mucous membrane continuous with that of the Eustachian tube, which invests the tympanic ossicles, ligaments, and muscles, and is prolonged backwards so as to line tho mastoid air-sinuses. The epithelium covering this membrane, where it lines the floor and the adjacent part of the anterior, posterior, and internal walls, consists of ciliated columnar cells; but the epithelium covering the roof, the promontory, the membrani tympani, and the tympanic ossicles, is tessellated. In the sub-epithelial connective tissue the blood and lymph vessels and nerves of the tympanum ramify. Kessel has recently described in it certain peculiar bodies, which consist of a central axial band with a series of capsules, possessing a fibrillar structure, arranged concentrically around the axis; the function of these bodies is not known.

The formation of the auricle and external meatus is well adapted for collecting and transmitting sound-vibrations inwards to the middle ear and labyrinth. These vibrations strike the membrana tympani, and are propagated by the chain of bones across the tympanic cavity to the labyrinth. The pressure of the vibrations on the tympanic membrane forces that membrane inwards, so that its inner surface presses on the handle of the malleus, the effect of which is to rotate the hammer about its axis; but by the ligamentous attachment of the malleus to the tympanic walls and to the incus, and, as Helmholtz has shown, by the interlocking of cog-like processes connected with the articular surfaces of the two bones, the range of movement is so limited that the pressure on the malleus is transmitted through the incus upon the stapes, which presses, therefore, on the membrane of the fenestra ovalis, so that the movements of the membrana tympani are thus transmitted to fluid within the labyrinth. The tensor tympani muscle tightens the tympanic membrane by drawing the handle of the malleus inwards, and still further adapts the structures for the transmission of sound-vibrations. An antagonistic muscle, the laxator tympani, has also been described. There is some difficulty in determining the action of the stapedius, but if, as is probable, it draws the stapes from the fenestra ovalis, it will diminish the pressure of the chain of bones on that membrane.

The Internal Ear, named the Labyrinth, from its complex construction, is the part of the auditory apparatus in which the nerve of hearing is distributed, and where the peripheral end-organs are situated. It is imbedded in the petrous bone, and is divided into three parts, viz., vestibule, semicircular canals, and cochlea, each of which consists of an osseous and a membranous portion (Pl. XIX. figs. 8, 9,10).

The Vestibule lies immediately internal to the tympanum, between it and the bottom of the internal auditory meatus; behind it are the semicircular canals, and in front is situated the cochlea. It is the part of the labyrinth which first appears in animals, and is therefore the most constant part of the organ. In the myxinoid fishes a single semicircular canal is superadded to the vestibule, in the lamprey two canals, but in other fishes and in the higher vertebrates three canals exist. In amphibia, reptiles, and birds the cochlea is small and rudimentary in comparison with its development in mammals. The osseous vestibule is an ovoid chamber about ∣th inch in diameter. In its outer or tympanic wall is the fenestra ovalis ; in its inner are small auditory foramina, which transmit from the internal meatus the vestibular branches of the auditory nerve; behind these holes is the opening of a minute canal, the aqueductus vestibuli; its anterior wall communicates with the scala vestibuli of the cochlea, and into its posterior wall open the five orifices of the three semicircular canals.

The Semicircular Canals are named superior, posterior, and external. The superior and posterior are sometimes called the vertical canals, and the external the horizontal canal, but, as Crum Brown has shown, the superior and posterior lie in planes equally inclined to the mesial plane of the head, and the external is in a plane at right angles to the mesial plane. Further, the canals in the two ears have definite relations to each other; for whilst the superior canal of each ear is nearly parallel to the posterior canal of the other, the external canals in both ears lie nearly in the same plane. The canals are bent, forming nearly fds of a circle, and would have had six openings into the vestibule had not the contiguous ends of the superior and posterior blended together to open by a common orifice. The opposite end of each of these canals and the outer end of the external canal dilate close to the vestibule to twice their usual diameter, and form an ampulla. The osseous vestibule and semicircular canals are lined by a periosteum invested by a tessellated endothelium, and contain a little fluid, the perilymph. In this fluid the membranous labyrinth is suspended.

The membranous vestibule is formed of two small sac-like dilatations, the walls of which are directly continuous with each other, though the cavities are separated by an intermediate partition. The upper and posterior dilatation, named utriculus, is larger than the lower and anterior, named sacculus. The sacculus is continuous with the ductus cochlearis of the membranous cochlea, and both sacculus and utriculus communicate by slender tubes with a short diverticulum lodged in the aqueductus vestibuli, to which the name of ductus vestibuli may be given. The membranous semicircular canals are about ⅜d the diameter of the osseous. Their walls are continuous with that of the utriculus, and they open by five orifices into it. Each has an ampulla within the ampulla of the osseous canal. Both the sacculus and utriculus are in places attached to the periosteal linings of the osseous vestibule, and delicatθ ligamentous bands connect the membranous semicircular canals to the periosteal lining of the tubes in which they are contained. The wall of the membranous vestibule and canals consists of a delicate fibrous membrane lined by a tessellated endothelium. The inner part of this membrane has a vitreous or hyaline lustre, and gives origin in the canals to short papillae which project into the lumen. The membranous vestibule and canals are distended with the fluid endolymph. The sacculus, utriculus, and ampullae are specially modified in connection with the peripheral termination of the vestibular branches of the auditory nerve. The membranous wall forms in each of these dilatations a projecting ridge, the crista acoustica, to which calcareous particles, the otoliths, which may be either amorphous or crystalline, are adherent. The endothelial investment of the crista is elongated into columnar cells, and intercalated between them are fusiform cells, the auditory cells, each of which, as Schultze and other observers have described, possesses a peripheral and a central process. The peripheral process projects beyond the plane of the free surface of the endothelium into the endolymph as the auditory hair, whilst the central process extends into the sub-endothelial tissue, where the nerve plexus belonging to the terminal branches of the auditory nerve ramifies, and with which it is probably continuous. These auditory cells are, therefore, the peripheral end-organs of the vestibular branches of the auditory nerve, and their general arrangement is not unlike that of the olfactory cells of the nose.

The Cochlea is by far the most complex part of the labyrinth. It is about {th inch long, and shaped like the shell of a common snail; its base lies near the internal meatus, and its apex is directed outwards. The osseous cochlea is a tube wound spirally two and ahalf times round a central pillar or modiolus. Both the pillar and the tube diminish rapidly in diameter from the base to the apex of the cochlea* The tube is imperfectly divided into two passages by a plate of bone, the osseous spiral lamina, which, springing from the modiolus, winds spirally around it, and projects into the tube. When the membranous cochlea is in its place the division is completed by a membrane, the membranous spiral lamina, or basilar membrane, which bridges across the interval between the free edge of the osseous spiral lamina and the outer wall of the tube, to which it is attached by the spiral cochlear ligament. These passages are called scala tympani and scala vestibuli. But another membrane, the membrane of Reissner, also arises from a denticulated spiral crest, limbus or crista spiralis, attached to the vestibular border of the free edge of the osseous spiral lamina, and extends to the spiral ligament at the outer wall of the tube, on the vestibular aspect of the basilar membrane, so as to enclose a passage between it and the basilar membrane, called scala intermedia or ductus cochlearis. The membrane of Reissner is formed of delicate vascular connective tissue, with an endothelial layer on each of its two surfaces. The scala tympani or lower passage, widest at the base of the cochlea, begins at the inner wall of the tympanum, into which it would have opened through the fenestra rotunda, had not the fenestra been closed up by a membrane. The scala vestibuli or upper passage, also widest at the base, communicates with the cavity of the osseous vestibule. At the apex of the cochlea these two scalae communicate with each other through a small hole, the helicotrema. As the scala vestibuli opens into the osseous vestibule, the perilymphis continued into it, and through the helicotrema into the scala tympani. The ductus cochlearis is the membranous cochlea, and its walls are formed of the basilar membrane next the scala tympani, of the membrane of Reissner next the scala vestibuli, and of the spiral ligament next the wall of the cochlea, which connects the two membranes together. It follows the spiral windings of the cochlea, terminates at the apex of the spiral in a closed end, whilst at the base it communicates with the sacculus of the membranous vestibule by a slender tube, the canalis reuniens; hence the membranous cochlea contains endolymph. The termination of the cochlear branches of the auditory nerve and the arrangement of the peripheral end-organs, in relation to them are to be looked for in the basilar membrane. These parts have been repeatedly investigated and described in elaborate monographs, the titles of which are given as an appendix to Waldeyer’s article on the cochlea in Stricker’s Handbuch der Lehre von den Geweben, Leipsic, 1872. The general results only of these investigations will be given here, and the original memoirs may be referred to for further details.

On the surface of the basilar membrane directed to the ductus cochlearis a remarkable arrangement of cells exists, which presents an appearance that has been compared with the key-board of a pianoforte, and has been named the organ of Corti; it consists of the following parts:—Some of these cells, distinguished by their elongated curved form, are arranged in two groups, an inner and an outer. The cells of the inner group rest by a broad foot on the inner part of the basilar membrane, close to its attachment to the spiral lamina, project obliquely forwards and outwards, and expand into a dilated head: the cells of the outer group also rest by a broad foot on the same membrane, incline forwards and inwards, and fit into a depression in the head of the cells of the inner group: these two groups of cells form the rods or pillars of Corti, and by their juxtaposition arch over an excessively minute canal enclosed between them and the basilar membrane, which may be named the canal of Corti. The inner rods are, however, more numerous than the outer, and Pritchard has shown that the rods increase in length from the base to the apex of the cochlea. Immediately internal and almost parallel to the inner group of these rods, and adjacent therefore to the crista spiralis, is a row of compressed conical cells, which possess at their anterior ends short stiff hair-like processes; they are the inner hair cells of Deiters. Immediately external and almost parallel to the outer group of rods are four or five rows of hair cells, the outer hair cells, which are attached by their bases to the basilar membrane, whilst from the opposite extremity a brush of hairs projects through the reticular membrane. The outer hair cells are, according to Waldeyer, relatively of large size in man. The reticular membrane of Kölliker is a delicate framework perforated by rounded holes. It extends parallel to the basilar mem brane from the inner rods of Corti to the external row of outer hair cells, and through the holes in it the hairs of the latter project. It obviously acts as a support for the anterior ends of these cells, and binds together these important elements of the organ of Corti The interval between the outer hair cells and the spiral ligament is occupied by cells of a more or less columnar form, the supporting cells of Hensen. Covering over the organ of Corti, and separating it from the endolymph of the ductus cochlearis, is the membrana tectoria, which springs from the crista spiralis close to the attachment of the membrane of Reissner, passes outwards superficial to the membrana reticularis, and ends externally at the spiral ligament.

