Physiologypart i

PHYSIOLOGYPART I.—GENERAL VIEW.

THE word “physiology” may be used either in a general or in a more restricted sense. In its more general meaning it was used largely of old, and is still occasionally used in popular writings, to denote all inquiry into the nature of living beings. A very slight acquaintance, however, with the phenomena of living beings shows that these can be studied from two, apparently very different, points of view.

The most obvious and striking character of a living being is that it appears to be an agent, performing actions and producing effects on the world outside itself. Accordingly, the first efforts of inquirers were directed towards explaining how these actions are carried on, how the effects of a living being upon its surroundings are brought about. And the dissection or pulling to pieces of the material body of a living being was, under the name of Anatomy (q.v.), regarded as simply an analysis preparatory and necessary to the understanding of vital actions. But it soon became obvious that this anatomical analysis gave rise of itself to problems independent of, or having only distant relations to, the problems which had to do with the actions of living beings. Hence in course of time a distinct science has grown up which deals exclusively with the laws regulating the form, external and internal, of living beings, a science which does not seek to explain the actions of living beings, and takes note of these actions only when they promise to throw light on the occurrence of this or that structural feature. Such a science, which is now known under the name of Morphology (q.v.), might be carried on in a world in which all living things had, in the ordinary meaning of the word, become dead. Were the whole world suddenly petrified, or were a spell to come over it like that imagined by Tennyson in his “Day Dream,” but more intense, so that not only the gross visible movements but the inner invisible movements which are at the bottom of growth were all stayed, the morphologist would still find ample exercise for his mind in investigating the form and structure of the things which had been alive, and which still differed from other things in their outward lineaments and internal build.

In its older sense physiology embraced these morphological problems, and so corresponded to what is now called Biology (g , .v.); in its more modern sense physiology leaves these matters on one side and deals only with the actions of living beings on their surroundings (the study of these necessarily involving the correlative study of the effect of the surroundings on the living being), and appeals to matters of form and structure only so far as they throw light on problems of action. Looking forward into the far future, we may perhaps dimly discern the day when morphology and physiology will again join hands, and all the phenomena of living beings, both those which relate to form and those which relate to action, will be seen to be the common outcome of the same molecular processes. But that day is as yet most distant; and, though occasionally even now the two sciences cross each other’s path, action explaining form and form in turn explaining action, the dominant ideas of the two are so distinct, the one from the other, that each must for a long time yet be developed along its own line. It is proposed to treat in the following pages of physiology in this narrower, more restricted sense.

If any one at the present day, making use of the knowledge so far gathered in, were to attempt a rough preliminary analysis of the phenomena of action of a living being, —for instance of one of the more complex, so-called higher animals, such as man—he might proceed in some such way as the following.

One of the first, perhaps the first and most striking fact about man is that he moves: his body moves of itself from place to place, and one part of the body moves on another. If we examine any one of these movements, such as the bending of the forearm on the arm, we find that it is brought about by certain masses of flesh, called muscles, which from time to time contract, that is, shorten; and these muscles are so disposed that, when they shorten, and so bring their ends nearer together, certain bones are pulled upon and the arm is bent. Upon further examination it will be found that all the gross movements of the body, both the locomotion of the whole body and the movements of parts upon parts, are carried out by the contraction or shortening of muscles. The muscles, together with bones, tendons, and other structures, are arranged in various mechanical contrivances, many of them singularly complex; hence the great diversity of movement of which an animal or man is capable; but in all cases the central fact, that which supplies the motive-power, is the contraction of a muscle, a shortening of its constituent fibres whereby its two ends are brought for a while nearer together.

When, pushing the analysis farther, we attempt to solve the question, Why do muscles contract? we find that the muscles of the body are connected with what is called the central nervous system by certain strands of living matter called nerves; and we further find that, with some few exceptions, which need not concern us now, the contractions of muscles are brought about by certain occult invisible changes called nervous impulses which travel along these nerves from the central nervous system to the muscles. Hence, when a nerve is severed, the muscle to which the nerve belonged, thus cut adrift from the central nervous system, no longer stirred by impulses reaching it from thence, ceases to contract, and remains motionless and as it were helpless. Pushing the problem still farther home, and asking how these impulses originate in the central nervous system, we find that this central nervous mass is connected, not only with the muscles by means of nerves which, carrying impulses outward from itself to the muscles and so serving as instruments of movement, are called motor or efferent nerves, but also with various surfaces and parts of the body by means of other nerves, along which changes or impulses travel inwards to itself in a centripetal fashion. Moreover, the beginnings or peripheral endings of these other nerves appear to be so constituted that various changes in the surroundings of the body, or internal changes in the body itself, give rise to impulses, which, thus originated, travel inwards to the central nervous system; hence these nerves are spoken of as sensory or afferent. Such sensory impulses reaching the central nervous system may forthwith issue as motor impulses leading to movement; but on many occasions they tarry within the central mass, sweeping backwards and forwards along particular areas of its substance, thus maintaining for a while a state of molecular agitation and leading to movement at some subsequent period only. Moreover, we have reason to think that molecular disturbances may arise within the central nervous system apart from the advent, either past or present, of any impulses along sensory nerves. Lastly, the presence of these molecular agitations in the central nervous system, whether the immediate result of some new afferent impulse, or the much delayed and complicated outcome of some impulse which arrived long ago, or the product of internal changes apparently independent of all disturbance from without and so far spontaneous, may be indicated by corresponding phases of what we speak of as consciousness. We are thus led to conceive of the central nervous system as, chiefly at least, the seat of a molecular turmoil maintained by multitudinous afferent impulses streaming in along the various afferent nerves, a turmoil which makes itself felt within as changes of consciousness, and produces effects without by movements wrought through motor nerves and muscles. And one large part of physiology has for its task the unravelling of the laws which govern this turmoil, which determine, in relation to the advent of afferent impulses and the occurrence of intrinsic changes, the issue of motor impulses, and thus the characters of the resulting movements.

The movements of man or of an animal are not, however, the only salient facts of his existence. Equally characteristic of him are the facts, (1) that he from time to time eats, and must eat in order to live, and (2) that a supply of fresh air containing a certain quantity of oxygen is indispensable to his remaining alive. Viewed from a chemical point of view, an animal body, whether dead or alive, is a mass of complex unstable chemical substances, combustible in nature, i.e., capable of being oxidized, and of being reduced by oxidation to simpler, more stable substances, with a setting free of energy. Combustible in the ordinary sense of the word an animal body is not, by reason of the large excess of water which enters into its composition; but an animal body thoroughly dried will in the presence of oxygen burn like fuel, and, like fuel, give out energy as heat. The material products of that combustion are fairly simple, consisting of water, carbonic acid, some ammonia or nitrogen compounds, and a few salts. And these same substances appear also as the products of that slower combustion which we call decay; for, whether the body be burnt swiftly in a furnace or rot away slowly in earth, air, or water, the final result is the same, the union of the complex constituent substances with the oxygen furnished from the air, and their reduction thereby to the above-named products, with a development of heat, which either as in the first case is rapid and appreciable, or as in the second is so slow and gradual as to be with difficulty recognized. Moreover, during life also the same conversion, the same oxidation, the same reduction of complex substances to simpler matters, the same setting free of the energy present in the former but absent in the latter, may be noted. The animal body dies daily, in the sense that at every moment some part of its substance is suffering decay, is undergoing combustion; at every moment complex substances full of latent energy are by processes of oxidation reduced to simpler substances devoid of energy or containing but little.

This breaking down of complex substances, this continued partial decay, is indeed the source of the body’s energy; each act of life is the offspring of an act of death. Each strain of a muscle, every throb of the heart, all the inner work of that molecular turmoil of the nervous system of which we spoke above, as well as the chemical labour wrought in the many cellular laboratories of glands and membranes, every throw of the vital shuttle, means an escape of energy as some larger compacted molecule splits into smaller simpler pieces. Within the body the energy thus set free bears many shapes, but it leaves the body in two forms alone, as heat and as the work done by the muscles of the frame. All the inner labour of the body, both that of the chemical gland-cells, of the vibrating nerve-substance with its accompanying changes of consciousness, and of the beating heart and writhing visceral muscles, is sooner or later, by friction or otherwise, converted into heat; and it is as heat that the energy evolved in this labour leaves the body. Manifold as seems the body’s energy, it has but one source, the decay of living material, i.e., the oxidation of complex substances diversely built up into various living matters, and but two ends, heat and muscular work. The continued setting free of energy which thus marks the living body, entailing as it does the continued breaking up and decay of living substance, constitutes a drain upon the body which must be met by constantly-renewed supplies, or otherwise the body would waste away and its energy flicker out. Hence the necessity on the one hand for that which we call food, which, however varied, is essentially a mixture of complex combustible energy-holding bodies, and on the other hand for that other kind of food which we call breath, and which supplies the oxygen whereby the complex oxidizable substances may be oxidized to simpler matters and their potential energy made to do work. Thus food supplies the energy of the body, but in. quantity only, not in quality. The food by itself, the dead food, can exhibit energy as heat only, with intervening phases of chemical action; before its energy can be turned into the peculiar grooves of nervous and muscular action it needs to be transmuted into living substance, and in that transmutation there is a preliminary expenditure of part of the food’s store of energy.

Here, then, we have a second view of physiological labour. To the conception of the body as an assemblage of molecular thrills—some started by an agent outside the body, by light, heat, sound, touch, or the like; others begun within the body, spontaneously as it were, without external cause: thrills which, travelling to and fro, mingling with and commuting each other, either end in muscular movements or die away within the body—to this conception we must add a chemical one, that of the dead food continually being changed and raised into the living substance, and of the living substance continually breaking down into the waste matters of the body by processes of oxidation, and thus supplying the energy needed both for the unseen molecular thrills and the visible muscular movements.

Hence the problems of physiology may in a broad sense be spoken of as threefold. (1) On the one hand, we have to search the laws according to which the complex unstable food is transmuted into the still more complex and still more unstable living flesh, and the laws according to which this living substance breaks down into simple, stable waste products, void or nearly void of energy. (2) On the other hand, we have to determine the laws according to which the vibrations of the nervous substance originate from extrinsic and intrinsic causes, the laws according to which these vibrations pass to and fro in the body acting and reacting upon each other, and the laws according to which they finally break up and are lost, either in those larger swings of muscular contraction whereby the movements of the body are effected, or in some other way. (3) And lastly, we have to attack the abstruser problems of how these neural vibrations, often mysteriously attended with changes of consciousness, as well as the less subtle vibrations of the contracting muscles, are wrought out of the explosive chemical decompositions of the nervous and muscular substances, that is, of how the energy of chemical action is transmuted into and serves as the supply of that vital energy which appears as movement, feeling, and thought.

Even a rough initial analysis, however, such as we have just attempted to sketch, simple as it seems with our present knowledge, is an expression of the accumulated and corrected inquiries of many ages; the ideas which it embodies are the results of long-continued investigations, and the residue of many successive phases of opinion.

In the natural hierarchy of the sciences, physiology fol- 1 lows after chemistry, which in turn follows physics, molar and molecular; and in a natural development, as indeed is evident from what we have just seen, the study of the two latter should precede that of the former. At a very early age, however, the exigencies of life brought the study of man, and so of physiology, to the front before its time; hence the history of physiology consists to a large extent, especially in its opening chapters, of premature vain attempts to solve physical and chemical problems before the advent of adequate physical or chemical knowledge. But no ignorance of these matters could hide from the observant mind, even in quite early times, two salient l>oints which appear also in the analysis just given, namely, that, while some of the phenomena of living beings seem due to powers wholly unknown in things which are not living, other phenomena, though at first sight special to living beings, appear to be in reality the peculiar outcome of processes taking place as well in things not alive. It was further early seen that, while the former are much more conspicuous, and make up a greater part of the life of the individual in those living beings which are called animals, especially in man, and in animals more closely resembling man, than in those which are called plants, the latter are common to both divisions of living things. Both sets of phenomena, however, were at first regarded as the products of certain special agencies; both were spoken of as the work of certain spirits; and the distinction between the two was formulated by speaking of the spirits as being in the former case animal and in the latter vital.

From the very outset even the casual observer could not fail to be struck with the fact that many of the processes of living beings appear to be the results of the various contrivances or machines of which a living body is largely built up. This indeed was evident even before the distinction between animal and vital spirits was recognized; and, when that differentiation was accepted, it was seen that the part played by these machines and contrivances in determining the actions of living beings was much more conspicuous in the domain of vital than of animal spirits. As inquiry was pushed forward the prominence and importance of this machinery became greater and greater, more especially since the phenomena supposed to be due to the agency of vital spirits proved more open to direct observation and experiment than those attributed to the animal spirits. It was found that the most fruitful path of investigation lay in the direction of studying the structure and independent action of the several constituent machines of the body, and of unravelling their mutual relations.

These machines received the names of organs, the work or action of an organ being at a later period spoken of as its function. And, when it became clear that many of the problems concerned with what was supposed to be the work of the vital spirits could be solved by the proper appreciation of the functions of certain organs, it was in ferred that the more difficult problems belonging to the animal spirits could be solved in the same way. Still later on it was found that the conception of organs and functions was not only quite separable from, but indeed antagonistic to, the hypothesis of the entities called spirits.

In this way the first great phase, as it may be called, of the science of physiology was evolved,—a phase which lasted till quite recent times. Under this conception every living being, plant or animal, was regarded as a complex of organs, each with its respective function, as an engine built up of a number of intricately contrived machines, each performing its specific work. The whole animal body was parcelled out into organs, each of which was supposed to have its appropriate function; and the efforts of investigators were directed, on the one hand, to a careful examination of the structural features of an organ with the view of determining by deduction what its function must be, and, on the other hand, to confirming or correcting by observation and experiment the conclusions thus reached by the anatomical method. And the fruitfulness of this line of inquiry proved so great that the ideas directing it became absolutely dominant. In many cases the problem to be worked out was in reality a purely mechanical one. This was notably so in the great question of the circulation so brilliantly solved by Harvey. Putting aside for a while the inquiry as to the origin of the force with which the walls of the heart press on the blood contained in its cavities, accepting the fact that the blood is thus pressed at each beat of the heart, all the other truths of the circulation which Harvey demonstrated are simply the outcome of certain mechanical conditions, such as the position and arrangement of the valves, the connexion of various patent tubes, and the like. And many other problems—as, for instance, those connected with respiration—proved to be similarly capable of solution by the application of ordinary mechanical principles to anatomical facts.

So fruitful, and consequently so adequate, seemed this conception of living beings as built up of contrivances or organs, in contrast with the lifeless world in whose monotonous masses no such structural disposition could be recognized, that the word “organic” came into use as a term distinctive of living things. The phrase was especially adopted by the chemists, who for a long time classified their material into “organic” substances, i.e., substances found only in living beings, and into inorganic substances, that is, substances occurring in lifeless bodies as well. Indeed, this nomenclature has not even yet been wholly abandoned. Triumphant, however, as was this mode of inquiry in these and similar instances, there remained in every investigation an unsolvable residue, like the question of the origin of the force exerted by the heart referred to above in speaking of Harvey’s work; and in many other instances the questions which could not be solved on mechanical principles formed a great part of the whole problem. Thus in the case of the liver careful dissection showed that minute tubes starting from all parts of the liver joined into one large canal, which opened into the small intestine, and observation and experiment taught that these tubes during life conveyed from the liver to the intestine a peculiar fluid called bile, which appeared on the one hand to originate in the liver, and on the other to be used up for some purposes in the intestine. But here the mere mechanical flow of the bile along the gallducts, instead of being of primary, was merely of secondary importance, and the problem of how the bile was generated and made its way into the small beginnings of the ducts was the greater part of the whole matter. This latter problem was left unsolved, and indeed for a while unattempted. Nevertheless the success in other directions attending the conception of organs and functions encouraged physiologists to speak of the liver as an organ whose function was to secrete bile, and further, led them to ignore to a large extent the great unsolved portion of the problem, and to regard the mere enunciation of the function as the chief end of physiological inquiry.

Moreover, whenever attempts were made to unravel these obscurer problems, the efforts of investigators were mainly confined to a fuller and more complete elucidation of the supposed function of an organ, and the method of inquiry adopted was in most cases one which regarded the finer elements of the part studied as minute organs making up the whole gross organ, and which sought to explain the functions of these smaller organs on the same mechanical principles which had proved so successful in the case of the whole organ. When the improvements in the microscope opened up a new world to the anatomist, and a wholly fresh mechanical analysis of the structure of living bodies became possible, great hopes were entertained that the old method applied to the new facts would soon solve the riddles of life by showing how the mysterious operations of the living substances out of which the grosser organs were built were the outcome of structural arrangements which had hitherto remained invisible, were in fact the functions of minute component organs. A vision of a grand simplicity of organic nature dawned upon the minds of physiologists. It seemed possible to conceive of all living beings as composed of minute organic units, of units whose different actions resulted from their different structural characters, whose functions were explicable by, and could be deduced from, their anatomical features, such units being built up into a number of gross organs, the functions of each of which could in turn be explained by the direction which its mechanical build gave to the efforts of its constituent units. Such a view seemed to have touched the goal, when, in the first half of this century, the so-called “cell-theory” was enunciated as a physiological generalization.

Long before, in the previous century, the genius of Caspar Wolff had led him to maintain that the bodies of living beings may be regarded as composed of minute constituent units, which, being in early life all alike and put together as an unformed mass, gradually differentiate and are ultimately arranged into the tissues and organs of the adult being. But, though Wolff was not unaware of the physiological bearing of his conception, his mind was chiefly bent towards morphological views, and his celltheory is essentially a morphological one. The cell-theory, however, which became famous in the third decade of the present century, and to which the twin names of Schwann and Schleiden will always be attached, was essentially a physiological one. The chief interest which these authors felt in the ideas that they put forth centred in the conviction that the properties of the cell as they described it were the mechanical outcome of its build; and for a time it seemed possible that all physiological phenomena could be deduced from the functions of cells, the anatomical characters of the various kinds of cells determining in turn their special functions. In the cell-theory the conception of organs and functions reached its zenith; but thenceforward its fall, which had been long prepared, was swift and great. Two movements especially hurried on its decline.

It had long been a reproach to physiologists that, wflιile to most orgaιis of the body an appropriate function had been assigned, in respect to certain even conspicuous organs no special use or definite work could be proved to exist. Of these apparently functionless organs the most notorious instance was that of the spleen, a large and important body, whose structure, though intricate, gave no sign of what its labours were, and whose apparent uselessness was a stumbling-block to the theological speculations of Paley. While in the case of other organs a definite function could be readily enunciated in a few words, and their existence therefore easily accounted for, the spleen remained an opprobrium, existing, as it appeared to do, without purpose, and therefore without cause.

The progress of discovery during the present century, by a cruel blow, instead of pointing out the missing use of the spleen, rudely shook the confidence with which the physiologists concluded that they had solved the riddle of an organ when they had allotted to it a special function. From very old times it had been settled that the function of the liver was to secrete bile; and the only problems left for inquiry as touching the liver seemed to be those which should show how the minute structure of the organ was adapted for carrying on this work. About the middle of this century, however, the genius of Claude Bernard led him to the discovery that the secretion of bile was by no means the chief labour of the liver. He showed that this great viscus had other work to do than that of secreting bile, had another “function” to perform, but a function which seemed to have no reference whatever to the mechanical arrangements of the organ, which could never have been deduced from any inspection however complete of its structure, even of its most hidden and minute features, and which therefore could not be called a function in the old and proper sense of that word. By a remarkable series of experiments, which might have been carried out by one knowing absolutely nothing of the structural arrangements of the liver beyond the fact that blood flowed to it along the portal vein, and from it along the hepatic vein, he proved that the liver, in addition to the task of secreting bile, was during life engaged in carrying on a chemical transformation by means of which it was able to manufacture and store up in its substance a peculiar kind of starch, to which the name of glycogen given. Bernard himself spoke of this as the glycogenic function of the liver, but he used the word “function” in a broad indefinite sense, simply as work done, and not in the older narrower meaning as work done by an organ structurally adapted to carry on a work which was the inevitable outcome of the form and internal build of the organ. In this glycogenic function organization, save only the arrangements by means of which the blood flows on from the portal to the hepatic channels in close proximity to the minute units of the liver-substance, the so-called hepatic cells, appeared to play no part whatever; it was not a function, and in reference to it the liver was not an organ, in the old senses of the words. This discovery of Bernard’s threw a great flash of light into the darkness hitherto hiding the many ties which bound together distant and mechanically isolated parts of the animal body. Obviously the liver made this glycogen, not for itself, but for other parts of the body; it laboured to produce, but they made use of, the precious material, which thus became a bond of union between the two.

