Digby

DIGESTIVE ORGANS. The organs of digestion, or alimentary apparatus, are for the purpose of receiving the food or aliment; of converting that portion of the food which is digestible into chyle, so that it may be absorbed and applied to the nourishment of the body; and of transmitting that which is indigestible onwards to be excreted.

In the Protozoa there is no special digestive apparatus, but the particles of food are introduced into the general substance of the body, where they undergo digestion and assimilation. But in animals generally there is a definite digestive cavity or stomach, which communicates with the surface by a distinct opening or mouth, through which the food is introduced into the stomach. As a rule a second opening, or anus, is also in communication with the stomach, at which the indigestible parts of the food are excreted. As animals increase in structural complexity the digestive apparatus has additional parts superadded to it. In man and all the more highly organized animals it consists of an elongated tube, the Alimentary Canal, divided into various compartments, into which numerous Glands pour their secretions to be used in the digestive process. In most vertebrates, the great class of birds being excepted, the compartment of the canal called the mouth, or oral cavity, contains a hard masticatory apparatus, the Teeth, which play an important part in breaking down the food.

As the digestive organs in the human body are so constructed as to illustrate one of the most perfect forms of an alimentary apparatus, they will form the special subject of description in this article.

The Alimentary Canal is a tube about 28 feet long, which traverses almost the entire length of the axial part of the body. In man and all other vertebrates, it lies in relation to the ventral surface of the bodies of the vertebrae. It commences on the face at the orifice of the mouth, and terminates on the surface of the lower part of the trunk at the orifice of the anus. It is divided into a series of segments, or compartments, which communicate with each other, from above downwards, in the longitudinal axis of the canal. These compartments are named mouth, pharynx, oesophagus, stomach, small intestine (subdivided into duodenum, jejunum, and ileum), and large intestine (subdivided into caecum, colon, and rectum). The canal is lined by a mucous membrane, called the alimentary mucous membrane, which is continuous with the nasal mucous membrane, with the respiratory mucous membrane, and at the anal and oral orifices with the integument, Outside this mucous membrane is the submucous coat, and external to it is the muscular wall of the canal. By the contraction of the muscular wall the food is propelled along the canal from above downwards. Opening on the surface of the mucous membrane are the orifices of the ducts of numerous glands, the secretions of which, mingling with the food, act chemically on it, so as to render it soluble and capable of being absorbed.

The Mouth, Oral Cavity, or Buccal Cavity, is the dilated commencement of the alimentary canal, in which the food is masticated and mingled with the secretion of the salivary and mucous glands. It is situated in the face, and extends from the lips in front to the pharynx behind. It is bounded above by the hard and soft palate, with the uvula; below by the lower jaw, the mucous membrane of the floor of the mouth, and the tongue; on each side by the cheek; and in front by the lips, between which is the aperture of communication with the surface of the face. Behind it freely communicates with the pharynx through the isthmus faucium. The muscles situated in the lips, cheeks, floor of the mouth, tongue, and soft palate enter into the formation of the walls of the mouth.

The mouth is lined by a red-coloured mucous membrane, which becomes continuous posteriorly with that of the pharynx, and at the margins of the lips with the skin of the face. The mucous membrane covering the alveolar portions of the jaws, and surrounding the necks of the teeth, is called the gum. From the outer surface of each jaw it is reflected to the inner surface of the cheeks and lips. From the inner surface of the lower jaw the mucous membrane is reflected to the floor of the mouth, and a broad band, called froenum linguae, is prolonged to the middle line of the under surface of the tongue.

In its structure the mucous lining of the mouth consists of a stratified pavement epithelium, and a sub-epithelial fibro-vascular corium, possessing numerous vascular papillae. The mucous membrane of the gum is characterized by its density and toughness, due to the numerous strongly developed bundles of connective tissue in the corium, many of which are continued into the fibrous tissue of the periosteum, which covers the alveolar surface of the jaw. The free surface of the corium of the gum possesses numerous broad papillae, and is covered by a stratified pavement epithelium similar to that in the lips and cheeks. The mucous membrane of the hard palate is also tense and tough, though not so much so as the gum; and the fibrous fasciculi of its corium blend with the connective tissue of the subjacent periosteum. The mucous lining of the mouth is a sensitive membrane, and receives its nervous supply from the fifth cranial nerve.

The mucous membrane of the mouth is specially modified on the dorsum of the tongue, in the interval between the circumvallate papillae and the epiglottis, and in the substance of the tonsils, by the development of collections of lymphoid tissue in the sub-epithelial connective tissue.

Thé Tonsils are two almond-shaped bodies, situated, one on each side of the posterior orifice of the mouth, in the fossa between the anterior and posterior pillars of the soft palate. Their normal size is not bigger than a hazel nut, but they are very apt to enlarge, grow inwards across the posterior aperture of the mouth, and diminish the size of that opening. The free surface is marked by several rounded holes, leading into shallow pits or crypts, which may be either simple or branched, in the substance of the tonsil. The pits are lined by the epithelial covering of the mucous membrane, into which minute papiIlae project. In the subepithelial connective tissue of the walls of the crypts numerous follicles of lymphoid tissue are situated, and lymph cells are infiltrated in great numbers in the connective tissue between the follicles. Interspersed amidst the crypts are small racemose mucous glands. The tonsils are very vascular, and capillary blood-vessels are distributed in connection w ith the papillae, the lymphoid tissue, and the racemose glands The tonsillar veins form a plexus in relation to the attached surface of the tonsil.

Mucous and Salivary Glands of the Mouth.— The ducts of numerous glands, engaged in secreting mucus and saliva, open on the free surface of the mucous membrane of the mouth. Their secretion not only keeps the mouth moist, and aids therefore in articulation, but by mingling with the food assists in mastication, deglutition, and the digestive process. Each gland is characterized by being divided into small lobules, and by possessing a duct or ducts, which branch off in an arborescent manner in the substance of the gland, and finally end in the minute lobules. They all belong to the compound racemose group of glands.

The mucous glands of the mouth are situated beneath its mucous lining in the following localities :—(a) labial glands, in the upper and lower lips, but absent at the angles of the mouth; (6) buccal glands, scattered on the inner surface of the cheek from the lips to the opening of the parotid duct; (c) palatine glands, on the oral surface of the hard palate, in the uvula, on both surfaces of the soft palate, and in the tonsils; (sh) molar glands, close to the last lower molar tooth on each side; (e) lingual glands, extending backwards from the tip of the tongue along its margin, and also on its dorsum between the circumvallate papillae and epiglottis. The ducts of these mucous glands consist of a delicate membrane lined by a single layer of columnar epithelial cells. The terminal branches of the ducts which enter the lobules end in a series of saccular dilatations, the acini, alveoli, or gland-vesicles, which contain rounded or polygonal secreting cells. A collection of such vesicles forms a lobule. The lobules are bound together by intermediate connective tissue, in which the blood-vessels divide into a capillary network, that ramifies on the outer surface of the delicate membrane forming the wall of the glandvesicles.

The salivary glands of the mouth are the parotid, submaxillary, and sublingual glands. The parotid gland is the largest salivary gland, and occupies the parotid hollow between the lower jaw and the external ear. Its anterior border overlaps the masseter muscle, and the excretory duct emerges out of this border. A prolongation of gland substance, the socia parotidis, frequently accompanies the duct for a short distance. The excretory duct of the gland, called Stenson , s duct, passes forwards superficial to the masseter muscle, then pierces the cheek, and opens on its inner surface opposite the second upper molar tooth. The duct is between 2 and 3 inches long, and about the thickness of a crow-quill. The submaxillary gland is situated immediately below the lower jaw. The excretory duct of the gland, called Wharton's duct, runs forwards and opens on the floor of the mouth by the side of the fraenum linguae. The sublingual is the smallest of the salivary glands, and lies under the mucous membrane of the floor of the mouth, close to the fraenum linguae. It possesses from ten to twenty small excretory ducts, the ducts of Rivinus, some of which join Wharton’s duct, though the greater number open directly on the floor of the mouth near the fraenum linguae.

Structure.— The ducts of the salivary glands branch and terminate in the lobules,—each terminal duct ending in a series of saccular dilatations, the acini, alveoli, or glandvesicles, the wall of which, formed apparently of a membrana propria, is continuous with the simple membranous wall of the terminal duct. The terminal ducts are lined by a layer of squamous epithelium, and the gland-vesicles contain the secreting cells.

The blood-vessels are distributed in the interlobular connective tissue, and form a capillary network on the wall of the gland-ducts, and on the wall of the gland vesicles.

The Pharynx is an irregularly dilated canal, which forms a common passage, connecting the mouth with the oesophagus, and the nose with the larynx, so as to be subservient to the processes both of deglutition and respiration. Its position and connections have been described under the heading Anatomy.

The wall of the pharynx consists of three coats-—an external muscular and an internal mucous coat, and an intermediate fibrous membrane, which blends with the submucous coat. The muscular coat consists of three pairs of circularly arranged muscles, the constrictors of the pharynx; and of two pairs of longitudinally-arranged muscles, the stylo-pharyngei and plato-pharyngei, with occasionally a third pair, the salpingo-pharyngei. The constrictor muscles extend from the lateral wall to the middle line of the posterior wall of the pharynx, and are named from below upwards the inferior, middle, and superior constrictors; they lie on three different planes, so that the inferior constrictor overlaps the middle, and the middle the superior.

The mucous coat of the pharynx lines the canal, and is continuous through the several openings with the mucous membrane lining the Eustachian tubes, nose, mouth, larynx, and oesophagus.

The epithelium covering the mucous membrane of the nasal part of the pharynx is columnar and ciliated over a considerable surface, but elsewhere the pharyngeal epithelium is tesselated and stratified: and in the latter localities, vascular papillae project into the epithelial layers. Small racemose glands lie beneath the mucous membrane, which is pierced by their ducts to open on the surface (fig. 2); they are most numerous in the nasal part of the pharynx. Collections of lymphoid tissue are found in the sub-epithelial connective tissue, more especially in the nasal part of the pharynx, where it forms a mass, extending across the posterior and upper wall, between the openings of the two Eustachian tubes, which Luschka has called the pharyngeal tonsil. The arteries of the pharynx are derived from the external carotid or some of its branches. The motor, sensory, and sympathetic nerves unite to form the pharyngeal plexus situated behind the middle constrictor muscle.

The Soft Palate forms an inclined plane, which projects, downwards and backwards into the pharynx, from the posterior border of the hard palate. It is less dependent at the sides than in the mesial plane, where it forms an elongated' body, the uvula. Its anterior or oral surface is smooth, and gives origin on each side to a fold, which curves downwards to the side of the root of the tongue, to form the anterior pillars of the palate or fauces. Its posterior or pharyngeal surface, also smooth, gives origin on each side to a fold, which, springing from the base of the uvula, curves downwards and backwards to be lost in the side-walls of the pharynx; this pair of folds forms the posterior pillars of the palate or fauces. The soft palate is complex in structure, and consists of muscles, mucous membrane, glands, blood and lymph vessels, and nerves. The muscles of the soft palate are arranged in two groups, those which elevate and make it tense, and those which constrict the fauces.