The origin, course, and distribution of the auditory nerve in the labyrinth will now be considered. The auditory nerve is the portio mollis of the seventh cranial nerve. It appears at the base of the brain at the lower border of the po∩3 Varolii. Traced to its origin its roots wind round the restiform body to the floor of the 4th ventricle, where they form the striae acousticae, and sink into the grey matter of the floor. Some of the fibres arise from an inner, others from an anterior collection of nerve cells, whilst others again are connected with the cells in the restiform body, and probably with the flocculus of the cerebellum. Where the nerve emerges at the lower border of the pons it contains a cluster of nerve cells. The auditory nerve passes down the internal meatus, and divides into a vestibular and a cochlear division. The vestibular division enters the vestibule, and divides into five branches for the sacculus, utriculus, and three ampullae of the membranous semicircular canals. Each branch enters a crista acoustica and forms a plexus, in the meshes of which nerve cells are imbedded. From this plexus fine non-medullated fibres arise, which enter the layer of cells on the surface of the crista, where they anastomose and form a very delicate plexus, from which fibres spring that in all probability join the central processes of the auditory cells.

The cochlear division enters a canal in the axis of the modiolus, and gives off lateral branches, which pass into the canals situated in the osseous spiral lamina. Here they radiate outwards to the membranous spiral lamina, and have connected with them collections of nerve cells forming the spiral ganglion. Beyond the ganglion they form a flat plexiform expansion, from which delicate nerves pass through a gap in the edge of the osseous lamina into the organ of Corti. In this organ the nerves, as Gottstein and Waldeyer have described, are arranged in two groups of fibres; the inner group become continuous with the deep end of the inner hair cells; the outer group pass across the canal of Corti and end in the outer hair cells. Hence these cells are the peripheral end-organs of the cochlear branch of the auditory nerve, or the auditory cells of the cochlea.

The perilymph of the labyrinth is set in vibration by the movements of the tympanic ossicles and the fenestra ovalis; motion is thus communicated to the membranous labyrinth and the endolymph which it contains. The auditory hairs and cells would thus be set in motion, and the vestibular branches of the auditory nerve would be stimulated to conduct sound-impulses to the brain. The movements of the perilymph in the scala tympani and of the endolymph in the ductus cochlearis would set in vibration the basilar membrane, and the auditory cells resting on it, by which the cochlear branch es of the auditory nerve would be stimulated to conduct sound-impulses to the brain. It has been customary for physiologists to regard the vestibule as the part of the labyrinth by which sound or mere noise is determined; the cochlea, as the part which determines variations and degrees of sound, as musical notes or harmony; the semicircular canals, as determining the directions from which sound proceeds. But within the last two years experiments and arguments have been advanced almost simultaneously by Crum Brown and Mach in favour of the view that the semicircular canals act as peripheral end-organs for the sense of rotation, by which sense the axis about which rotation of the head takes place, the direction of that rotation, and its rate, are determined.

In the account of the development of the skeleton, p. 831, it was stated that the external meatus, tympanum, and Eustachian tube are the remains of the first branchial cleft of the embryo, that the tympanic ossicles are formed in the first and second visceral arches, and that the petrous bone is ossified in the cartilaginous basis cranii. The membranous labyrinth apparently arises as an invagination of the integument at the upper end of the second branchial cleft. The invaginated fold then closes in to form a shut sac, the primary auditory vesicle. Out of this vesicle the three divisions of the labyrinth are successively produced, and become enclosed by the petrous cartilage, which when ossified forms the osseous labyrinth. The epidermal investment of the invaginated tegumentary sac becomes transformed into the special cell structures within the membranous labyrinth, and the sub-epidermal connective tissue forms its fibrous wall· The cochlear and vestibular nerves form at the same time as the labyrinth, and become connected through the trunk of the auditory nerve with the brain.

The Tongue, situated on the floor of the cavity of the mouth, is the chief organ provided for the excitation of the special sense of taste, but the under surface of the soft palate participates to some extent in this property. The tongue is also highly endowed with the sense of touch. The structures concerned in the excitation of taste and touch are situated in the mucous membrane which envelopes the tongue. The tongue is also a muscular organ, and plays an important part in articulation, mastication, and deglutition. Its shape is flattened from above downwards, so that it presents an upper surface or dorsum and a lower surface. Its posterior part is broad, forms the base or root of the organ, and is attached to the hyoid bone. Its anterior extremity or tip is more or less pointed, and its lateral margins or sides are rounded.

The muscles connected with the tongue are arranged in pairs, and form three distinct groups, viz., accessory, extrinsic, and intrinsic muscles. The accessory muscles are the stylo-hyoid, digastric, mylo-hyoid, genio-hyoid, omo-hyoid, sterno-hyoid, and thyro-hyoid, already referred to on page 836, which act upon the hyoid bone, and thus indirectly are concerned in the movements of the tongue. The extrinsic muscles pass from adjacent parts into the substance of the tongue, and are as follows :—The stylo-glossus arises from the tip of the styloid process and the stylo-maxillary ligament; it runs forwards along the side of the tongue to the tip. The hyo-glossus is divided into three parts; a, basi-glossus, which arises from the body of the hyoid; b, ceratoglossus, from the great cornu of the hyoid; c, chondroglossus, from the small cornu of the hyoid. The fibres from these origins ascend into the side of the tongue. The genio-hyo-glossus arises from the upper tubercle of the symphysis of the lower jaw, its fibres radiate into the substance of the tongue along its whole length from base to tip; this muscle is separated from the corresponding muscle of the opposite half of the tongue by a mesial septum of fibrous tissue. The palato-glossus arises in the substance of the soft palate, and descends to the tongue in the anterior pillar of the fauces. The intrinsic muscles lie in the substance of the tongue itself, and are as follows :—The lingualis superior (noto-glossus), consisting of longitudinal fibres, which extend from the base to the tip beneath the mucous membrane of the dorsum; the lingualis inferior, consisting of longitudinal fibres, which extend from the base to the tip along the under surface between the hyo-glossus and genio-hyo-glossus; transverse muscular fibres, which spring from the mesial fibrous septum and curve outwards and upwards to the sides of the tongue; vertical fibres, which pass through the substance of the tongue from the dorsum to the under surface. The extrinsic and intrinsic muscles can not only move the entire tongue within tho cavity of the mouth, protrude it between the lips, and again retract it, but can modify its form; thus the dorsum can be flattened, made convex or concave, the margins can be raised or depressed, and the tip elevated or depressed.

' ( The mucous membrane of the tongue forms a part of the general mucous lining of the mouth; it covers the dorsum, tip, sides, and under surface; is reflected from the under surface to the floor of the mouth, where it forms the fraenum or bridle of the tongue, and is reflected also from the base to the epiglottis as the froena of the epiglottis, as well as over the tonsils and anterior palatine pillars. This membrane has its free surface elevated into multitudes of fine processes, called the papillae of the tongue, some of which are simple, others compound. The simple papillae are situated on the back part of the dorsum and the under surface of the mucous membrane, as well as scattered between the compound papillae; they are simple conical elevations of the membrane. The compound papillae are arranged in three groups, named filiform, fungiform, and circumvallate papillae. The filiform papillae, elongated and thread-like, are the smallest and most numerous, and cover the dorsum in front of the circumvallate papillae. The fungiform or club-shaped are scattered over the anterior and middle parts of the dorsum, and at the tip and sides. The circumvallate papillae, seven to twelve in number, form a V-shaped figure on the dorsum towards its base; a depression in the mucous membrane, called foramen caecum, marks the apex of the V. These are the largest papillae; each is sunk in a vallum or trench-like depression of the mucous membrane, which isolates it from the surrounding surface. The compound character of these papillae is due to each having projecting from it numerous small secondary papillae. The epithelial covering of the filiform papillae is characterised by the peculiar modification which the tessellated epithelium of the mouth has undergone'^ the cells have become cornified and elongated into dense, imbricated brush-like processes. In the carnivora the epithelium is so hardened as to form sharp spines, with the points turned backwards, which give to the tongues of these animals a rough prickly character. In the fungiform and circumvallate papillae the inequalities between the secondary papillae, which project from them, are filled up by the^tessellated epithelium, so that the surface of the compound papillae has a smooth appearance. Both the simple and compound papillae are highly vascular; the lingual artery not only supplies the muscular substance of the tongue, but gives off fine branches to the mucous membrane. These branches end in capillaries, which form simple loops in the simple papillae, but in the compound papillae the capillaries are so multiplied that each secondary papilla has a capillary loop within it. The tongue is provided with several nerves. The hypo-glossal nerve supplies its muscular structure, but the inferior lingualis apparently receives a branch from the chorda tympani of the facial· The lingual branch of the fifth is distributed to the mucous membrane of the anterior two-thirds of the tongue: it breaks up into minute branches, which enter the fungiform and filiform papillae, but their exact mode of termination has not been precisely ascertained, though end-bulbs and gustatory bodies are said to have been seen in connection with some of the terminal branches. The glossal branch of the glosso-pharyngeal is distributed to the mucous membrane of the root of the tongue and of the circumvallate papillae. In connection with its terminal branches peculiar flask-shaped organs, called gustatory bulbs or bodies, have recently been described by Lovén, Schwalbe, and Engelmann, in the sides of the circumvallate papillae. These have been found in large numbers in lamellated folds of the mucous membrane of the posterior part of the side of the rabbit’s tongue, which folds may appropriately therefore be called gustatory lamellae. When sections aro made through one of these folds, or through a circumvallate papilla and the trench which surrounds it, numerous flask-shaped gustatory bulbs may be seen in the epithelium, which covers the side of the papilla and the opposite side of the trench. The bottom of each flask is next the sub-epithelial tissue, whilst its short neck opens on the surface by a mouth, the gustatory pore; similar bodies, though in much smaller numbers, have also been seen in the fungiform papillae. Each gustatory body consists of two different forms of cells, named covering cells and gustatory cells. The covering cells are elongated, nucleated spindles, which, arranged in layers, form the envelope of each gustatory bulb, and reach from the bottom to the mouth of the flask; they enclose the gustatory cells. The gustatory cells are attenuated, homogeneous, and highly refractile cells, which possess an elliptical nucleated body with two processes, a central and peripheral. These cells occupy the axis of the gustatory bulb. The peripheral process, broader than the central, sometimes ends in a short hair-like tip, which almost reaches the gustatory pore; the central process extends to the base of the flask, and often divides into small branches. This process is varicose, and not unlike the axial cylinder of a nerve fibre. The branches of the glosso-pharyngeal nerve, which are distributed to the back of the tongue, enter the circumvallate papillae, and form a minute plexus, with groups of nerve cells interspersed in it, from which bundles both of medullated and non-medullated fibres pass to the basis of the gustatory bulbs; and it is believed that the finest non-medullated fibres are continuous with the peripheral processes of the gustatory cells, which are therefore regarded as the peripheral end-organs of the nerve of taste, and by the excitation of these bodies gustative or taste sensations are produced. As the glosso-pharyngeal is the nerve distributed to the circumvallate papillae, where these gustatory bulbs are especially found, it is therefore the special nerve of taste; but as these bulbs have also been sparingly seen in the other papillae, where the lingual nerve is distributed, that nerve probably acts in a minor degree as a nerve of taste, though its special function is undoubtedly that of a nerve of touch. The gustatory bulbs are not penetrated by blood-vessels, but, as Fig. 85 shows, the vascular sub-epithelial tissue is prolonged upwards along the sides of the bulbs almost as far as the plane of the gustatory pore. Key, Beale, and other observers have described special modifications of the epithelium in connection with the terminations of the gustatory nerves in the frog. The mucous membrane of the tongue contains numerous smau tubular or branched glands, more especially on the dorsum near its root, which secrete mucus. Depressions also occur in this part of the mucous membrane, around the walls of which groups of lymphoid cells are collected in the sub-epithelial connective tissue, which have an arrangement closely resembling the structure of the adjacent tonsils, and form an example of adenoid tissue.