The glycogenic labours of the simple hepatic substance carried out independently of all intricate structural arrangements, and existing in addition to the hepatic function of secreting bile, being thus revealed, men began to ask themselves the question, May not something like this be true of other organs to which we have allotted a function and thereupon rested content 1 And further, in the cases where we have striven in hope, and yet in vain, to complete the interpretation of the function of an organ, by finding in the minute microscopic details of its structure the mechanical arrangements which determine its work, may we not have followed throughout a false lead, and sought for organization where organization in our sense of the word does not exist? The answer to this question, and that an affirmative one, was hastened by the collapse of the cell-theory on its physiological side, very soon after it had been distinctly formulated.

The “cell,” according to the views of those who first propounded the cell-theory, consisted essentially of an envelope or “cell-membrane,” of a substance or substances contained within the cell-membrane, hence called cellcontents, and of a central body or kernel called the “nucleus,” differing in nature from the rest of the cellcontents. And, when facts were rapidly accumulated, all tending to prove that the several parts of the animal or vegetable body, diverse as they were in appearance and structure, were all built up of cells more or less modified, the hope arose that the functions of the cell might be deduced from the mutual relations of cell-membrane, cellcontents, and nucleus, and that the functions of an organ might be deduced from the modified functions of the constituent modified cells. Continued investigation, however, proved destructive of this physiological cell-theory. It soon became evident that the possession of an investing envelope or cell-membrane was no essential feature of a cell, and that even the central kernel or nucleus might at times be absent. It was seen in fact that the anatomical unit need have no visible parts at all, but might be simply a minute mass, limited in various ways, of the material spoken of as cell-contents. Under the cell-theory, the cell was supposed to be the first step in organization, the step by which a quantity of formless unorganized plasm became an organized unit; this plasm was further supposed still to form the chief part of the cell-contents, and soon became recognized under the name of protoplasm. Hence the destructive anatomical researches which deprived the cell of its cell-membrane, and even of its nucleus, left nothing except a mass of protoplasm to constitute an anatomical unit. For such a unit the word “cell” was a misnomer, since all the ideas of organization denoted by the word had thus vanished; nevertheless it was retained with the new meaning, and up to the present time the definition of a cell is that of a limited mass of protoplasm, generally but not always containing a modified kernel or nucleus.

With this anatomical change of front the physiological cell-theory was utterly destroyed. The cell was no longer a unit of organization; it was merely a limited mass of protoplasm, in which, beyond the presence of a nucleus, there was no visible distinction of parts. It was no longer possible to refer the physiological phenomena of the cell to its organization; it became evident that the work done by a “cell” was the result not of its form and cellular structure but simply of the nature and properties of the apparently structureless protoplasm which formed its body. A new idea pressed itself on men’s minds, that organization was a concomitant and result of vital action, not its condition and cause; as Huxley in one of his earliest -writings put it, “They [cells] are no more the producers of the vital phenomena than the shells scattered in orderly lines along the sea-beach are the instruments by which the gravitative force of the moon acts upon the ocean. Like these, the cells mark only where the vital tides have been, and how they have acted.” Hence arose the second of the two movements mentioned above, that which may be called the “protoplasmic” movement, a movement which, throwing overboard altogether all conceptions of life as the outcome of organization, as the mechanical result of structural conditions, attempts to put physiology on the same footing as physics and chemistry, and regards all vital phenomena as the complex products of certain fundamental properties exhibited by matter, which, either from its intrinsic nature or from its existing in peculiar conditions, is known as living matter, —mechanical contrivances in the form of organs serving only to modify in special ways the results of the exercise of these fundamental activities and in no sense determining their initial development.

Long before the cell-theory had reduced to an absurdity the “organic” conception of physiology, the insight of the brilliant Bichat, so early lost to science, had led him to prepare the way for modern views by developing his doctrine of “tissues.” That doctrine regarded the body as made up of a number of different kinds of living material, each kind of material having certain innate qualities proper to itself as well as certain structural features, and the several kinds of material being variously arranged in the body. Each of these body-components λvas spoken of as a tissue, muscular tissue, nervous tissue, and the like; and the varied actions of the body were regarded as the result of the activities of the several tissues modified and directed by the circumstance that the tissues were to a great extent arranged in mechanical contrivances or organs which largely determined the character and scope of their actions.

The imperfection of microscopic methods in Bichat’s time, and, we may perhaps add, his early death, prevented him from carrying out an adequate analysis of the qualities or properties of the tissues themselves. During the middle portion of this century, however, histological investigation, i.e., inquiry into the minute structure of the tissues, made enormous progress, and laid the basis for a physiological analysis of the properties of tissues. In a short time it became possible to lay down the generalization that all the several tissues arise, as far as structure is concerned, by a differentiation of a simple primitive living matter, and that the respective properties of each tissue are nothing more than certain of the fundamental properties of the primordial substance thrown into prominence by a division of labour running to a certain extent parallel to the differentiation of structure. Developed in a fuller manner, this modern doctrine may be expounded somewhat as follows.

In its simplest form, a living being, as illustrated by some of the forms often spoken of as amoebae, consists of a mass of substance in which there is no obvious distinction of parts. In the body of such a creature even the highest available powers of the microscope reveal nothing more than a fairly uniform network of material, a network sometimes compressed, with narrow meshes, sometimes more open, with wider meshes, the intervals of the meshwork being filled, now with a fluid, now with a more solid substance or with a finer and more delicate network, and minute particles or granules of variable size being sometimes lodged in the open meshes, sometimes deposited in the strands of the network. Sometimes, however, the network is so close, or the meshes filled up with material so identical in refractive power with the bars or films of the network, and at the same time so free from granules, that the whole substance appears absolutely homogeneous, glassy or hyaline. Analysis with various staining and other reagents leads to the conclusion that the substance of the network is of a different character from the substance filling up the meshes. Similar analysis shows that at times the bars or films of the network are not homogeneous, but composed of different kinds of stuff; yet even in these cases it is difficult if not impossible to recognize any definite relation of the components to each other such as might deserve the name of structure; and certainly in what may be taken as the more typical instance, where the network seems homogeneous, no microscopic search is able to reveal to us a distinct structural arrangement in its substance. In all probability optical analysis, with all its aids, has here nearly reached its limits; and, though not wholly justified, we may perhaps claim the right to conclude that the network in such case is made up of a substance in which no distinction of parts will ever be visible, though it may vary in places or at times in what may be spoken of as molecular construction, and may carry, lodged in its own substance, a variety of matters foreign to its real self. This remarkable network is often spoken of as consisting of protoplasm, and, though that word has come to be used in several different meanings, we may for the present retain the term. The body of an amoeba, then, or of a similar organism consists of a network or framework which we may speak of as protoplasm, filled up with other matters. In most cases it is true that in the midst of this protoplasmic body there is seen a peculiar body of a somewhat different and yet allied nature, the so-called nucleus; but this we have reason to think is specially concerned with processes of division or reproduction, and may be absent, for a time at all events, without any injury to the general properties of the protoplasmic body.

Now such a body, such a mass of simple protoplasm, homogeneous save for the admixtures spoken of above, is a living body, and all the phenomena which we sketched out at the very beginning of this article as characteristic of the living being may be recognized in it. There is the same continued chemical transformation, the same rise and fall in chemical dignity, the same rise of the dead food into the more complex living substance, the same fall of the living substance into simpler waste-products. There is the same power of active movement, a movement of one part of the body upon another giving rise to a change of form, and a series of changes of form resulting eventually in a change of place. In what may be called the condition of rest the body assumes a more or less spherical shape. By the active transference of part of the mass in this or that direction the sphere flattens itself into a disk, or takes on the shape of a pear, or of a rounded triangle, or assumes a wholly irregular, often star-shaped or branched form. Each of these transformations is simply a rearrangement of the mass, without change of bulk. When a bulging of one part of the body takes place there is an equivalent retraction of some other part or parts; and it not unfrequently happens that one part of the body is repeatedly thrust forward, bulging succeeding bulging, and each bulging accompanied by a corresponding retraction of the opposite side, so that, by a series of movements, the whole body is shifted along the line of the protuberances. The tiny mass of simple living matter moves onward, and that with some rapidity, by what appears to be a repeated flux of its semi-liquid substance.

The internal changes leading to these movements may begin, and the movements themselves be executed, by any part of the uniform body; and they may take place without any obvious cause. So far from being always the mere passive results of the action of extrinsic forces, they may occur spontaneously, that is, without the coincidence of any recognizable disturbance whatever in the external conditions to which the body is exposed. They appear to be analogous to what in higher animals we speak of as acts of volition. They may, however, be provoked by changes in the external conditions. A quiescent amoeba may be excited to activity by the touch of some strange body, or by some other event,—by what in the ordinary language of physiology is spoken of as a stimulus. The protoplasmic mass is not only mobile but sensitive. When a stimulus is applied to one part of the surface a movement may commence in another and quite distant part of the body; that is to say, molecular disturbances appear to be propagated along its substance without visible change, after the fashion of the nervous impulses we spoke of in the beginning of this article. The uniform protoplasmic mass of the amoeba exhibits the rudiments of those attributes or powers which in the initial sketch we described as being the fundamental characteristics of the muscular and nervous structures of the higher animals.

These facts, and other considerations which might be brought forward, lead to the tentative conception of protoplasm as being a substance (if we may use that word in a somewhat loose sense) not only unstable in nature but subject to incessant change, existing indeed as the expression of incessant molecular, that is, chemical and physical change, very much as a fountain is the expression of an incessant replacement of water. We may picture to ourselves this total change which we denote by the term “metabolism” as consisting on the one hand of a downward series of changes (katabolic changes), a stair of many steps, in which more complex bodies are broken down with the setting free of energy into simpler and simpler waste bodies, and on the other hand of an upward series of changes (anabolic changes), also a stair of many steps, by which the dead food, of varying simplicity or complexity, is, with the further assumption of energy, built up into more and more complex bodies. The summit of this double stair we call “protoplasm.” Whether we have a right to speak of it as a single body, in the chemical sense of that word, or as a mixture in some way of several bodies, whether we should regard it as the very summit of the double stair, or as embracing as well the topmost steps on either side, we cannot at present tell. Even if there be a single substance forming the summit, its existence is absolutely temporary: at one instant it is made, at the next it is unmade. Matter which is passing through the phase of life rolls up the ascending steps to the top, and forthwith rolls down on the other side. But to this point we shall return later on. Further, the dead food, itself fairly but far from wholly stable in character, becomes more and more unstable as it rises into the more complex living material. It becomes more and more explosive, and when it reaches the summit its equilibrium is overthrown and it actually explodes. The whole downward stair of events seems in fact to be a series of explosions, by means of which the energy latent in the dead food and augmented by the touches through which the dead food becomes living protoplasm, is set free. Some of this freed energy is used up again within the material itself, in order to carry on this same vivification of dead food; the rest leaves the body as heat or motion. Sometimes the explosions are, so to speak, scattered, going off as it were irregularly throughout the material, like a quantity of gunpowder sprinkled over a surface, giving rise to innumerable minute puffs, but producing no massive visible effects. Sometimes they take place in unison, many occurring together, or in such rapid sequence that a summation of their effects is possible, as in gunpowder rammed into a charge, and we are then able to recognize their result as visible movement, or as appreciable rise of temperature.

These various phenomena of protoplasm may be conven-ientlyspoken of under the designation of so many properties, or attributes, or powers of protoplasm, it being understood that these words are used in a general and not in any definite scholastic sense. Thus we may speak of protoplasm as having the power of assimilation, i.e., of building up the dead food into its living self; of movement, or of contractility as it is called, i.e., of changing its form through internal explosive changes; and of irritability or sensitiveness, i.e., of responding to external changes, by less massive internal explosions which, spreading through its mass, are not in themselves recognizable through visible changes, though they may initiate the larger visible changes of movement.

These and other fundamental characters, all associated with the double upward and downward series of chemical changes, of constructive and destructive metabolism, are present in protoplasm wherever found; but a very brief survey soon teaches us that specimens of protoplasm existing in different beings or in different parts of the same being differ widely in the relative prominence of one or another of these fundamental characters. On the one hand, in one specimen of protoplasm the energy which is set free by the series of explosions constituting the downward changes of destructive metabolism may be so directed as to leave the mass almost wholly in the form of heat, thus producing very little visible massive change of form. Such a protoplasm consequently, however irritable and explosive, exhibits little power of contractility or movement. In another specimen, on the other hand, a very large portion of the energy similarly set free may be spent in producing visible changes of form, the protoplasm in this instance being exquisitely mobile. Such differences must be due to different internal arrangements of the protoplasm, though, since no vision, however well assisted, can detect these arrangements, they must be of a molecular nature rather than of that grosser kind which we generally speak of as structural. It is true that, as the differences in properties become more and more prominent, as the protoplasm becomes more and more specialized, features which we can recognize as structural intervene; but even these appear to be subsidiary, to accompany and to be the result of the differences in property, or to be concerned in giving special directions io the activities developed, and not to be the real cause of the differences in action. We are thus led to the conception of protoplasm as existing in various differentiated conditions while still retaining its general protoplasmic nature, a difference of constitution making itself felt in the different character of the work done, in a variation of the results of the protoplasmic life. We have a division of physiological labour going hand in hand with a differentiation of material, accompanied ultimately by morphological results which may fairly be spoken of as constituting a differentiation of structure.

Some of the simpler and earlier features of such a division and differentiation may be brought out by comparing with the life of such a being as the amoeba that of a more complex and yet simple organism as the hydra or freshwater polyp. Leaving out certain details of structure, which need not concern us now, we may say that the hydra consists of a large number of units or cells firmly attached to each other, each cell being composed of protoplasm, and in its broad features resembling an amoeba. The polyp is in fact a group or crowd of amoeba-like cells so associated together that, not only may the material of each cell, within limits, be interchanged with that of neighbouring cells, but also the dynamic events taking place in one cell, and leading to exhibitions of energy, may be similarly communicated to neighbouring cells, also within limits. These cells are arranged in a particular way to form the walls of a tube, of which the body of the hydra practically consists. They form two layers in apposition, one an internal layer called the endoderm, lining the tube, the other an external layer called the ectoderm, forming the outside of the tube. And, putting aside minor details, the differences in structure and function observable in the organism are confined to differences between the ectoderm on the one hand, all the constituent cells of which are practically alike, and the endoderm on the other, all the cells of which are in turn similarly alike. The protoplasm of the ectoderm cells is so constituted as to exhibit in a marked degree the phenomena of which we spoke above as irritability and contractility, whereas in the endoderm these phenomena are in abeyance, those of assimilation being prominent. The movements of the hydra are chiefly brought about by changes of form of the ectoderm cells, especially of tail-like processes of these cells, which, arranged as a longitudinal wrapping of the tubular body, draw it together when they shorten, and lengthen it out when they elongate, and it is by the alternate lengthening and shortening of its body, and of the several parts of its body, that the hydra changes its form and moves from place to place. Inaugurating these changes of form, the products of contractility, are the more hidden changes of irritability; these also are especially developed in the ectoderm cells, and travel readily from cell to cell, so that a disturbance originating in one cell, either from some extrinsic cause, such as contact with a foreign body, or from intrinsic events, may sweep from cell to cell over the surface of the whole body. The animal feels as well as moves by means of its ectoderm cells. In the endoderm cells the above phenomena, though not wholly absent, are far less striking, for these cells are almost wholly taken up in the chemical work of digesting and assimilating the food received into the cavity, the lining of which they form.

Thus the total labour of the organism is divided between these two membranes. The endoderm cells receive food, transmute it, and prepare it in such a way that it only needs a few final touches to become living material, these same cells getting rid at the same time of useless ingredients and waste matter. Of the food thus prepared the endoderm cells, however, themselves use but little; the waste of substance involved in the explosions which carry out movement and feeling is reduced in them to a minimum; they are able to pass on the greater part of the elaborated nourishment to their brethren the ectoderm cells. And these, thus amply supplied with material which it needs but little expenditure of energy on their part to convert into their living selves, thus relieved of the greater part of nutritive labour, are able to devote nearly the whole of their energies to movement and to feeling.

Microscopic examination further shows that these two kinds of cells differ from each other to some extent in visible characters; and, though, as we have seen, the differences in activity appear to be dependent on differences in invisible molecular arrangement rather than on gross visible differences such as may be called structural, still the invisible differences involve or entail, or are accompanied by, visible differences, and such differences as can be recognized between endoderm and ectoderm, even with our present knowledge, may be correlated to differences in their work; future inquiry will probably render the correlation still more distinct.

The ectoderm cells together constitute what we have spoken of above as a tissue, whose function in the modern sense of the word is movement and feeling, and the endoderm cells constitute a second tissue, whose function is assimilation; and the phenomena of the whole being result from the concurrent working of these two functions. Of organs, in the old sense of the word, of mechanical contrivances, there is hardly a trace. The performances of the being are, it is true, conditioned by its being moulded in the form of a long tubular sac with a crown of like tubular arms, but beyond this the explanation of every act of the hydra’s life is first to be sought in the characters of the endoderm and ectoderm. The physiology of the hydra is, for the most part, a series of problems, dealing on the one hand with the intimate nature of the ectodermic protoplasm and the changes in that protoplasm which give rise to movement and feeling, as well as with the laws whereby those changes are so regulated that movement and feeling come and go as the needs of the organism may require, and on the other hand with the intimate nature of the endodermic protoplasm and the changes in that protoplasm whereby the dead food is, also according to the needs of the economy, transformed into living substance. Whereas the older physiology dealt almost exclusively with mechanical problems, the physiology of to-day is chiefly busied with what may be called molecular problems.

The physiology of the higher animals, including man, is merely a development of the simpler physiology of the hydra, which has been rendered more complex by a greater division of physiological labour, entailing greater differentiation of structure, and been varied by the intercalation of numerous mechanical contrivances.

In the hydra each ectoderm cell—for, broadly speaking, they are all alike—serves three chief purposes of the body. (1) It is sensitive, that is, it is thrown into peculiar molecular agitations, with expenditure of energy, when acted upon by external agents. In man and the higher animals certain cells of the original ectoderm of the embryo are differentiated from their fellows (which, losing to a large extent this sensitiveness, remain as a mechanical covering to the body) by a more exquisite development of this power of reaction, and moreover are differentiated from each other in their relative sensitiveness to different agents, so that one set of cells becomes peculiarly susceptible to light, another set to pressure, and the like. Thus the uniform ectoderm of the hydra, uniformly susceptible to all agencies, is replaced by a series of special groups of cells forming the basis of sensory organs, each group being specially sensitive to one agent, and having the nature of its constituent cells correspondingly modified. (2) In each ectoderm cell of the hydra the agitations primarily induced by the exciting agent become so modified by changes taking place in the cell that the outcome is not always the same. According to processes taking place in the cell, movement of one kind or another, or no movement at all, may result, and such movement as results may take place immediately or at some other time; it may be at a time so distant that the connexion between the exciting disturbance is lost, and the movement appears to be spontaneous. In man and the higher animals these more complex “neural” processes are carried on, not by the simple sensory cells which receive the primary impression, but by a group of cells set apart for the purpose. These cells constitute a central nervous system, in which a still further division of labour and differentiation of structure takes place, the simple neurotic processes of the hydra, with its dim volition and limited scope of action, being developed in a complex manner into processes which range from simple elaboration of the initial additional agitation of the sensory cell into what we speak of as intelligence and thought. (3) Each octoderm cell, by its tail-like prolongation, or by its whole body, contributes to the movement of the animal while still carrying on the two other actions just described. In man and the higher animals the material of the sensory cell and of the central nervous cells is too precious to be wasted in movements; these accordingly are carried out by groups of cells constituting the muscular tissue, in which both the sensitiveness and the higher neurotic processes of the primitive cell are held in abeyance; indeed, the latter have almost disappeared in order that the energy of the protoplasm may be more completely directed to producing those changes of form which determine the movements of the animal.

Further, the separation in space of these three groups of cells or tissues necessitates the introduction of elements whereby the agitations set up in the sensory cell should be communicated to the central nervous cells, where these agitations are further elaborated, as well as of elements whereby the muscular tissue may receive vibrations from the central nervous cells, so that the movements of the body may be determined by these. Hence strands of irritable protoplasm whose energy is not spent in movement, but wholly given up to the rapid and easy transmission of molecular vibrations, unite, as sensory nerves, the sensory cells with the central nervous cells, and, as motor nerves, these with the muscles.