The mucous membrane of the soft palate is continuous with that of the mouth and pharynx. The epithelium covering the anterior or oral surface is a stratified pavement epithelium. That on the posterior or pharyngeal surface is in infancy a lammated cylindrical and ciliated epithelium, with isolated areas of pavement epithelium, but in adults it is a laminated pavement epithelium. Numerous racemose mucous glands lie beneath the mucous membrane, but much more abundantly on the oral than on the pharyngeal aspect. Collections of lymphoid tissue, similar to those found in the tonsils, are also met with. The arteries are branches of the internal maxillary, facial, and ascending pharyngeal. The veins of the soft palate often assume a dilated character, and are continuous with the pharyngeal veins. Lymphatics are also distributed beneath the mucous membrane.

The (Esophagus, or Gullet, is an almost cylindrical tube, about 9 or 10 inches long, which transmits the food from the pharynx to the stomach. It commences in the neck opposite the body of the sixth cervical vertebra, where it is continuous with the pharynx. It passes down the lower part of the neck, traverses the cavity of the thorax, pierces the diaphragm at the oesophageal opening, enters the abdomen, and becomes continuous with the cardiac end of the stomach close to that opening.

Structure.— The wall of the oesophagus consists of three coats, named, from without inwards, muscular, submucous, and mucous coats.

The muscular or external -coat is divided into two layers, an external and an internal. The external layer is composed of fibres arranged longitudinally in the wall. The internal layer consists of fibres arranged in a series of rings around the tube, which lie sometimes horizontally, at others obliquely. The muscular coat in the upper fourth of the oesophagus is red, and its fibres are transversely striped; in the second fourth numerous non-striped fibres are mingled with the striped; whilst in the lower half the coat consists exclusively of non-striped fibres. By the contraction of the fibres of the muscular coat the food is propelled downwards into the stomach.

The submucous coat connects the muscular and mucous coats with each other. It consists of bundles of white fibrous tissue intermingled with elastic fibres, and the nerves and blood-vessels passing to the mucous coat ramify in it.

The mucous or internal coat lines the interior of the tube, and is continuous above with the mucous lining of the pharynx, and below with that of the stomach. When the oesophagus is empty it is thrown into longitudinal folds. Its free surface is covered by a thick layer of stratified squamous epithelium, which terminates abruptly at the cardiac orifice of the stomach in an irregular line. Projecting into the epithelium are multitudes of minute conical papillae. Opening on the surface of the membrane are the ducts of numerous small racemose glands similar to those in the pharynx (fig. 2). Collections of lymphoid tissue, forming solitary follicles, are also found in the mucous membrane. The deep surface of the mucous membrane consists of a layer of non-striped muscular tissue, the bundles of which run longitudinally; it forms the muscular layer of the mucous coat, or muscularis mτιcosce.

The oesophagus is supplied with blood by the inferior thyroid artery, the oesophageal branches of the thoracic aorta, and the ascending branch of the coronary artery of the stomach. The nerves are derived from the pneumo-gastrics, which form plexuses containing nerve-cells, not only in the muscular coat, but in the muscularis mucosae. A network of lymphatic vessels also occurs in both the mucous and submucous coats.

Abdominal Cavity and Peritoneum. —As the remaining portions of the alimentary canal are situated in the abdominal cavity, it will be advisable, before describing their anatomy, to give an account of the form and boundaries of that cavity, of its division into regions, and of the general arrangement «f the peritoneum, which constitutes its lining membrane.

The Abdominal Cavity, Abdomen, or Belly, is the largest of the three great cavities of the body. It occupies about the lower two-thirds of the trunk, and extends from the diaphragm above to the pelvic floor below. As its walls, except in the pelvic region, are chiefly formed of muscles and of fibrous membrane, they are much more distensible than those of the thorax, and permit considerable modifications to occur in the size of the viscera contained within the cavity. The abdomen is elongated in form; its vertical diameter is greater than either the transverse or the antero-posterior diameter. The superior boundary is formed by the concave vault of the diaphragm, and by the seven lower pairs of ribs and costal cartilages; in this boundary occur the opening through which the oesophagus passes into the abdomen, and also the apertures for the transmission of the great blood-vessels, the nerves, and the thoracic duct. The inferior boundary is formed by the levatores ani and coccygei muscles, and the pelvic fascia; in relation to this boundary are the termination of the rectum and anal orifice, the termination of the urethra, and in - the female that of the vagina also. The anterior boundary is formed above by the muscles of the anterior abdominal wall and the fascia transversalis; the linea alba occupies its middle fine, and about the middle of the linea alba is the umbilicus or navel; the anterior wall below is formed by the two pubic bones with the symphysis. The lateral walls, or flanks, arc formed above by the flat muscles of the abdominal wall and the fascia transversalis, and below on each side by the ilium and ischium with the muscles attached to them. The posterior wall is formed by the lumbar spine, sacrum, and coccyx, and by the muscles attached to these bones with their accompanying fasciae. The abdomen is primarily divided into the pelvis and abdomen proper. The pelvis is subdivided into the false pelvis, or the part above the pelvic brim, and true pelvis, or part below the pelvic brim.

The Peritoneum is the largest and most complicated serous membrane in the body. Like the other serous membranes, it not only lines the walls of the cavity in which it is situated, but gives a more or less complete investment to the contained viscera. It is arranged, therefore, so as to form a parietal and a visceral part, which are continuous with each other in the various regions where the part lining the wall is reflected as a covering upon the viscera. A space or cavity, called the peritoneal cavity, is inclosed between the parietal and visceral layers. This cavity, as in other serous membranes, is a closed or shut sac, without any communication externally, except in the female, where the two Fallopian tubes open into it.

Through these openings the mucous membrane lining the tubes becomes continuous with the serous membrane, and a communication is established between the lumen of each tube and the peritoneal cavity. That surface of both the parietal and visceral portion of the peritoneum which lies next the cavity is free, smooth, covered by an endothelium, and lubricated by a little serous fluid, which under some pathological conditions may be greatly increased in quantity, so as to cause abdominal dropsy. The moistening of the two free surfaces by the serum permits them to glide smoothly on each other, during the movements of the viscera, and the changes which take place in their size and position. The opposite surface of the peritoneum is attached—that of the parietal part to the fasciae situated internal to the abdominal muscles, that of the visceral part to the subjacent coat of the several organs.

Special names are applied to the folds or duplicatures of the peritoneum, which pass from the wall of the abdomen to the viscera. In the case of the liver, spleen, bladder, and uterus, these folds are named ligaments, whilst the corresponding folds which pass to the intestine have received the name of mesenteries. Folds of peritoneum also pass between certain of the viscera themselves, and these are called omenta.

The Stomach is the bag-like dilatation of the alimentary canal, connecting the oesophagus with the duodenum, in which the food is mingled with the gastric juice, and converted into a pulpy substance—the chyme. The stomach is situated in the costal zone of the abdominal cavity; three-fourths of its volume being contained in the left hypochondrium, whilst the remaining fourth extends into the epigastrium. About five-sixths of the organ lies to the left of the mesial plane, and one-sixth to the right. The stomach varies in size, shape, and somewhat in position, according as it is empty or full of food. When moderately full it is about 1 foot in length, whilst its greatest transverse diameter is 4 to 5 inches. Its general shape is pyriform, and it may be described as possessing two extremities, two surfaces, and two borders. The larger extremity, called the fundus, cardiac extremity, or great cul-de-sac, is directed upwards so as to be in contact with the under surface of the diaphragm, whilst the smaller end, the pyloric or duodenal extremity, is directed downwards, curves to the right, and becomes continuous with the duodenum. The surfaces form the anterior and posterior walls of the stomach. When the organ is empty, the walls are flattened, and in apposition with each other by their inner surfaces; but when it is distended they are convex, The borders of the stomach are curved and unequal in size; one is convex, about three times as long as the other, and is named the greater curvature; the other is concave, and forms the lesser curvature. The curvatures are so arranged that the greater has its convexity directed downwards and to the left, where it lies in relation to the transverse colon and the splenic flexure of the colon. The lesser curvature has ks concavity directed upwards and to the right, and the oesophagus opens into the stomach at the upper end of the lesser curvature. Above this orifice the stomach expands into the fundus, which is situated in the highest part of the left hypochondrium, and occupies therefore the summit of the vault of the left half of the diaphragm. At the lower and right end the two curvatures lie almost horizontally in the epigastrium and terminate at the pylorus, where the stomach becomes continuous with the duodenum. The pylorus, or gate of the stomach, is situated in the epigastrium about three fingers’ breadth below the ensiform cartilage, and immediately to the right of the mesial plane. The junction of the stomach with the duodenum is marked by a circular constriction externally, called the pyloric constriction, and by a valve internally, the pyloric valve. At its pyloric end the stomach presents a small bulging, the lesser cul-de-sac, or antrum pylori.

The stomach is retained in position, partly by its connections with the aesophagus and duodenum, partly by the pressure of the surrounding abdominal walls and viscera, and partly by folds of peritoneum which pass from it to the adjacent structures. These folds are as follows :—The gastro-phrenic ligament extends from the diaphragm to the stomach in the angle between the aesophagus and the cardiac extremity; the gastro-hepatic or small omentum passes from the lesser curvature of the stomach to the lips of the transverse fissure of the liver; the gastrosplenic omentum from the cardiac end of the stomach to the spleen; the gastro-colic or great omentum descends from the greater curvature of the stomach in front of the coils of the small intestine, and then ascends to inclose the transverse colon.

Structure of the Stomach. —The wall of the stomach consists of four coats, named, from without inwards, serous, muscular, submucous, and mucous coats.

The external or serous coat is that part of the peritoneal membrane which incloses the stomach,—one layer covering the anterior, the other the posterior surface. It leaves the stomach at the curvatures, where it forms the great and small omenta, and along these borders the two layers inclose between them the blood-vessels and nerves which supply the organ.

The muscular coat consists of non-striped fibres arranged in three layers from without inwards. The outer layer consists of longitudinal fasciculi, which are continuous with the external longitudinal layer of the aesophagus. They form scattered fasciculi extending longitudinally over the surface of the stomach from cardia to pylorus; but along the two curvatures, more especially the lesser, they are collected into stronger bundles, and at the pylorus they become continuous with the longitudinal fibres of the duodenum. The middle layer consists of circular fasciculi, which form a ring-like arrangement transversely to the long axis of the stomach. These fasciculi are comparatively thin and scattered at the cardiac end, but as they approach the pylorus they become more closely aggregated, so as to form a thick layer, which at the pylorus extends into the pyloric valve, and forms the sphincter pylori muscle. The circular fibres of the stomach are in the same morphological plane as the circular fibres of the aesophagus and duodenum. The inner layer consists of oblique fasciculi, which are not found over the entire organ; the greater number spring from the left side of the cardiac orifice, and radiate on the anterior and posterior surfaces towards the pylorus and greater curvature. These oblique fibres by their contraction approximate the cardia to the pylorus, the great curvature to the smaller, and the anterior to the posterior wall; they are thus the true grinding muscles of the stomach, and have been compared to the muscular gizzard of the bird. From the relation of the two groups of oblique fibres to the cardiac orifice they probably close that opening during gastric digestion. The longitudinal and circular fibres occasion a longitudinal shortening and transverse constriction of the stomach. By the action of the muscular coat the food is churned about in the stomach, so as to become thoroughly intermingled with the gastric juice. The contraction of the sphincter pylori closes the pyloric orifice, and prevents the passage of the food into the duodenum, before it is converted into chyme.

The submucous coat consists of the areolar variety of connective tissue, and lies immediately subjacent to the oblique layer of the muscular coat.