The Skin, or Integument, invests the entire outer surface of ⅛e body, and contains structures by the excitation of which the properties of things are determined by the sense of touch. The skin also contains accessory structures, as the nails, hairs, sebaceous glands, and sweat glands. The skin consists of a non-vascular. cuticle or epidermis, and of a vascular and sensitive corium, or cutis vera.

The Cuticle, Epidermis, or scarf skin, forms the outer covering of the skin, and protects the cutis. It is a laminated structure, and consists of numerous layers of cells superimposed on each other. As these cells cover a free surface exposed to the air, they belong to the epithelium group. The thickness of the cuticle varies in different localities from τ ⅛th to 7 ⅜∙ δ ∙th inch; where the skin is frequently exposed to pressure, as in the soles of the feet, the cuticle is the thickest and hardest; and the hands of those accustomed to manual labour have a hard and horny cuticle. The increase in thickness in these localities is for the purpose of protecting the highly sensitive cutis from injury. The outer surface of the cuticle in many parts of the body, especially the palm of the hand and the fingers, is marked by ridges and furrows; the ridges indicate the position and arrangement of the papillae of the cutis, whilst the furrows are due to the sinking of the cuticle into the spaces between the rows of papillae. The mouths of the sweat glands open on the surface of these ridges. The cuticle is divided into two strata. The superficial homy stratum consists of layers of flat, polygonal scales like a tessellated epithelium; the cells in the superimposed layers firmly adhere to each other by their surfaces, and in vertical sections this stratum presents a fibrous appearance; but the cells may be readily isolated by digestion in a caustic alkali. The deeper or mucous stratum, or rete Malpighii, lies next the cutis, and closely follows the undulations of its papillary surface. The cells forming the layer next the cutis are columnar in shape, those in the layers immediately succeeding are rounded or cubical, whilst those next in order are polygonal, and not unfrequently possess pointed processes or prickles projecting from them, hence the name, prickle cells, employed by Schultze. The cells which lie next the horny stratum assume the scale-like form. It is in the cells of the mucous stratum that the colouring matter of the skin is found, which in the fair races of men forms the isolated coloured spots called freckles and moles, but in the dark races the pigment granules are uniformly distributed through the cells of this stratum. The superficial cells of the homy stratum of the cuticle are continually being shed, so that the cells of the deeper layers gradually approach the surface, and new cells are continually being formed in the deeper part of the rete "Malpighii. The cuticle is closely adherent to the cutis in the healthy living skin, but on the application of a blister, or when putrefaction sets in after death, it separates from it.

The Cutis vera.— When the cuticle is removed the surface of the cutis is seen to be studded with multitudes of minute elevations, the papillae of the skin. These papillae are either simple conical structures, or compound with two or three branches. They are largest in the palm and sole, being from τ ⅜ 7 th to ∙ 2 -⅞∙ δ ∙th of an inch high, and are arranged in ridges, but more usually they are much shorter and irregularly distributed. The cutis is formed of connective tissue, in which stellate connective tissue corpuscles and elastic fibres are abundant. The deeper surface of tho connective tissue of the cutis is reticulated, and is continuous with the bundles of connective tissue that form the areolar subcutaneous tissue. In the papillae themselves the fibres of the connective tissue are not so well marked, and the surface of the papillae possesses more of a homogeneous aspect, which gives rise to the appearance described as a basement membrane. The cutis is highly vascular; the small arteries which go to the skin give off branches to the lobules of fat in the subcutaneous tissue, then penetrate the cutis, and form a plexus from which capillaries arise, which enter the papillae, and form vascular loops within them. The lymphatic vessels of the skin are numerous; they form a plexus in the cutis, which lies beneath the vascular plexus, forms, as Neumann’s injections show, a network around both the sebaceous and sweat glands, and gives off capillary loops into the papillae. The nerves of the skin are the cutaneous branches both of the spinal and of certain of the cranial nerves, the origin and distribution of which have already been described. They run through the subcutaneous tissue, and enter the deep surface of the cutis, where they divide into branches. As these pass towards the papillae they unite to form a nerve plexus, from which smaller branches arise to enter the papillae, and terminate, more especially in the skin of the palm of the hand, fingers, and sole, which are the surfaces most sensitive ’to touch impressions, in the tactile or touch corpuscles. The touch corpuscles discovered by Wagner and Meissner are the peripheral end-organs of the nerves of touch. They may be single or compound; are usually ovoid inform, not unlike a minute fir cone; and are transversely marked, from the transverse direction of the nuclei of fusiform cells which form an investing capsule. Each single corpuscle and each division of a compound corpuscle is penetrated by one, and, according to Thin, by never more than one, medullated nerve fibre, but the exact mode of termination of the axial cylinder of the fibre has not been ascertained. Virchow and other German observers have stated that the papillae which contain capillaries do not contain nerves or touch corpuscles, and vice versa; but Dalzell and Thin have shown that certainly the majority of papillae that contain nerve fibres and touch corpuscles are also vascular papillae. Non-medullated nerve fibres ascend to the surface of the cutis, and, according to Langerhans, pass into the rete Malpighii between the cells of the mucous layer.

Nails.— On the back of the last phalanx of each thumb, finger, and toe is situated a firm horny curved plate, the naü. Each nail rests on a bed, the surface of which is formed of the cutis, which also overlaps the side and root of the nail; thus the nail fits into a groove formed of the cutis something after the manner in which a watch-glass fits into its rim. A nail is merely a special modification of the cuticle, the cells of the superficial stratum of which are more homy, harder, and more firmly adherent to each other than in the cuticle proper. Deeper than the horny stratum is the rete Malpighii of the nail, the cells of which are soft, as in the cuticle itself. The cutis forming the bed of the nail is studded with papillae, which are arranged in almost parallel rows, and are highly vascular. Nails grow both in length and thickness: the increase in thickness is due to the formation of nerve cells on the bed of the nail; the increase in length takes place through the formation of nail cells at its root, and as the nail is thus slowly pushed forward it requires to be cut at intervals. At the root, sides, and below the free border of the nail the cuticle is continuous with the substance of the nail itself.

Hair.— Projecting from the surface of the skin are multitudes of elongated cylindrical horny structures, the hairs. In the skin of the scalp, the armpits, and the pubis, they are long and numerous; but in the eye-brows, eye-lashes, vibrissae of the nostrils, and surface of the body generally, they are short. They are stronger and thicker in the skin of man than of woman, more especially on the cheeks, lips, and chin. Hairs do not grow from the skin of the palms and soles, the back of the ungual phalanges, and the surface of the upper eye-lids. Each hair is partially embedded in a depression of the skin, called a hair follicle. The deeper end of the follicle is somewhat dilated, and has in it a papilla, the hair papilla. The wall of the hair follicle is formed of the constituent structures of the skin; the outer part of the wall belongs to the cutis, and has been described as arranged in three layers, the external, middle, and inner layer of the hair follicle. The external and middle layers are formed of connective tissue, with blood-vessels; whilst the inner, sometimes called the vitreous layer, is transparent and homogeneous, and continuous with the so-called basement membrane of the cutis. The inner part of the wall of the hair follicle, or the root-sheath, belongs to the cuticle, and consists of two layers, the outer and inner root-sheaths. The outer root-sheath is continuous with the rete Malpighii, and consists of cells similar to those of that stratum. The inner root-sheath is continuous with the horny stratum of the cuticle, and consists of elongated scale-like translucent cells in which no nuclei can be seen.

A hair possesses a root, a shaft, and a tip; the root is embedded in the hair follicle, whilst the shaft and tip form the free projecting part of the hair. In the human hair the substance of the hair is composed of a ^δrozw-looking horny material, which by the action of strong sulphuric acid is resolved into elongated, closely compacted, fusiform cells, which in coloured hairs contain pigment granules. In the thicker hairs the cells in the axis of the hair are polygonal, contain air, and form a central pith or medulla. The hair is invested by imbricated scale-like cells, which form the hair cuticle. In different animals the size and relative proportion of the cells of the cuticle, medulla, end fibrous part of the hair present many modifications. The wool of the sheep has its cuticle scales, with well-defined serrated margins, so that the hair of this animal is well adapted for felting into cloth; in the bat, also, the cuticle cells are large and strongly serrated. The bristles of the pig, again, have the fibrous part of the hair largely developed. In the deer tribe the hair consists of polygonal medulla-like cells, which contain air. The root of the hair dilates at its deeper end into a bulb which embraces the hair papilla. It is softer in texture than the shaft, so that the cellular structure of the hair is more easily demonstrated. Next the papilla the cells are like those of the rete Malpighii, but when traced onwards to the shaft they are seen to become differentiated, both in structure and composition, into the proper hair cells. The root is enveloped in a special sheath, termed the sheath of Huxley, composed of nucleated cells, which sheath, in the more superficial part of the follicle, blends with the internal root-sheath. The hair papilla bears to the hair the same relation as a papilla of the cutis has to its investing cuticle, so that a hair is to be regarded as a specially modified cuticular structure. . The human hair papilla is vascular, but no nerves have been traced into it. In the tactile hairs of the mammalia, however, nerves have been traced into their papillae.

The bristles, feathers, claws, hoofs, the horny envelope of the horn cores in the hollow horned ruminants, and various tegumentary spines and scales, present in many animals, are, like hairs and nails, special modifications of the tegumentary system.

Each hair follicle has opening into it the excretory duct of a small gland, named a sebaceous gland. This gland consists of the excretory duct, and of from two to twenty grape-like saccular expansions which open into the duct. The wall of the sacculi and of the duct is continuous with the vitreous layer of the outer wall of the hair follicle. Capillary blood-vessels are distributed on the outer wall of the sacculi. The sacculi are almost entirely filled with polygonal cells containing drops of fat, which cells are continuous with the epithelial lining of the gland duct and the cells of the outer root-sheath. These glands secrete a fatty materia], which lubricates the surface of the hair. Sometimes a small parasite, called Acarus folliculorum, is found in a sebaceous gland.

Some years ago Kölliker described one or two bundles of smooth muscular fibres extending from the wall of the hair follicle to the deep surface of the cutis; these muscles, named arrectores pili, by their contraction erect the hairs, that is, cause them to become more prominent, and produce the condition of skin, called cutis anserina or goose skin, well known to occur when cold is applied to the surface of the body.