Lastly, for the adequate carrying out of complex movements, the contractile cells, elongated into specially constructed fibres and constituting the muscles, are arranged, with inert tissues such as bones, cartilages, tendons, and the like (tissues of mechanical virtues, manufactured by an active protoplasm, but themselves passive, no longer active), into various mechanical contrivances. Similarly the sensory cells, as notably those of the eye and the ear, set apart to be acted upon by special agents, are provided with special mechanisms in order that the agent may act with more complete precision. Thus the sensory cells constituting the retina of the eye, in which alone sensory, visual impulses are generated, are provided with an intricate dioptric mechanism, formed partly of inert tissues such as the lens, partly of peculiarly arranged muscular and nervous elements.

In this way the simple ectoderm of the hydra is replaced by a complicated system composed of organs, some of them of extremest intricacy. But the whole system may be reduced to two sets of factors. On the one hand there are organs in the old sense of the word, that is, mechanical arrangements, some connected with the muscles and others connected with the sensory cells, organs whose functions have for the most part to be interpreted on mechanical principles, since their most important factors, putting aside intervening muscular and nervous elements, are the inert products of protoplasm doing simple mechanical work. On the other hand there are organs in the later sense of the word, namely, sensory cells differentiated to be sensitive to special influences, central nervous cells differentiated to carry on the inner nervous work, muscles differentiated to contract, and nerves differentiated to bind together these three other factors. The work of these latter organs is dependent on the nature of their protoplasm; mechanical arrangements play but little part in them; and the results of the⅛ activity can in no way be explained on simple mechanical principles.

Corresponding with this differentiation of the ectoderm cells runs a somewhat similar differentiation of the endoderm cells. In the hydra each endoderm cell appears to receive some of the food bodily into itself and there to elaborate it into what may be spoken of as prepared nutritive material. Some of this material the cell retains within itself in order to renew its own protoplasm; the rest oozes out to the ectoderm cells, the replenishment of whose protoplasm is thereby effected with a saving of labour. In the higher animals the preparation of food is far more complicated. The endodermic sheet of the alimentary canal is folded and arranged into organs called glands, with the mechanical advantage that a large amount of surface is secured within a small bulk; and the constituent endodermic cells of their glands pour out, or secrete, as is said, divers fluids into the cavity of the canal, so that much preliminary preparation of digestion of the food takes place before the food really enters the body. Further, these secreting glandular cells are so differentiated as to pour out special juices acting on special constituents of a meal, and the food subjected in turn to the action of these several juices becomes thoroughly prepared for reception into the body. This reception is carried out by other endoderm cells, which in receiving the digested food probably act upon it so as still further to heighten its nutritive value; and the absorbed food, before it is presented to the muscular and nervous tissues, for whose use it is largely, though of course not exclusively, intended, is subjected to the action of other cells, such as those forming the lymphatic glands and the liver, in order that it may be still further elaborated, still further prepared for the final conversion into living protoplasm.

As in the case of the tissues and organs of ectodermic origin, so also here, the wide separation in space of the masses of differentiated cells constituting tissues necessitates the introduction of mechanical contrivances for the carriage of material from place to place. In the simple minute hydra the nutritive material can permeate the whole body by simply oozing from cell to cell. In the higher animal a hydraulic system for the distribution of nutritive material is introduced. A fluid is distributed in a ceaseless flow all over the body by a mechanical arrangement, consisting of a pump with branching tubes, worked on mechanical principles, and capable of being imitated artificially, save that the power which drives the machine is the energy set free by living muscle. As this circulating fluid or blood rushes past the endoderm cells which have gorged themselves from the rich contents of the alimentary canal, it receives from them some of the material which they have absorbed and elaborated, and carries this nutritive supply to muscles, nerves, and all parts of the body. Similarly it carries away from muscles, nerves, and other tissues the waste-products of their activity, those broken fragments of simpler stuffs into which, as we have seen, the complex protoplasm, wherever it exists, is for ever splitting up, and bears them back to differentiated endodertn and other cells, whose work has become, so to speak, inverted, since their activity is directed to casting things out of the body, instead of receiving things into the body. And lastly, by a special arrangement, by a peculiar property of those red corpuscles which make blood red, this circulating material at one and the same time carries to each corner of the body, not only the nutritive material required for building up protoplasm, but also the oxygen by which the constructed protoplasm may suffer oxidation, and in being oxidized set free that energy the manifestation of which is the token of life. Blood is in fact the medium on which all the various parts of the body live. Just as an amoeba finds in the water which is its home both the food with which it builds itself up and the oxygen with which it breaks itself down, and returns to the water the waste-products of its continued disintegration, so each islet of the living substance of the higher animal, be it muscle or nerve or gland, draws its food and its oxygen from the red blood-stream sweeping past it, finding therein all its needs, and sheds into the same stream the particles into which it is continually breaking up, and for which it has no longer any use. Hence the blood becomes, as it were, a chemical epitome of the body: from it each tissue takes something away; to it each tissue gives something back. As it sweeps by each tissue, losing and gaining, it makes the whole body common, and when working aright brings it about that each tissue is never in lack of the things which it wants, never choked up with the things with which it has done.

This vascular system, consisting of a force-pump and branching tubes, constitutes, as we have said, a mechanical arrangement worked on mechanical principles. Nevertheless occult protoplasmic processes intervene as factors in its total work. Not only is the force-pump itself a living muscular organ, not only are the walls of the tubes muscular in nature, so that the mere mechanical working of the system is modified by changes not of mechanical origin taking place in them, but the living material which lines the tubing throughout, especially in the minuter channels, finds work to do, also not of a mechanical nature. The gross phenomena of the flow of blood through the capillary channels may (see Vascular System) be interpreted on simple hydraulic principles; but no appeal to the ordinary physical laws of dead material will explain the phenomena of the interchange between the blood on the inside of a capillary wall and the tissue-elements on the outside. In every tissue, be it gland, muscle, or nerve, the blood, so far from being actually in contact with the active protoplasmic units of the tissue, is separated by the protoplasmic film of the capillary wall, and by a space or spaces, greater or smaller, filled with the fluid called lymph and lined to a greater or less extent with protoplasmic cells, which lining, often at least, parts the tissue-units from the lymph. Hence the tissue lives upon the lymph, while the lymph is replenished from the blood; and the interchange between the tissue-unit and the blood is determined, not only by the direct action of the tissue-unit on the lymph, but also by the relations of the lymph to the blood, as regulated by the capillary wall and the cellular lining of the lymph-spaces. We may speak of the interchange as broadly one of diffusion or osmosis through filmy membranes; but diffusion is not the lord in the matter: it is rather a humble servant directed hither and thither by occult molecular processes in the protoplasmic structures concerned.

The foregoing rough analysis leads to a conception of the physiology of the animal body which may be expressed somewhat as follows. The body is composed of different kinds of matter; each kind of matter, arranged in units more or less discrete, constitutes a tissue; and the several tissues, though having a common likeness in token of their origin from a common primordial protoplasm, have dissimilar molecular constitutions, entailing dissimilar modes of activity. Nor is each tissue homogeneous, for two parts of the body, though so far alike as to be both examples of the same general tissue, may be different in molecular constitution, more or less distinctly expressed by microscopic differences of structure, and correspondingly different in action. Thus a liver-cell and a kidney-cell, though both examples of glandular tissue, are quite distinct; so also several varieties of muscular tissue exist; and in the dominant nervous tissue we have not only a broad distinction between nerve-fibres and nerve-cells, but the several groups of nerve-cells which are built up into the brain and spinal cord, and indeed probably the single nerve-cells of these, though all possessing the general characters, both in structure and function, of nervous protoplasm, differ most widely from each other. These several tissues of diverse constitution and activity, ranging as regards the rapidity of the molecular changes taking place in them from the irritable, unstable, swiftly-changing nerve-cell to the stable, slowly-changing, almost lifeless tendon or bone, are disposed in the body in various mechanical arrangements constituting organs or machines, whereby the activities of the constituent tissue-elements are brought to bear in special directions. These organs range from those in which the mechanical provisions are dominant, the special activity of the tissue-elements themselves being in the background, and supplying only an obscure or even unimportant factor, as in the organs of respiration, to those in which the mechanical provisions are insignificant, as in the central nervous system, where the chief mechanical factor is supplied by the distribution in space of the nerve fibres or cells.

Hence it is obvious that almost every physiological inquiry of any large scope is, or sooner or later becomes, of a mixed nature. On the one hand, investigation has to be directed to the processes taking place in the actual tissue-elements, in the protoplasmic cells and modifications of cells. These are essentially of a molecular, often of a chemical or chemico-physical nature; in the problems thus raised matters of form and structure, other than that of molecular structure, which no microscope can ever reveal, are of secondary moment only, or have no concern in the matter at all. These may be spoken of as the purely physiological or as the molecular problems. On the other hand, the natural results of these tissue-activities are continually being modified by circumstances whose effect can be traced to the mechanical arrangements under which the tissue in question is acting, whence arise problems which have to be settled on simple mechanical principles.

We may take as an illustration the physiology of the kidney. In the old language the function of the kidney is to secrete urine. When we come to inquire into the matter, we find, in the first place, that the secretion of urine —that is, the quantity and quality of the urine escaping from the duct of the kidney in a given period—is partly determined by the quantity of blood passing through the kidney and the circumstances of its passage. Now the quantity of blood reaching the kidney at any one time is dependent partly on the width of the renal arteries, partly on the general pressure of the blood in the arterial system. The width of the renal arteries is in turn dependent on the condition of their muscular walls, whether contracted or relaxed; and this condition is determined by the advent of nervous impulses, the so-called vaso-motor impulses, arising in the central nervous system and passing down to the renal arteries along certain nerves. The emission of these vaso-motor impulses from the central nervous system is further determined, on the one hand by the condition of certain parts of the central nervous system, the so-called vaso-motor centres, and on the other by the passage of certain afferent sensory impulses to those vaso-motor centres from sensory surfaces such as the skin. Similarly the general blood-pressure is dependent on the condition, patent or narrowed, of the small arteries generally, this being likewise governed by the vaso-motor system and on the coincident work done by the heart in driving blood into the great blood-vessels, this work being also governed by the nervous system. Hence in attacking such a problem as to how any particular event, such as the exposure of skin to the cold, influences the flow of blood through the kidney and thus the secretion of urine, the investigator, without staying to inquire into the nature of nervous impulses, or into the nature of changes taking place in vaso-motor centres, &c., directs his attention to determining what impulses are generated under the circumstances, what paths they take, to what extent they are quantitatively modified, how far they and their effects react upon each other, and so on. His inquiry in fact takes on to a large extent the characters of an attempt to unravel an intricate game, in which the counters are nervous impulses, muscular contractions, and elastic reactions, but in which the moves are determined by topographical distribution and mechanical arrangements.

But there are other problems connected with the physiology of the kidney of quite a different nature. The kidney is, broadly speaking, constructed of living protoplasmic cells so arranged that each cell is on one side bathed with blood and lymph, and on the other forms the boundary of a narrow canal, which, joining with other canals, ultimately opens into the urinary bladder. Here the question arises how it is that these protoplasmic cells, having nothing to draw upon but the common blood, which is distributed to other organs and tissues as well, are able to discharge on the other side of them into the canal the fluid urine, which is absolutely distinct from blood, which contains substances wholly unknown in blood, as well as substances which, though occurring in blood, are found there in minute quantities only, and, moreover, are not found to escape from the blood into any other tissues or organs. In attempting to answer this question we come upon an inquiry of quite a different nature from the preceding, an inquiry for the solution of which mechanical suggestions are useless. We have to deal here with the molecular actions of the protoplasmic cell. We must seek for molecular explanations of the questions, why a current sets across the cells from blood-capillary and lymph-space to the hollow canal; why the substances which emerge on the far side are so wholly unlike those which enter in on the near side; why, moreover, the intensity of this current may wax and wane, now flooding the canal with urine, now nearly or quite drying up; why not only the intensity of the current but also the absolute and relative amount of the chemical substances carried along it are determined by events taking place in the cell itself, being largely independent of both the quantity and quality of the blood which forms the cell’s only source of supply. These and other like questions can only be solved by looking with the mind’s eye, by penetrating through careful inferences, into those inner changes which we call molecular, and which no optical aid will ever reveal to the physical eye.

These two lines of inquiry, which we may call the mechanical and the molecular, obtain in all parts of physiology, sometimes the one and sometimes the other being dominant. A study of the special articles dealing with the several parts of physiology (see “Nervous System” below, Nutrition, Reproduction, Respiration, Vascular System) will perhaps sufficiently show this; but it may be worth while to give a very brief survey of the whole field from this point of view.

The master tissues and organs of the body are the nervous and muscular systems, the latter being, however, merely the instrument to give effect and expression to the motions of the former. All the rest of the body serves simply either in the way of mechanical aids and protection to the several parts of the muscular and nervous systems, or as a complicated machinery to supply these systems with food and oxygen, i.e., with blood, and to keep them cleansed from waste matters throughout all their varied changes. The physiology of the muscular system is fairly simple. The mechanical problems involved have been long ago for the most part worked out, and the molecular problems which touch on the nature of muscular contractions, their dependence on the blood-supply, and their relations to nervous impulses are being rapidly solved. The physiology of the nervous system, on the other hand, is in its infancy. The mechanical side of the inquiry is here represented, inasmuch as the various actions of the system are conditioned by the distribution and topographical arrangement of the constituent fibres and cells; and even these simple problems, as may be seen from the article “Nervous System” below, are as yet largely unworked. The deeper molecular problems, those which deal with the real nature of the processes taking place in cell and fibre, even the simpler of these, such as the one which asks why the neural protoplasm of one cell, or group of cells, seems quiescent until stirred by some foreign impulse, its own vibrations being otherwise retained and lost within its own substance, while the neural protoplasm of another cell is continually, or from time to time, discharging vibrations, as rhythmic molecular pulses, along adjoining fibres,— these, at the present day, can hardly be said to be touched. The physiology of the nervous system is emphatically the physiology of the future.

The rest of the body may, from a broad point of view, be regarded as a complex machinery for supplying these master tissues with adequately-prepared food and oxygen, for cleansing them from the waste-products of their activity, and for keeping them at a temperature suitable for the development of their powers. As we have already said, the blood is the agent which not only supplies both food and oxygen but sweeps away all refuse, and, we may add, is the instrument for maintaining an adequate temperature. All the rest of the body may in fact be looked upon as busied in manufacturing food into blood, in keeping up the oxygen supply of the blood, in sifting out from the blood all waste material, and in maintaining the blood at a uniform heat. This work, of which blood is, so to speak, the centre, is, as we have already seen, carried out by protoplasmic cells, many of which are themselves of a muscular nature, often forming part of complicated mechanical contrivances, built up partly of inert tissues, partly of active tissues, such as muscle and nerve. In tracing the food and oxygen into the blood and the waste matters out of the blood, in studying the distribution of the blood itself and the means adopted to maintain its even temperature, we come, as before, on problems partly mechanical or chemical and partly molecular. The changes which the food undergoes in the intestine can be, and have been, successfully studied as a series of purely chemical problems conditioned by anatomical arrangements, such as the existence of an acid fluid in the stomach, succeeded by alkaline fluids in the intestine, and the like; but the questions concerned in the discharge of the digestive juices into the alimentary canal, in the secretory activity of the digestive glands, raise up protoplasmic molecular inquiries. In the reception or absorption of the digested food we similarly find the purely physical processes of diffusion and the like overridden by the special protoplasmic activities of the constituent cells of the lining of the canal. In the further elaboration of the digested products the action of cells again intervenes, as it similarly does in the, so to speak, inverted action by which waste matters are cast out of the body, though in both cases the results are in part conditioned by mechanical contrivances. The circulation of the blood is carried on by means of an intricate mechanical contrivance, whose working is determined and whose effects are conditioned by molecular changes occurring in the constituent muscles and other protoplasmic cells; the work done by the heart, the varying width of the channels, the transit of material through the filmy capillary walls, all these are at once the results of protoplasmic activity and factors in the mechanical problems of the flow of blood. The oxygen passes into and carbonic acid out of the blood, through simple diffusion, by means of the respiratory pump, which is merely a machine whose motive-power is supplied by muscular energy, and both oxygen and carbonic acid are carried along in the blood by simple chemical means; but the passage of oxygen from the blood into the tissue and of carbonic acid from the tissue into the blood, though in themselves mere diffusion processes, are determined by the molecular activity of the constituent cells of the tissue. Lastly, the blood, however well prepared, however skilfully driven to the tissue by the well-timed activity of the vascular system, even when it has reached the inner network of the tissue-elements, is not as yet the tissue itself. To become the tissue it must undergo molecular changes of the profoundest kind: it must cross the boundary from dead material to living stuff. The ultimate problems of nutrition are of the molecular kind. All the machinery, however elaborate, is preparatory only, and it is the last step which costs the most.

Of the many problems concerned in these several departments of physiology the one class which we have spoken of as being mechanical in nature is far too varied to be treated of as a whole. The problems falling under it have but few features in common; each stands, as it were, on its own bottom, and has to be solved in its own way. The problems of the other class, however—those which we have spoken of as being molecular in nature—have a certain common likeness; and it may be worth while to consider, in a brief and general manner, some of their most striking characters.

For this purpose we may first of all turn to the changes taking place in a secreting cell, for these have of late years been studied with signal success. They illustrate what may be called the chemical aspects of vital actions, jαst as the changes in a muscular fibre, on the other hand, seem to present, in their simplest form, the kinetic aspects of the same actions. If we examine a secreting gland, such as a pancreas or a salivary gland, we find that it is composed of a number of similar units, the unit being a secreting cell of approximately spheroidal form, one part of the surface of which borders a canal continuous with the duct of the gland, while another part is bathed in lymph. The process of secretion consists in the cell discharging into the canal a fluid which is of a specific character, insomuch as, though it consists partly of water and other substances common to it and other fluids of the body, these are present in it in special proportions; and it also contains substances or a substance found in itself and nowhere else. To enable it to carry on this work the cell receives supplies of material from the lymph in which it is bathed, the lymph in turn being replenished from neighbouring capillary blood-vessels. The secreting cell itself consists of a soft protoplasmic “body,” of the nature previously described, in the midst of which lies a “nucleus.” The consideration of the actions carried out by the nucleus may, for simplicity’s sake, be left on one side for the present; and we may regard the cell as a mass of protoplasm consisting, as we have seen, of a network of a particular nature, and of other substances of different nature filling up the meshes or interstices of the network.

Such a cell may exist under two different conditions. At one time it may be quiescent: although the bloodvessels surrounding it are bathing it with lymph, although this lymph has free access to the protoplasm of the cell, no secretion takes place, no fluid whatever passes from the cell into the canal which it borders. At another time, under, for instance, some influence reaching it along the nerve distributed to the gland, although there may be no change in the quantity or quality of the blood passing through the adjacent blood-vessels, a rapid stream of material flows from the protoplasmic cell-body into the canal. How is this secretion brought about 1

If we examine certain cells, such, for instance, as those of the pancreas, we find that during a period of rest succeeding one of activity the cell increases in bulk, and further that the increase is not so much an enlargement of the protoplasmic network as an accumulation of material in the meshes of the network; in fact, there appears to be a relative diminution of the actual protoplasm, indicating, as we shall see, a conversion of the substance of the network into the material which is lodged in the interstices of the network. This material may, and frequently does, exist in the form of discrete granules, recognizable under the microscope; and in the pancreas there is a tendency for these granules to be massed together on the side of the cell bordering the lumen of the canal. During activity, while the cell is discharging its secretion into the canal, these granules disappear, so that the protoplasmic network is after prolonged activity left with a very small burden of material in its meshes; at the same time there also appears to be an accompanying absolute increase of growth of the mass of the protoplasm itself. We have further evidence that the substance which is thus stored up in the meshes of the cell, forming the granules, for instance, just spoken of, is not, as it exists in the cells, the same substance as that which occurs in the secretion as its characteristic constituent. Thus the characteristic constituent of pancreatic juice is a peculiar ferment body called “trypsin,” and we f>ossess evidence that the granules in the pancreatic cells are not trypsin. But we have also evidence that these granules consist of material which, upon a very slight change, becomes trypsin, of material which is an antecedent of trypsin, and which has accordingly been called “trypsinogen.” Thus the cell during rest stores up trypsinogen, and the change which characterizes activity is the conversion of trypsinogen into trypsin, and its consequent discharge from the cell. These are facts ascertained by observation and experiment, viz., that trypsinogen appears in the protoplasm of the cell, and that in the act of secretion this trypsinogen is discharged from the cell in the form of the simpler trypsin. When, however, we come to consider the origin of the trypsinogen we pass to matters of inference and to a certain extent of speculation.