The mucous or internal coat lines the cavity of the stomach, and is continuous with the mucous membrane of the oesophagus and duodenum. It is a soft, pulpy membrane, of a pink colour, which becomes redder during digestion, owing to turgescence of the blood-vessels. At the pyloric end it is often stained yellow or green with bile, and in old people it has a brown colour, from formation of pigment. In the empty stomach it is thrown into folds or rugae, which have usually a longitudinal direction, but when distended the rugae are obliterated, and the surface of the mucous membrane is smooth.. This membrane is commonly said to be thicker at the pyloric end than in the fundus; but Brinton, who had opportunities of examining the stomach of healthy young adults immediately after death, found the cardiac mucous membrane to be more than twice as thick as the pyloric. He ascribes the thinning of the cardiac mucous membrane to the effects of postmortem digestion, owing to the gravitation of the gastric juice, in the recumbent position of the dead body, into the fundus of the stomach.

If the free surface of the gastric mucous membrane be examined with a pocket lens it will be seen to be pitted with shallow depressions or alveoli, polygonal in form, and varying from τ ¾ iy th to ¶¾ð-th inch in diameter. In the sides and bottom of each of these pits numerous rounded orifices may be seen, which are the mouths of the gastric secreting glands. If vertical sections be now made through the mucous membrane, these glands will be seen to be tubular in form.

In the human stomach the tubular glands are, for the most part, simple, almost straight cylinders, and possess an average length of ⅛ffi inch, and a breadth of about - 3 -¾ σ th inch. They are somewhat dilated at their orifices, and at their closed ends give rise to caecal pouches. For about the upper fourth or fifth of their length the tubes are lined by a single layer of columnar epithelium, continuous with the columnar epithelium covering the free surface of the gastric mucous membrane. In the rest of the gland-tube Brinton found two kinds of cells. The one,'the so-called peptic cells, about - t 2 1 00 th inch in diameter, and of an ovoid or somewhat polygonal form, lay next to the wall of the gland. The other kind, somewhat cubical in form, lined the very narrow central canal of the gland, and formed an axial layer, which was continuous above with the columnar epithelium lining the upper end of the tube.

It is in the dog and cat, however, that the structure of the gastric mucous membrane has especially been studied, and two kinds of glands have been described. The one, situated especially in the region of the pylorus, consists for the most part of simple tubes, which may, however, branch at their deeper end; they have’ been called the mucus glands. They are lined by a columnar epithelium, the cells of which at the deeper end of the gland are more cubical in form, and have a clouded granular appearance. The other kind of gland is situated in the remaining part of the gastric mucous membrane, and consists of tubes which divide usually into four branches; they have been named the peptic glands. The cellular lining of these peptic glands closely corresponds with the dimorphous arrangement in the human stomach already referred to. Heidenhain states that in a fasting dog the glands are shrunken, and the axial cells are transparent, whilst during digestion the peptic glands are swollen out and the cells are clouded and granular.

The gastric glands are separated from each other by slender prolongations of the muscularis mucosae, and by the vascular inter-glandular connective tissue, which is soft and delicate, and contains a small proportion of lymphoid corpuscles diffused in it. In some localities the lymphoid tissue may be collected into solitary follicles, forming the lenticular glands of the stomach. Beneath the glands is a well-defined muscularis mucosae, arranged in two layers, which gives off bundles that pass between the gastric glands.

The gastric mucous membrane is highly vascular; small arteries enter it from the submucous coat, and terminate in a capillary plexus, situated in the inter-glandular connective tissue surrounding the gastric glands; a vascular capillary ring surrounds the orifice of each gland.

The pyloric valve is the name given to the circular fold, situated at the junction of the stomach and duodenum, which surrounds the pyloric orifice. This fold is covered on its free surface by mucous membrane, which incloses the submucous coat and the circular layer of the muscular coat, but not the longitudinal layer, or the serous coat. That portion of the mucous membrane which covers the gastric surface of the valve possesses the structure of the mucous membrane of the stomach; whilst that which covers the duodenal surface is studded with villi, and possesses the structure of the intestinal mucous membrane.

The arteries of the stomach form arches along the greater and lesser curvatures, and anastomose in the anterior and posterior walls of the stomach. The veins of the stomach are rootlets of the portal vein. The lymphatics are numerous, and form a superficial and a deep set. The nerves of the stomach are derived from the epigastric plexus of the sympathetic and from the pneumogastric nerves.

The Intestinal Canal, Intestine, Gut, or Bowel, is situated in the abdominal cavity, and extends from the pyloric opening, or gate, of the stomach to the orifice of the anus. In it the chyme becomes mingled with the bile, the pancreatic fluid, and the secretions of the intestinal glands, and is converted into chyle. In it also the absorption of the chyle takes place, and the insoluble part of the food' is passed onwards to be excreted in the form of faeces. The intestine is the longest division of the alimentary canal, and measures on an average about 25 feet. It is primarily divided into two parts, called small intestine and large intestine; the length of the small is about 20 feet, that of the large about 5 feet.

Φhe Small Intestine is the upper of the two divisions of the canal, and consists of a convoluted, almost cylindrical tube, which reaches from the pylorus to the caecum, or commencement of the large intestine. It is subdivided into three portions, named duodenum, jejunum, and ileum.

The Duodenum is the commencement of the small intestine, and has received its name from its length being regarded as about equal to the breadth of twelve fingers. It forms the shortest and widest of the three sub-divisions of the small bowel; it curves, in the form of a horse-shoe, from the pylorus to opposite the left side of the body of the second lumbar vertebra, where it becomes continuous with the jejunum. The duodenum is distinguished from the rest of the small intestine by having the ducts of the liver and pancreas opening into its canal, by containing in its wall a collection of compound racemose glands, named the glands of Brunner, and by being developed from the primitive fore-gut, and not, like the jejunum and ileum, from the primitive middle gut. Like the stomach, it should be regarded as a distinct segment of the alimentary canal.

The Jejunum and Ileum form by far the longest part of the small intestine, and are not separated from each other by any sharp line of demarcation —the upper two-fifths being called jejunum, on account of its being usually empty after death, the lower three-fifths being termed ileum, from its convoluted arrangement. They occupy the umbilical, hypogastric, right and left iliac regions of the abdomen, in which they are arranged in a series of coils or convolutions; one or two coils of the ileum sometimes lie in the cavity of the pelvis, between the bladder and rectum. The coils are attached to the posterior wall of the abdomen, along a line from the body of the first lumbar vertebra to the right sacro-iliac joint, by the fold of peritoneum called the mesentery. Owing to the extent of the mesentery, the coils of the jejunum and ileum can be freely moved about in the abdominal cavity, so that they are apt to be displaced from their natural position, and, when a rupture occurs, to become the most usual contents of the hernial sac. The lower end of the ileum passes into the right iliac fossa, where it becomes continuous with the large intestine, at the junction of the caecum and ascending colon. Though the line of demarcation between jejunum and ileum is an arbitrary one, yet the upper end of the jejunum may be distinguished from the lower end of the ileuπι by being wider, and having a thicker mucous membrane, in which the folds called valvulae conniventes are larger and more numerous.

Structure of the Small Intestine.— The wall of the small intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, submucous, and mucous coats.

The serous or external coat, derived from the peritoneum, forms a complete investment for the jejunum and ileum, and is continuous with the mesentery along a line of attachment, named the mesenteric border of the intestine; but the serous covering of the duodenum is incomplete.

The muscular coat consists of non-striped fibres arranged in two layers from without inwards. The outer layer consists of longitudinal fasciculi, which form a thin layer parallel to the long axis of the intestine. The inner layer consists of circular fasciculi arranged around the gut transverse to its long axis; this layer is thicker, stronger, and more highly coloured than the longitudinal layer. By the contraction of the muscular coat, the peristaltie or vermicular movement is produced, which propels the ingested materials along the intestine.

The submucous coat lies immediately subjacent to the circular layer of the muscular coat, and consists of areolar connective tissue; in it the blood-vessels ramify before they pass into the mucous membrane.

The mucous or internal coat is a soft, velvety-looking membrane, which lines the wall of the small intestine, and possesses a complex appearance and structure. The inner surface is not smooth, but is thrown into strongly-marked, transverse folds, the valvulae conniventes, which are not obliterated during distension'of the gut. They are very numerous in the duodenum and jejunum, but then decrease in size and numbers, until at the lower end of the ileum they have disappeared. Each valvula consists of a fold of the mucous membrane with its submucous coat. Owing to their presence, the extent of the mucous surface is much greater than if it were a plane-surfaced membrane.

In its more minute structure the mucous coat may be regarded as composed of numerous projecting bodies, a glandular layer, and a muscular layer.

The projecting bodies are the intestinal Villi, which jut out into the lumen of the intestine from the free surface of the mucous membrane, not only of the valvulae, but of the intermediate surface. They are delicate, minute processes, varying in length from a fourth to half a line, and in number amount to several millions.

They are best examined when the mucous surface is placed in water or spirit, when they may be seen with the naked eye, or, still better, with a pocket lens; when the chyle-vessels or blood-vessels are injected, they become erected, and stand out more prominently from the surface. They vary in form, being filiform, or cylindrical, or conical, or club-shaped, or leaf-shaped. They are more numerous in the duodenum and jejunum than in the ileum, and to their presence is due the velvety appearance of the mucous surface. They are not found elsewhere than in the small intestine.

As they are the parts of the mucous membrane directly concerned in the absorption of the chyle, their structure is interesting and important. Each villus is invested by a cap of epithelium continuous with the general epithelial covering of the mucous membrane. The epithelium consists of a single layer of columnar cells, compactly arranged side by side. Scattered amidst the columnar cells are cells which possess the form of microscopic goblets, and are named goblet cells. The free end of each goblet cell appears to have an open mouth on the surface of the villus, through which a mucus-like substance exudes. Various opinions have been expressed as to the nature of these goblet cells. Some regard them as special structures engaged either in the absorption of chyle, or the secretion of mucus; others look upon them as merely modifications of the columnar epithelium; whilst others again consider them to be post-mortem productions, due to the swelling out of the columnar epithelium by the imbibition of fluid. There can be no doubt, however, that they are not specially concerned in the absorption of chyle, as cells of the same character are found in the respiratory mucous membrane, and on other surfaces, where the absorption of chyle does not take place.

The sub-epithelial tissue of a villus forms its matrix or basis substance, and consists of the sub-epithelial connective tissue of the mucous membrane. When thin sections through a villus are examined, the matrix is seen to be composed of a delicate retiform tissue, which îorms a network, in the meshes of which numbers of colourless lymphoid corpuscles are imbedded. These cells were described and figured by Goodsir, as the absorbing cells or vesicles of the villus. In the axis of the villus one, or perhaps two, minute lacteals or chyle vessels are situated, which serve as rootlets of origin of the lacteal division of the lymph vascular system. The lacteal is a capillary tube, which ends near the apex of the villus, as a dilated microscopic cul-de-sac. By its opposite extremity it becomes continuous with a plexus of lacteals in the submucous coat. In the matrix substance, around the lacteal vessel of the villus, is a layer of non-striped muscular fibre-cells, which is continuous with the general muscular, layer of the mucous coat, and extends as far as the apex of the villus. By the contraction of this layer the chyle during absorption is propelled along the lacteal vessel. The villus also contains blood-vessels; a small artery enters at its attached base, and terminates in a capillary plexus, situated in the peripheral part of the matrix, close to the cap of epithelium; from the plexus a vein arises, which leaves the villus at its base, and joins the veins in the submucous coat.