Hairs are developed about the 4th month of embryo life, within depressions in the cutis, which form the future hair follicles, filled with cells similar to, and continuous with, those of the rete Malpighii. A papilla forms at the bottom of this depression, around which the cells become arranged in a bulbous expansion. The cells, in line with the bulb, elongate and harden, and group themselves so as to form the shaft of the young hair, which at this stage is completely buried within the follicle. A rapid production of new cells takes place at the bulb, the hair consequently increases in length, and is pushed outwards through the superficial horny stratum of the cuticle, which had closed in the mouth of the depression or follicle in which the hair is produced. At the same time, the more external cells within the follicle are pushed outwards towards its wall, and form the cells of the root-sheath. When a hair is pulled out of its follicle the cells of the root-sheath are drawn out along with it. A new hair will be developed at the bottom of a follicle from which the hair has been shed as long as cells continue to be formed around the papilla. When the growth of cells ceases within the hair follicles then permanent baldness is the result.

The sebaceous glands are developed as bud-like offshoots from the hair follicles, filled, like the follicles themselves, with cells continuous with those of the rete Malpighii. Instead of the cells in these buds differentiating into a hair, they become filled with fatty particles, and the wall of the bud assumes the characteristic sacculated form of the gland.

Sweat Glands, or sudoriparous glands, are found generally distributed throughout the skin, but are most abundant in the palms and soles, where they number 2500 to 3000 in •each square inch. In the skin of the back, again, there are only between 4β0 and 500 in the square inch. Each gland consists of a ball-like body lying in the subcutaneous .tissue, from which a tubular duct proceeds through the skin to open on its free surface. The ball is composed of a convoluted tube continuous with the tubular duct, and terminating in a blind end. The wall of the gland tube consists of a delicate nucleated membrane lined by columnar secreting cells. It is surrounded by connective tissue containing capillary blood-vessels. As the gland-duct pierces the cutis it passes between the papillae; in its course through the cuticle it pursues a spiral direction, and has its walls formed, not of a distinct membrane, but of the cuticle cells themselves. The epithelial lining of the duct is continuous with the cells of the rete Malpighii of the cuticle. In the axilla and groin the sweat glands are much larger than in the skin generally. The sweat glands arise as flask-shaped pouches of the rete Malpighii projecting into the cutis, which in course of time become elongated into tubes, and the cells contained in which become the secreting cells of the gland.

Vascular System.

The human body and the bodies of all the more highly organised animals are traversed by numerous tubes or pipes, technically called Vessels, some of which in man are nearly an inch in diameter, others so small as to require a microscope for their examination, others again of every intermediate size. In connection with the vessels is a central organ, the Heart. The heart and the vessels collectively constitute the Vascular System. Of these vessels some contain blood, and form the Blood-vascular system; others contain lymph, and form the Lymph-vascular system. The lymph-vascular system is not independent of the blood-vascular system, but communicates with it at several points. The vascular system is a hydraulic apparatus, possessing a pump, pipes, and valves. The heart is the pump, which works, not by the movements of a piston, but by the contraction of its muscular walls; the vessels are the pipes, which convey the contained fluid, and they are provided in certain localities with valves for modifying its flow.

Blood-Vascular System. —The movement of the blood in the blood-vascular system is called the circulation of the blood. In the lower Vertebrata the heart is a single organ, and the blood flows from it through the vessels back again into the part of the heart from which it had proceeded, forming a simple circulation. In man and the higher vertebrates the heart is a double organ, i.e., it consists of a right and left portion, intimately united to, but not directly communicating with, each other. The blood which flows from its right side passes through vessels which traverse the lungs, and is conveyed to the left side of the heart; whilst the blood which flows from the left side passes through vessels which traverse the body generally, and is conveyed to the right side of the heart. This is called a double circulation; that which appertains to the lungs is the pulmonic circulation; that which appertains to the body generally is the systemic circulation. The vessels which carry the blood away from the heart are called arteries; those which convey it back to the heart are veins. The arteries and veins do not communicate directly with each other, but through the intermediation of a network of extremely minute vessels, the capillaries. Hence, both in the pulmonary and systemic circulation, the blood in its passage from the arteries into the veins must go through capillaries. The blood which flows from the left side of the heart into the systemic arteries is pure or arterial blood; as it traverses the systemic capillaries it parts with certain of its constituents to nourish the organs and tissues, and as it receives from them waste products it becomes impure blood; in which condition it flows back to the right side of the heart by the systemic veins as venous blood; hence the right side of the heart is often called the venous side. The blood which flows from the right side of the heart along the pulmonary artery is this impure blood; as it traverses the pulmonary capillaries it is purified by the action of the air in the lungs, and is changed into arterial or pure blood, in which condition it flows back by the pulmonary veins to the left side of the heart, which consequently is called the arterial side. The object of the pulmonary circulation, therefore, is to reconvert into pure blood the blood which has been rendered impure during its passage through the systemic capillaries.

The Heart.— The heart is a hollow muscle contained in the cavity of the chest, and enclosed within a bag called the Pericardium. The pericardium, with its enclosed heart, occupies the space called mediastinum, between the twc lungs; it lies therefore behind the sternum, and in front of the spinal column, but projects more to the left than to the right side of the mesial plane. The bag of the pericardium is formed externally of a strong fibrous membrane, which is attached below to the central tendon of the diaphragm, but blends above with the sheaths of the great vessels which pass to and from the heart. When the bag is cut open its inner surface is seen to possess a smooth glistening serous aspect, for it is lined by a layer of squamous endothelium, which layer is continuous with the serous membrane that invests the heart, and forms the visceral layer of the pericardium. The continuity of the serous lining of the bag with the §erous investment of the heart takes place where the great blood-vessels pierce the fibrous bag.

The heart lies obliquely from above downwards, from right to left, and from behind forwards. For descriptive purposes it may be regarded as possessing a base, an apex, an anterior and a posterior surface, a right and left border. The base lies backwards, upwards, and to the right, opposite the 4th to the 8th dorsal vertebrae. The apex is directed forwards, downwards, and to the left, opposite to the interval between the 5th and Gth left ribs. The heart has on its surface grooves which indicate its division internally into four chambers, two in its right half, two in its left half. The right chambers are the right auricle and right ventricle. The left chambers are the left auricle and left ventricle. All these chambers are lined by a smooth membrane, the endocardium,, which is continuous on the one hand with the lining membrane of the veins, on the other with the lining membrane of the arteries.

The Right Auricle occupies the right part of the base of the heart. It consists of a large dilated; portion, the sinus venosus, and of a small ear-shaped appendage, the auricula. Its muscular wall is smooth internally, except in the auricula and adjacent anterior wall of the sinus venosus, where it is thrown into parallel ridges like the teeth of a comb, and named musculi pectinati. Into the sinus venosus open the great systemic veins or venae cavae. The superior vena cava conveys to the auricle the systemic blood that has been circulating in the body above the diaphragm; it opens by a patent mouth into the upper and back part of the sinus venosus. The inferior vena cava conveys to the auricle the blood that has been circulating in the parts of the body below the diaphragm; it opens into the lower and back part of the auricle, and at its mouth is a rudimentary valve, the Eustachian valve. Close to its orifice is the mouth of another large vein, the coronary venous sinus, which also possesses a small valve. Several minute openings, the foramina Thébesii, scattered over the inner wall of the auricle, are the mouths of small veins ramifying in the wall itself. Through these various orifices the venous blood pours into the auricle, and then flows into the right ventricle through a large orifice of communication between them. The light auricle is separated by a partition, the auricular septum, from the left auricle. On the surface of this septum is a depression, the fossa ovalis, surrounded by a raised border, the annulus ovalis, with which border the inner end of the Eustachian valve is continuous. Before the birth of the child the septum is perforated by a hole, called foramen ovale, through which the blood flows directly into the left auricle, but this foramen is obliterated after the birth of the child.

The Right Ventricle forms the right border, a large part of the anterior surface, but only a small part of the posterior surface of the heart. It is shaped somewhat like a flattened cone, its apex being directed downwards towards the apex of the heart, its base to the corresponding auricle. The inner surface of its wall is very irregular, owing to the muscular bundles being elevated into strong ridges, called columnoe carneoe. Two, or it may be three, of these fleshy columns project like nipples or big papillae into the cavity of the ventricle, and are called musculi papillares. Attached to the free apex of each papillary muscle are several fibrous threads, the chordae tendineae, which, by their opposite extremities, are connected to the segments of a large valve situated around the opening between the right auricle and ventricle. The right auriculo-ventricular opening, situated at the base of the ventricle, is sufficiently large to admit three fingers, and possesses a valve which consists of three large pointed segments or cusps (hence the name tricuspid given to it), between which three small intermediate cusps lie. One of the large cusps lies opposite the anterior wall of the ventricle, another opposite the posterior, whilst the third is between the auriculo-ventricular and pulmonary openings. The cusps are flattened triangular folds of membrane connected by their bases around the opening; when the valve is not in action the apex of each cusp hangs pendulous in the ventricle: one surface is smooth, and looks to the cavity of the ventricle, the other surface is rough and directed to its wall; to this rough surface, to the apex, and to the edges of the cusp, the chordae tendineae are attached. As tho musculi papillares, from which the chordae tendineae spring, lie opposite the intervals between the cusps, the chordae tendineae from any given papillary muscle divide themselves into two groups, one for each of the two cusps between which it is situated. Attention has recently been drawn by Rolleston to a band which passes from the base of the anterior papillary muscle to the septal wall of the ventricle. As it prevents over-distension of the ventricle, he has named it the moderator band. The base of this ventricle forms to the left and in front of the auriculo-ventricular opening, a funnel-shaped prolongation, the conus arteriosus, from which the pulmonary artery arises, through the intermediation of a strong fibrous ring. Surrounding the mouth of this artery is a valve called semilunar, which consists of three semilunar segments. Each segment is attached by its convex border to the artery where it springs from the ventricle. The opposite border is free, and possesses at its centre a minute nodule, the corpus Arantii, from which slender threads curve outwards at the free border and in the substance of the valve to strengthen it. A thin lunated portion lies immediately within the free border. One surface of the valve is convex, and directed to the lumen (i.e., the space contained by the walls) of the artery; the other is concave, and directed to the wall of the artery, and between it and the wall is a pouch named sinus of Valsalva. The pulmonary artery extends upwards and to the left for about If inch, and then divides into two branches, one for each lung. The right ventricle is completely separated from the left by the ventricular septum, which passes obliquely from left to right, and from before backwards, so that it forms the posterior wall of the right ventricle and the anterior wall of the left.

The Left Auricle occupies the left part of the base of the heart, and, like the right auricle, consists of a dilated sinus venosus and an ear-shaped auricula. Its muscular wall forms a smooth surface internally, except in the auricula, where the ridge-shaped musculi pectinati occur. Opening into the sinus are the orifices of.the four pulmonary veins, two from the right, two from the left lung: these orifices are without valves. At the lower part of the auricle is the large orifice of communication between it and the base of the left ventricle.