Two views seem open to us. On the one hand, we may adopt an old theory, once generally accepted, and suppose that the cell picks out from the lymph which bathes it particles of trypsinogen, or particles of some substance which is readily transformed into trypsinogen, and deposits them in its substance. This may be called the “selective” theory. On the other hand, we may suppose that the trypsinogen results from the breaking down, from the katabolic or destructive metabolism of the protoplasm, being thus wholly formed in the cell. This may be called the “metabolic” theory. Our present knowledge does not permit us wholly to prove or wholly to disprove either of these theories; but such evidence as we possess is in favour, and increasingly in favour, of the metabolic theory. All efforts to detect in the blood or in the lymph such substances as trypsinogen, or analogous substances in the case of other glands, have hitherto failed; and, although such a negative argument has its weakness, still it is of avail as far as it goes. On the other hand, the diminution of the protoplasm in the pancreatic cell, pari passu with the increase of trypsinogen, and its subsequent renewal previous to the formation of new trypsinogen, strongly support the metabolic theory, and a number of other facts drawn from the history of various animal and vegetable cells all tend strongly in the same direction. We have further a certain amount of evidence that trypsinogen arises from an antecedent more complex than itself, as it in turn is more complex than trypsin. So, although clear demonstration is not as yet within our reach, we may with considerable confidence conclude that trypsinogen and other like products of secreting cells arise from a breaking down of the cellsubstance, are manufactured by the protoplasm of the cell out of itself.

We are thus led to the conception that the specific material of a secretion, such as the trypsin of pancreatic juice, comes from the protoplasm of the cell, through a number of intermediate substances, or mesostates as they are called; that is to say, the complex protoplasm breaks down into a whole series of substances of decreasing complexity, the last term of which is the specific substance of the secretion. Now the protoplasm is undoubtedly formed at the expense of the material or pabulum brought to it from the blood through the medium of the lymph; the pabulum becomes protoplasm. Here also two views are open to us. On the one hand, we may suppose that the crude pabulum is at once by a magic stroke, as it were, built up into the living protoplasm. On the other hand, we may suppose that the pabulum reaches the stage of protoplasm through a series of substances of increasing complexity and instability, the last stage being that which we call protoplasm. And here, too, no absolute decision between the two views is possible, but such evidence as we do possess is in favour, and increasingly in favour, of the latter view.

We may therefore with considerable confidence anticipate the future arrival of evidence which will demonstrate the as yet only probable view that in the secreting cell there are two series of events, two staircases, as it were, of chemical transformation,—one an ascending staircase of synthetic, anabolic processes through which the pabulum, consisting of several substances, some of them already complex and unstable, is built up into the still more complex and still more unstable protoplasm; the other a descending staircase, consisting of a series of katabolic processes giving rise to substances of decreasing complexity and increasing stability. The substances or mesostates appearing in the former we may speak of as “anastates,” those of the latter we may call “katastates.” At each step of the former, by which a simpler anastate becomes, or by which simpler anastates become, a more complex one, energy is absorbed; at each step of the latter energy is set free. And, since in the animal-cell the initial anastates seem always or at least generally more complex than the final katastates, the total life of the animal-cell is virtually a giving forth of energy.

So far we have spoken of the secreting cell, but we have evidence that in the activity of a muscle a similar series of events takes place. Reduced to theoretical simplicity, the unit a number of which go to form a muscle is a protoplasmic cell, undergoing, like the secreting cell, a continual metabolism, with a change in the results of that metabolism at the moment of functional activity. Put in a bald way, the main difference between a secreting cell and a muscle-cell, or elementary muscle-fibre as it is often called, is that in the former the products of the metabolism constitute the main object of the cell’s activity, a change of form being of subordinate importance, whereas in the latter the change of form, an increase of one axis at the expense of another, a shortening with corresponding thickening, is the important fact, the products of the metabolism which thus gives rise to the change of form being of secondary value.

Now we have evidence, which, as in the case of the secreting cell, though not demonstrative, is weighty and of daily increasing weight, that the change of form, the contraction of a muscle, is due to a sudden metabolism, to an explosive decomposition of what may be called “contractile substance,” a substance which appears to be used up in the act of contraction, and the consumption of which leads with other events to the exhaustion of a muscle after prolonged exertion. We know as a matter of fact that when a muscle contracts there is an evolution of a considerable quantity of carbonic acid, and a chemical change of such a kind that the muscle becomes acid. This carbonic acid must have some antecedent, and the acidity must have some cause. It is of course possible that the protoplasm itself explodes, and is the immediate parent of the carbonic acid and the direct source of the energy set free in the contraction; but evidence analogous to that brought forward in relation to the secreting cell leads to the conclusion that this is not so, but that the explosion takes place in, and that the energy is derived from, a specific contractile substance. And there is further evidence that this hypothetical substance, to which the name of “inogen” has been provisionally given, is, like its analogue in the secreting cell, a katastate. So that the contracting activity of a muscular fibre and the secreting activity of a gland-cell may be compared with each other, in so far as in each case the activity is essentially a decomposition or explosion, more or less rapid, of a katastate, the inogen in the one instance, the trypsinogen or some other body in the other instance, with the setting free of energy, which in the case of the secreting cell leaves the substance wholly as heat, but in the case of the muscle partly as movement, the activity being followed in each case by the discharge from the fibre or cell of the products, or some of the products, of this decomposition.

Further, we may carry on the parallel to nervous substance. As a nervous impulse travels along a nerve-fibre or pursues its intricate course along the fibrillar tracts of a nerve-cell, the amount of chemical change is too slight to be satisfactorily appreciated by the methods at present under our command. There is certainly no massive explosion like that of a muscular fibre, and the most striking phenomena attending the passage of a nervous impulse are of an electrical nature. We cannot indeed distinctly prove that any chemical change does really accompany nervous activity; but from many considerations it is extremely probable that a chemical change, an explosive decomposition of more complex into more simple substances, is the basis of a nervous impulse. The energy, however, which is set free by this decomposition is not discharged from the nervous substance to so great an extent as is the case in the muscle-fibre, but is largely consumed in conveying or conducting the decomposition from one particle of the nervous substance to succeeding ones,—that is to say, in carrying out that which is the essential work of the nervous substance. Moreover, even in a muscle, while the explosion of inogen and the consequent shortening and thickening of the muscle-substance travel along the fibre from particle to particle in the form of a wave starting from the junction of the nerve-fibre with the muscular fibre, or from some other point of stimulation, this wave of visible contraction is preceded by invisible molecular changes also travelling along the fibre in the form of a wave, changes which manifest themselves by no massive explosions, which are indicated by electrical phenomena chiefly, and which are exceedingly like the nervous impulses of proper nervous structures. In the very substance of the muscular fibre there appears to be a material which is not inogen, but which is capable of undergoing changes, probably of the nature of an explosive decomposition, and it is these which in turn induce the more massive decomposition of the inogen. It is possible, indeed probable, that the constituent particles of inogen are not able to communicate their explosions to each other, so that the presence in the muscular fibre of an impulse-carrying material is a necessity. Be this as it may, a change antecedent to the explosion which is the cause of the actual contraction does occur in every particle of the muscle which contracts, and, as we have urged, the change is probably one taking place in a special substance. This substance may be the veritable protoplasm itself of the fibre, but considerations analogous to those urged before would lead us to suppose that it too is a katastate, but a katastate different in qualities from inogen; and we may furthur suppose that a very similar katastate is manufactured by nervous protoplasm, and by its decomposition gives rise to nervous energy. This katastate is, as it were, the fuse or trigger whose action fires the massive charge of the muscular gun, and might receive the name of “apheter.”

If we accept this view as to the nature of the simple nervous impulses which sweep along nerve-fibres—and in this respect motor and sensory nerves would seem wholly alike—there is no great difficulty in extending the conception to the more complicated processes taking place in the central nervous system. An ordinary reflex act, so called, is perhaps one of the simplest labours of that system, and we have evidence that in a reflex act sensory impulses arriving along a sensory fibre at the protoplasm of a nervecell induce in that protoplasm changes which, though in certain respects differing from, are fundamentally analogous to, those changes in motor and sensory nerve-fibres which constitute their respective nervous impulses. The chief difference is that, whereas along sensory and motor fibres the impulses pursue an even course, possibly undergoing some augmentation, but one which is not sufficiently marked to be beyond doubt, in the nerve-cell, on the contrary, an unmistakable augmentation accompanied by a systematic dispersion takes place. So great is the augmentation in some cases that a gentle short series of sensory impulses reaching one of a group of nerve-cells along a single sensory fibre may throw the whole group of nerve-cells into such profound agitation that repeated series of even violent impulses may be discharged along a multitude of motor fibres. Allowing for this increase in the energy set free, the changes in the nerve-cells do not seem to differ fundamentally from those in the nerve-fibres (which indeed differ to a certain extent among themselves), and may, like them, be regarded as due essentially to the decomposition of some katastate or katastates.

Further, just as the apheter of the muscular fibre, that which inaugurates the explosion of the contractile inogen, differs from the apheter of the nerve-fibre, so we may suppose that in the various nerve-cells of different parts of the central nervous system difference of function, while partly due to the mere arrangement and distribution of nervous impulses of the same kind, is also and more largely due to difference in the kind of impulses brought about by difference in the composition and mode of decomposition of the nervous material. For instance, certain changes in the nervous system are accompanied by distinct changes of consciousness, while others are not. And, while we may justly refuse to attempt any explanation of consciousness, it is nevertheless within our right to suppose that, in that nervous substance which is the highest development of protoplasm and to whose service the whole body ministers, amid the many substances of increasing complexity and dignity which enter into its composition there should be a substance or some substances the changes in which are, or may be, accompanied by consciousness. The doctrine of evolution compels us to admit that consciousness must be potentially present in the simple protoplasm of the amoeba, and must be similarly present in all the tissues of the highly-developed animal, instead of being confined to some limited portion of the nervous system. Evolution refuses to admit a sharp line of demarcation between a “conscious” and a “non-conscious” part, and this decision is increasingly supported as our knowledge of the nervous system advances. But a great deal of the earlier part of this article was directed to show that all the powers of the complex animal are the outcome of the differentiation of a primordial protoplasm, while the discussion concerning the molecular changes of tissues in which we are now engaged is simply an attempt to trace out how that differentiation has taken place. And, as far as we can see, there are no just reasons why the differentiation which sets apart the nervous tissue from other parts of the body should not obtain in the nervous tissue itself, and the obscure rudiments of consciousness present in all nervous material become by differentiation developed, in some particular kinds of nervous substance, into consciousness more strictly so called.

In the case, then, of secreting cells, of muscular tissue, and of the various forms of nervous tissue the tendency of inquiries into the molecular processes taking place in them is to lead us to regard the varied activities of these tissues as due to molecular disruptive changes in their several katastates, these being various stages of the downward metabolism or katabolism of protoplasm.

Similar considerations might be extended to other tissues of the body which are neither nervous nor muscular, and, though engaged in chemical work, are not distinctly secretory or excretory, such, for instance, as the hepatic cells engaged in the elaboration of glycogen. They might also be extended to those tissues in which the katastates are not exploded and discharged, but retained and massed up in the body for mechanical or other purposes, to cartilage, for instance, the chondrigenous basis or groundsubstance which many considerations show to be a product or katastate of protoplasm. We are thus led to the conception, brought forward in an earlier part of this article, that all over the body protoplasm is continually building itself up out of the pabulum supplied by food, and continually breaking down, giving rise in different tissues and different parts of the body to different katastates with different composition and different properties, the various activities of the body being the outcome of the various properties of the various katastates. If this be admitted, it almost inevitably follows that what we have called protoplasm cannot be always the same thing, that there must be many varieties of protoplasm with different qualities and with correspondingly different molecular structure and composition. If this be so, the question naturally arises, why use the word protoplasm at all, since, by the showing, it seems to have no exact meaning? But it has an exact meaning. All the evidence at our disposal goes to show that a katastate of any given degree cannot form a fresh katastate of the same degree; any one katastate can only arise from the decomposition of a preceding more complex katastate, and that in turn from a katastate still more complex. Passing upwards, we come at last to something which, instead of proceeding from a more complex substance, builds itself up out of a less complex, more simple substance, and it is this something, whatever its exact composition, into whatever katastates it is destined to fall asunder, to which the generic name “protoplasm” should be given. Possibly another new name were better, but there are advantages in retaining the old term. It is protoplasm in this sense which is alone living; it is its synthetic power which is its token of being alive. That synthetic power is, we must admit, exercised along the ascending series of anastates. But here our knowledge is a blank; and it would be simply waste of time to speculate as to the details of the constructive processes. Using the word “protoplasm” in this sense, it is obvious that the varieties of protoplasm are numerous, indeed almost innumerable. The muscular protoplasm which brings forth a contractile katastate must differ in nature, in composition—that is, in construction—from glandular protoplasm, whose katastate is a mother of ferment. Further, the protoplasm of the swiftly contracting striped muscular fibre must differ from that of the torpid smooth unstriated fibre; the protoplasm of human muscle must differ from that of a sheep or a frog; the protoplasm of one muscle must differ from that of another muscle in the same kind of animal; and the protoplasm of Smith’s biceps must differ from that of Jones’s.

We may, for a moment, turn aside to point out that this innate difference of protoplasm serves to explain the conclusions to which modem investigations into the physiology of nutrition seem to be leading. So long as we speak of muscle or flesh as one thing, the step from the flesh of mutton which we eat to the flesh of our body which the mutton, when eaten, becomes, or may become, does not seem very far; and the older physiologists very naturally assumed that the flesh of the meal was directly, without great effort and without great change, as far as mere chemical composition is concerned, transformed into the muscle of the eater. The researches, however, of modern times go to show that the substances taken as food undergo many changes and suffer profound disruption before they actually become part and parcel of the living body, and conversely that the constructive powers of the animal body were grossly under-rated by earlier investigators. If one were to put forward the thesis that the proteid of a meal becomes reduced almost to its elements before it undergoes synthesis into the superficially similar proteid of muscle, the energy set free in the destruction being utilized in the subsequent work of construction, he might appeal with confidence to modem results as supporting him rather than opposing him in his views. It would almost seem as if the qualities of each particle of living protoplasm were of such an individual character that it had to be built up afresh from almost the very beginning; hence the immense construction which inquiry shows more and more clearly every day to be continually going on as well in the animal as in the vegetable body.

Taking into consideration all the fine touches which make up the characters of an individual organism, and remembering that these are the outcome of the different properties or activities of the several constituent tissues of the body, working through.a delicately-balanced complicated machinery, bearing in mind the far-reaching phenomena of heredity by which the gross traits and often the minute tricks of the parents’ body are reproduced in the offspring, if there be any truth at all in the views which we have urged, tracing the activities of the organism to the constitution of its protoplasm, this must be manifold indeed. The problems of physiology in the future are largely concerned in arriving, by experiment and inference, by the mind’s eye, and not by the body’s eye alone, assisted as that may be by lenses yet to be introduced, at a knowledge of the molecular construction of this protean protoplasm, of the laws according to which it is built up, and the laws according to which it breaks down, for these laws when ascertained will clear up the mysteries of the protean work which the protoplasm does.

And here we may venture to introduce a word of caution. We have, in speaking of protoplasm, used the words “construction,” “composition,” “decomposition,” and the like, as if protoplasm were a chemical substance. And it is a chemical substance in the sense that it arises out of the union or coincidence of certain factors, which can be resolved into what the chemists call “elements,” and can be at any time by appropriate means broken up into the same factors, and indeed into chemical elements. This is not the place to enter into a discussion upon the nature of so-called chemical substances, or, what is the same thing, a discussion concerning the nature of matter; but we may venture to assert that the more these molecular problems of physiology, with which we are now dealing, are studied the stronger becomes the conviction that the consideration of what we call “structure” and “composition” must, in harmony with the modem teachings of physics, be approached under the dominant conception of modes of motion. The physicists have been led to consider the qualities of things as expressions of internal movements ; even more imperative does it seem to us that the biologist should regard the qualities (including structure and composition) of protoplasm as in like manner the expression of internal movements. He may speak of protoplasm as a complex substance, but he must strive to realize that what he means by that is a complex whirl, an intricate dance, of which what he calls chemical composition, histological structure, and gross configuration are, so to speak, the figures; to him the renewal of protoplasm is but the continuance of the dance, its functions and actions the transferences of figures. In so obscure a subject it is difficult to speak otherwise than by parables, and we may call to mind how easy it is to realize the comparison of the whole body of man to a fountain of water. As the figure of the fountain remains the same though fresh water is continually rising and falling, so the body seems the same though fresh food is always replacing the old man which in turn is always falling back to dust. And the conception which we are urging now is one which carries an analogous idea into the study of all the molecular phenomena of the body. We must not pursue the subject any further here, but we felt it necessary to introduce the caution concerning the word “substance,” and we may repeat the assertion that it seems to us necessary for a satisfactory study of the problems on which we have been dwelling for the last few pages to keep clearly before the mind the conception that the phenomena in question are the result not of properties of kinds of matter, in the vulgar sense of these words, but of kinds of motion.

In the above brief sketch we have dealt chiefly with such well-known physiological actions as secretion, muscular contractions, and nervous impulses. But we must not hide from ourselves the fact that these grosser activities do not comprise the whole life of the tissues. Even in the simple tissues, and more especially in the highly-developed nervous tissues, there are finer actions which the conception outlined above wholly fails to cover.

Two sets of vital phenomena have hitherto baffled inquirers,—the phenomena of spontaneous activity, rhythmic or other, and the phenomena of “inhibition.” All attempts to explain what actually takes place in the inner working of the tissues concerned when impulses passing down the pneumogastric nerve stop the heart from beating, or in the many other analogous instances of the arrest of activity through activity, have signally failed; the superficial resemblance to the physical “interference of waves” breaks down upon examination, as indeed do all other hypotheses which have as yet been brought forward. And we are wholly in the dark as to why one piece of protoplasm or muscular fibre or nervous tissue remains quiescent till stirred by some stimulus, while another piece explodes into activity at rhythmic intervals. We may frame analogies and may liken the phenomena to those of a constant force rhythmically overcoming a constant resistance, but such analogies bring us very little nearer to understanding what the molecules of the part are doing at and between the repeated moments of activity.

Further, if the ingenious speculations of Hering, that specific colour-sensations are due to the relation of assimilation (anabolism) to dissimilation (katabolism) of protoplasmic visual substances in the retina or in the brain, should finally pass from the condition of speculation to that of demonstrated truth, we should be brought face to face with the fact that the mere act of building up or the mere act of breaking down affects the condition of protoplasm in other ways than the one which we have hitherto considered, viz., that the building up provides energy to be set free and the breaking down lets the energy forth. In Hering’s conception the mere condition of the protoplasm, whether it is largely built up or largely broken down, produces effects which result in a particular state of consciousness. Now, whatever views we may take of consciousness, we must suppose that an affection of consciousness is dependent on a change in some material. But in the case of colour-sensations that material cannot be the visual substance itself, but some other substance. That is to say, according to Hering’s views, the mere condition of the visual substance as distinct from a change in that condition determines the changes in the other substance which is the basis of consciousness. So that, if Hering’s conception be a true one (and the arguments in favour of it, if not wholly conclusive, are at least serious), we are led to entertain the idea that, in addition to the rough propagation of explosive decompositions, there are continually passing from protoplasm to protoplasm delicate touches compared with which the nervous impulses which with such difficulty the galvanometer makes known to us are gross and coarse shocks. And it is at least possible, if not probable (indeed present investigations seem rapidly tending in this direction), that an extension of Hering’s view, with such modifications as future inquiry may render necessary, to other processes than visual sensations, more especially to the inner working of the central nervous system, may not only carry us a long way on towards understanding inhibition and spontaneous activity but may lay the foundation of a new molecular physiology. This, however, is speculative and dangerous ground. But it seemed desirable to touch upon it since it illustrates a possible or probable new departure. What we have said of it and of the more manageable molecular problems of physiology will perhaps show that, vast and intricate as is the maze before the physiologist of to-day, he has in his hand a clue which promises, at least, to lead him far on through it.