Various theories have been put forward to account for the mode of passage of the chyle, during digestion, from the lumen of the intestine into the lacteal vessels of the villi; but the question cannot even yet be regarded as definitely settled. The appearance of a network of minute tubules within the matrix, extending from the epithelial investment to the lacteal, which Letzerich supposed to be the channels along which the chyle flowed, is doubtless produced by the arrangement of the strands of the tetiform tissue. There seems little doubt that both the cells of the epithelial investment and those of the retiform tissue of the matrix become distended with the particles of chyle previous to its passage into the lacteal. The view advanced by Schafer, that the corpuscles in the meshes of the retiform tissue may serve as carriers of the fatty particles of the chyle into the lacteals, is but another mode of expressing the function of these cells advocated thirty years ago by Goodsir.

The mucous membrane of the small intestine is abundantly provided with secreting glands, named the glands of Brunner and of Lieberkühn.

Brunner's glands are confined to the duodenum; they belong to the compound racemose group of glands, and resemble generally in structure the mucous and salivary glands. The minute lobules of these glands lie in the submucous coat, and the excretory duct pierces the mucous membrane to open on the surface. The wall of the duct is formed of connective tissue lined by columnar epithelium. The finest branches of the duct are continuous with the acini or gland-vesicles, and the gland-vesicles contain the secreting cells, which are columnar in form. A plexus of capillary blood-vessels is distributed outside the membrana propria of the gland-vesicles, and lymphatic vessels ' lie around the lobules. Into the duodenum, about the junction of its descending and horizontal portions, the duct of the pancreas, and the bile duct from the liver, open by a common orifice. These glands may be regarded, therefore, as accessory glands to this portion of the small intestine.

The glands of Lieberkuhn are distributed throughout the whole length of the mucous coat of the small intestine. They are simple tubular glands, in shape like test tubes, which lie vertically in the mucous membrane, and form its proper glandular layer (figs. 5 and 6). The tubes are microscopic in size, vary in length from ⅛th to - 5 ⅛th of a line, and are sometimes closely set together, but in the localities where the solitary and Peyer’s glands occur they are more widely separated. The glands open on the surface of the mucous membrane between the villi; and the opposite end of the tubes is closed and rounded, and reaches close to the muscular layer of the mucous coat. They are lined by a layer of columnar' epithelium cells, continuous with the epithelial investment of the villi. The glands are separated from each other by retiform connective tissue, in the meshes of which colourless lymphoid corpuscles exist in considerable numbers; the plexus of capillary blood-vessels, which is distributed outside the membrana propria of the gland tube, lies in this connective tissue.

The connective tissue of the mucous coat is characterized generally by its retiform character, and by the diffusion of colourless lymphoid corpuscles in the meshwork. But in some parts of the mucosa these corpuscles, with their supporting framework of retiform tissue, are collected into distinct masses or follicles, visible to the naked eye, and known as the solitary and Peyer’s glands or follicles.

The solitary glands are scattered throughout the whole length of the intestinal mucous membrane. They are about the size of millet seeds, and vary in number and distinctness in different individuals. They are globular or ovoid in form, and occasion a slight elevation of the mucous membrane. One pole of the gland lies next the free surface of the mucous membrane, and is in relation to the columnar epithelium covering the mucosa, whilst the opposite pole rests on the submucous coat.

Peyer's, glands, or the agminated glands, consist of an aggregation of solitary glands or follicles, which are crowded together, so as to form distinct elongated patches, which may vary in length from ¾ inch to 3 or 4 inches. The long axis of each patch corresponds to the long axis of the intestine, and the patches are placed opposite to the mesenteric attachment of the bowel. Villi either may or may not be situated on the surface of the patch, in the intervals between the individual follicles, but Lieberkühnian glands are always found opening on the surface, and frequently forming a ring of orifices around each follicle. Peyer’s patches are most abundant in the lower end of the ileum, but diminish in size and numbers in its upper end and in the jejunum, and are absent in the duodenum.

These follicles are lymphoid organs, and are composed of lymphoid or adenoid tissue. The solitary and Peyer’s glands, as is the case generally with the lymphoid organs, are more distinct and perfect in structure in infancy and childhood, than in adults or in advanced age.

The muscular layer of the mucous membrane lies next to the submucous coat, aud consists of non-striped fibres which lie parallel to the surface of the membrane. It passes into the substance of the villi, and lies around the closed end of the glands of Lieberkühn.

Of the blood-vessels of the small intestine, the arteries enter the wall of the jejunum and ileum at its attached or mesenteric border, and are branches from the arcades of the superior mesenteric artery. They run in the sub-serous tissue around the wä,ll of the intestine; then pierce the muscular coat and supply it; they then enter the submucous coat, and a form a network from which branches pass into the mucous coat. The veins accompany the arteries, and form rootlets of the superior mesenteric vein.

The lymph-vessels, or lacteals, may be traced into the wall of the intestine at the mesenteric border; they form a network in the muscular coat, and then enter the submucous coat, where they are very abundant; from this submucous layer offshoots pass through the retiform tissue, which lies between the Lieberkühnian glands, into the villi. Where the solitary and Peyer’s glands are situated, the lacteals, as Frey has pointed out, form a system of anastomosing vessels around the base and mesial part of each follicle.

The nerves are derived from the plexuses of the sympathetic, which accompany the branches of the superior mesenteric artery. They form between the two layers of the muscular coat an important plexus, named, after its discoverer, Auerbach’s plexus, in which large stellate nervecells are intermingled with nerve-fibres, and a similar nervous plexus is found in the muscular coat of the other divisions of the alimentary canal. It supplies and regulates the movements of the muscular coat.

The Large Intestine, though not nearly so long as the small intestine, is of much greater diameter. It reaches from the end of the ileum to the orifice of the anus, and is divided into the caecum with the appendix vermiformis, the colon, and the rectum; whilst the colon is subdivided into the ascending colon, the hepatic flexure, the transverse colon, the splenic flexure, the descending colon, and the sigmoid flexure.

The Caecum, the dilated commencement of the large intestine, lies below the ileum, and occupies the right iliac fossa. It forms a large cul-de-sac, closed in below, but communicating freely above with the ascending colon. Opening on the inner and posterior wall of the caecum is the appendix vermiformis, which is a slender hollow prolongation of the bowel, varying in length from 3 to 6 inches. It has the calibre of the stem of a common tobacco pipe, and ends in a free closed extremity, so that, like the caecum, it is a cul-de-sac. It is not generally found in mammals, but is present in man, the orang, certain lemurs, and the marsupial wombat.

The Colon extends from the caecum to the rectum, and forms the longest part of the large intestine. The transverse part of the colon lies immediately below the great curvature of the stomach, but owing to the length of the transverse meso-colon, which forms its peritoneal attachment, it not unfrequently undergoes some change in its position, and may hang downwards towards the pelvis, or be elevated in front of the stomach, or thrown to the right or left side.

The sigmoid flexure of the colon is situated in the left iliac fossa, but as the sigmoid meso-colon, which forms its peritoneal attachment, is of some length, it is freely movable, and not unfrequently hangs into the pelvis, or even extends across into the right iliac fossa.

The Rectum is the terminal segment of the large intestine, and extends from the sigmoid flexure to the orifice of the anus. It lies in the cavity of the pelvis. It commences opposite the left sacro-iliac joint, and passes at first obliquely downwards and to the right until it reaches the middle line of the sacrum; secondly, it closely follows the curvature of the sacrum and coccyx, lying in relation to their anterior surface; thirdly, when it reaches the tip of the coccyx its terminal or third part inclines downwards and backwards for about 1 ‡ inch to the anal orifice. The anus opens on the surface of the middle line of the perineum, midway between the two ischial tuberosities, and the skin surrounding the orifice is thin, and wrinkled when the opening is closed. Immediately beneath the skin is the sphincter ani externus muscle, which forms a thin layer of fasciculi, arranged in a series of ellipses around the orifice. The sphincter in its normal condition of contraction simply closes the opening, but, under the influence of the will, a more powerful contraction can be induced, so as to resist the entrance of foreign bodies into the rectum.

The large intestine is arranged in the abdominal cavity in the form of an arch, the summit of which is the transverse colon, whilst the caecum and rectum are the right and left piers. Within the concavity of this arch the coils of the jejunum and ileum are situated. The large intestine is not, except in the rectum, a cylindriform tube, but is dilated into three parallel and longitudinal rows of sacculi, which rows are divided from each other by longitudinal muscular bands, whilst the sacculi in each row are separated externally by intermediate constrictions. In the rectum the sacculi have disappeared, and the intestine assumes a cylindrical form, but at its lower end it dilates into a reservoir, in which the faeces accumulate prior to being excreted.

At the junction of the large with the small intestine a valvular arrangement, termed the ileo-caecal or ileo-colic valve, is found. This valve is due to the peculiar manner in which the ileum opens into the large intestine.

The opening is bounded by two semi-lunar folds, which project into the large bowel. These folds are the two segments of the valve; one situated above the opening is the ileo-colic segment, the other, below the opening, the ileo-caecdl. The two segments become continuous with each other at the ends of the elongated opening, and are prolonged for some distance around the inner wall of the large intestine as two prominent ridges, named the fraena of the valve. The use of the ileo-caecal valve is to impede or prevent the reflux of the contents of the large into the small intestine. When the caecum and colon are distended the fraena of the valve are put on the stretch, and the two segments are approximated, so that the opening is reduced to a mere slit, or even closed, if there is great distension of the bowel.

Structure of the Large Intestine.— The wall of the large intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, sub mucous, and mucous coats.

The serous or external coat, derived from the peritoneum, forms a complete investment for. the flexures of the colon, the transverse colon, and the first part of the rectum, but not for the caecum, or the ascending and descending colon. The second part of the rectum has only a partial serous investment, and the third part has no serous coat. Numerous pedunculated ’processes invested by the serous membrane, and containing lobules of fat, named appendices epiploicae, are attached to the large intestine.

The muscular coat consists of non-striped fibres arranged in two layers from without inwards. The outer layer consists of longitudinal fasciculi, which are not as a rule distributed uniformly in the wall, but in the caecum and colon are collected into three longitudinal bands, which start from the caecum, where it is joined by the appendix vermiformis, and extend along the colon to the rectum. As these bands are not so long as the colon itself, they occasion the puckerings which separate the sacculi, so that when the bands are cut through the sacculi disappear. The colon then becomes more elongated and cylindriform.

In the appendix vermiformis the longitudinal layer is not collected into bands, but arranged uniformly along the wall. In the rectum, also, the longitudinal layer is spread uniformly along the wall, and forms a well-defined red-coloured layer.

The inner layer of the muscular coat consists of circular fasciculi distributed around the wall of the large intestine. In the rectum this layer increases in thickness, and in proximity to the anus forms a circular muscle, the sphincter ani internus, which is a strong band, about half an inch broad, around the lower end of the rectum. In the large, as in the small intestine, the muscular coat occasions the peristaltic movements, and its increased thickness in the rectum is for the purpose of expelling the faeces.

The submucous coat has similar relations and Structure to the corresponding coat in the small intestine.

The mucous or internal coat is not thrown into valyulae conniventes, but presents a series of well-marked permanent ridges, lying transversely or somewhat obliquely to the long axis of the gut, and corresponding internally to the constrictions, which, on the outer surface of the colon, separate the sacculi from each other. The mucous membrane of the large intestine is covered by a layer of columnar epithelium. It is devoid of villi, and consists of a glandular and a muscular layer. The secreting glands of the glandular layer have the form and structure of the Lieberkühnian glands of the small intestine (fig. 7); they open on the free surface of the mucous coat, and, owing to the absence of villi, their mouths are more closely set together than is the case with the corresponding glands in the small intestine; the tubular glands are separated by a retiform tissue with lymphoid corpuscles. Solitary glands, similar to those in the small intestine, are also present, but no Peyer’s patches. The muscularis mucosae resembles generally that of the small intestine.