The Left Ventricle forms the left border, the apex, a large part of the posterior surface, but only a small part of the anterior surface of the heart. It is conical in form, its apex is at the apex of the heart, the base at the corresponding auricle. As in the right ventricle, the inner surface of its wall is elevated into fleshy columns, two of which project like nipples into the cavity and form musculi papillares, which have chordoe tendineoe connected with them. The left auriculo-ventricular opening is large enough to admit >two fingers. It possesses a valve, which consists of two large pointed segments or cusps, between which two small intermediate cusps lie, hence it is called the bicuspid valve; and as these cusps are placed one in front of the other like the segments of a bishop’s mitre, the name mitral valve is often given to it. The cusps agree in shape, general arrangement, and mode of attachment with those of the tricuspid valve, but they are stronger; and as the more anterior segment lies obliquely between the auricular and aortic orifices, both its surfaces are smooth. From the base of this ventricle the great systemic artery or aorta arises through the intermediation of a strong fibrous ring. The mouth of the aorta is surrounded by a three segmented semilunar valve, similar to the semilunar pulmonary valve, but with thicker and stronger segments, and possessing more strongly marked sinuses of Valsalva. The base of each ventricle has therefore two openings in it, one for communication with the auricle, the other with the great artery arising from the ventricle. The auriculo-ventricular openings are the most posterior, and almost in the same plane; the aortic opening lies in front of the interval between the two auriculo-ventricular, and the pulmonary opening is in front of the aortic.

The walls of the cavities of the heart are formed of striped muscular fibre, over the contractions of which the will exercises no control. The fibres are collected into fasciculi, which have a reticulated arrangement, and the fibres themselves branch and again unite to form a complicated network. The fibres of the walls of the auricles are distinct from those of the ventricles, so that the auricular and ventricular compartments are connected together, not by an interchange of muscular tissue, but by an intermediate ring-like arrangement of fibres of connective tissue. The muscular fasciculi of the auricles are arranged in two strata. Tho deeper stratum consists of fibres proper to each auricle, some of which run obliquely in the wall, others form the musculi pectinati, surround the auricula, and are prolonged in rings into the coats of the venae cavae and pulmonary veins, whilst fibres extend longitudinally and obliquely along the wall of the coronary venous sinus. The superficial stratum consists of fasciculi, which run obliquely from one auricle to the other on both the anterior and posterior surfaces, and are said to be prolonged into the auricular septum.

The muscular wall of the ventricles is much thicker than that of the auricles, and the wall of the left ventricle is about three times thicker than the right. The fibres vary in their direction in different parts of the thickness of the ventricular walls. The superficial external fibres run obliquely from above downwards, and from right to left, and on the anterior surface of the ventricles dip into the anterior ventricular groove to enter the septum, whilst on the posterior surface they extend across the posterior ventricular groove; at the apex of the heart they turn inwards in a whorl-like manner, and, as was known to Lower and Gerdy, become continuous with superficial fibres on the inner wall of the ventricle; at the base of the ventricles they turn round the border of the auriculo-ventricular openings, and, as Pettigrew has shown, become continuous with these superficial internal fibres, which run in the reverse direction. The internal fibres are also prolonged into the musculi papillares, the chord® tendineae springing from which serve therefore as tendons of insertion for these muscles. If the substance of the wall be now dissected the fibres situated in the centre of the wall are seen to lie in the horizontal plane. Various anatomists have described these fibres of the ventricles as arranged in layers. Lower recognised two layers spirally crossing each other; Haller, three; Wolff, three in the right and six in the left ventricle. Pettigrew at one time believed he could dissect nine layers, but has subsequently reduced the number to seven—three external, a fourth or central, and three internal. He conceives that the fibres of the three external layers run in a spiral direction from left to right downwards, the first layer being more vertical than the second, and the second than the third, whilst the fibres of the fourth or central layer are horizontal. The three internal layers also run spirally, but in the reverse direction from the external, with which they become continuous both at the base and apex. The subdivision of the ventricular wall into such precise and determinate layers as is implied in the descriptions of Pettigrew is, however, to some extent an artificial procedure. There can be no doubt, as his dissections so beautifully show, that the direction of the fibres in the ventricular wall varies at different depths; but owing to the reticulated arrangement of the fibres, not only are those connected together which lie in one of the so-called layers, but they also anastomose with the fibres in the layer contiguous to it on either aspect. Hence when one layer is peeled off, that immediately subjacent exhibits, not a smooth face, which it would have done had the definition of the layers been distinct, but a rough appearance, due to the tearing through of intermediate connecting muscular fibres. Owing to these connections the substance of the wall of the ventricle, as Henle’s dissections show, may, with the exceptioħ of the superficial internal and external fibres, be split up into lamellae, which extend either horizontally, obliquely, or in an arched manner through the wall between its two surfaces; and the surfaces of those lamellae are not parallel to the wall of the ventricle, but are directed upwards and downwards.

Except at the fibrous rings, where both the white and yellow fibres are distinct, the connective tissue of the heart is small in quantity. The endocardial lining consists of connective tissue with elastic fibres, with a layer of endothelium on the free surface; and Schweigger-Seidel has also described smooth muscular fibres in it. Hence, as Luschka has stated, the endocardium represents not merely the inner coat of the blood-vessels but all the structures of the vascular wall. Purkinje described fibres beneath the endocardium, which are now regarded as imperfectly formed striated muscular fibres. The valves are folds of the endocardium, enclosing fibres continuous with those in the fibrous rings: the cuspidate auriculo-ventricular valves receive fibres from the chordae tendineae.

The heart is well supplied with blood, not by the blood which flows through its cavities, but by two special coronary arteries which ramify in its walls, and end in numerous capillaries lying between the fibres. From these capillaries the coronary veins arise, which join to form the coronary venous sinus. Lymphatic vessels occur both in the endocardium and pericardium, and apparently ramify in the muscular wall. The nerves of the heart have been dissected especially by Scarpa, Remak, Lee, and Pettigrew, and numerous small ganglia described in connection with them (see p. 883).

The blood flows along the great veins into the auricles.

and is forced by the contraction of their muscular walls through the auriculo-ventricular openings, the valves of which open outwards, into the ventricles. When the ventricles are distended their muscular walls contract and force the blood into the arteries—the right ventricle into the pulmonary artery, the left into the aorta—the valves at the mouth of each artery opening outwards to allow of the free passage of the fluid. To prevent, during the ventricular contraction, the regurgitation of blood into the auricles, the auriculo-ventricular valves are floated away from the sides of the ventricle across their respective openings, and by the apposition and slight overlapping of their edges temporarily close the openings. The tilting upwards of the valves into the auricles is prevented by the contraction of the musculi papillares, and their connection with the cusps of the valve through the chordae tendineae. Pettigrew has shown that casts of the ventricular cavities, more especially of the left, have the form of a double cone, spirally twisted from right to left, and has described the blood as forced in spiral streams against the under surface of‘the segments of the valve, which are twisted and wedged into each other so as to prevent regurgitation. The propulsion of the blood into the arteries distends the elastic walls of those tubes; but when the ventricular contraction has ceased, the elastic wall recoils, and the blood is propelled onwards in the circulation. The regurgitation of the blood into the ventricles is prevented by the closure of the semilunar valves, the segments of which are thrown across the arterial orifices through the pressure exercised on the column of blood in the lumen of the artery and in the sinuses of Valsalva.

The Arteries.— These vessels were named arteries by the older anatomists, on the supposition, now known to be erroneous, that they contained air. The term is now employed to express a blood-vessel, which, arising either directly or indirectly from the heart, conveys blood away from that organ. Arteries divide and subdivide into smaller vessels in their course, and to the individual branches descriptive names are applied. Some of these names express the position of an artery, as subclavian, axillary; others, the organ in which it is distributed, as pulmonary, hepatic; others a peculiarity in its course, as circumflex, coronary. The branches of arteries may be either collateral or terminal. The collateral branches arise from the sides of the parent artery either at an acute, a right, or an obtuse angle. Terminal branches arise at an acute angle by the bifurcation of the parent artery, which is the most common form, or by the breaking up of the artery into a cluster of branches. Branches which arise either from the same artery or from different arteries may be distributed in a common locality, may there unite together, and form what is called an inosculation or anastomosis, so that the blood from one artery may thus flow from it into another. The most common anastomosis is by the formation of loops between adjacent branches, but sometimes, as when the + ⅛vo vertebral arteries join to form the basilar, a convergence of two almost straight arteries takes place; and in other cases, as where the two anterior cerebral arteries are joined together by the anterior communicating, a connecting branch passes transversely across the mesial plane. A more complex form of anastomosis is when an artery (and a similar arrangement is sometimes found in veins) rapidly subdivides into numerous branches, which may again join to form a trunk either with or without the formation of a plexus. This is called a rete mirabile, an arrangement not uncommon in the cetacea, in the internal carotid arteries of ruminants, in the mesenteric arteries of the pig, in the arteries of the limbs of the sloths and lemurs, and in the arterial system of fishes. The only examples of a rete in the human body are the convoluted Malpighian tufts of the kidney and the arterial distribution in the coccygeal body.

The distribution of the pulmonary artery will be considered in the anatomy of the lungs. That of the aorta will now be briefly described.

The Aorta (Plate XX. figs. 2, 3, a) lies in the cavities of the thorax and abdomen,and arises from the base of the left ventricle. It ascends forwards, upwards, and to the right as far as the level of the second right costal cartilage, then runs backwards and to the left to reach the left side of the body of the 4th dorsal vertebra, and then descends almost vertically to reach the left side of the body of the 5th dorsal vertebra. It forms, therefore, an arch, well known as the arch of the aorta, which arches over the root of the left lung, and which has attached to its concave surface a fibrous cord, known as the obliterated ductus arteriosus, which connects it with the left branch of the pulmonary artery. The aorta continues its course downwards in close relation to the bodies of the lower dorsal vertebrae, then passes through an opening in the diaphragm, enters the abdomen, and descends in front of the bodies of the lumbar vertebrae as low as the 4th, where it is usually described as dividing into the two terminal branches, the common iliac arteries. At the angle of bifurcation, however, a long slender artery, called the middle sacral, is prolonged downwards in front of the sacrum to the end of the coccyx. In animals with long tails this artery can be recognised as a direct continuation of the aorta, prolonging it downwards in front of the caudal vertebrae, whilst the iliacs are seen to be collateral branches; but in man, where the coccyx is rudimentary, and the lower limbs largely developed, the iliac arteries which supply those limbs are so big as to obscure the true signification of the middle sacral artery, and appear themselves to be the terminal branches of the aorta. The branches which arise directly from the aorta may be arranged in four groups.—1st, Branches for the supply of the viscera of the thorax and abdomen proper; 2d, branches for the walls of the thorax, abdomen, and pelvis; 3d, branches for the head, neck, and upper limbs; 4th, branches for the lower limbs, pelvic walls, and viscera.

The branches of the aorta which supply the viscera of the thorax are the coronary, the oesophageal, the bronchial, and the pericardial. The coronary arteries, two in num ber, are the first branches of the aorta, and arise opposite the right and left segments of the semilunar valve, from the wall of the aorta, where it dilates into the sinuses of Valsalva. The mouths of these arteries are closed by the opening outwards of the aortic valves during the ventricular contraction. The elastic recoil of the aorta following that contraction not only closes the aortic valves, but drives the blood into the coronary arteries. These arteries break up into branches in the muscular walls of the heart, and the sudden turgescence of its walls, which results from the filling of these vessels, is, according to Brücke and Garrod, the cause of the dilatation of the ventricular cavities.