Space forbids our entering upon a discussion concerning the methods of physiology; but, accepting the truth of the preceding discussion as to the nature of physiological problems, the means of solving these problems speak for themselves.

From the earliest times the methods of physiological inquiry have belonged to one of two categories: they have been anatomical or experimental. And the same distinction holds good to-day, though both methods are often joined together in one inquiry, and indeed at times may be said to merge the one into the other. By the anatomical method the observer ascertains the gross outlines, the minute structure, and if necessary the physical characters and the chemical composition of an organism or part of an organism; and by comparison of these with those of different organisms, or of the same organism placed by nature —that is, not by himself—in different circumstances, he draws conclusions as to the actions taking place in it while it was alive. In early times the comparison of gross structures gave important results, but they have now been to a great extent exhausted; and the most valuable conclusions reached at the present day by the anatomical method are those arrived at by histological investigation of minute structures and by chemical analysis. The marks of this method are that on the one hand it deals for the most part with things which are no longer alive, and hence must necessarily fail to make touch with the inner workings of which we have spoken above, and on the other hand in its comparison of organisms under different conditions it has to wait till Providence brings about what it requires, and has to be satisfied with such differences as the chapter of accidents provides. In the experimental method the observer places the organism or part of the organism under conditions of his own choosing, and applies to the organism under those conditions the same analysis as in the former methods. He ascertains changes in the gross features, minute structure, physical characters, and chemical composition, as before. So that in reality the two methods are in part identical, and differ chiefly by the fact that in the latter the observer chooses the conditions in which to place the organism. But an important corollary follows, viz., that by choosing his own conditions the observer is able to bring his analysis to bear on an organism or part of an organism while still alive.

The history of physiology, especially in recent times, shows that this method is the one not only of the greatest fertility but one becoming more and more essential as inquiry is pushed deeper and deeper into the more abstruse parts of physiology. If there be any truth in the sketch given above of the modern tendencies of molecular physiology, it will be clear to every mind that the experimental method alone can in the future give adequate results. It might indeed be urged that when molecular physics has advanced far enough the molecular problems of physiology will be interpreted by its light without recourse to experiment. It will be a long waiting till that comes. Meanwhile, all the power over not only the body but, what is more important, the mind of man which the physiology of the future unmistakably promises must lie unused. Nor is it simply a matter of waiting, for it is at least within the range of possibility that when the molecular problems of physiology are fairly grasped conclusions may be reached which will throw back a light on the molecular processes of inanimate masses, revealing features of what we call “matter” which could not be discovered by the examination of bodies which had never lived.

It would not be a hard task to give chapter and verse for the-assertion that the experimental method has, especially in these later times, supplied the chief means of progress in physiology; but it would be a long task, and we may content ourselves with calling attention to what is in many respects a typical case. We referred a short time back to the phenomena of “inhibition.” It is not too much to say that the discovery of the inhibitory function of certain nerves marks one of the most important steps in the progress of physiology during the past half-century. The mere attainment of the fact that the stimulation of a nerve might stop action instead of inducing action constituted in itself almost a revolution; and the value of that fact in helping us on the one hand to unravel the tangled puzzles of physiological action and reaction, and on the other hand to push our inquiries into the still more difficult problems of molecular changes, has proved immense. One cannot at the present time take up a physiological memoir covering any large extent of ground without finding some use made of inhibitory processes for the purpose of explaining physiological phenomena.

Now, however skilfully we may read older statements between the lines, no scientific—that is, no exact—knowledge of inhibition was possessed by any physiologist until Weber, by a direct experiment on a living animal, discovered the inhibitory influence of the pneumogastric nerve over the beating of the heart. It was of course previously known that under certain circumstances the beating of the heart might be stopped; but all ideas as to how the stoppage was or might be brought about were vague and uncertain before Weber made his experiment. That experiment gave the clue to an exact knowledge, and it is difficult, if not impossible, to see how the clue could have been gained otherwise than by experiment; other experiments have enabled us to follow up the clue, so that it may with justice be said that all that part of the recent progress of physiology which is due to the introduction of a knowledge of inhibitory processes is the direct result of the experimental method. But the story of our knowledge of inhibition is only one of the innumerable instances of the value of this method. In almost every department of physiology an experiment or a series of experiments has proved a turning-point at which vague nebulous fancies were exchanged for clear decided knowledge, or a starting-point for the introduction of wholly new and startling ideas. And we may venture to repeat that not only must the experimental method be continued, but the progress of physiology will chiefly depend on the increased application of that method. The more involved and abstruse the problems become, the more necessary does it also become that the inquirer should be able to choose his own conditions for the observations he desires to make. Happily, the experimental method itself brings with it in the course of its own development the power of removing the only valid objection to physiological experiments, viz., that in certain cases they involve pain and suffering. For in nearly all experiments pain and suffering are disturbing elements. These disturbing elements the present imperfect methods are often unable to overcome; but their removal will become a more and more pressing necessity in the interests of the experiments themselves, as the science becomes more exact and exacting, and will also become a more and more easy task as the progress of the science makes the investigator more and more master of the organism. In the physiology of the future pain and suffering will be admissible in an experiment only when pain and suffering are themselves the object of inquiry. And such an inquiry will of necessity take a subjective rather than an objective form. (μ. f.) PART II.—NERVOUS SYSTEM.

To supplement the foregoing general sketch some detailed account must be given of the physiology of the several functions. N utrition {q.v.) has received separate treatment; a sketch of the “Nervous System” is now appended; and Respiration and Reproduction will be dealt with in their places.

However complex may be the anatomical arrangements in man and the higher animals, the nervous system consists essentially of three portions: (1) central masses of nervous matter, or ganglia, constituting the brain and spinal cord, and containing invariably nerve-cells; (2) peripheral or terminal arrangements, existing in the organs of sense, in muscle, and in electric organs; and (3) nerves, or internuncial cords connecting the central "with the peripheral organs. The nerves may be regarded as conductors of a mode of energy which, for want of a better term, is termed “nerve-force,” originating either in the nerves themselves on the application of a stimulus or in the terminal organs or in the central organs. Thus, if a nerve be irritated at any point of its course, a change is set up in the nerve-fibres at the point of irritation, and this change is propagated along the nerve-fibres to a central or terminal organ, thus producing a characteristic phenomenon,—it may be a sensation of pain or of pleasure, an involuntary movement, the contraction of a muscle, or a discharge of electricity. Again, the stimulus may act on a terminal organ, such as the retina, setting up a change which is then propagated or conveyed to the brain by the optic nerve, there giving rise to a sensation of light or colour. Finally, the nervous action may originate in a central organ, as is the case when a voluntary movement is made. The voluntary impulse, in this instance, originates in the brain; a change passes along nerve-fibres from the brain to the muscles, and as a result the muscles contract. We have therefore to discuss the general properties and modes of action of nerves, terminal organs, and central organs.

1.— Nerves. Structure of Nerves.— A general description of the structure of nerves and of nerve-fibres will be found in vol. i. p. 859 sq.; but there are a few points of physiological importance still to be noticed. Two kinds of nerve-fibres exist in the body, white or medullated fibres, so called because each fibre has a sheath indicated by a double contour (see fig. 1), and the pale or non-medullated. The medullated nerve-fibres form the white part of the brain, spinal cord, and nerves. They vary in diameter from the T5⅛πth to the π ⅜τ iι th of an inch, and when of very small size often show varicosities or swellings. Each fibre consists of three parts: (1) an external sheath, or primitive sheath;

(2) within this the medullary sheath or white substance of Schwann; and (3) in the centre an axial fibre, the cylinder axis of Purkinje, or band of Remak. The axiscylinder in a fresh nerve seems to be homogeneous; but with high powers and proper illumination, and more especially by the action of perosmic acid, it is seen to be formed of extremely fine fibrillae. It is continuous from end to end of the nerve. The medullary sheath shows at certain intervals interruptions called the “nodes of Ranvier” (see fig. 2). In the middle of each internode an oval nucleus is found in the medullary sheath. The pale or non-medullated fibres, sometimes called the “fibres of Remak,” are found chiefly in branches of the sympathetic nerve. They have no double contour, and they often give oft’ branches which anastomose, a condition never found in the course of medullated nerves. If the axis-cylinder of a nerve-fibre be traced inwards to a nerve-centre it is found to end in the pole or process of a nerve-cell, whilst in the periphery it ends in a terminal organ, in muscle, blood-vessel, gland, skin, or electric organ. It may therefore be presumed that the axis-rod is the conducting and so far the essential constituent of the nerve-fibre.

Chemical Constitution.— Little is known of the chemical constitution of the nerve-fibre. The medullary matter contains fatty substances such as lecithin (C 44 H 90 NPO 9 ). The axis-rod appears to contain albuminous substances. Cholesterin (C 26 H 44 O + H g O) and creatin (C 4 H 9 N 3 O i ) have been obtained from nerves along with a small amount of gelatin and a horny matter, neurokeratin (Hermann). Nerves contain about 30 per cent, of water. Their reaction to test-paper is neutral. Nothing is known regarding the chemical changes occurring in a nerve-fibre during a state of activity, nor of the chemical products formed at that time. It has been stated that a nerve becomes acid after death (Funke).

As to the micro-chemistry of the nervous elements, it may be stated that the axis-cylinder contains an albuminous substance different from myosin; it gives a red colour on boiling with Milton’s reagent (an acid nitrate of mercury); it is dissolved in weak solutions of ammonia, caustic potash, and chloride of sodium; it is hardened by solutions of chromic acid, bichromate of potash, and corrosive sublimate; it reduces the chloride of gold; and it shows transverse markings on the action of nitrate of silver. The white substance of Schwann is blackened by perosmic acid and is soluble in alcohol, ether, and oil of turpentine, thus showing its fatty character, χhe primitive sheath is a substance of a collagenous nature,—that is, it yields gelatin on boiling. According to some, digestion in gastric or pancreatic juice leaves an insoluble matter like that found after the digestion of corneal or epidermic tissues, and hence called “neurokeratin.”

Excitability.— The special property of a nerve-fibre is termed “excitability.” Every kind of living protoplasmic matter is irritable,—that is, it responds in some way to a stimulus. Thus, if a morsel of protoplasm, such as an amoeba or a cartilage-cell or a white blood-corpuscle, be stimulated mechanically or by shocks of electricity, it will contract or change its form. Again, if living muscular fibre be thus stimulated it also will contract. This property of responding to a stimulus is termed “irritability”; and in the structures mentioned the property is manifested by movement as the obvious phenomenon, but it is well known that more obscure phenomena follow the application of the stimulus. In the case of living nerve there is irritability also,—that is, the nerve responds to a stimulus; but in a portion of isolated nerve no change is visible. If, however, the nerve be connected at one end with its ≡riate central or terminal organ we may have evidence of ing having been transmitted along the nerve. Thus there may be sensation or movement, or both. Nerves are more irritable than contractile matter, and the term “excitability” is applied to the special irritability of nerve. The same strength of stimulus will act more powerfully on a nerve than on a muscle (Rosenthal). The use of the term “property” in physiology does not imply the idea of any kind of inherent force or entity, but simply that, in the case of muscle and nerve, irritation is followed by certain phenomena seen only in living matter.

(1. ) The excitability of nerves is affected by certain conditions. Injury to the nerve, the application of caustics, and drying quickly destroy it. When a nerve is divided the excitability is increased for a short time, then rapidly diminishes, and finally disappears near the point of section. The end of the nerve still connected with central organs undergoes these changes in excitability more quickly than the portion cut off. As shown by Augustus Waller, when a nerve is separated from its central organ, such as the grey matter of the spinal cord for the motor roots, and the ganglia on the posterior roots for the sensory roots, the end of the nerve separated from the centre undergoes fatty degeneration. If, however, the cut ends of the nerve be brought into accurate contact, union soon takes place. Surgeons have frequently observed a return of sensibility to a part within a few days after the sensory nerve had been divided and the cut ends again brought into contact. Continued or excessive activity of a nerve soon lowers and may abolish excitability, thus producing exhaustion. On the other hand, a lengthened period of absolute repose lowers excitability, and if the nerve be inactive beyond a certain time it wastes, becomes thinner, and fatty degeneration occurs in its substance. Heat increases, whilst cold diminishes, excitability. In the case of frogs’ nerves temperatures above 45° C. destroy excitability the more rapidly as they approach 70°, at which point it is almost instantaneously destroyed. Below 45° a rise of temperature first increases and then diminishes excitability, and it has been observed that whilst increasing its intensity it diminishes its duration (Afanasieff, Heπnann). Finally, a diminished supply of blood quickly causes a fall of excitability.

(2.) Nerves may be excited by various kinds of stimuli: (a) mechanically, as by intermittent pressure, beating, section, pricking, &c.; (b) thermally, by variation of temperature; (c) chemically, by the application of such substances as acids, alkalis, or metallic salts; (d) electrically, by continuous or induced currents; and (e) normally, by changes in the central or terminal organs. Mechanical irritation is applied during life when the trunk of a nerve is pressed upon. Radiant heat acts on the nerves of the skin, or heat may oe applied by conduction from a hot body in contact with the surface. Little is known as to the specific effects of heat on the nerves of the human being. In the frog it has been found that a temperature of from 34° to 45° C. stimulates the motor nerves; about 40° C. sudden alterations of temperature may cause tλvitching of the connected muscles (Hermann). Many chemical substances in sufficient concentration will quickly destroy a nerve; but if they are in weak solutions the result may be stimulation. Thus, concentrated solutions of the mineral acids, alkalis, alkaline salts, concentrated lactic acid, and concentrated glycerin may act as strong stimulants (Kiihne).

(3.) The influence of electrical stimulation of nerves demands more elaborate description. The effects, as already indicated, can be observed only when the nerve is connected with a muscle or with a central organ. In the first case electrical stimulation is followed by contraction of the muscle, in the second by a sensation if the central organ is the brain. Consequently we have to consider the phenomena following electrical stimulation (a) of a motor nerve and (b) of a sensory nerve.

(a.) Electrical Stimulation of a Motor Nerve.— A perfectly constant current of electricity, of moderate quantity and intensity, flowing through a portion of nerve produces no evident effect on the muscle, but any variation in the intensity or density of the current causes irritation, and the muscle gives a twitch. The effect is most apparent when the current is allowed to flow into the nerve and when it is suddenly cut off, or, in other words, at the moment of opening and of closing the circuit. The rapidity with which the variation in the density of the current is effected also has an important influence. Thus the shocks of frictional electricity stimulate strongly, because, although the amount of electricity is small, the currents are extremely rapid in appearing and disappearing. In like manner the quick shocks from induction-coils produced by rapidly opening and closing the primary circuit are strongly stimulating. Again, a very powerful current may pass through a nerve without exciting it, if it pass gradually. Occasionally a very weak current sent through a portion of nerve will cause a contraction, whilst a very strong current may fail to do so. In fact, the phenomenon of contraction of a muscle is influenced (a) by the direction and (∕3) by the strength of the current sent through the nerve. When the current is transmitted from the muscle in the direction of the spinal cord it is called an “upward” or “centripetal” current, when from the cord in the direction of the muscle it is called a “downward” or “centrifugal” current. Its strength is graduated by employing small Grove’s cells, one cell giving a weak current, two or three giving a medium current, and four to six or seven a strong current. To graduate its amount more precisely resistance-coils may be introduced into the circuit, or we may employ a rheochord, by which a portion of the current is shunted back to the battery, whilst the remainder is allowed to pass to the nerve. In the circuit a key or interrupter is interposed, and so arranged that when the key is opened the current is broken or interrupted, and when the key is closed the circuit is completed and the current passes to the nerve. With these arrangements, and employing the sciatic nerve of a frog attached to the limb, the following results are readily obtained. [table]

That is to say, on beginning with a very feeble current neither opening nor closing causes a contraction, but on strengthening it up to a certain point contraction appears first on closing, whilst opening produces no effect. By increasing the strength of the current a contraction is obtained both on opening and on closing the key, and by and by, when a certain strength of current is reached, the closing contraction becomes weaker and finally disappears, leaving only a contraction on opening the key. Thus the effects of a strong current are usually the reverse of those caused by a weak current. These facts, usually included under the term “Pfltiger’s Law of Contraction,” have been specially investigated by Pflüger, and the following is the explanation offered by him and generally accepted by physiologists.

Suppose that the sciatic nerve of a frog connected with the isolated limb is stretched over two wires passing from the positive and negative poles of a combination of Grove’s elements, with the distance of an inch and a half between the wires. If a key be interposed in the circuit a current will thus pass along one an<l a half inches of nerve when the key is closed, and be cut off when the key is opened. By having also a commutator or reverser in the circuit we can send the current up or down the nerve at pleasure. Arrangements can also be made for irritating the nerve by another couple of wires coming from an induction-machine, either near the negative or near the positive pole of the current coming from the Grove's elements. It will then be found that near the negative pole the excitability of the nerve is increased, whilst near the positive pole it is diminished,—that is to say, a stimulus from the induction-coil, not sufficient to excite the nerve so much as to cause a muscular contraction if applied near the positive pole, will at once do so if applied near the negative pole; or, a stimulus so strong as to cause tetanus in the muscle when applied near the negative pole may produce no effect when applied near the positive pole. In other words, the nerve near the negative pole is more excitable than in the normal state, whilst near the positive pole it is less so, indicating that at least one of the physiological properties of the nerve has been changed by the action of the continuous current. But a nerve-fibre has also the property of conducting the effects of an impression, or the nerve-force travels with a certain velocity along a nerve, as will be shown lower down. It has been ascertained that near the negative pole the rate of conductivity is increased, whilst near the positive pole it is diminished. Finally, a piece of living nerve, when connected with the terminals of a galvanometer, so that the one terminal touches the surface whilst the other touches the transverse section of the nerve, shows the existence of a current of electricity travelling from the surface of the nerve through the galvanometer to the transverse section,—that is, the surface is positive to the transverse section. This condition is also modified by the transmission through the nerve of a continuous current, so that the difference of potential is increased near the positive pole and diminished near the negative. These results are thus summed up. [table]

The properties of the nerve, therefore, are altered by the passage through it of a continuous current, and the altered condition is termed the ‘‘electrotonic state,” the condition in the neighbourhood of the positive pole, or anode, being termed “anelectrotonic,” whilst that near the negative pole or katode is called “katelectrotonic.” A certain portion of nerve near each pole is thrown during the passage of a continuous current into these conditions of anelectrotonus and of katelectrotonus, whilst the amount of nerve thrown into the one condition or the other depends on the strength of the current. Further, there is always between the two poles a point of indifference, in which the properties of the nerve seem to be unaltered, and the position of this point depends on the strength of the current. Thus, with a current of medium strength the point is midway between the poles; with a weak current the point is neaι∙ the positive pole,— that is, a large portion of the nerve near the negative pole is in the katelectrotonic state in which the excitability is increased; and with a strong current the point is near the negative pole,—that is, a large portion of the nerve near the positive pole is in the anelectrotonic state in which the excitability is diminished. Now, according to Pflüger, the stimulating effect of closing the current occurs at the katode only, whilst the stimulating effect of opening the current occurs at the anode only, or a nerve is stimulated by a current on the appearance or increase of katelectrotonus, on closing the circuit, or by the disappearance or diminution of anelectrotonus on opening the circuit. If we suppose that this depends on the modification of excitability near the negative pole, by the molecules of the nerve becoming more mobile, the matter is intelligible. Thus the passage of the molecules from the normal stable condition to the katelectrotonic less stable condition acts as a stimulus, whilst the passage backwards has no effect. On the other hand, the passage from the more stable condition in anelectrotonus to the normal stable condition acts as a stimulus, whilst, again, the reverse action has no effect. This explains why it is that a weak current gives contraction on closing, because on closing a large portion of the nerve near the negative pole passes from the normal into the katelectrotonic state, and this acts as a stimulus. On the other hand, a strong current causes contraction on opening, because on opening a large portion of nerve near the positive pole passes back from the anelectrotonic state into the normal state, and this acts as a stimulus. Again, with currents of medium strength, as both states are equally produced, there is contraction both on opening and on closing. Thus Pfluger’s theory accounts for most of the facts; but its weak point is that no reason can be given why a nerve is stimulated only by the appearance of katelectrotonus and by the disappearance of anelectrotonus. It remains only to add that currents passing transversely through nerves produce no stimulating effect. In ascending currents the shorter the piece of nerve between the electrodes the greater the stimulating effect, whereas in descending currents the reverse holds good (Hermann).