Of the blood-vessels of the large intestine, the arteries are principally derived from branches of the superior and inferior mesenteric arteries, but the lower end of the rectum receives the haemorrhoidal branches of the internal iliac and the pudic. The veins which correspond to these arteries for the most part join the superior and inferior mesenteric veins, and are consequently rootlets of the portal. But the veins which belong to the middle and inferior haemorrhoidal arteries forma plexus about the anal orifice, which partly joins the superior haemorrhoidal vein, and through it the portal vein, and is partly connected through the middle and inferior haemorrhoidal veins with the internal iliac vein, and through it with the inferior vena cava. The veins about the anus are very apt to become varicose, and to form the excrescences termed haemorrhoids or piles. The lymph vessels are arranged as in the small intestine, except that they are not prolonged into villi. Nervous plexuses with ganglion cells are fcund in both the muscular and submucous coats. They proceed from the superior and inferior mesenteric plexuses, but the rectum receives branches from the hypogastric plexus, and from the third and fourth sacral spinal nerves.

The Liver is the biggest of the abdominal viscera, and the largest gland in the body. It is the organ in which the secretion of bile takes place, and is the chief seat in the body of the formation of glycogen, a substance like dextrin, which readily undergoes conversion into sugar. It lies in the costal zone of the abdomen, fills up the greater part of the right hypochondrium, and extends, through the epigastrium, into the left hypochondrium. In its long or transverse diameter it averages about 12 inches, in its antero-posterior diameter about 6 inches, in the vertical diameter of its thickest part about 3 inches. Relatively to the size of the body the liver is bigger and heavier in the foetus than in the adult; soon after birth the relative weight declines, and that of the left lobe diminishes much more rapidly than the right lobe. Frerichs states that the relative weight of the healthy liver fluctuates in adults between √ τ th and 1 1 π th of that of the body, and the absolute weight varies from 1 , 8 to 4√6 pounds avoird. During the digestion of the food the liver increases both in size and weight, partly from the greater quantity of blood flowing through it, and partly from the new material in the secreting cells; whilst after a long fast it becomes smaller and lighter.

For descriptive purposes the liver may be regarded as having two surfaces, two borders, and two extremities.

The superior or diaphragmatic surface is smooth and convex, and attached to the diaphragm by the falciform ligament.

The posterior or vertebral bordeι ’ is comparatively thick, and attached by the coronary ligament to the diaphragm. The anterior border of the liver is unattached, thin, and attenuated, and is marked by a deep notch, opposite the anterior edge of the falciform ligament, which lodges the round ligament of the liver.

Of the two extremities of the liver the right is thick and massive, and lies deep in the right hypochondrium, in contact with the diaphragm; the left is thin and attenuated, and overlaps the oesophageal opening and fundus of the stomach.

The inferior or visceral surf ace of the liver is much more complex in form than the upper. The longitudinal or umbilical fissure, continuous with the notch in the anterior border of the liver, and much nearer to the left than the right extremity of the gland, divides it into a large right and a small left lobe. In the anterior part of the fissure the round ligament, formed by the obliteration of the umbilical vein of the foetus, is lodged; whilst the posterior part contains a slender fibrous cord formed by the obliteration of a vein of the foetus, named ductus venosus. The longitudinal fissure is often bridged across by a band of liver substance called pons hepatis. The under surface of the left lobe is smooth, and overlaps the anterior surface of the stomach. The under surface of the right lobe is divided into smaller lobes by fissures and fossae. Starting from about the middle of the longitudinal fissure is the portal or transverse fissure, which extends for from 3 to 4 inches across the under surface of the right lobe. It is the gate (porta) of the liver, the hilus or fissure of entrance into the organ of the portal vein, hepatic artery, hepatic duct, and hepatic nerves and lymphatics. A short distance to the right of that part of the longitudinal fissure in which the round ligament lies, is the fossa for the gall bladder, which is a depression on the under surface of the right lobe extending from the anterior border to the transverse fissure: in it the gall bladder lies. Extending somewhat obliquely from the posterior border of the liver, towards the transverse fissure, is a deep fossa for the inferior vena cava. Opening into the vena cava as it lies in this fossa are the trunks of the large hepatic veins from the substance of the liver. A portion of liver substance, which is bounded by the gall bladder, the longitudinal fissure, the transverse fissure, and the anterior border, forms a four-sided lobe called lobus quadratus. Another portion, bounded by the transverse fissure, the posterior border, the vena cava, and the longitudinal fissure, is the lobus Spigelii. A thin prolongation of liver substance continuous with the lobus Spigelii, and running obliquely between the fossa for the inferior cava and the transverse fissure, is the lobus caudatus. Structure of the Liver.— The liver is a solid Organ, of a brownish-red colour. It is composed of the ramifications of the portal vein, of the portal capillaries, the hepatic vein, the hepatic artery, the hepatic duct, of secreting cells, nerves, and lymphatics. These several structures are bound together by connective tissue, and the organ is invested by the peritoneum. The liver possesses two coats, a serous and a fibrous.

The serous or external coat is a part of the peritoneal membrane, and forms an almost complete investment for the liver. It is reflected from the transverse fissure as the gastro-hepatic omentum, and from the upper surface and the posterior border as the falciform, coronary, and right and left lateral ligaments of the liver.

The fibrous coat, or tunica propria, is immediately subjacent to the serous coat. When carefully raised from the liver delicate processes of areolar tissue may be seen to pass from its deep surface into the substance of the organ. At the transverse fissure it is prolonged into the liver as a very distinct sheath, enveloping the portal vein, hepatic artery, hepatic duct, nerves, and lymphatics. This sheath is named the capsule of Glisson, and is prolonged throughout the substance of the organ, along the ramifications of the portal vein and the structures that accompany it.

Lobules of the Liver.— To the naked eye the substance of the liver doss not present a homogeneous aspect, but is mottled, and mapped out into multitudes of small areas or lobules,—the hepatic lobules or leaflets. The lobules of the liver are irregular polygons, and vary in size from ^⅛th to ⅛ths of an inch. In man and the mammalia generally the lobules are imperfectly separated from each other by the interlobular vessels and duct, and a scarcely appreciable quantity of areolar connective tissue. In the pig, camel, and polar bear, each lobule is circumscribed by a definite capsule of connective tissue.

As a lobule of the liver is a liver in miniature, and as the structure of the entire liver is the sum of the structure of its lobules, it will be necessary to examine with care the constituent parts of a lobule, and the arrangement of the vessels, duct, and nerves which pass to and from it. An hepatic lobule is composed of blood-vessels, secreting cells, and bile-ducts, with perhaps nerves and lymphatics. The blood-vessels will first be considered.

The portal vein conveys to the liver the venous blood from the stomach, spleen, pancreas, gall bladder, and small and large intestine. It ascends to the transverse fissure, and before it enters the liver divides into two branches, one for the right and one for the left lobe. In its course within the liver, the portal vein divides and subdivides after the manner of an artery. It is closely accompanied by the hepatic artery and duct, and, along with them, is invested by the fibrous sheath, called Glisson’s capsule. The terminal branches of the portal vein run between the lobules, and are named, from their position, the interlobular branches. The interlobular branches lie around the circumference of a lobule, and anastomose with each other. They partly terminate directly in a capillary network situated within the lobule, and partly give off fine branches, which enter the lobule before they end in the capillary network. The intralobular capillaries form a close network, and converge from the periphery of the lobule, where they spring from the interlobular branches of the portal vein, to the centre of the lobule, where they terminate in the intralobular or central vein, one of the rootlets of the hepatic vein. In man, where the lobules are not separated from each other by a distinct capsule, the capillaries of one lobule to some extent communicate with those of adjacent lobules.

The hepatic artery closely accompanies the portal vein, and divides into two branches, for the right and left lobes. It is the nutrient artery of the liver, and gives off three series of branches :— (a) vaginal branches, which are distributed to the walls of the portal vein, the hepatic duct, and to Glisson’s capsule, probably also to the wall of the hepatic vein; they end in a capillary network in these structures, from which vaginal veins arise that terminate in the portal vein; (6) capsιdar branches, which are distributed to the fibrous coat of the liver, and end in a capillary network, from which arise capsular veins that join the portal vein; (c) interlobιdar branches of the hepatic artery lie along with the interlobular branches of the portal vein, and end in the capillary network within the lobules.

The hepatic vein arises within the substance of the liver from the intralobular capillaries. In the centre of each lobule is the intralobular or central vein. It traverses the axis of the lobule, and leaves it to join a small vein running immediately under the bases of adjacent lobules, which, from its position,is named the sublóbular vein.

Adjacent sublobular veins then join together, and form larger vessels, which are the trunks of the hepatic vein, or the hepatic venous canals These trunks run towards the posterior border of the liver, and open into the inferior vena cava.

From this description of the vascular arrangements within the liver, it will be seen that the intralobular capillaries are continuous with three vascular trunks,—two which carry blood to them, the portal vein and the hepatic artery, and one which conveys the blood away from them, the hepatic vein. The communication in each case is so free that the capillaries can be artificially injected from any one of these vessels.

The secreting cells of the liver, hepatic cells, form the proper parenchyma of the organ. They are situated within the lobules, and occupy the spaces of the capillary network. The cells vary in diameter from g¾ σ th to ττ ¾ rσ th inch; they have the form of irregular polyhedrons, with from four to seven sides, and with the angles sometimes sharp, at other times rounded. They do not appear to possess definite walls, but have a distinct nucleus. The cell protoplasm is granular, and usually contai ns fat drops, and yellow particles, apparently bile pigment. The general arrangement of the cells is in rows or columns, and when sections are made through a lobule, transverse to the long axis of the central vein, the columns of cells are seen to converge from the periphery to the centre of the lobule, and to form a network.

By many observers the cells are regarded as in contact with the intralobular capillaries, without the intervention of an intermediate membrane. By others, and more especially by Lionel Beale, the secreting cells are regarded as inclosed in a tubular network, the wall of which is formed by a basement membrane. Beale states that the diameter of the network is usually about - 1 - ⅜ 1 og th of an inch in most mammals. According to this view, the cells are not in direct contact with the capillary blood-vessels, but separated from them by the basement membrane. In some parts of the lobule Beale has been able to demonstrate the basement membrane as distinct from the wall of the capillaries, but usually they are incorporated together. At the periphery of the lobule the membrane becomes continuous with the wall of the interlobular duct.

The hepatic or bile duct is the tube that conveys the bile out of the liver. It leaves the transverse fissure as two branches, one from the right, another from the left lobe, which almost immediately unite at an acute angle. It closely accompanies within the liver the ramifications of the portal vein and hepatic artery, and its terminal branches pass between the lobules to form the interlobular branches of the duct. If the hepatic duct be injected, not only does the injection fill the interlobular ducts, but it flows into a set of excessively minute passages w ithin the lobules themselves. These passages are arranged so as to form a polygonal network, which may appropriately be called the intralobular biliary network. This network has a most intimate relation to the polyhedral hepatic calls, for the passages lie between the flattened sides of adjacent cells, so that each cell is inclosed in a mesh of the network. The German observers, who first directed attention to these passages, named them bile-capillaries, but it is probable that they are merely intercellular passages bounded by the protoplasm of the hepatic cells.