The bronchial arteries are two in number ; one accompanies each bronchial tube, and supplies the tissues of the lung.

The oesophageal arteries, three or four in number, supply the coats of the oesophagus.

The pericardial branches are very small arteries which supply the back of the bag of the pericardium.

The branches of the aorta which supply the viscera of the abdomen arise either singly or in pairs. The single arteries are the coeliac axis, the superior mesenteric, and the inferior mesenteric, which arise from the front of the aorta; the pairs are the capsular, the two renal, and the two spermatic or ovarian, which arise from its sides. The single arteries supply viscera which are either completely or almost completely invested by the peritoneum, and the veins corresponding to them are the roots of the vena portae. The pairs of arteries supply viscera developed behind the peritoneum, and the veins corresponding to them are rootlets of the inferior vena cava.

The coeliac axis is a thick, short artery, which almost immediately divides into the coronary, hepatic, and splenic branches. The coronary artery subdivides into an oesophageal branch for the lower end of the oesophagus, and a gastric branch for the coats of the stomach. The hepatic artery ends in the substance of the liver; but gives off a cystic branch to the gall bladder, a pyloric branch to the stomach, a gastro-duodenal branch, which divides into a superior pancreatico-duodenal for the pancreas and duodenum, and a right gastro-epiploic for the stomach and omentum. The splenic artery ends in the substance of the spleen; but gives off pancreatic branches to the pancreas, vasa brevia to the great end of the stomach, and a left gastro-epiploic to the stomach and omentum.

The superior mesenteric artery gives off an inferior pancreatico-duodenal branch to the pancreas and duodenum; about twelve intestinal branches to the small intestines, which form in the substance of the mesentery a series of arches before they end in the wall of the intestines; an ileocolic branch to the end of the ileum, the caecum, and beginning of the colon; a right colic branch to the ascending colon; and a middle colic branch to the transverse colon.

The inferior mesenteric artery gives off a left colic branch to the descending colon, a sigmoid branch to the sigmoid flexure of the colon, and ends in the superior hcemorrhoidal artery which supplies the rectum. The arteries which supply the coats of the alimentary tube from the oesophagus to the rectum anastomose freely with each other in the wall of the tube, or in its mesenteric attachment, and the anastomoses are usually by the formation of arches or loops between adjacent branches.

The capsular arteries, small in size, run outwards from the aorta to end in the supra-renal capsules.

The renal arteries pass one to each kidney, in which they for the most part end, but in the substance of the organ they give off small perforating branches, which pierce the capsule of the kidney, and are distributed in the surrounding fat.

The spermatic arteries are two long slender arteries, which descend, one in each spermatic cord, into the scrotum to supply the testicle. The corresponding arteries in the female, called the ovarian, do not leave the abdomen; they supply the ovaries.

The branches of the aorta which supply the walls of the thorax, abdomen, and pelvis, are the intercostal, the lumbar, the phrenic, and the middle sacral.

The intercostal arteries arise from the back of the thoracic aorta, and are usually ten pairs. They run down the sides of the vertebral bodies as far as the commencement of the intercostal spaces, when each divides into a dorsal and a proper intercostal branch; the dorsal branch passes to the back of the thorax to supply the deep muscles of the spine; the proper intercostal branch runs outwards in the intercostal space to supply its muscles, and the lower pairs of intercostals also give branchée to the diaphragm and wall of the abdomen.

The lumbar arteries arise from the back of the abdominal aorta, and are usually four pairs. They run down the sides of the lumbar vertebrae, and divide into a dorsal branch, which supplies the deep muscles of the back of the loins, and an abdominal branch which runs outwards to supply the wall of the abdomen. The distribution of the lumbar and intercostal arteries exhibits a transversely-segmented arrangement of the vascular system, similar to the transversely-segmented arrangement of the bones, muscles, and nerves met with in these localities, but more especially in the thoracic region.

The phrenic arteries, two in number, pass to supply the under surface of the diaphragm.

The middle sacral artery, as already stated, is rather the continuation of the aorta than a branch. As it runs down the front of the sacrum it gives branches to the back of the pelvic wall.

The statement has frequently been made that the visceral and parietal branches of the aorta do not anastomose with each other. Injections made by Turner have, however, shown that, both in the thoracic and abdominal cavities, slender anastomosing communications exist between the two sets of branches. In the abdominal cavity a wide meshed plexus of small arteries, named by him sub- or extra-peritoneal plexus, lies in the fat outside the peritoneum. It communicates, on the one hand, with the perforating branches of the renal arteries and with slender branches of the capsular, spermatic, colic, and pancreatic arteries, and in the region of the diaphragm with the hepatic artery. On the other hand, it communicates with the phrenic arteries, the lower intercostals, the lumbar branches of the aorta, and with the ilio-lumbar, circumflex ilii, and epigastric branches of the iliac arteries, which also go to the wall of the abdomen. In the pelvis also the visceral superior haemorrhoidal artery anastomoses with the middle and lateral sacral arteries. The extra-peritoneal plexus supplies the fat and lymphatic glands lying outside the peritoneum, and it also gives origin to vasa vasorum for the coats of the aorta and vena cava. This plexus may, when the visceral branches of the aorta are obstructed, aid in an important manner in carrying on the circulation. In a subject examined by J. Chiene, in the dissecting room of the University of Edinburgh, where the coeliac axis and the superior and inferior mesenteric arteries were obliterated at their origins, the blood flowed into these arteries and the viscera they supplied through a great enlargement of the arteries of this plexus. In the thoracic cavity a similar plexus, named the extra-pleural plexus, lies between the pleura and pericardium, which communicates on the one hand with the internal mammary arteries, and on the other passes in front of the root of the lung to join the bronchial system of vessels. Another portion of this plexus joins on the one hand the intercostal arteries near the dorsal vertebrae, and on the other passes to the lung at the back of its root.

The branches for the head, neck, and upper limbs arise as three large arteries from the transverse part of the aorta; they are named arteria innominata, left common carotid, and left subclavian. The arteria innominata is the largest; it passes, upwards and to the right, to the root of the neck, and then divides into the right common carotid and the right subclavian. The carotid arteries supply the two sides of the head and neck; the subclavian arteries the two upper extremities.

The subclavian artery is the commencement of the great arterial trunk for the upper limb. It passes across the root of the neck and under the clavicle, when it enters the armpit, and becomes the axillary artery; by that name it extends as far as the posterior fold of the axilla, when it enters the upper arm, takes the name of brachial or humeral artery, and courses as far as the bend of the elbow, where it bifurcates into the radial and ulnar arteries. From the subclavian part of the trunk the following branches arise :— a, Vertebral, which enters the foramen at the root of the transverse process of the 6th cervical vertebra, ascends through the corresponding foramina in the vertebrae above, lies in a groove on the arch of the atlas, and enters the skull through the foramen magnum, where it joins its fellow to form the basilar artery; it gives off muscular branches to the deep muscles of the neck, spinal branches to the spinal cord, meningeal branches to the dura mater, and an inferior cerebellar branch to the under surface of the cerebellum. The basilar artery, formed by the junction of the two vertebrals, extends from the lower to the upper border of the pons Varolii; it gives off collaterally transverse branches to the pons, auditory branches which accompany the portio mollis to the internal ear, inferior cerebellar branches to the under surface of the cerebellum, whilst it breaks up into four terminal branches, viz., two superior cerebellar to the upper surface of the cerebellum, and two posterior cerebral which supply the tentorial aspect of the temporo-sphenoidal lobes, the occipital lobes, and the posterior convolutions of the parietal lobes, b, Thyroid axis, which immediately divides into the inferior thyroid, the suprascapular, and the transverse cervical branches; the inferior thyroid supplies the thyroid body, and gives off an ascending cervical branch to the muscles of the neck; the suprascapular supplies the muscles on the dorsum scapulae; the transverse cervical supplies the trapezius and the muscles attached to the vertebral border of the scapula, c, Internal mammary, supplies the anterior surface of the walls of the chest and abdomen, and the upper surface of the diaphragm, d, Superior intercostal supplies the first intercostal space, and by its deep cervical branch the deep muscles of the back of the neck.

The axillary artery supplies long and short thoracic branches to the wall of the chest and the pectoral muscles; an alar thoracic branch to the fat and glands of the axilla; an acromial thoracic to the parts about the acromion; anterior and posterior circumflex branches to the shoulder joint and deltoid muscle; a subscapular branch to the muscles of the posterior fold of the axilla.

The brachial artery supplies muscular branches to the muscles of the upper arm; a nutrient branch to the humerus; superior and inferior profunda branches and an anastomotic to the muscles of the upper arm and the region of the elbow joint.

The ιdnar artery extends down the ulnar side of the front of the fore-arm to the palm of the hand, where it curves outwards towards the thumb, and anastomoses with the superficial volar and radial index branches of the radial artery to form the sτιperficial palmar arterial arch. In the fore-arm the ulnar gives off the interosseous arteries, which supply the muscles of the fore-arm and give nutrient branches to the bones; two recurrent branches to the region of the elbow; carpal branches to the wrist joint: in the hand it gives a deep branch to the deep muscles of the hand, and from the superficial arch arise digital branches to the sides of the little, ring, and middle fingers, and the ulnar border of the index finger.

The radial artery extends down the radial side of the front of the fore-arm, turns round the outer side of the wrist to the back of the hand, passes between the 1st and 2d metacarpal bones to the palm, where it joins the deep branch of the ulnar, and forms the deep palmar arterial arch. In the fore-arm it gives off a recurrent branch to the elbow joint; carpal branches to the wrist joint; and muscular branches, one of which, named superficialis voice, supplies the muscles of the thumb and joins the ulnar artery: in the hand it gives off a digital branch to the thumb, and one to the radial side of the index, interosseoτιs branches to the interosseous muscles, perforating branches to the back of the hand, and recurrent branches to the wrist.

The common carotid artery runs up the neck by the side of the windpipe, and on a level with the upper border of the thyroid cartilage divides into the internal and external carotid arteries.

The internal carotid artery ascends through the carotid canal in the temporal bone into the cranial cavity. It gives off an ophthalmic branch to the eyeball and other contents of the orbit, and then divides into the anterior and middle cerebral arteries. The middle cerebral artery extends outwards into the Sylvian fissure, and supplies the island of Reil, the orbital part, and the outer face of the frontal lobe, the parietal lobe, and the temporo-sphenoidal lobe; it also gives a choroid branch to the choroid plexus of the velum interpositum. The anterior cerebral artery supplies the inner face of the hemisphere from the anterior end of the frontal lobe as far back as the internal parieto-occipital fissure. At the base of the brain not only do the two internal carotids anastomose with each other through the anterior communicating artery, which passes between their anterior cerebral branches, but the internal carotid on each side anastomoses with the posterior cerebral branch of the basilar, by a posterior communicating artery. In this manner a vascular circle, the circle of Willis, is formed, which permits of freedom of the arterial circulation by the anastomoses between arteries not only on the same side, but on opposite sides of the mesial plane. The vertebral and internal carotid arteries, which are the arteries of supply for the brain, are distinguished by lying at some depth from the surface in their course to the organ, by having curves or twists in their course, whereby the force of the flow of blood is retarded, and by the absence of large collateral branches. Further, as the ophthalmic artery is a branch of the internal carotid, the circulation in the eyeball is in sympathy with that in the brain.