(b. ) Electrical Stimulation of Sensory Nerves.— The effect of stimulating sensory nerves as distinguished from the direct stimulation of sensory or terminal organs has not been sufficiently studied, but, so far as is known, the laws seem to be the same as those relating to motor nerves. When a sensory nerve is stimulated the test must be the resulting sensation. As stimulation of the motor nerve in the condition of anelectrotonus or of katelectrotonus may or may not be followed by a contraction, so stimulation of the sensory nerve may or may not be followed by a sensation, or the character of the sensation may vary just as the muscular contraction may be weak or strong. Further, Donders has shown that electrical stimulation of the vagi or ∙pneumogastric nerves is attended by analogous phenomena, so far as the movements of the heart are concerned. In this case, however, as will be shown lower down in discussing the phenomena of nervous inhibition, the result is not movement but arrest of movement.

(c.) Chauveau’ s Researches mi Unipolar Excitation.— Chauveau has studied the comparative influence of the two poles of any arrangement supplying a continuous current,—that is, he has tried the stimulating effect, supposing either the positive or the negative pole be applied to the nerve whilst the other is in contact with another part of the body. He has found, amongst other more abstruse and less practical results, that there is in each case a certain intensity of current corresponding to the physiological condition of the nerve by which the influence of one pole is the same as that of the other. If the intensity of the current be below this medium strength the effect of the negative pole on motor nerves is greater than that of the positive; but, if the intensity be above, the reverse is the case,—that is, the positive pole is the stronger excitant. In the case of sensory nerves Chauveau found that application of the negative pole with a moderately strong current was more painful than application of the positive pole. Thus the influence of unipolar excitation with a strong current on motor nerves is the reverse of that on sensory nerves,—that is, the positive pole is the more powerful on motor nerves, the negative pole on sensory nerves.

(<Z.) Production of Tetanus.— Tetanus or cramp of a muscle is produced when its nerve is stimulated by successive irritations at intervals so short that the muscle has no time to relax between them, and consequently it passes into a state of more or less firm contraction. A single muscular contraction may be called a twitch of the muscle, but in tetanus or cramp the individual contractions are fused together so as to maintain a rigid state of the muscle for some time. A rapid series of induction shocks, each of short duration, always produces tetanus, even if they are sent to the muscle at the rate of 15 per second. A continuous current, on the other hand, usually causes contraction only at the moment of opening and closing the circuit, but occasionally tetanus may be seen during the passage of the current. Tetanus during the passage of a constant current has been attributed to electrolytic changes in the nerve. Pflüger holds that this is a normal production of tetanus and may be seen even with feeble currents; but certainly it is very difficult to demonstrate. Long ago Ritter showed that, if a constant current of sufficient intensity be sent up a nerve for a considerable time, say half an hour, and then be suddenly interrupted, tetanus lasting for eight or ten seconds may be seen, which disappears on again closing the current. Ritter’s tetanus, according to Pflüger, is really due to the stimulation caused by the disappearance of anelectrotonus, which occurs, as we have seen, when the current is opened, and the proof he offers is that the tetanus disappears when the muscle is cut off from the anelectrotonic portion. Tetanus may also be caused by the mechanical irritation of the nerve, 01 by heat, or by chemical substances.

Nervous Conductivity.— When a nerve is irritated at any point in its course a change is produced which is propagated along the nerve,—that is, the nerve conducts, and the phenomenon is called the “nerve-current.” The velocity of transmission can be measured only by the use of delicate apparatus, as the time occupied is too short to directly affect consciousness. For example, when the tip of the finger is touched the mind apparently perceives the contact without any loss of time. But it can be shown that an appreciable interval of time elapses between the instant the finger is touched and the instant the mind perceives the impression. During this time a change passes along the nerve from the point touched to the brain. The method usually employed for determining the velocity of the nerve-current consists in preparing the gastrocnemius muscle of a frog with the sciatic nerve attached, and connecting it with a recording apparatus, so that if the muscle be caused to contract by irritating the nerve the record of the contraction may be made on a rapidly-moving surface. If, then, the nerve be irritated in two consecutive experiments, first close to the muscle, and secondly at a distance from it, and the muscle be caused to contract in each case, it will be found that it does not contract so soon when the nerve is irritated at a distance from the muscle as when it is irritated close to it; in other words, if the nerve be irritated at a distance from the muscle the transmission of the nervous impression from the point irritated to the muscle occupies an appreciable time. If, then, we know the length of nerve between the two points irritated, we can determine the length of time the nerve-current took in passing along that distance of nerve.

(1.) Measurement of Velocity in Motor Nerves.— Many ingenious methods have been devised for this purpose, but the simplest is the use of the “spring myographion” of Du Bois-Reymond (see fig.3). The apparatus consists of a smoked-glass plate, which is driven in front of the recording stylet of the myograph by the recoil of a steel spring C. Underneath the frame carrying the glass plate are two binding screws 1 and 2, to one of which is attached a rectangular arm of brass 1, which can so move horizontally as to establish metallic connexion between the two binding screws (marked Break, F). By means of these binding screws the myograph is interposed in the circuit of a galvanic element and the primary coil I of an induction-machine, and the brass arm is so placed as to connect both binding screws, thus completing the circuit. From underneath the frame carrying the smoked-glass plate there descends a small flange, which (when the glass plate, by releasing a catch not seen in the figure, but close to C, is driven across by the spiral spring from left to right) pushes the brass arm aside and thus interrupts the circuit of the primary coil. When this occurs an opening shock is transmitted from the secondary coil II to a commutator E, an instrument by which electric currents may be transmitted to the nerve either at a point close to the muscle at A, or at a distance from it at B. Suppose the apparatus all arranged so as to send the shock to the nerve at a point close to the muscle A, the muscle stimulated contracts, and draws by means of the stylet, on the smoked surface of the glass, the curve seen in the lower j>art of it at A. This leaves the horizontal line (which would be drawn by the stylet were the muscle at rest) at A. Arrangements are then made for another experiment, in which the nerve will be stimulated at a distance from the muscle, at the point B in the upper j>art of the diagram. This is done by again placing the smoked-glass plate in proper position, closing the primary circuit by the brass arm at the binding screws, as already described, and reversing the commutator so as to send the shock along the wires to B. The muscle again contracts when the primary circuit is opened, and this time it describes on the smoked surface the curve B, seen to the left of the curve A. It will be perceived that this curve leaves the horizontal line at B,—that is, a little later than when the nerve was stimulated close to the muscle. It follows, therefore, that the distance on the horizontal line from A to B represents the time occupied by the transmission of the nervous impulse from B to A of the nerve. With suitable arrangements, the rate of movement of the glass plate can be measured by bringing into contact witlι it a marker on one of the prongs of a vibrating tuning-fork. The waves thus recorded enable the experimenter to measure with accuracy the rate of movement of the glass plate, and consequently the minute interval of time between A and B. In the diagram it will be observed that there are 2J waves between A and B; each represents ,i∏th of a second; therefore the ⅛th of a second is the time represented by the distance A, B; or, in other words, the s ⅛th of a second was occupied by the nerve-current in passing along the portion of nerve from B to A.

(2.) Measurement of Velocity in Sensory iVerves.—Suppose a sensory nerve to be excited in the hand; the theory of nervous conduction is that a change is propagated along the nerve to the brain, and that in the brain the molecular changes occur which result in a sensation. The individual having the sensation may feel it and make no sign by which any one else might be made aware that he has felt it, or the subject of the sensation might, by a muscular movement, such as the motion of an’ arm, let any one else see that he has felt the sensation. We have no means of knowing whether or not an individual has felt a sensation except by the individual making some kind of gesture or muscular movement. Now it is clear that, if we regard the brain as the seat of the changes resulting in sensation, the nearer any stimulated portion of skin is to the brain the sooner will the brain feel and respond to the stimulu⅛ Thus, if the skin on the big toe of the right foot be stimulated, the effect of the stimulus passes to the brain and there calls forth a sensation, but if the stimulus be applied to the skin at the top of the thigh it is evident the effect has to pass along a shorter length of nerve and that the sensation in the brain will be aroused sooner. If we suppose that in each case the individual who is the subject of the experiment indicates the moment he feels the sensation, and that the instant the stimulus is applied successively to the skin on the toe and on the thigh is also accurately recorded, it is clear that he will signal the sensation of stimulation of the toe a little later than when he signals stimulation of the skin on the thigh, and that the difference will indicate the time required by the change in the nerve to pass along the length of nerve from the toe to the thigh. In the observation it is assumed that the time required for the changes in the brain resulting in sensation and volition, for the transmission along the motor nerve, and for the muscular contraction required to signal is the same in each experiment. Thus, supposing the total time between the moment of stimulating to the moment when the signal that the sensation has been felt and responded to is x, it is clear that this time is composed of a, the time required for the passage of the nerve-current in the first experiment from the toe to the brain, of b, the time required for the changes in the brain involved in sensation and volition, and of c, the time required for the transmission along the motor nerves and for the muscular contraction to move the signal, — that is, z=α+δ-t-c. But, if the time between the moment of stimulating the thigh to the moment of signalling be shorter, and supposing that b and c are constant, then a varies according to the length of the nerve. Suppose the difference of time between the registration of stimulating at the toe and at the thigh to be y, then in the second experiment x=a-y+b+c,— that is, y=the time occupied by the passage of the nerve-current from the toe to the thigh. This method has also been used to measure the time required for signalling a nervous impression in various circumstances, or what is usually called the “reaction period.” The most convenient apparatus for the purpose is a chronograph made by König of Paris, the instrument being fully described in Μ‘Kendrick’s Outlines of Physiology, pp. 538-542.

The general result of measurements made by these methods is that the nerve-current travels slowly compared with the velocity of electricity or of light. In the motor nerves of the frog the velocity is about 87 feet (26 to 27 metres) per second, and in man and warm-blooded animals somewhat faster, 115 to 130 feet (35 to 40 metres) per second. The results as to velocity in sensory nerves vary from 50 to 100 metres per second. Cold retards, heat accelerates, the velocity. As already stated, the velocity is also retarded in a nerve in an anelectrotonic, and accelerated in a katelectrotonic state. The remarkable point is that the transmission of the nervecurrent is slow, and that events appearing to our consciousness instantaneous require a considerable time for their occurrence. It may be laid down as a general truth that all kinds of nervous actions, even those considered as purely psychical, require time.

Production of Heat by Nerves.— It is extremely doubtful whether the production of heat by a nerve in action has been detected, although theoretically one would expect heat to be so produced. Schiff observed an increase of temperature on tetanization in the nerves of warm-blooded animals that had been artificially cooled; on the other hand, Helmholtz and Heidenhain’s experiments yielded only negative results.

Electrical Phenomena of Nerve.— When a piece of nerve is properly brought into contact with the terminals of a sensitive galvanometer, a current flows through the galvanometer from the surface of the nerve to its transverse section (see fig. 4).

If metallic conductors, composed (say) of zinc, from the galvanometer were brought into connexion with a piece of nerve removed from an animal newly killed, little or no current would be obtained, and even if there were a current it might be due to contact of the metallic conductors with the living tissue exciting electrolytic decomposition. Hence it is necessary to have a fluid interposed between the metal and the animal tissue, say, for example, the zinc wire or plate forming the terminals of the galvanometer is immersed in a saturated solution of sulphate of zinc. But as sulphate of zinc solution would have the effect of irritating the living muscle it is necessary to have an inactive substance between the tissue and the sulphate of zinc solution. All these conditions are fulfilled by the non - polarizable electrodes of Du Bois-

Reymond, of which there are various forms. Two zinc troughs, mounted on insulating plates of vulcanite, have the inner surfaces carefully amalgamated. These are tilled with a saturated solution of sulphate of zinc, and in each trough is placed a small cushion of clean blotting or filter paper, which quickly becomes j>ermeated with the solution. Finally, a small plate of sculptor’s clay, or kaolin, moistened with a half per cent, solution of common salt, or, still better, with saliva, is laid on each paper pad. These clay pads are for guarding the tissue from the irritant action of the sulphate of zinc. Wires are carried from the troughs to the galvanometer, and a key is interposed in the circuit. The object of these careful arrangements is to secure that no current is formed by the apparatus itself. If now a small piece of nerve be so placed on the clay pads that the transverse section touches one pad and the longitudinal surface the other, and the key is opened, a current passes through the galvanometer, as indicated by the swing of the needle. Suppose that the needle is allowed to come to rest, the amount of deflexion of course indicating the strength of* the current, and the nerve is now irritated so as to call forth its physiological activity, then the needle swings back towards zero. This backward swing is called the negative variation of the nerve-current. The electromotive force of the current obtainable from a frog’s sciatic nerve is about '022 of a volt, and somewhat more from the sciatic nerve of a rabbit. This is somewhat less than the electromotive force of a frog’s muscle, which varies from ∙035 to ∙075 of a volt. According to the views of Hermann, the negative variationcurrent is a true current indicating, and indeed preceding, the physiological activity of the nerve. He denies that the currents pre-exist in nerve or muscle, and states that the first current observed when the nerve is laid on the pads is simply due to the lower potential of the transverse section, caused by the rapid death of the nerve-substance. The nerve-current excited by a series of irritations, say feeble induction-currents, may be regarded as composed of a wave-like series of momentary currents, each of which is preceded by a negative variation-current. Thus the electrical phenomena of nerve are similar in kind to those manifested by living muscle.

Nutrition of Nerves.— Probably nerves are nourished by the plasma reaching the axis-cylinder at the nodes of Ranvier; but it would appear from the researches of Waller that the nutrition of the nerve-fibre is influenced by the nerve-cell with which it is connected. The so-called “law of Waller” is well illustrated in the case of division of the roots of the spinal nerves. Each of these nerves has two roots,—a posterior, sensory, on which there is a ganglion; and an anterior, motor. If the anterior root be divided, in the course of a few days the end of the nerve cut off from the spinal cord is found to be undergoing degeneration, whilst the end attached to the cord is still normal. Again, if the posterior root be divided between the ganglion and the cord, the end remaining in connexion with the ganglion remains unaffected, whilst the other end undergoes degeneration. This degeneration, in the case of a motor nerve, affects the nerve to its very terminations. The axiscylinder disintegrates into drops of fatty matter, and the medullated structure entirely disappears. It is well known that when a nerve is cut the ends may reunite so completely as to ensure a return of the normal function in from two to five weeks. According to Ranvier, the axis-cylinders in connexion with the central portion play an important part in this regeneration. They become larger, striated, and by and by form new axis-cylinders, which pass into the cicatricial tissue and come into contact with the other end of the divided nerve. This is a remarkable confirmation of the view of Waller that the nutritional activity of a nerve-fibre is in the direction of its physiological activity.

Nature of Nerve-currents.— The intrinsic nature of the change in a nerve-fibre effected by a stimulus is quite unknown; but it is important to appreciate clearly the view that a nerve is both a receiver and a conductor of impressions. It can be stimulated in any part of its course, and from the stimulated point some kind of change is propagated along the nerve. This change is analogous to the passage of electricity along a conductor, or to the rapid passage onwards of a series of chemical decompositions, as when a long thin band of gun-cotton properly prepared is seen to slowly burn from end to end, or to the quick transmission of isomeric changes; but the analogy is not complete in any case. Whatever the change may be, however, it does not appear to pass from one nerve-fibre to another running alongside of it. Each fibre conducts only its own impression, and there is nothing analogous to the inductive effect of one electrical conductor upon an adjacent one. Another question much debated is whether sensory and motor nerves act in the same way; or, in other words, is there any essential difference between them? There appears to be no difference in mode of action; the difference in the effect produced depends on the apparatus in which the nerve ends. Thus there may be contraction of a muscle if the nerve terminates in a muscle, change of the calibre of a blood-vessel if the nerve ends in that structure, secretion from a gland if the nerve is in connexion either with the vessel or the nerve-cells of a gland, an electrical discharge if the nerve ends in the electrical organ of a Torpedo or Gymnotus, and a feeling or sensation if the nerve-fibres go to a sentient brain. In all these instances the nature of the change in the nerve and the mode of its transmission are the same, and the results are different because the nerves terminate in different kinds of structure. It would appear from experimental evidence that, when a nervefibre is irritated, say about the middle of its length, a change is simultaneously propagated towards each end; but, as only one end is in connexion with an apparatus capable of responding, the effect at this end’ is the only one observed. Thus, if a motor nerve be irritated, there is muscular contraction, in consequence of the stimulus rousing the muscular substance into activity, probably through the agency of the end - plates; but there will be at the same time a backward wave along the nerve to the motor centres in the cord or brain. It is doubtful whether the nerve-energy becomes weaker or gathers intensity as it passes along a nerve; but the balance of evidence is in favour of the view that the so-called “avalanche theory” of Pflüger, according to which the energy gathers intensity as it passes along, is incorrect Classification of Nerves.— Functionally, nerves may be classified into motor, sensory, vascular, secretory, and inhibitory. The original meaning attached to the term “motor” nerve was a nerve entirely composed of fibres by the excitation of which influences were conveyed to a muscle which caused the muscle to contract. As these influences passed outwards from a nerve-centre towards the periphery of the body they were also termed “efferent” nerves. On the other hand, nerves were found which, when stimulated, gave rise to sensations of pleasure or of pain, and these were called “sensory” nerves. Finally, it was shown that a third class of nerves were composed both of sensory and of motor fibres, and they were called “senso-motor” nerves. Sensory nerves were also subdivided into those of general and those of special sensibility. This was an artificial classification based on the fact that when a nerve of so-called special sensibility, such as the optic, was stimulated in any way the same kind of sensation followed. Thus stimulation of the optic nerve by cutting, pricking, pressure, or electricity is always followed by a luminous sensation. But the progress of research showed that when certain nerve-fibres were stimulated the result was not necessarily a muscular contraction: it might be contraction of a blood-vessel, modified secretion of a gland, or a diminution or arrest of some kind of nervous action. These facts demand another classification of nerves such as the following. [table]

In addition there are nerve-fibres connecting nerve-cells in the great centres, to which no special functions can be attributed.

2.— Terminal Organs.

Although, as has been shown, a nerve may be stimulated in any part of its course, the stimulus is usually applied to a special structure adapted physiologically for the reception of the particular kind of stimulus. Such a special structure may be termed a “terminal organ.” For example, in the mechanism of vision (see Eye, vol. viii. p. 821 sq. ) there are the retina or terminal organ, the optic nerve or conductor, and the brain or a portion of it, the recipient of the impression. The fibres of the optic nerve are not affected by light, but when they are mechanically or electrically irritated the result is a luminous sensation, because the action of the fibres of the optic nerve is to call forth in the brain the mechanism connected with luminous sensations. But light has a specific action on the retina, and in turn the activity of the retina stimulates the fibres of the optic nerve. The retina is therefore the terminal organ adapted for the reception of rays of light. In like manner, each sense has its appropriate terminal apparatus, and these are described under the headings of the various senses, Ear, Eye, Smell, Taste, Touch. To understand the true nature of nervous action it is necessary to be clear as to the functions of the terminal organs. They are liberating mechanisms. They do not transform the outer energy into the physiological energy, nervous action; but they call it into action. Thus light acting on the retina is not directly transformed into nervous energy, but it excites changes in the retina, which in turn produce activity of the optic nerve. The structure of each of these terminal organs need not be here described, but it may be stated that they all essentially consist of modified epithelium-cells, or what may be called “nerveepithelium.” In tracing their development throughout the animal kingdom it will be found that the simplest terminal organs are epithelium-cells on the surface of the body; but during evolutionary progress from lower to higher forms these cells become more and more modified and more and more protected by descending deeper into the structure of the animal, until we meet with the complicated organs of special sense in the higher animals. Another class of terminal organs is that comprehending the forms at the ends of motor nerves. Such are the end-plates found in muscle, and described in vol. i. pp. 861, 862. The different modes of nerve-termination may be here briefly classified. [table]

3.— Central Organs. A.—General Physiology of Central Organs. General Structure.— The central organs consist of a special kind of cells called “nerve-cells,” of nerve-fibres, both medullated and non-medullated, and of a variety of connective tissue, termed “neuroglia.” On cutting into any central nervous organ, such as the spinal cord or brain, two kinds of nervous matter are seen, the white and the grey. The grey consists of nerve - cells, nerve - fibres, and neuroglia, whilst the white is composed chiefly of nerve-fibres with a small amount of neuroglia and no nerve-cells. Nerve cells vary much in form, as will be seen by referring to fig. 5. They may be spheroidal, ovoidal, or irregularly triangular.