The intralobular biliary network differs from the intralobular blood capillary network, not only in the character of the fluid conveyed, but in other important particulars. The bile passages have a tranverse diameter of about 1 ⅛th of that of the blood capillaries; the passages are in relation to the sides of the cells, the blood capillaries to their angles, so that the two systems of networks are not in contact with each other, but are separated by intervening hepatic cell substance; the passages have not, in all probability, an independent wall, such as is possessed by the blood capillaries. As these passages can be injected from the hepatic duct, and as they convey bile from the interior of the lobule into the duct, it is obvious that they must be continuous with the lumen of the interlobular branches of the duct, at the periphery of the lobules.

The wall of the larger bile ducts is formed of a fibroelastic tissue, with a proportion of non-striped muscular fibre; it is lined by a columnar epithelium. Opening into the larger ducts are numerous orifices, which communicate with branched caecal tubes and follicles, situated within and clustered around the walls of the larger ducts, often in considerable numbers. Some of these appendages to the duct doubtless serve as glands for the secretion of mucus, but others are probably, as Beale supposed, mere diverticula of the duct, in which the bile may be temporarily retained, as in the gall bladder.

The lymphatics of the liver form a superficial and a deep set. The superficial set ramifies beneath the serous coat, where they form a network. The deep lymphatics accompany the portal vein and hepatic artery as far as the intervals between the lobules, where they form interlobular lymphatics, which, like the corresponding branches of the portal vein, run around the lobule.

The nerves of the liver arise from the caeliac plexus of the sympathetic and from the left pneumogastric. They accompany the portal vessels in their distribution, and supply the muscular coats of the vessels.

The Gall Bladder is a reservoir for the bile, situated in a fossa on the under surface of the right lobe of the liver, and in a notch in its anterior border (fig. 8). It is pyriform in shape; its larger end, or fundus, projects beyond the anterior border; its opposite end, or neck, gives origin to the cystic duct, which is directed towards the transverse fissure; after a course of l¾ inch it joins the hepatic duct, and forms the common bile duct, ductus communis choledochus. At its neck, the gall bladder bends on itself in a sigmoid curve. The gall bladder is 3 or 4 inches long, and can hold from one to two ounces of bile. It is attached to the liver partly by areolar tissue, and partly by the peritoneum, which is reflected over its free surface.

Structure.— In addition to its partial serous coat, the gall bladder has a fibrous and mucous coat. The fibrous coat consists of interlacing bands of connective tissue, with which non-striped muscular fibres are sparingly intermingled. The mucous membrane lining the gall bladder is deeply bile-stained, and presents on its free surface an alveolar appearance, due to the presence of multitudes of minute folds, which form a reticulum with intermediate depressions. The surface is covered by columnar epithelium. The mucous lining of both the neck of the gallbladder and cystic duct is thrown into folds, which in the duct have an oblique direction, and form the spiral valve. Racemose glands, for the secretion of mucus, occur in the wall of the gall bladder, cystic duct, and common bile duct. The gall bladder is supplied with blood by the cystic branch of the hepatic artery. It receives lymphatics and nerves continuous with those which belong to the liver.

The common bile duct, formed by the junction of the cystic and hepatic ducts, is about 3 inches long, and conveys the bile into the duodenum. It lies in the gastro-hepatic omentum between its two layers, having the hepatic artery to its left, and the portal vein behind it. It then inclines behind the duodenum to the inner side of its descending part, where it comes into relation with the pancreatic duct. The two ducts then run together in an oblique direction through the wall of the duodenum, and open on the summit of a papilla, by a common orifice, about the junction of the descending and transverse portions of the duodenum.

The Pancreas is an elongated gland which lies in relation to the posterior wall of the abdomen, in front of the first lumbar vertebra, and extends obliquely from the right lumbar region through the epigastrium into the left hypochondriac region. It is from 6 to 8 inches long, and whilst its dilated right extremity, or head, occupies the horse-shoe curve of the duodenum, and is attached by areolar tissue to the descending and transverse portions, its attenuated left extremity, or tail, is in relation to the spleen. A prolongation of the gland, named the accessory or lesser pancreas, usually surrounds the superior mesenteric artery at its origin.

Structure. —The pancreas is one of the compound racemose glands, and resembles generally in structure the mucous and salivary glands of the mouth and the glands of Brunner (fig. 5). It is sometimes called the abdominal salivary gland, and its secretion flows into the duodenum, and assists in the process of chylification. It has a yellowish creamy colour, and is divided into distant lobules by septa of connective tissue. The excretory duct, or duct of Wirsung, is completely surrounded by the lobules, and extends from the tail to the head of the gland, receiving in its passage the numerous secondary ducts, and increasing gradually in size. It leaves the head of the gland, comes into relation with the common bile duct, and with it pierces obliquely the posterior wall of the descending part of the duodenum, to open by a common orifice about the junction of the descending and transverse portions. Sometimes the duct from the accessory part of the pancreas opens independently into the duodenum, a little above the common hepatico-pancreatic orifice. The finest ducts within the gland terminate in the acini, or gland-vesicles, of the lobules. These acini contain the secreting cells, which have a somewhat cubical form. The ducts are lined by a columnar epithelium, and mucous glands are situated in the mucous membrane lining the duct of Wirsung. The pancreas receives its supply of blood from the splenic, superior mesenteric, and hepatic arteries. Its veins join the splenic and superior mesenteric veins, and through them contribute to the formation of the portal vein. Its blood capillaries are abundantly distributed on the walls of the gland vesicles. Lymph vessels are found in the connective tissue between the lobules. The nerves are derived from £he solar plexus, and accompany the arteries.

The Teeth. —The teeth are calcified organs developed in connection with the mucous membrane of the mouth. Their primary use is that of biting and grinding the food; but in man they serve as aids to speech, and in many animals act as instruments of offence and defence.

Arrangement and Form of the Teeth.— Teeth are present in the greater number of the Mammalia, in which class they are implanted in sockets in the alveolar arches of the bones of the upper and lower jaws, and form only a single row in each arch. In a few mammals, as the toothed whales and the sloths, only one generation of teeth is produced, and when these drop out they are not replaced by successors; these animals are called Monophyodont. In the majority of the Mammalia, however, there are two generations of teeth,—a temporary or milk set, which are deciduous, and are replaced by a permanent or adult set; these animals are called Diρhyodont. But in speaking of two generations of teeth it is not to be supposed that all the teeth in the adult jaw have had temporary predecessors, for the molar or back teeth have only a single generation. A few mammals, as the toothed whales, have the teeth uniform in size, shape, and structure, and are named Homodont; but, in the majority of the Mammalia, the teeth in the same jaw vary in size, form, and structure, and they are therefore called Heterodont. In every Heterodont mammal, possessing a complete dentition, four groups of teeth are found, which are named incisor, canine, premolar, and molar teeth. Each of these teeth possesses a crown, which projects into the cavity of the mouth, and a fang lodged in the socket in the jaw; at the junction of the crown and fang there is usually a constriction named the neck of the tooth.

In man the dentition is Diphyodont and Heterodont. The single row of teeth in each alveolar arch of the human jaw is characterized by the crowns of the teeth being of almost equal length, and by the absence of any great interspace, or diastema, between the different teeth, or of irregularities in the size of the interspaces, so that the teeth form an unbroken series in each jaw. The span of the upper dental arch is slightly bigger than that of the lower, so that the lower incisors fit within the upper, and the lower molars, being inclined obliquely upwards and inwards, are somewhat overlapped by the upper molars. The upper and lower dental arches terminate behind in line with each other, and the teeth are equal in number in the two jaws.

Man possesses 32 teeth in his permanent dentition, arranged in four groups, viz.—8 incisors, 4 canines, 8 premolars or bicuspids, and 12 molars. The number and arrangement of the permanent teeth in the two jaws is expressed in the following formula :— [table]

Man possesses only 20 teeth in his milk or temporary dentition, and their arrangement is expressed in the following formula :— [table]

If the temporary and permanent formulae be compared with each other, it will be seen that, while the incisors and canine teeth correspond in numbers in both dentitions, in the temporary dentition there is an absence of premolars, and the molar teeth are only eight, instead of twelve, in number. The characters of the permanent teeth will now be considered.

The incisor teeth, eight in number, are lodged in the front of the jaws, two on each side of the mesial plane. The upper incisors project downwards and forwards, the lower are directed almost vertically upwards. The oblique direction of the upper incisors in the Negroes, Kaffres, and Australians adds to the prognathic form of the face possessed by these races. The central pair of upper incisors are larger than the lateral; whilst the lateral pair of lower incisors are larger than the central pair, which are the smallest incisor teeth. The crowns of the incisor teeth are chisel-shaped, and adapted for biting and cutting the food. When the crown is first erupted the cutting edge is minutely serrated, but the serrations soon wear down by use. The fangs are long and simple,—being in the upper incisors round and fusiform, in the lower laterally compressed, and sometimes marked by a longitudinal groove. Although the human incisors are, as the name implies, cutting, chisel-shaped teeth, in many mammals the incisors are greatly modified in form, as for example in the tusks of the elephant. The determination of the incisor teeth does not depend, therefore, on their form, but on their position in the jaws. The name incisor is given to all the teeth situated in the pre-maxillary portion of the upper jaw, and in the anterior end of the lower jaw, whatever their shape may be.

The canine or unicuspid teeth, four in number, one on each side of the mesial plane of each jaw, are placed next the lateral incisors. They are bigger than the incisor teeth, and the upper canines, which are sometimes called the eyeteeth, are larger than the lower; the fangs of the upper canines are lodged in deep sockets in the superior maxillae, which extend towards the floor of each orbit. The crowns of these teeth are thick and conical; the fangs are long, single, conical, compressed on the sides where they are marked by a shallow groove. In many ιhammals these teeth are developed into large projecting tusks.

The premolar or bicuspid teeth, eight in number, two on each side of the mesial plane of each jaw, lie immediately behind the canines, and the upper bicuspids are somewhat larger than the lower. The crown is quadrilateral in form, and convex both on the inner and outer surfaces. It possesses two cusps, of which the outer or labial is larger and more projecting than the inner, palatal, or lingual cusp. The fangs of the upper bicuspids are single and laterally compressed, often bifid at the point into an outer and inner segment; in the lower bicuspids the fangs are rounded, and taper to a single point.

The molar or multicuspid teeth, twelve in number, are placed three on each side of the mesial plane of each jaw. They are the most posterior teeth, are the largest of the series, and as a rule decrease in size from the first to the last; the crowns of the lower molars are somewhat bigger than those of the upper molars. The last molar tooth does not erupt until the end of puberty, and is called dens sapientiae, or wisdom tooth. The crowns are broad, quadrilateral, and convex both on the inner and outer surfaces. The first and second upper molars have four cusps projecting from the angles of the grinding or masticating surface, and an oblique ridge often connects the large anterior internal cusp with the posterior external cusp; in the upper wisdom teeth, the two inner or palatal cusps are frequently conjoined. The first lower molar has five cusps, the fifth being interposed between the two posterior cusps; in the second lower molar the fifth cusp is usually absent, or only rudimentary in size, but in the lower wisdom tooth it is often present. The fangs of the first and second upper molars are three in number, and divergent; two on the outer or buccal side, one on the inner or palatal side; in the upper wisdom the fangs are frequently partially conjoined, though trifid at the point. The fangs of the first and second lower molars are two in number, an anterior and a posterior, of which the anterior is the larger; they usually curve backwards in the jaw; in the lower wisdom the fangs are usually conjoined, but bifid at the point.