The external carotid artery ascends through the upper part of the side of the neck, and behind the lower jaw into the parotid gland, where it divides into the internal maxillary and temporal branches. This artery gives off the following branches :— a, Superior thyroid to the larynx and thyroid body; b, Lingual to the muscles and mucous membrane of the tongue, and to the sublingual gland; c, Facial to the face, palate, tonsil, and sub-maxillary gland; d, Occipital to the sterno-mastoid muscle and back of the scalp; e, Posterior auricular to the back of the ear and the adjacent part of the scalp; f, Superficial temporal to the scalp in front of the ear, and by its transverse facial branch to the back part of the face; g, Internal maxillary, giving muscular branches to the muscles of mastication, meningeal branches to the dura mater, dental branches to the teeth, and other branches to the nose, palate, and tympanum; A, Ascending pharyngeal, which gives branches to the pharynx, palate, and tonsils.

. The common iliac artery, after a short course, divides into the internal and external iliac arteries. The internal iliac enters the pelvis and divides into branches for the supply of the pelvic walls and viscera, including the organs of generation, and for the great muscles of the buttock. The external iliac descends behind Poupart’s ligament into the thigh, where it takes the name of femoral artery. The femoral descends along the front and inner surface of the thigh, gives off a profunda or deep branch, which, by its circumflex and perforating branches, supplies the numerous muscles of the thigh; most of these extend to the back of the limb to carry blood to the muscles situated there. Tho femoral artery then runs to the back of the limb in the ham, where it is called popliteal artery. The popliteal divides into two branches, of which one, called anterior tibial, passes between the bones to the front of the leg, and then downwards to the upper surface of the foot; the other, posterior tibial, continues down the back of the leg to the sole of the foot, and divides into the internal and external plantar arteries; branches proceed from the external plantar artery to the sides of the toes, and constitute the digital arteries. From the largo arterial trunks in the leg many branches proceed, to carry blood to the different structures in the limb.

The wall of an artery consists of several coats. The outermost is the hιnica adventitia, composed of connective tissue; immediately internal to this is the yellow elastic coat; within this again the muscular coat, formed of involuntary muscular tissue, the contractile fibro-cells of which are for the most part arranged transversely to the long axis of the artery; in the larger arteries the elastic coat is much thicker than the muscular, but in the smaller arteries the muscular coat is relatively strong; the vaso-motor nerves terminate in the muscular coat. Internal to the muscular coat is the elastic fenestrated coat, formed of a smooth elastic membrane perforated by small apertures. Most internal of all is a layer of endothelial cells, which form the free surface over which the blood flows. The arteries are not nourished by the blood which flows through them, but by minute vessels, vasa vasorum, distributed in their external, elastic, and muscular coats.

The Capillaries.— These are the minute tubes which connect together the terminal branches of the arteries and the rootlets of the veins. They vary in diameter in different localities from τ ∙g⅛ σ th to 33⅛ 7 th inch. They are arranged in more or less compact networks, which lie in the interstices between the tissues of the part or organ. The vascularity of a tissue depends upon the relative proportion of the capillaries that it contains. Some tissues, as adult cartilage, the cornea, epithelium, and endothelium, are destitute of capillaries, i.e., are non-vascular. The capillary wall is very simple in structure: in the smallest capillaries it consists merely of a layer υi endothelial cells, continuous with the endothelial lining of the arteries and veins; in the larger capillaries a delicate tunica adventitia is superadded. The transition from a capillary to a small artery or a small vein is marked by the development of a muscular and an elastic coat in the wall of the blood-conveying tube.

The Veins.— The veins convey the blood from the periphery back to the heart, and in their course increase in size, by junction or anastomosis with each other. In most of the veins delicate valves are found, each of which consists of two semicircular segments, and a pouch-like dilatation of the wall of the vein is opposite each segment. When the blood flows along the veins, the valves lie against the wall of the vessel, but if pressure be applied to a vein so as to obstruct the onward flow of the circulation, then the blood passes into the pouch between the wall of the vein and the valve adjacent to the seat of pressure, when the valve closes so as to stop regurgitation. The valves are found especially in those veins where the circulation is likely to be interfered with either by the pressure of the muscles on the veins during their action, or by the pressure of blood caused by gravity, and are usually seated at the points of confluence of veins. They are absent in the veins of the lungs, of the brain, and of several of the abdominal viscera. Some of the veins lie in the subcutaneous fat, and are called superficial veins, others lie amidst the muscles, and form the deep veins. The deep veins accompany the arteries and are named after them; the superficial veins do not accompany arteries; frequent anastomoses take place between the superficial and deep veins.

The veins are arranged primarily into two groups—the Pulmonary veins and the Systemic veins. The distribution of the pulmonary veins will be given in the anatomy of the lungs.

The Systemic veins consist of the coronary venous system; of the system of the superior vena cava; of the system of the inferior vena cava; and associated with the inferior vena cava is the portal venous system. The arrangement of the coronary vein has been described in the anatomy of the heart.

The system of the Superior Vena Cava consists of both superficial and deep veins, and is arranged as follows :—

The superficial veins of the hand commence at the tips and sides of the fingers, from which they proceed along tho back of the hand, beneath the skin of which they may be distinctly seen. They then ascend along the fore-arm, forming three large veins: the radial, on the outer side; the ulnar, on the inner; and the median, in the middle of the front of the fore-arm. At the bend of the elbow the median divides into two branches, of which one joins the radial to form the cephalic, the other joins the ulnar to form the basilic. Into one or other of the two branches of the median the surgeon generally makes an opening when he is desirous of drawing blood from the patient. The cephalic and basilic veins terminate by joining the deep or axillary vein. The communications between the superficial and deep veins are not, however, confined to the point of termination of the former, but occur at various parts of their course.

The deep veins of the hand commence at the tips of the fingers, and pass as digital veins up the sides of the fingers to the palm of the hand, where they form an arch corresponding to the arterial arch of the palm; from this they extend upwards along the front of the fore arm, as far as the bend of the elbow, closely accompanying the arteries of the fore-arm, and receiving from the muscles numerous small branches corresponding to the small arteries sent to those muscles. At the bend of the elbow two brachial veins result from the junction of these different veins of the fore-arm, which pass up the inner side of the upper-arm, closely accompanying the brachial artery as far as the armpit, where they join to form a single large vein, the axillary. They receive in their course many small branches from the muscles. The axillary vein also receives the cephalic and basilic veins. Thus, a single large trunk conveys away all the blood that has been circulating through the upper limb. This large vein passes as the subclavian Fein behind the clavicle, and reaches the lower part of the aide of the neck, where it is joined by the large veins that return the blood from the head and neck.

The veins that return the blood from the inner and outer parts of the head and neck are called the external and internal jugular veins. The external is the smaller, and may commonly be seen beneath the skin on the side of the neck. It returns the blood that has been circulating on the outer part of the head, and must be regarded as a superficial vein. The internal jugular returns the blood that has been circulating on the face, in the brain, and cranial blood-sinuses, and in the deeper parts of the neck. It accompanies the carotid artery, and must thus be regarded as a deep vein. By the junction of the jugular and subclavian veins at the root of the neck a large brachio-cephalic vein on each side is formed; these gradually converge, join, and form a single trunk, the superior vena cava, which, after a short course, enters the upper part of the right auricle of the heart. The veins corresponding to the intercostal arteries, which run between the ribs, do not open directly into either the superior or inferior vena cava, but pass to form the azygos vein, which begins in the cavity of the abdomen, then enters the cavity of the chest, and, as it courses upwards, gradually increases in size by receiving the various intercostal veins, until it finally terminates by joining the superior vena cava.

The system of the Inferior Vena Cava consists of both superficial and deep veins, and is arranged as follows :— The superficial veins of the foot are separated from the deep veins by the strong membrane or fascia which binds down the muscles. They commence by very fine branches arising from the capillaries of the skin. On the back of the foot the digital veins proceeding from the skin of the toes form an arch, from the inner side of which a vein, called the long saphena, arises. This passes upwards along the inner side of the leg and thigh, increasing considerably in size in its course, owing to the number of veins joining it from the extensive surface of the skin of the limb. It terminates, at the upper part of the thigh, by passing through a hole in the fascia, and joins the femoral vein. From the outer side of the same arch arises the external saphenous vein, which runs up the back of the leg to the ham, and pierces the fascia to join the popliteal vein. The deep veins begin both on the back of the foot and in the sole. Those which arise on the back of the foot form the anterior tibial veins, and accompany the anterior tibial artery; they receive a considerable number of branches in their upward course, which proceed from the great mass of muscles lying on the outer side of the leg. The veins which begin in the sole of the foot accompany the plantar arteries, and then pass upwards, along the inner side, of the ankle-joint, to reach the back of the leg, along which they ascend as the posterior tibial veins, closely accompanying the posterior tibial artery, and receiving in their course numerous small veins that proceed from the muscles of the calf of the leg. At the upper part of the leg the anterior tibial veins pass to the back of the leg, and join the posterior tibial veins. The large popliteal vein ? formed by their junction, ascends behind the knee-joint, lying in the ham, along with the popliteal artery. It leaves the upper part of this space, and, passing to the inner side of the thigh, ascends as the femoral vein along with the femoral artery as far as Poupart’s ligament, when it enters the cavity of the abdomen. At the upper part of the thigh it receives the profunda vein, corresponding to the deep artery of the thigh, which conveys bacl the blood that has been carried by that vessel to the numerous large and important muscles of the thigh. The femoral vein is also joined at this spot by the long saphena vein. When the femoral vein enters the cavity of the abdomen it becomes the external iliac vein. The external iliac vein receives the smaller veins which ramify in the lower part of the walls of the abdomen, as well as the large internal iliac vein, which corresponds to the internal iliac artery, and by their junction the common iliac vein is formed. The two common iliac veins gradually converge, and, about the level of the last vertebra of the loins, join to form a single large vein, the inferior vena cava. The inferior vena cava ascends at the back of the abdominal cavity lying on the right side of the aorta. Several veins open into it; some corresponding with the parietal branches of the abdominal aorta, others with the capsular, renal, and spermatic arteries. The greater number of the veins proceeding from the organs contained in the cavity of the abdomen do not open directly into the vena cava, but form a large vein called portal. The vena cava passes through the diaphragm, enters the cavity of the chest, and terminates by opening into the right auricle of the heart.

The Portal system of veins is formed by the veins which proceed from the large and small intestines, from the stomach, pancreas, and spleen; they form the inferior mesenteric, superior mesenteric, splenic, and gastric veins, which join together in the neighbourhood of the pancreas to form the portal venous trunk. The portal vein then ascends to the under surface of the liver, which it enters at the portal transverse fissure. In the substance of the liver it subdivides into branches just like an artery, and the finest branches terminate in the lobules of the liver in a plexus of capillaries.- From this plexus the rootlets of the hepatic veins arise, which joining together form the large hepatic vein, which opens into the inferior vena cava before it pierces the diaphragm. Retzius has pointed out that an extra-peritoneal venous plexus exists in the abdominal cavity, which connects the rootlets of the portal vein with those of the veins of the parietes of the abdomen.