The cells of the spinal ganglia are usually rounded; those of the sympathetic more angular; those of the spinal cord multipolar,— that is, having many processes or poles connected with them; those of the cerebrum triangular or pyramidal; and those of the cerebellum flask-shaped, having processes at each end. A nerve-cell shows a large clear nucleus and a small nucleolus, whilst the cellsubstance is very granular. Some observers think they have traced into the substance of the cell a fibrillated structure from the axiscylinder of the nerve-fibre ending in the pole or process; but this is doubtful, and the appearance may be accounted for by the action of the reagent employed and by the great difficulty of correctly interpreting optical apj>earances under very high powers. The neuroglia is a delicate interstitial connective substance having small connective-tissue corpuscles imbedded in it.

I Chemical Constitution of Grey and White Matter.— This is still imperfectly known, and throws almost no light on the functions of the central organs. By various chemical processes the following substances have been obtained from nervous matter: cerebrin, lecithin, albumin, neurokeratin, cholesterin and fats, creatin, xanthin, hypoxanthin, inosite, lactic acid, volatile fatty acids, salts, and water. The grey matter of the brain is distinguished chemically from the white chiefly by containing more water, albumin, lecithin, and lactic acid, and less cholesterin, fat, and protagon (Hermann). Doubtless many of these substances are derived from the disintegration of a more complex chemical substance not yet isolated in a pure state from nervous matter. Petrowsky gives the composition of grey and white matter as follows. [table]

The salts found in nervous matter are similar to those in blood, and it would appear that phosphates, or rather combinations in which phosphorus exists, are the most prominent products of analysis. Thus about 40 per cent, of the salts consist of phosphates of soda and of potash—that is, the ash, on analysis, gives this result; but it must not be inferred that in nervous tissue phosphates of the alkalis exist to this amount, as there is every reason to think that phosphorous compounds, along with alkalis, exist in nervous matter, although not in the form usually called phosphates. The remarkably large amount of water, amounting to no less than from 70 to 80 per cent., indicates matter in a condition suitable for rapid molecular changes, on which, no doubt, the functions of the tissue depend.

Excitability of Grey Matter.— As grey matter contains both nervefibres and nerve-cells, and as these cannot be separated in any experiment, it is clear that no precise results can be obtained from any effort to distinguish the excitability of grey matter from that of white. The excitability of the grey matter must depend on bloodsupply and on the rapid removal of waste-products. If the first be deficient either in quantity or quality, or if the second be not carried on so rapidly as to get rid of the waste-products as they are formed, the activity of the nerve-cells must suffer. The sudden deprivation of blood, as when the heart ceases to beat for even half a second, will cause unconsciousness; the mixture with the blood of a small quantity of bromide of potassium, or of alcohol, or of chloroform or other anaesthetic, or of morphia, will affect the activity of the brain. And it is well known that, when disease of the kidney, or such a disease as an acute fever, affects the body, matters may accumulate in the blood which so contaminate it as to make it unfit to carry on the vital changes on which activity of brain depends, and the result is delirium or unconsciousness. There is every reason to believe that the activity of nerve-cells is delicately attuned to surrounding conditions. A small excess per cent, of carbonic acid, or a small amount of what we call a poison, is sufficient to modify or arrest their action. The rhythmic action of various centres, such as those controlling the movements of respiration, is in favour of the view that the activity of such centres depends on delicate equipoises. If during expiration there is for the moment a deficiency of oxygen in the blood, or an accumulation of carbonic acid, the result will be an attempt at inspiration. This gets rid of the carbonic acid and introduces oxygen, and an expiration ensues. It is not pretended here to state wdιat exactly happens, as these phenomena of respiration are still obscure, but they are brought forward with the view of showing that the actions of the rhythmic centres of respiration depend on the delicate balance established between the external conditions and those centres. If this be the case there is little doubt that a similar effect is produced on other centres by the nature of the blood supplied, and that the quality and quantity of the supply are important factors in the production of all conscious conditions.

General Phenomena manifested by Nervous Centres.— Before entering on a detailed description of the functions of the great centres such as spinal-cord and brain, it is well to take a survey of some of the general phenomena manifested by such centres. These may be grouped under the heads of (1) reflex actions, (2) inhibitory actions, (3) accelerating actions, (4) va-so-motor actions, (5) secretory actions, (6) sensations, and (7) intellectual acts.

Reflex Actions.— Impressions made on sensory nerves are conveyed to nerve-centres, where they may or may not awaken consciousness. A sensation may be defined as the consciousness of an impression, and may or may not be followed by motion. Either motion may be voluntary, or it may be caused by direct stimulation of the motor nerve distributed to the muscles. The latter kind of action in the living body is not common. Usually motor nerves are acted on by the will or by emotional states; but it not unfrequently happens that physical stimuli occasion motion in an indirect manner, the impressions being carried along sensory nerves to a central organ, where changes are excited which result in a discharge of nervous energy along motor nerves to various muscles. Thus a frog in which the brain and medulla oblongata have been destroyed will draw up its limbs if the foot be pinched. Such actions, taking place without consciousness, are called “reflex actions,” and the mechanism required for their performance may be thus described: (1) excitation of a sensory or afferent nerve, (2) excitation of an intermediate nervous or reflex centre, and (3) excitation of a motor or efferent nerve, which causes a muscular contraction. The diagram in fig. 6 shows the simplest mechanism; but it is rare to find the arrangements so simple, and the mechanism may become more complex (see fig. 7) either by the existence of a number of cells or groups of cells in the nerve-centre, or by the existence of numerous afferent or efferent nerves. The essence of a reflex action is the transmutation by means of the irritable protoplasm of a nerve-cell of afferent into efferent impulses (Foster). The following is a brief summary of the leading facts relating to reflex action.

(a) The initial excitation may occur both in nerves of general sensibility and in those of the special senses; but certain nerves more easily excite reflex actions than others. Thus when light falls on the retina there is contraction of the pupil, the afferent nerve in this case being the optic (see vol. viii. p. 821 sq.). (b) A reflex movement may occur whether we excite a sensory nerve at its commencement or at some point in its course, but in the latter case the action is less intense than in the former.

(c) Grey matter containing nerve-cells constitutes the chief portion of reflex centres, and groups of such reflex centres are frequently associated by internuncial fibres. The excitability is increased when these centres are severed from communication with psychical centres which preside over voluntary movements. Thus, after decapitation, reflex movements occur with greater intensity than in the injured animal; they are also more active during sleep. It is evident, therefore, that reflex actions may be restrained or hindered in their development by the action of higher centres. This is termed the “inhibition of reflex action.”

(d) Reflex movements may occur in one muscle, or in many muscles or groups of muscles. One or more groups of muscles may be involved according to the strength of the stimulus applied to the sensory surface and the degree of excitability of the reflex centre at the time (see fig. 8). The facts are thus summarized by Pflüger. Unilateral action: if in a decapitated frog we excite the skin of the hind foot p, the excitation is transmitted from the centre a to the muscles 1 of the foot on the same side. Symmetrical action; if the excitation be more intense, it is transmitted to a centre on the opposite side b, and contractions may occur in the muscles of the hind limbs on both sides 1, 2. Irradiation; if the excitation be still increased in intensity, it affects higher centres c, d, and there may be contraction of the fore-limbs 3, 4. General action; if the excitation be still further increased, it may pass to a still higher reflex centre e, and the result is general convulsions.

(e) Reflex centres may be so arranged in the body as to constitute a series in which those of the cerebrum govern or control others in the deeper ganglia of the brain, while these, in turn, have an influence over still lower centres in the spinal cord. This arrangement is termed the “superposition of reflexes” (see fig. 9).

(∕) Stimulation of a sensory surface may simultaneously produce, by a reflex mechanism, movement, secretion, and consciousness. Thus a condiment in the mouth may cause involuntary twitchings of the muscles, secretion of saliva, and a sensation (see fig. 10).

(g) Certain substances, in particular strychnin, increase reflex excitability, so that the slightest external stimulation of the sensory nerves of the skin is sufficient to cause severe convulsions. On the other hand, bromide of potassium, hydrate of chloral, and atropin diminish reflex excitability.

(Ae) Individual stimuli only excite a reflex act when they are very powerful, but stimuli applied at frequent intervals act the more quickly and powerfully the more rapidly they succeed each other. To produce the reflex change in the centre, therefore, a summation or addition of centripetal excitations is required. When these reach a certain number the centre responds (Stirling).

(ι) Reflex actions involve time. Thus the time between the stimulation and the movement can be measured, and, if we take into consideration the time occupied by the passage of the nervecurrent along the nerves involved, and the latent period of muscular contraction, and subtract this from the total time, the remainder will represent the time occupied by the changes in the centre or the reflex-time. This has been found to be from - 0555 to Ό471 of a second. It is lengthened by cold and shortened by increasing the strength of the stimulus and by strychnia.

(½) In compound reflex acts the initial excitation may occur in psychical centres, as when the recollection of an odour causes nausea, or when a feeling of ennui is followed by a yawn.

(Z) Some reflex movements are the result of inherited peculiarities of structure, as those made by a new-born child when it seizes the breast. Other· reflex movements are acquired during life. Such are at first voluntary, but they become automatic by repetition.

The following are some of the more common examples of reflex movements. Motions of the muscles in any part of the limbs or trunk under the influence of sensory impressions on the skin, such as tickling, pricking, &c.; shuddering from cold, shuddering caused by grating noises, &c.; contraction of the pupil under the influence of light on the retina; winking, from irritation of the sensory nerves of the conjunctiva; sneezing, from irritation of the Schneiderian membrane, or by a glaring light on the eye; spasm of the glottis and coughing, from irritation of the larynx or trachea; laughing, caused by tickling the skin; the first respiration of the child at birth, from the impression of cold upon the nerves of the skin, and especially those of the chest; respiratory movements in the adult, from the impression caused by the afferent nerves of the lungs (sympathetic or vagus), by the presence of carbonic acid in the air cells and passages, or in those of the general system,—also, occasional modifications of the respiratory movements from impressions of cold, &c., on the surface of the body; sucking in infancy; deglutition or swallowing, with all the complicated movements then occurring in the tongue, fauces, larynx, and gullet; vomiting, caused by irritation in stomach, or in fanc.es, or following nausea; forced contractions of the sphincter muscles of the anus, urinary bladder, and vagina, under local irritation; erection and emission under the influence of irritation of the nerves of the penis and other parts in the vicinity; rhythmic, movements of lymphatic hearts in reptiles; rhythmic movements of the heart by the action of cardiac ganglia; peristaltic motions of the stomach and alimentary canal, in digestion and in defecation, &c., under the influence of impressions conveyed to the ganglia by the splanchnic and intestinal nerves; action of the bladder in expelling urine; expulsive action of the uterus in parturition; contractions and dilatations of the blood-vessels under the influence of the vaso-motor system of nerves; many obscure and complex morbid actions, such as palpitations of the heart, cramps in the limbs caused by irritation in the stomach or intestinal canal, fainting from concussion or from peculiar odours, dilatation of the pupil and grinding of the teeth from the irritation of worms, and the convulsive seizure sometimes occurring during teething.

Inhibitory and Accelerating Actions.— As we have seen, stimulation of a nerve may cause a sensation, a reflex action, or the direct contraction of a muscle; but in some instances when the nerve is stimulated movements may be arrested. This occurs where rhythmic and apparently spontaneous or automatic actions are restrained or inhibited by the activity of certain nerves. The most striking instance of inhibition is offered by the heart. The subject will be readily understood with the aid of fig. 11, which illustrates the innervation of the heart and bloodvessels and especially the inhibition and action of the depressor nerve.

It is well known that the heart of a warm-blooded animal ceases to beat almost immediately after removal from the body, but the heart of a cold-blooded animal, such as the frog, will beat for hours or even days, especially if it be supplied with defibrinated blood. The rhythmic beat depends to some extent on the existence in the heart of ganglia or small nerve-centres (fig. 11, I, A, R). It is quite tπιe, however, that rhythm may go on in a portion of the heart containing no ganglionic structure. Now the heart receives nerves from two sources, from the vagus or pneumogastric nerve and from fibres derived from the spinal cord through the sympathetic. If the vagus be cut and the lower end stimulated by feeble inductionshocks, the heart beats more slowly, and will probably be brought to a stand-still in a dilated condition. A strong stimulation of the vagus will invariably arrest the action of the heart, and the organ will be found dilated, or, as it is said, in a state of diastole. On removal of the stimulus the heart will soon resume its beats. It is clear that the vagus cannot be regarded as the motor nerve of the heart, because, if so, stimulation would have arrested the action of the heart in a state of contraction or systole. The question, then, is whether the fibres of the vagus possessing this remarkable power of inhibiting or restraining the action of the heart terminate in the muscular fibres, or in some intermediate structures, such as ganglia The influence of various poisons has shed light on this question. A minute dose of atropin injected into the blood paralyses this inhibitory action. After such a dose stimulation of the vagus is followed by no effect, and the heart beats as usual.

“Again, in slight urari poisoning the inhibitory action of the vagus is still present; in the profounder stages it disappears, but even then inhibition may be obtained by applying the electrodes to the sinus. In order to explain this result, it has been supposed that what we may call the inhibitory fibres of the vagus te∏ninate in an inhibitory mechanism (probably ganglionic in nature) seated in the heart itself, and that the urari, while in large doses it may paralyse the terminal fibres of the vagus, leaves this inhibitory mechanism intact and capable of being thrown into activity by a stimulus applied directly to the sinus. After atropin has been given inhibition cannot be brought about by stimulation either of the vagus fibres or of the sinus, or indeed of any part of the heart. Hence it is inferred that atropin, unlike urari, paralyses this intrinsic inhibitory mechanism. After the application of muscarin or pilocarpin the heart stops beating, and remains in diastole in perfect standstill. Its appearance is then exactly that of a heart inhibited by profound and lasting vagus stimulation. This effect is not hindered by urari. Tire application, however, of a small dose of atropin at once restores the beat. These facts are interpreted as meaning that muscarin (or pilocarpin) stimulates or excites the inhibitory apparatus spoken of above, which atropin paralyses or places hors de combat’’ (Foster, Text-Book of Physiology, 4th ed,, p. 186).

Thus physiologists are satisfied that, when the vagus is stimulated, currents of nerve-enèrgy pass along its fibres to some of the intrinsic ganglia of the heart and inhibit or restrain these, so that the heart beats more slowly or is arrested altogether. But the centres in the heart may also be stimulated. After division of both vagi in a mam mal/say a rabbit, the heart’s beat may be quickened or accelerated by stimulation of the cervical spinal cord. Fibres having this power of accelerating the action of the heart have been traced from the cervical spinal cord through the last cervical and first thoracic ganglia of the sympathetic. Stimulation of these accelerator nerves causes a quickening of the heart’s beat, in which, however, “what is gained in rate is lost in force” (Foster). They are referred to here as showing another kind of nervous action, quite different from inhibition. Thus any nerve-centre concerned in reflex movements may have, by impulses reaching it from the periphery, its action inhibited or restrained or accelerated.

Influence of Nerves on Blood-Vessels. — If the sympathetic nerve be divided in the neck, there is a dilatation of the vessels and an increase of temperature on the same side; but irritation by weak induction - currents of the cephalic end will cause the vessels to contract and the temperature to fall. In the sympathetic, therefore, there are nerve-fibres which influence the contractile coats of the blood-vessels. These fibres, called “vaso-motor,” originate from a vaso-motor centre in the medulla oblongata between the point of the calamus scriptorius and the lower border of the corpora quadrigemina, in the floor of the fourth ventricle. From this chief vaso-motor centre nervous influences emanate which tend to keep the smaller vessels in a more or less contracted condition. If it be injured, paralysed, or destroyed there is at once great dilatation of the vessels, more especially those in the abdominal cavity, and the blood collects in these dilated vessels. This of course diminishes the arterial pressure in the larger vessels. Consequently, by observations on blood-pressure, it has been found possible to study the conditions of vaso-motor action. By connecting a kymograph (a recording manometer) with a large vessel, say the carotid, observing for a time the mean blood-pressure, and afterwards injuring the supposed vaso-motor centre, Ludwig and his pupil OwsjannikofΓ at once observed an enormous fall of blood-pressure, to be explained by the paralysis of the smaller and a consequent emptying of the larger vessels.

The vaso-motor centre may be influenced—that is, inhibited, or possibly strengthened—by impulses coming from the periphery. Such impulses may be sent to the centre along any sensory nerve, but in 1866 Cyon and Ludwig discovered a nerve which apparently exercises this function to a remarkable extent. In the rabbit it originates by two roots from the superior laryngeal and from the vagus. Stimulation of the distal end of this nerve produces no effect, but stimulation of the cephalic end causes at once a great fall of blood-pressure in the arterial system and a diminution in the frequency of the pulse. As this nerve, therefore, inhibits, restrains, or depresses the activity of the vaso-motor centre it has received the name of the “depressor nerve of Cyon and Ludwig.” It appears to influence chiefly the vaso - motor arrangements of the abdomen and lower extremities. Thus, after section of the splanchnics, which control the vessels of the abdominal viscera, it is said that excitation of the depressor does not produce nearly the same diminution of pressure in the carotid vessels. By the influence of the depressor a balance is kept up between the central and the peripheric circulations. Imagine the heart to be pumping blood through the vessels. If from some cause the smaller vessels become constricted so as to offer greater resistance to the passage of the blood, the arterial pressure in the larger vessels is increased, and the heart has more work to do to overcome this resistance. When the resistance reached a certain amount the heart would be in danger of exhaustion in endeavouring to overcome it. But by the depressor this danger is removed, as an influence may pass from the heart along the fibres of the depressor to the vaso-motor centre, the effect of which is to inhibit the activity of this centre, and thus allow the smaller vessels to dilate. When this occurs, either locally, as in the abdominal region, or generally, the result is a depletion of the larger vessels, a consequent fall of pressure in them, and therefore less resistance to the efforts of the heart. Thus it would appear that in the heart itself there is an arrangement by which, to a certain extent, it governs its own work, and there is an adjustment between the activity of the heart and distribution of blood throughout the body (see flg. 11).

The vaso-motor nerves causing contraction of vessels have been called “vaso-constrictors”; but there are other nerve-fibres possessing the property of causing a dilatation instead of a contraction. These have been called “vaso-dilators.” Excitation of the chorda-tympani nerve, for example, causes the vessels of the sub-maxillary gland to dilate (see vol. xvii. p. 672). Erection, as it occurs in the penis, has long been known to depend on dilatation of vessels and consequent increased afflux of blood. Stimulation of the nerves of the sacral plexus may cause erection. But how do such nervefibres act? It cannot be that they directly cause relaxation of the muscular fibres in the walls of the vessels. These contain layers of involuntary muscular fibres in the transverse and longitudinal directions, and it is difficult to understand how any contraction of fibres in either of these directions could possibly cause dilatation of the vessel. Probably the effect is brought about by the action of some kind of inhibitory mechanism. Ganglia abound in the walls of the vessels. From these, fibres pass to and from the muscular elements of the vessel. Such ganglia or local reflex centres may be supposed to be under the influence of two sets of nerve-fibres: (1) accelerating or strengthening, corresponding to the accelerating fibres that influence the heart; and (2) inhibitory, like the fibres of the vagus distributed to the heart, having the power of restraining the action of the local ganglia. According to this view, the fibres in the chorda which cause dilatation of the vessels of the sub - maxillary gland on stimulation are vaso-inhibitory nerves.

Influence of Nerves on Glands.— This has already been described under Nutrition (voL xvii. p. 672), but the facts may be here briefly summarized. A secreting gland is supplied with three sets of nerve-fibres,—vaso-constrictor, vaso-dilator or vaso-inhibitory, and secretory. The first two regulate the distribution of blood in the gland, whilst the third set directly affects the activity of the secreting cells. According to Heidenhain, in addition to the vascular nerves supplying a gland there are secretory and trophic nerves. “Stimulation of secretory fibres leads to an increased flow of water, stimulation of the trophic to an increased secretion of specific substances and to an increased production of protoplasm” (Gamgee). The vaso-constrictor fibres of a gland are derived from the sympathetic, and the vaso-dilator and secretory from the cerebro-spinal system.