The crowns of all the teeth become more or less flattened by use, so that the incisors lose their sharp cutting edge, and the cusps of the premolars and molars are worn away.

The temporary or milk teeth are smaller than the permanent teeth. They are more constricted at the neck, where the crown joins the fang, especially in the milk molars, the fangs of which also diverge more widely than in the permanent set. The second temporary molar is bigger than the first. The crown of the first upper molar has three cusps, two buccal, one palatal; that of the second four cusps. The crown of the first lower molar has four cusps; that of the second five, three of which are buccal, two lingual. The temporary teeth lie more vertically in the jaws than the permanent.

The alveolus, or socket for the lodgment of the single fanged teeth, is a single socket; in the multi-fanged teeth, the socket is divided into two or three compartments, according to the number of the fangs. The socket is lined by the alveolo-dental periosteum, which is continuous at the mouth of the socket with the periosteal covering of the jaw, and with the deeper fibrous tissue of the gum, where it embraces the neck of the tooth. The alveolo-dental periosteum is formed of retiform connective tissue, on the one hand connected with the surface of the cement, on the other with the more fibrous periosteum lining the bony wall of the socket (fig. 15). It is vascular, its vessels being continuous with those of the gum, the pulp-vessels, and the bone. It receives nerves from those going to the pulp. The fang fits accurately in the socket, and through a hole at the tip of the fang the blood-vessels and nerves of the tooth pass into the pulp-cavity of the tooth.

Structure of the Teeth.— Each tooth is composed of the following hard structures—dentine, enamel, and cement or crusta petrosa; occasionally other substances, named osteodentine or vasodentine, are present. In a tooth which has been macerated, an empty space exists in its interior, called the pulp-cavity, which opens externally through the hole at the tip of the fang; but in a living tooth this cavity contains a soft, sensitive substance named the pulp.

The Dentine, or Ivory, makes up the greater part of each tooth; it is situated both in the crown, where it is covered by the enamel, and in the fang, where it is invested by the crusta petrosa; whilst the pulp cavity in the centre of the tooth is a cavity in the dentine. The dentine is composed of an intimate admixture of earthy and animal matter in the proportion of 28 of the animal to 72 of the earthy. The animal matter is resolved on boiling into gelatine; the earthy matter consists mostly of salts of lime.

If thin slices through the dentine of a macerated tooth be examined microscopically, it will be seen to consist of a hard, dense, yellowish-white, translucent matrix, penetrated by minute canals, called dentine tubes. The dentine tubes commence at the pulp cavity, on the wall of which they open with distinct orifices. They radiate in a sinuous manner from the pulp cavity through the thickness of the dentine, and terminate by dividing into several minute branches; this division takes place in the crown of the tooth immediately under the enamel, and in the fang of the tooth immediately under the crusta petrosa, lu their course the dentine tubes branch more than once in a dichotomous manner, and give off numbers of extremely minute collateral branches. The transverse diameter of the dentine tubes near the pulp cavity is - r ⅛ σ th inch, but that of their terminal branches is much more minute.

If the dentine be examined in a fresh tooth, the tubes will be seen to be occupied by soft, delicate, thread-like prolongations of the pulp. The passage of processes of the pulp into the dentine tubes was first seen by Owen in the examination of the tusk of an elephant; but the soft contents of the dentine tubes have been made the subject of special investigation by J. Tomes in the human and other mammalian teeth, and have been named the dentinal fibrils.

In sections through the dentine of dried teeth, it is net uncommon to find, near its periphery, irregular, black spaces containing air. These spaces freely communicate with each other. As the dentine which forms their boundary has not unfrequently the appearance of globular contours, they were named by Czermak the interglobular spaces. In a fresh tooth they are not empty, but are occupied by a soft part of the matrix, which is traversed in the usual manner by the dentine tubes. This matrix is apparently imperfectly calcified dentine, which shrinks up in a dried tooth, and occasions an air-containing space. A layer of small irregular spaces situated in the peripheral part of the dentine in the fang, immediately under the crusta petrosa, and sometimes named the granular layer, is apparently of the same nature as the interglobular spaces.

The Enamel is the brilliant white layer which forms a cap on the surface of the crown of a tooth. It is thickest on the cutting edge or grinding surface of the crown, and thins away towards the neck, where it disappears. It is not only the hardest part of a tooth, but the hardest tissue in the body, and consists of 96 ’ 5 per cent, of earthy and of 3 ’ 5 per cent, of animal matter.

The earthy matter consists almost entirely of salts of lime. The great hardness of the enamel admirably adapts it as a covering for the cutting edge, or grinding surfaces, of the crowns of the teeth.

The enamel is composed of microscopic rods,—the enamel fibres, or enamel prisms. These rods are set side by side in close contact with each other; one end of each rod rests on the surface of the dentine, the other reaches the free surface of the crown. The rods do not all lie parallel to each other, for whilst some are straight, others are sinuous, and the latter seem to decussate with each other. The rods are marked by faint transverse lines, and are solid structures in the fully, formed enamel. When cut across transversely, they are seen to be hexagonal or pentagonal, and about - gl ⅛ i jth inch in diameter.

The free surface of the enamel of an unworn tooth is covered by a thin membrane, named the cuticle of the enamel, or Nasmyihss membrane. This membrane can be demonstrated by digesting an unworn tooth in a dilute mineral acid, when it separates as a thin flake from the free surface of the crown. It is a horny membrane, which resists the action of acids. Its deep surface is ’pitted for the ends of the enamel rods. As the crown of the tooth comes into use, Nasmyth’s membrane is worn off, and the enamel itself by prolonged use is thinned and worn down. In persons who live on hard food, that requires much mastication, it is not uncommon to find the grinding surface of the crowns of the molar teeth worn down quite flat, and the dentine exposed.

The Cement, Crusta Petrosa, or Tooth Bone, forms a thin covering for the surface of the fang of a tooth, and extends upwards to the neck. It is of a yellowish colour, and is usually thickest at the point of the fang; though in the multifanged teeth it sometimes forms a thickish mass at the point of convergence of the fangs. It possesses the structure of bone, and consists of a lamellated matrix with perforating fibres, lacunae, and canaliculi. The lacunae are irregular in size and mode of arrangement, and vary also in the number of the canaliculi proceeding from them. Sometimes the canaliculi anastomose with the branched terminations of the dentine tubes. In the thin cement situated near the neck of the tooth the lacunae are usually absent. If the jaw with its contained teeth be softened in acid, and sections be made so as to show the teeth in situ, there is no difficulty in recognizing the cellular masses of nucleated protoplasm within the lacunae, which resemble in appearance the corresponding structures in the adjacent bone. Haversian canals are only found in the cement when it acquires unusual thickness. In old teeth he cement thickens at the tip of the fang, and often closes up the orifice into the pulp cavity; the passage of the nerves and vessels into the pulp is thus cut off, and the nutrition of the tooth being at an end, it loosens in its socket and drops out.

Osteo-dentine and Vaso-dentine do not exist as normal structures in human teeth, though they occur in various animals. They may appear, however, as abnormalities in the human teeth, and are found on the inner wall of the pulp cavity. Osteo-dentine consists of dentine structure, intermingled with lacunae and canaliculi. If vascular canals, like the Haversian canals of bòne, are formed in it, then the name vaso-dentine is applied.

The Pulp of the tooth is one of its most important constituents. It is a soft substance occupying the cavity in the dentine, or the pulp cavity, and is destroyed in a macerated and dried tooth. It consists of a very delicate gelatinous connective tissue, in which numerous cells are imbedded. Those which lie at the periphery of the pulp are in contact with the dentine wall, and form a layer, named by Kölliker the membrana eboris. As the cells of this layer play a part in the formation of the dentine similar to that performed by the osteoblast cells in the formation of bone, Waldeyer has named them odontoblasts. The odontoblasts are elongated in form, and their protoplasm gives off several slender processes; some enter dentine tubes to form the soft dentinal fibres already described; one passes towards the centre of the pulp, to become connected with more deeply-placed pulp cells; whilst others are given off laterally to join contiguous cells of the odontoblast layer. The pulp contains the nerves and blood-vessels of the tooth, which pass into the pulp, through the foramen at the point of the fang. The vessels form a beautiful plexus of capillaries. The nerves are sensory branches of the fifth cranial nerve. They enter the pulp as medullated fibres, which divide into very fine non-medullated fibres, that form a network in the peripheral portions of the pulp. The pulp of the tooth is the remains of the formative papilla, out of which the dentine or ivory has been produced. In adult teeth changes that lead to the production of osteo-dentine and vaso-dentine may take place in it. Through the dentinal fibres an organic connection is preserved between the dentine and the puip, and the sensitiveness exhibited by the dentine in some states of a tooth is not necessarily due to the passage of nerves into it, but to its connection with the sensitive dentine pulp.

Development of the Teeth.— In studying the development of the teeth, not only has the mode of formation of the individual teeth to be examined, but the order of succession of the different teeth both in the temporary and permanent series.

The teeth are developed in the mucous membrane or gum, which covers the edges of the jaws of the young embryo, and their formation is due to a special differentiation in the arrangement and structure of portions of the epithelial and sub-epithelial tissues of that membrane. The enamel is produced from the epithelium, and the dentine, pulp, and cement from the sub-epithelial connective tissue.

The development of the temporary teeth will first be consideied. If a vertical section be made th rough the mouth of a young human embryo about the sixth or seventh week, its cavity may be seen to be lined by a stratified epithelium, continuous with the layer of stratified epiblast forming the cuticle of the face. Along the edge of the gum, corresponding in position to that of the future jaws, the epithelium is of some thickness, and an involution of the epithelium into the subjacent connective tissue has taken place. Owing to this involution a narrow furrow or groove in the connective tissue is produced, which constitutes the primitive dental groove of Goodsir. This groove is not, however, an empty furrow, but is occupied by the involuted epithelium. The sub-epithelial connective tissue is soft and gelatinous, and abounds in corpuscles, which are especially abundant in the connective tissue at the bottom of the groove, where the dental papillae are produced. These papillae are formed, at the bottom of the groove, by an increased development and growth of the corpuscles óf the subjacent connective tissue. The base of each papilla is continuous with the-subjacent connective tissue, and the apex projects into the deeper parts of the involuted epithelium. As a papilla increases in breadth and length the groove widens an<f deepens, and the involuted epithelium, increasing in quantity, expands over the apex and sides of the papilla, so as to form a hood-like covering or cap for it. The cap of epithelium constitutes the enamel organ, whilst the papilla is the formative pulp for the dentine and permanent pulp. Whilst these changes are taking place in the epithelium and the connective tissue at the bottom of the groove, no commensurate widening occurs at its upper part, which remains for a time relatively narrow, but retains within it a narrow string of epithelial cells, continuous on the one hand with the epithelial lining of the mouth, and on the other with the enamel organ. This epithelial string forms the nedc of the enamel organ. After a time, however, the growth of the connective tissue forming the lips of the primitive groove causes the neck of the enamel organ to atrophy, so that all communication between the enamel organ and the superficial epithelium is cut off; and the embryo tooth, being now completely inclosed in a cavity or sac, formed by the gelatinous connective tissue of the gum, has entered on what Goodsir termed its saccular stage of development.

When inclosed in its sac the embryo tooth, though perfectly soft, acquires a shape which enables one to recognize to what group of teeth it belongs. After a time it begins to harden and to exhibit the characteristic tooth structure.