The wall of a vein possesses the same number of coats as that of an artery, but the coats are thinner. Veins are also extensively provided with valves, which are absent from the arteries except at the mouths of the aorta and pulmonary artery.

Lymph-Vascular System. —This subdivision of the! vascular system consists partly of small tubes or vessels, t the lymph vessels, and partly of collections of lymphoid or adenoid tissue (p. 849), the lymph glands. The lymph vessels or lymphatics are tubes with delicate transparent walls, which convey the fluid called lymph and chyle. They arise in the tissues and terminate by joining the venous system, so that their contained fluid flows towards the heart. They resemble veins in having a course from periphery to centre; in possessing valves, which are generally two in number and semilunar in shape; in being divided into a superficial and a deep set—the superficial lymphatics being situated, like the superficial veins, in the subcutaneous tissue; the deep lymphatics accompanying the arteries and deep veins. Lymphatics differ, however, from veins in possessing in their course glandular enlargements, in having thinner coats, in being almost uniform in size, and not uniting into larger vessels as they pass onwards in their course. As a rule they are like fine threads, and their main trunk, the thoracic duct, is not bigger than a crow-quill. The lymph-vessels are divided into lacteal or chyle vessels and lymphatics proper.

The lacteal or chyle vessels, named from the milk-like chyle which they contain, arise in the minute processes called intestinal villi, which project from the free surface of the mucous membrane of the small intestine into the lumen of the boweL The lacteals from adjacent villi form a network in the submucous coat of the intestine, from which larger lacteals arise, which pierce the muscular coat, and then run between the folds of the mesentery to the posterior wall of the abdomen, where, opposite the body of the first lumbar vertebra, they join the deep lymphatic vessels of the abdomen to form the thoracic duct.

The lymphatic vessels proper correspond so closely in their distribution in the extremities and in the head and neck with the veins of those parts, that a special description of their arrangement is nob necessary, the more so as a general representation of these vessels is given in Plate XXI. The superficial and deep lymphatics of the lower limbs enter the abdominal cavity, and are joined by the lymphatics of the pelvis. They ascend in front of the bodies of the lumbar vertebrae, join the lacteal vessels to form the thoracic duct, the place of junction being marked by a dilatation of the duct called receptaculum chyli. The thoracic duct passes through the opening in the diaphragm which transmits the aorta, ascends in front of the bodies of the dorsal vertebrae, receives in its course the deep lymphatics of the left half of the chest, reaches the root of the neck on the left side, is joined there by the deep and superficial lymphatics of the left upper limb and left side of the head and neck, and opens into the great veins at the angle of junction between the left internal jugular and subclavian. This duct conveys, therefore, the chyle during digestion, and the lymph contained in the lymph-vessels below the diaphragm and in the lymph-vessels situated to the left side of the mesial plane in the parts of the body above the diaphragm. The lymph-vessels on the right side of the supra-diaphragmatic parts of the mesial plane do not join the thoracic duct, but converge to the root of the neck on the right side, where they join to form the right lymphatic duct, which opens into the angle of junction of the right internal jugular and subclavian veins.

The mode of origin of the lymph-vessels has long been a vexed question amongst anatomists. The lacteal vessels were at one time supposed to arise by open mouths on the free surface of the intestinal villi, and this idea has been revived in a modified form by some recent observers, who conceive that the lacteals are continuous with a network formed by the anastomoses of processes proceeding from the deep ends of the goblet cells, the mouths of which cells open on the free surface of the villus. The lymph-vessels proper are in some localities continuous with the serous cavities (p. 848); in others they arise within the textures and organs. The most minute lymph-vessels, called lymphcapillaries, like the blood-capillaries, have walls formed of a single layer of elongated endothelial cells. These capillaries take their rise in the connective tissue of a part or organ, and probably spring from spaces, or juice-canals, between the bundles of the connective tissue, which bundles are invested by an endothelial layer of cells. The juice canals are, therefore, a network of minute canals, situated outside the blood-vessels, which allow the tissues to be permeated by a nutrient juice derived from the blood.

In some localities, as the brain and eyeball, the bloodvessels have been described as enclosed in tubular spaces, called peri-vascular canals, in which cells like the corpuscles of the lymph have been seen, and which are believed to be continuous with the lymphatic system. The researches of Ludwig and some of his pupils into the minute structure of the lachrymal gland, the glands of the skin, and the testis, have shown that lymph-capillaries lie in close relation to the secreting structures of these glands.

The coats of the lymph-vessels resemble in structure those of the veins, but they are thinner and more transparent. The valves are small and numerous.

The lymphatic glands are small bodies, varying in size from a pea to an almond, situated in the course of the lymph-vessels in several regions of the body. They are found especially in the groin, armpit, mesentery, back of the abdomen, roots of the lungs, and side of the neck (Plate XXI.) Entering one end of each gland are lymph-vessels, named vasa afferentia, and emerging from the opposite end of the gland are the lymph-vessels named vasa efferentia. Each gland is invested by a capsule of connective tissue, which sends processes into the substance of the gland to divide it into compartments; it consists of adenoid tissue, and the meshes of its retiform connective tissue contain multitudes of lymph corpuscles. Each -gland is permeated by a network of minute canals, which are continuous with both the vasa afferentia and efferentia; the gland, therefore, is traversed by a stream of lymph which washes the lymph corpuscles out of the meshes of the reticulum, and in this manner these corpuscles find their way into the lymph. The lymph glands are, therefore, centres of origin for the lymph corpuscles. The collections of adenoid tissue, forming the solitary and Peyer’s glands of the intestine, and found in the tonsils and other localities (p. 849), are also without doubt centres of formation for the lymph corpuscles.

Blood-Vascular Glands. —Intimately associated with the vascular system are certain organs to which the names of blood-vascular glands, or glands without ducts, are applied. These organs are the spleen, the thyroid gland, the thymus gland, the suprarenal capsules, and portions of the pituitary and pineal glands. The Spleen is situated in the cavity of the abdomen between the stomach and the diaphragm. It is invested by peritoneum, and has a fibroelastic coat in which involuntary muscular fibro-cells are formed. This coat sends multitudes of fine trabeculae-into the interior of the organ, which subdivide it into numbers of minute compartments, in which the red, highly vascular spleen pulp is contained. This pulp consists of collections of small spherical masses of adenoid tissue, forming the Malpighian corpuscles, of the terminal branches of the splenic blood-vessels, and of the lymph-vessels, together with numerous cells, some of which are red blood corpuscles, others lymph corpuscles, others contain pigment granules or fat, others contain in their interior numerous blood corpuscles. The arteries of the spleen in part end in capillaries from which the veins arise, but more frequently they open into lacunae or blood spaces, which give origin to the veins. The Thymus gland, best seen in infancy and childhood, lies in the cavity of the thorax near the base of the heart. It consists of two lobes, each of which is composed of lobules of adenoid tissue, to which numerous lymphvessels may be traced. In the adult it is converted into a mass of fat. The Thyroid gland is situated in the neck at the front and sides of the windpipe. It consists of multitudes of minute closed follicles, each of which is lined by a layer of cells. The Suprarenal capsules, two in number, lie in the abdomen one above each kidney. They contain cells, some of which are arranged in columns, others in a reticulated manner, and are well provided with bloodvessels, nerves, and lymphatics.

Development of the Vascular System.— The vascular system is formed in the middle or mesoblast layer of the early embryo. The cells of the mesoblast lose their original spherical form and become stellate’, the processes of adjacent cells unite together and form a network, and the nuclei rapidly increase in numbers. The peripheral part of the protoplasm of the stellate cells differentiates into a wall of nucleated protoplasm, and forms the wall of the blood-vessels, whilst the central part of the protoplasm liquefies, and the nuclei contained in it become the bloodcorpuscles. If the vessel remains as a capillary, its wall assumes merely the character of a single layer of endothelial cells; but if it becomes an artery or a vein, a further differentiation of the mesoblast cells into the muscular and elastic coats and the tunica adventitia takes place. The heart appears immediately below the head in the form of a collection of cells in the splanchnopleure layer of the mesoblast. It is believed that these cells form in the first instance a solid mass, the central part of which liquefies to form blood and blood-corpuscles, whilst the peripheral cells form the wall of a tube. The heart tube now presents two constrictions, which indicate its division into an auricle, a ventricle, and a bulbus arteriosus. The single ventricle then subdivides into two by the gradual growth of the septum from the apex to the base, and about the eighth week of embryo-life the right and left ventricles are completely separated from each other. A septum then begins to form in the originally single auricle, but its growth is not completed until after the birth of the child, so that during foetal life the cavities of the right and left auricles communicate with each other through a hole in the septum, named foramen ovale. The primitive aortoe, right and left, arise from the ductus arteriosus, and extend upwards to the 1st pair of visceral arches, into which they pass and arch backwards to the sides of the spinal column, where they form the dorsal aortoe. Four additional pairs of arterial arches then spring from the primitive aortae below the 1st pair, and the whole are enumerated from above downwards as the 1st, 2d, 3d, 4th, and 5th pairs of vascular arches. Each arch communicates behind with the dorsal aorta of its own side. The two dorsal aortae then approximate and blend with each other to form the descending thoracic and tho abdominal aorta. A longitudinal septum also forms within the bulbus arteriosus itself, which divides it into two vessels: the one, the ascending aorta, becoming continuous with the cavity of the leftventricle and with the 1st, 2d, 3d, and 4th pairs of vascular arches, the other, the pulmonary artery, becoming continuous with the cavity of the right ventricle and with the 5th pair of vascular arches. The 4th left vascular arch enlarges to form the transverse part of the arch of the aorta, and the left subclavian artery springs as a collateral branch from it. The 4th right arch forms the innominate, and the com mencement of the right subclavian artery. The 3d pair of arches form the two internal carotid arteries; each common carotid is formed from the part of the primitive aorta which connects the 3d and 4th arches with each other; whilst the external carotid is an enlargement of that part of the primitive aorta which runs upwards from the 3d to the 1st visceral arch. From the 5th left vascular arch, which is now continuous with the pulmonary artery, two collateral branches arise, which proceed one to each lung, and form the right and left pulmonary arteries, whilst the terminal part of this arch joins the end of the transverse part of the arch of the aorta, and forms the ductus arteriosus. During foetal life, the lungs being inactive, the blood of the right ventricle which passes into the pulmonary artery almost entirely flows through the ductus arteriosus into the aorta. But when the child is born, and the lungs come into play as respiratory organs, then the blood of the right ventricle flows into the lungs through the right and left pulmonary arteries, and the ductus arteriosus, being no longer required, shrivels up into a slender fibrous cord. The capillaries, veins, and lymphatics are also produced by a histological differentiation of the cells of the mesoblast.

In order to complete the exposition of the subject, the Digestive, Respiratory, Reproductive, and Urinary systems of organs have still to be considered. These will be dealt with in detail under other headings in the succeeding volumes.

(w. t.) [table]

END OF VOLUME FIRST