The dental papilla is more vascular than the surrounding connective tissue, from the blood-vessels of which its vessels are derived. The papilla abounds in cells, which are, in the first instance, rounded and ovoid in shape. Changes then take place in the cells situated at its periphery, which become elongated and branched, and form layers of cells (odontoblasts). Calcification of the protoplasm of these odontoblasts then oc curs, and the peripheral layer of the dentine is produced. In contact with the inner surface of the thin film of dentine, a second layer of odontoblast cells is then arranged, which in their turn calcify, and as the process goes on in successive layers of odontoblasts, the entire thickness of the matrix of the dentine and the dentinal sheaths are produced. But the process of calcification does not apparently take place throughout the whole thickness of the protoplasm of the odontoblasts, for, as Waldeyer pointed out, the axial part of the cells remains undifferentiated as the soft dentinal fibrils of the dentine tubes. As these changes are going on in the peripheral layers of the odontoblasts, the central part of the dental papilla increases in quantity, apparently by a proliferation of its cells; nerve fibres are developed in it, and it persists as the soft pulp of the tooth. The papilla of the tooth has essentially, therefore, the same relation to the formation of dentine that the cellulo-vascular contents of the medullary spaces, in intra-cartilaginous ossification, have to the formation of bone. In both instances the hard matrix is due to a special differentiation of the protoplasm of the formative cells; the dentinal fibrils are the equivalent structures to the soft contents of the lacunae and canaliculi, and the persistent pulp is equivalent to the cellulo-vascular contents of the Haversian canals.

Prior to the embryo tooth becoming sacculated, changes had taken place in the enamel organ. Those cells of the enamel organ which lie next the dental papilla are continuous, through the neck of.the enamel organ, with the deepest layer of cells of the oral epithelium, which cells are elongated columns set perpendicularly to the surface on which they rest. Similarly the cells of the deepest layer of the enamel organ are columns set perpendicularly to the surface of the dental papilla. They undergo a greater elongation, and form six-sided prismatic cells, which Kölliker has named the internal or enamel epithelium. The cells of the most superficial layer of the enamel organ lie in contact with the vascular connective tissue which encloses the embryo tooth. They form the external epithelium of the enamel organ, and slender papillary prolongations of the connective tissue frequently project into this epithelial layer. The cells of the enamel organ, situated between its external and its internal epithelium, become stellate, and form with each other an anastomosing network of cells like those sometimes seen in the gelatinous connective tissue.

After the tooth has become sacculated, and coincident with the transformation of the odontoblast cells of the dental papilla into dentine, calcification begins in the elongated prismatic cells of the internal or enamel epithelium; their protoplasm becomes calcified, and they become the rods or prisms of the enamel. As the hardening takes place from the periphery to the centre of each cell, the axial portion may, as Tomes pointed out, remain soft for some time in the axis of the enamel rod. With the increase in length, and with the calcification of the cells of the enamel epithelium, the stellate gelatinous cells disappear, and the outer ends of the enamel rods come in contact with the cells of the external enamel epithelium. By some observers the external epithelium is supposed to disappear without undergoing any special differentiation, but by others it is believed to undergo conversion into Nasmyth’s membrane.

In this manner the crown of a tooth is formed, and it is lodged in a membranous sac formed by the differentiation into a fibro-vascular membrane of the surrounding connective tissue. Whilst within its sac, the crown of the tooth possesses the characteristic form of the group of teeth to which it belongs. After the calcification of the enamel rods is completed, it can undergo no further change either in shape or in increase of size.

Whilst the crown of the tooth is being formed, ossification of the jaws has been going on, and the tooth, with its membranous sac, has become lodged in an alveolus or socket in the jaw, which alveolus is closed in by the gum.

In order that the crown of the tooth may come into use as a masticatory organ, it has to be elevated to the level of the gum, which is absorbed by the pressure, and the crown then erupts into the cavity of the mouth. The process of eruption is due to the development of the fang, which, as it grows in length, elevates the crown of the tooth and forces it outward. The dentine of the fang is developed from the odontoblast cells of the pulp in a manner similar to that already described for the development of the dentine of the crown. The cement or crusta petrosa is developed from the connective tissue lining the alveolus, which forms the alveolo-dental periosteum. It is therefore an ossification in membrane.

As the temporary or milk teeth precede the permanent teeth, their papillae are naturally the first to form. The series of milk-papillae are not, however,, simultaneously produced. From the observations of Goodsir, it has been shown that the milk-papilla of the anterior molar in the upper jaw appears about the seventh week; then the canine papilla, the two incisor papillae, and the posterior molar papilla are sucessively formed, the last making its appearance about the end of the tenth week. The dental papillae in the upper jaw immediately precede the papillae of the corresponding teeth in the lower jaw.

The eruption of the milk teeth into the mouth does not begin to take place until the latter half of the first year of extra-uterine life, and is not completed until betweeen the second and third year. Though variations occur in the date of eruption of each tooth in different children, it may be stated that the incisors usually appear from the seventh to the ninth month, the anterior molars from the twelfth to the sixteenth month, the canines during the seventeenth or eighteenth month, the posterior milk molars from two to two and a half years. The milk teeth begin to be shed about the sixth year by the dropping out of the incisors. The last to be shed are the canines, which do not fall out till the tenth or eleventh year. The shedding of the milk teeth is preceded by the absorption of the fangs. This is effected, as was satisfactorily shown by J.

Tomes, by the agency of a group of cells situated at the bottom of the sockets. As these cells occasion absorption of the tooth tissue, similar to that occurring in the bone tissue from the action of the large multi-nucleated osteo-klast cells, they may appropriately be called odonto-klasts.

The development of the permanent teeth will now be considered. In the description of the arrangement of the teeth it has been pointed out that the number of teeth in the permanent set exceeds that of the temporary set. The permanent incisors and canines come into the place of the temporary incisors and canines, and the permanent bicuspids succeed the temporary molars, but the permanent molars have no milk predecessors, and are superadded at the back of the dental series.

The development of the successional permanent teeth, which are the ten anterior teeth in each jaw, will first be examined. Prior to the period when the lips of the primitive dental groove meet, to produce the saccular stage of dentition of the severa’ temporary teeth, an indentation, or furrow, takes place in the connective tissue adjoining the string of epithelial cells which form the neck of the enamel organ. This furrow constitutes what Goodsir termed the cavity of reserve, and it is filled up by epithelial cells continuous with the epithelium of the neck of the enamel organ. As a cavity of reserve is formed immediately behind (t'.β., on the lingual side of) each milk tooth, they are ten in number in each jaw, and, except that for the anterior molar, are formed successively from before backwards.

The cavities of reserve are concerned in the production of the permanent successional teeth, and each temporary tooth is replaced by the permanent tooth formed in connection with the cavity of reserve situated immediately behind it (fig 21).. The cavities of reserve become elongated, and widened, and pass above the temporary teeth in the upper jaw, and below those in the lower jaw. At the bottom of each a dental papilla forms, the apex of which indentates and becomes covered by the epithelium contained in the cavity, which forms a cap for the papilla, and constitutes the enamel organ for the permanent tooth. The cavity becomes completely closed by the growth of the surrounding connective tissue, and the embryo permanent tooth becomes sacculated. The process of calcification then goes on, in both the enamel organ and dental papilla, in a manner similar to that already described in the temporary teeth. The permanent teeth then become lodged in sockets in the jaw distinct from those of the temporary teeth. The sac of each permanent tooth remains connected with the fibrous tissue of the gum by a slender fibrous band, or gubernaculum, which passes through a hole in the jaw immediately behind the corresponding milk tooth. Before the successional permanent tooth erupts, not only should the temporary tooth be shed, but the bony partition between their respective sockets must be absorbed.

The supeτadded, permanent teeth, or permanent molars, three in number on each side, lie behind the successional teeth. Their mode of origin is similar to that of the temporary teeth. The primitive groove, occupied by an involution of the epithelial covering of the gum, is prolonged backwards. Three dental papillae successively appear at the bottom of this groove, and the epithelium covering each papilla forms its enamel organ. Legros and Magitot, however, state that the second permanent molar arises in connection with a diverticulum (cavity of reserve) proceeding from the epithelial string of the enamel organ of the first permanent molar, and that the wisdom tooth is formed in connection with a similar diverticulum from the second permanent molar. The embryo tooth becomes sacculated, and goes through the process of calcification similar to what has been described in the other teeth The germ of the first permanent molar appears about the sixteenth week of embryo life; that of the second permanent molar not until about the seventh month after birth; Whilst that of the wisdom tooth is not formed until about the sixth year The crown of the first molar is the first of the permanent teeth to erupt into the mouth, which it usually does in the sixth year. The incisors appear when the child is seven or eight; the bicuspids when it is nine or ten; the canines about twelve; the second molars about thirteen; and the wisdom teeth from seventeen to twenty five.

In his dentition man is diphyodont as regards his incisor, canine, and premolar teeth, but monophyodont in the molar series.

From the description of the development of the teeth, it will have been seen that a tooth is made up of three hard tissues—enamel, dentine, and· cement—and of the soft vascular and nervous pulp. These tissues are not developed from one layer only of the blastoderm. The enamel is of epiblast origin, whilst the dentine, cement, and pulp are derived from the mesoblast. A tooth in its fundamental development, as was long ago pointed out by Goodsir, must be referred to the same class of organs as the hairs and feathers. The enamel of the tooth, like the hair, is produced by a differentiation of the involuted epithelium of the epiblast, whilst the dentine and pulp resemble the papilla of the hair, in proceeding from the mesóblast. The tooth-sac, like the hair-follicle, is also of mesoblast origin. Whether the cement, as Robin and Magitot have described, be developed by means of a special cement organ, in the interior of the tooth-sac, or be formed, as has been stated in this description, by the alveolo-dental periosteum, it is on either view derived from the mesoblast. As to the origin of Nasmyth’s membrane, there is a difference of opinion; some regard it as a special cornification of the external cells of the enamel organ, in which case it would be from the epiblast; whilst others consider it to be continuous with though structurally different from, the cement—homologous, therefore, with the layer of cement, which in the horse, ruminants, and some other mammals covers the surface of the crowns of the teeth.

The tissues of a tooth have not all the same importance in the structure of a tooth. The dentine is apparently always present, but the enamel, or the enamel and cement, may be absent in the teeth of some animals. For example, the tusks of the elephant and narwhal, and the teeth of the Edentata, are without enamel, and in the Rodentia enamel is present on only the anterior surface of the incisors. But though the enamel is not developed, or forms only an imperfect covering for the crowns of these teeth, yet an enamel organ is formed in the embryo jaws. In 1872 W. Turner described a structure homologous with the enamel organ in relation with each of the dental papillae in the lower jaw of a foetal narwhal; but this organ did not exhibit a differentiation into the three epithelial layers, such as occurs in those teeth in which enamel is developed. Since then C. S. Tomes has seen an enamel organ in the embryo armadillo, and has also pointed out that, in teeth generally, enamel organs exist, quite irrespective of whether enamel subsequently does or does not form.

But further, the involution of the oral epithelium, and the coincident formation of a primitive groove, take place not only where the teeth subsequently arise, but along the whole curvature of the future jaws; whilst the production of dental papillae is restricted to the spots where the teeth are formed. Hence it would seem that the inflection of the oral epithelium is not so essential to the development of a tooth as the formation of a papilla. The inflected epithelium marks only a preliminary stage, and it may or may not be transformed into tooth structure. But that which is essential to the formation of a tooth is the production of the papilla which appears at the bottom of the primitive groove. (W. T. )