Classification of Nerve-Centres. — Although these are usually classified anatomically, according to the organ in which they are situated, they may also be arranged according to their functions, as follows:—(1) receptive centres, to which influences arrive which may excite sensations (in grey matter of brain), or some kind of activity not associated with consciousness (reflex centres of the cord and of the brain); (2) psychical centres, connected with sensation in the sense of conscious perception, emotion, volition, and intellectual acts (in the grey matter of the brain); (3) discharging centres, whence emanate influences which, according to structures at the other ends of the nerves connected with them, may cause movements, secretions, or changes in the calibre of vessels (in brain and spinal cord); (4) inhibitory centres, which inhibit, restrain, or arrest the actions of other centres.

B.—Special Physiology of Central Organs. General Physiological Anatomy.— The central organs of the nervous system consist of ganglia or of what is called a “cerebrospinal axis.” The anatomy of the latter is described under Anatomy, and some account of the gangliated cords in invertebrates and of the rudimentary nervous systems of the lower forms of vertebrates will be found under the articles Crustacea, Insects, Amphibia, Birds, Ichthyology, &c. But, as one of the most effective ways of obtaining an intelligent conception of the complicated nervous system of man and of the higher animals is to trace its various forms in the scale of animal existence, and to observe the close correspondence between complexity of structure and complexity of function, a short introductory review of its comparative anatomy, from the physiological side, will here be given. In the first place, we find that the different forms of nervous systems may be divided into (a) those consisting of ganglia or chains of ganglia, as found throughout the invertebrates, and (b) those having a great axis of nervous matter forming a brain and spinal cord, the cerebro-spinal axis, as seen in vertebrates.

Comparative View of Nervous System of Invertebrates.— In the simplest forms of animals the protoplasmic cell is the seat of sensation and of motion; but as the contractile or muscular layers become more marked sensation is relegated to the cells of the ectoderm, or outer layer of the body. As portions of this sensory layer become of higher value to the organism, their protection is accomplished by some of the sensory cells sinking into the body of the organism so as to be covered by less important structures. The portions, originally of the surface, thus differentiated and protected become ganglia, and processes pass from them on the one hand to cells in the periphery, so that they may still be influenced by external energies, and on the other to the contractile parts of the organism by which movements are accomplished. Still higher in the scale of life, the ganglia are connected by internuncial fibres, and the plan of the primitive nervous system bears a relation to the general type of structure of the animal. Thus in radiate animals the gangliated cords show a radiated arrangement, and when the animal form is bilateral and symmetrical the nervous arrangements are on the same type. It is also to be noted that the ganglion specially connected with the rudimentary organs of sense attains a size and importance proportionate to the development of the sense-organs. The nerves of the sense-organs are chiefly connected with the supra-oesophageal ganglion, which thus may be looked on as a rudimentary brain. When the body of the animal becomes more complicated by the development of similar segments (or metameres), we find that by a reduplication, as it were, of the suboesophageal ganglion a ventral chain of ganglia is formed, a pair of ganglia for each segment, the individual ganglia being connected by longitudinal commissures. Such an arrangement is seen in the ringed worms and in arthropods. The next step is a fusion of ganglia into masses, according to the size and importance of the part of the body to be innervated (see vol. vi. p. 636, figs. 7 and 9).

No trace of a nervous system can be detected in Protozoa. The Scyphomedu-soid forms of Hydrozoa show nerve-flbres and ganglion-cells (Schafer) in the sub-umbrella and around the tentaculo-cysts (see vol. xii. p. 652, flg. 15), and in the Hydromedusoid forms the nerve-ganglion cells form a ring round the margin of the disk. In some of the Λctiιιozoa (anemones, <fcc.) fusiform ganglionic cells united by nerve-fibres are said to exist (P. Μ. Duncan). In all the worms (Vermes) the most important central organs of the nervous system are placed in the anterior part of the body near the beginning of the alimentary canal. If they have a distinct head the nervous organ is in it and supplies branches to the sense-organs. From thence nerve-trunks radiate to the periphery of the body, often in the form of two longitudinal trunks on the ventral surface. Frequently there is a nervous ring round the oesophagus. Nerveorgans have been found in all the Platyhelminthes, Rotatoria, and liryozoa. The Nematħelminthes show a further advance. The central organ is placed on the oesophagus, surrounding it as a ring, from which nerves radiate forwards and backwards. Often six strands of nerve run forwards, whilst a dorsal and a ventral trunk pass backwards. The size of these trunks depends on the length of the body. The cephalic ganglion is bilateral and is largely developed. In the Hirudinia and Annelida the cerebral ganglia are connected by commissures with a ventral cord, which, in turn, shows individual ganglia connected by commissures. Each ganglion consists of two equal portions with a transverse commissure, and in the higher forms they are so close as to form almost a single cord. It is also evident that the cerebral ganglia are composed of several ganglia fused together, and acquire functional importance as the sense-organs are more highly developed. In the Echinodermata the nervous system consists of a number of trunks placed ventrally and having a radial arrangement. Each of these trunks corresponds to the ventral ganglionic chain of the Annulata. In Asterida (star-fishes) each radial nerve consists of two bands thickened in the middle, and at the end there is a swelling connected with an optical apparatus placed there. In the Echinus (sea-urchin) the nervous ring lies above the floor of the oral cavity, between the oesophagus and the lips of the ossicles of the masticatory apparatus. From this ring lateral branches issue which accompany the branches of the ambulacra! vessels. In IIolothuroida (sea-cucumbers) the nervous ring lies in front and near the mouth, and is thicker than the five nerves which it gives off, thus differing from the Asteroida and Echinoida. The nervous system of the Arthropoda resembles that of the Annelida. There is a large ganglion above the oesophagus, the cerebral ganglion, united to a ventral ganglion by two commissures so as to form a nervous ring. From the ventral ganglion a series of ganglia united by commissures extends along the ventral surface of the body. The increased size of the cerebrum is the most striking characteristic, and no doubt bears a relation to the higher degree of development of the sense-organs, more especially those of sight. In some Crustacea the optic nerves arise from distinct lobes. As pointed out by Gegenbauer, when the optic organs are reduced or lost the cerebrum becomes so small as to be represented by nothing but a commissure. In the individuals having a large portion of the body composed of similar metameres the ganglia are regular in size, appearing in pairs. On the other hand, in the Thoracostraca (crabs, &c.) the anterior ganglionic masses are fused into larger masses so as to correspond to the concrescence of the anterior metameres into a cephalo-thorax. In the abdominal portion of the body, where the metameres are small, distinct, and more or less regular, the ganglia are also distinct and in pairs. In the Protracheata (Peripatus) the nervous system is simpler, and consists of the oesophageal collar with a double ventral cord having no ganglia or swellings on it, although nerve-cells are distributed through it. In the Myriapoda there is a well-marked ventral cord, with ganglia corresponding to the metameres. In the Arachnida the ventral ganglia are often reduced in number and fused. They are characterized by the close connexion between the cerebral ganglia and the ventral cord, owing to the extreme shortness of the commissures (Gegenbaucr). In the Scorpions the nervous system is richly segmented, and remarkable for the large size of the ganglion giving off the pedal nerves. The Spiders have a single large ganglion in the cephalo-thorax, no doubt consisting of several ganglia. In the √fccιrin<ι(mites) the cerebral ganglion is extremely small, and the other ganglia are fused so as to form one single mass, giving off nerves all round. These minute animals show a remarkable degree of concentration of the nervous system. In Insecta (see fig. 12) the ventral cord traverses the whole length of the body, the ganglia being at equal distances, and all united by commissures. This condition is well seen in the larval condition, and is like the permanent state of the Myriapoda. When the insect passes into the adult condition changes occur, consisting essentially of the fusion o£ ganglia and a Shortening of the commissures. The cerebral ganglion is composed primitively of three pairs, and in most cases does not unite with the rest of the ventral cord. It shows hemispheres and a complicated structure. The first ganglion of the ventral cord supplies the organs of the mouth; the three succeeding send nerves to the appendages, feet, and wings; the re maining ganglia are small, except the last, which supplies the generative organs. There is great variety among the Insecta in the number of ganglia in the ventral cord, but coalescence always indicates a higher type of structure. The nervous system of the Brachiσpoda is formed of masses of ganglia near the oesophagus. From these nerve-fibres pass to various parts of the body. There is an oesophageal ring, but the superior ganglion is very small, owing to the absence of higher sensory organs. In Mollusca (see vol. xvi. p. 635, fig. 1) the nervous system is divided into a superior ganglionic mass, which lies above the commencement of the oesophagus—the supra-oesophageal or cerebral ganglia—and a ventral mass which is connected with the other by commissures, and forms the inferior or pedal ganglia. They are both paired. The cerebral ganglion is connected with the sense-organs. Both the cerebral and the pedal ganglionic masses really consist of ganglia fused together. This is well shown in some of the lower forms, in which the pedal ganglia are divided, and form an arrangement like the ventral cord of the Annulata. The remarkable feature in the nervous system of Mollusca is the great development of the visceral ganglia and nerves supplying the heart, branchial apparatus, and generative organs (see vol. xvi. p. 643, figs. 17,18; p. 644, figs. 20, 21, 22; p. 647, figs. 34, 35; p. 648, fig. 36). In the iM-mellibranchia the cerebral ganglia are very small, owing to the absence of a head and its sense-organs. In some forms they arc placed so much to the side as to be united by a long commissure. There are also two pedal ganglia, of a size proportional to the degree of development of the foot. The visceral ganglionic mass is often the largest. It lies behind the posterior adductor muscle, and is united by long commissures to the cerebral ganglion (vol. xvi. p. 693, fig. 144). The nervous system of the Gastropoda is remarkable for the large size of the cerebral ganglia. In the Pteropoda the cerebral ganglia either retain their lateral position or approach the pedal ganglia, with which the visceral ganglia are also fused. The three ganglionic masses, cerebral, pedal, and visceral, are also represented in the Cephalopoda, but they are more approximated by the shortening of the commissures. The ganglionic masses consequently are of great size, and they are more differentiated than any other ganglia in invertebrates. It is possible to distinguish an outer grey layer, formed of ganglionic cells, surrounding a white layer, composed of fibres (vol. xvi. p. 679, figs. 113, 114, 115). Lastly, in Tunicata the nervous system is dorsal, instead of ventral, as in other invertebrates. It is developed from the ectoderm, or outermost layer of the embryo, by an infolding so as to form at first a groove and afterwards a tube. In the Aseidian larvae this nervous tube reaches throughout the length of the tail, and we have thus the remarkable condition of a dorsal-median nerve-cord, analogous to the cerebro-spinal system of vertebrates. Further, embryologists are of opinion that this rudimentary nervous system is the true central organ, although the greater portion of it disappears by the atrophy of the tail in the passage from the larval to the adult state. (Gegenbauer.)

Comparative View of Nervous System of Vertebrates.— To understand the structure of the complicated central nervous system of vertebrates, and to appreciate the physiological importance of its various parts, it is necessary to trace its development in the embryo and to note the various forms it presents from the lowest to the highest vertebrates. A consideration of the embryological and morphological aspects of the subject clears up many difficult problems which a study of the human nervous system, by far the most complicated physiological system in the body, fails to do, and in particular it gives an intelligent conception of its architecture, as seen both in simple and complex forms. The cerebro-spinal axis begins in the embryo as a tube of nervous matter produced by an infolding of the epiblast, or outermost embryonic layer. The tube widens at its anterior end, and, by constrictions in its wall, three primary cerebral vesicles are formed, which afterwards become the anterior, middle, and posterior parts of the brain. In the fully-developed condition the cavity of the tube remains as the central canal of the spinal cord and the ventricles of the brain, whilst the various parts of the brain and cord are formed by thickenings in its walls. The three cerebral vesicles. have been called the forebrain, the mid-brain, and the hind-brain. A protrusion from the anterior cerebral vesicle, at first single, but afterwards divided by a median cleft, becomes the rudiment of the cerebral hemispheres (prosencephala), the cavity remaining in the adult condition as the lateral ventricle on each side. From each cerebral vesicle another hollow process protrudes ’which constitutes the olfactory lobe (rhinen-cephalori). What remains of the cavity of the first vesicle becomes the third ventricle (thalamencephalon). In the ’ outer and under walls of the prosencephala a thickening is formed which becomes the corpora striata, two large bodies in the floor of the lateral ventricles of the adult brain, whilst the roof is modified into the substance of the cerebral hemispheres. Immediately behind the corpora striata, and in the floor of the thalamencephalon, two similar thickenings occur which become the optic thalami, a thin layer between the two constituting the taenia semicircularis, and the Y-shaped canal passing from the cavity between the thalami to the cavities in the cerebral hemispheres (lateral ventricles) is the foramen of Monro. The floor of the third ventricle is produced into a conical process, the infundibulum, at the blind end of which is the pituitary body, or hypophysis cerebri. The roof of this ventricle is very thin, and in connexion with it is developed the pineal gland, or epiphysis cerebri. Transverse fibres pass from the one corpus striatum to the others, constituting the white commissure, whilst the two optic thalami are connected by two grey commissures. In mammals the two cerebral hemispheres are connected by a large and important set of commissural fibres, forming the corpus callosum. In addition there are certain sets of longitudinal commissural fibres. Thus two sets of fibres arise from the floor of the third ventricle, arch upwards, and form the anterior pillars of the fornix. These are continued over the roof of the third ventricle and run backwards, constituting the body of the fornix. Behind this the bands diverge so as to form the posterior pillars of the fornix. In the higher vertebrates the upper lip of the foramen of Monro thickens, and becomes converted into a bundle of longitudinal fibres, which is continuous anteriorly with the anterior pillars of the fornix. These are continued back between the inner boundary of the cerebral hemisphere and the margin of the corpora striata and optic thalami, and project into the lateral ventricle, forming the hippocampus major. As in highly - formed brains the corpus callosum passes across considerably above the level of the fornix, a portion of the inner wall of the hemisphere on each side and a space between are intercepted. The two inner walls constitute the septum lucidum, and the space the cavity of the fifth ventricle. By a thickening of the floor of the middle cerebral vesicle {mesencephalon) two large bundles of longitudinal fibres, the crura cerebri, are formed, whilst its roof is modified into the optic lobes, corpora bigemina or corpora quadrigemina. The cavity, reduced to a mere tube, is the iter a tertio ad quartum ventriculum, or the aqueduct of Sylvius. The third cerebral vesicle, mylcncephalon, undergoes less modification than the others. The upper wall is exceedingly thin before the cerebellum so as to form a lamina, the valve of Vieussens, whilst the part behind is covered only by membrane, and opens into the posterior subarachnoid space. The cerebellum makes its appearance as a thin medullary lamina, forming an arch behind the corpora quadrigemina across the wide primitive medullary tube. The portion forming cerebellum, pons Varolii, and the anterior part of the fourth ventricle is termed the cpencephalon, whilst the remaining portion, forming the medulla oblongata and fourth ventricle, is the metencephalon. These facts are briefly summarized as follows (Quain, vol. ii. p. 828). [table]

The general architecture of the brain considered in this way will be understood by the diagram in fig. 15, whilst details as to the exact anatomy of the human brain will be found under Anatomy (vol. i. p. 869 sq. ).

The complex structure of the brain in the higher animals arises to a large extent from the great development of the cerebral hemispheres. At a very early period these glow forward and project more and more beyond the region of the first primary vesicle, which, as has been noticed, never advances farther forward than the pituitary fossa {lamina terminalis) ; in expanding upwards they take the place previously occupied by the mid-brain, and fill the most prominent part of the head; and by a downward and lateral enlargement they form the temporal lobes. Thus frontal, parietal, and temporal lobes come to be distinguishable, and somewhat later, by a farther increase posteriorly, the hindmost lobes constitute the occipital lobes, and the cerebrum at last covers completely all the lower parts of the brain. The hemispheres, therefore, which are small in the early embryo of all animals, and in adult fishes permanently, attain so large a size in man and in the higher animals as to conceal all the other parts. Whilst this general development is going on the layer of grey matter on the surface of the hemispheres increases to such an extent as to throw the surface into folds or convolutions. The upper surface of the hemispheres is at first smooth (see fig. 16). The first appearance of division into lobes is that of a blunt notch between the frontal and temporal parts below, in what afterwards becomes the Sylvian fissure. In the fourth and fifth months there appear the vertical fissure, separating the parietal and occipital lobes, and the transverse fissure, called the fissure of Rolando, which divides the frontal and parietal lobes superiorly, and which is peculiarly characteristic of the cerebral type of man and of the apes (Allen Thomson). Then the convolutions appear from the formation of secondary grooves or sulci, for even at birth they are not fully perfected; and by the deepening of the grooves and the formation of subordinate ones the process goes on during the first years of infancy. For the convolutions see voh i. p. 873; also Phrenology, vol. xviii. p. 847.

The evolution of the brain throughout the animal kingdom shoλvs a graduated series of increasing complication proceeding out of the same fundamental type; so that the forms of brain found permanently in fishes, amphibians, reptiles, birds, and in the lower mammals are repetitions of those shown in the stages of the embryonic development of the brain of one of the higher animals.

In the whole class of fishes the brain retains throughout life more or less of the elementary form,—that is, it consists of a series of enlargements, single or in pairs (see fig. 17, C). The simplest of all forms is in the lancelet {Branchiostoma), in which there is no distinction between brain and cord, there being no anterior enlargement to form an encephalon. In the Cyclostomata, as the lampreys, the form is nearer that of the embryo when the five fundamental parts of the brain can be distinguished. At this stage the cerebrum and cerebellum are extremely small, whilst the ganglia chiefly developed are those connected with the organs of sense, more especially those of vision. In the sharks and skates {Selachii, or cartilaginous fishes) the cerebral portion is considerably larger. In osseous fishes (Teleostei) the thalamencephalon is so fused with the mesencephalon as to make the homology of the parts difficult to trace, but both cerebellum and cerebrum are still small relatively to the rest of the brain. The most important part of the brain of a fish is the part behind the mesencephalon, as from it all the cerebral nerves originate. Thus not only are the optic lobes relatively important as being the centres of vision, but the medulla oblongata is usually very large. In many sharks it forms the largest part of the brain (Gegenbauer). The spinal lobes of the electric fishes are differentiations of this portion of the encephalon.

In the Amphibia the hemispheres are larger, and are divided into two parts (see fig. 17, B). In the Urodela (siren, proteus, triton, newt) the mesencephalon remains small, and consists of one lobe, but in the Anura (frogs, toads, &c.) there is an advance in this part, it being divided into two. In reptiles there is still an advance in the size of the thalamencephalon and mesencephalon, and the prosencephalon is so large as to pass backwards and overlap the thalamencephalon. The cerebellum (metencephalon) is still small, especially so in Ophidii (serpents) and Saurii (lizards), but in the Chelonii (tortoises, &c.)and in Crocodilini (crocodiles, alligators) it is larger. In the crocodile there is a transverse grooving of the cerebellum, giving rise to foliation or laminar division, which is carried much farther in birds and mammals, indicating a greaterpower of co-ordination or regulation of movement

In birds (fig. 18) the vesicles of the mid-brain are partially hidden by development of the cerebral hemispheres. These are connected by a fine anterior commissure, and they contain a large amount of ganglionic matter bulging into the primitive cavity or ventricles, which are of very small size. The middle portion of the cerebellum shows a distinctly laminated structure and a differentiation into white and grey matter. But there is no pons Varolii, nor corpus callosum, nor fornix, nor hippocampus. In the floor of the lateral ventricles may be seen a ganglionic mass corresponding to corpus striatum and optic thalamus. The optic lobes are relatively large and show considerable differentiation of structure.

Mammals, even the lower orders, not only show a general enlargement of the cerebral hemispheres, but we find a commissure, the corpus callosum, uniting them. This commissure is of small size, and is confined to the fore part of the hemispheres in Monotremata ( Omithorhynchus, Echidna) and Marsupialia (kangaroos, &c.), and in some of the Edentata (ant-eaters, sloths, &c. ), but it gradually extends farther and farther back as we ascend to the higher orders. The chief changes thus occur in the prosencephalon. In the lower orders of mammals the hemispheres are comparatively small and simple, and do not present any division into convolutions, and very little distinction even of lobes. The cerebral hemispheres gradually grow backwards, covering mid-brain, cerebellum, and medulla oblongata, as we find in the higher Primates (monkeys, apes, and man). There is also a general enlargement of the brain and of the cranial cavity. The development of a posterior lobe only takes place in the higher orders, and in these also the enlargement of the frontal lobes brings the front of the cerebrum more and more over the nasal cavities, causing a development of forehead. This also explains how the olfactory bulbs in more highly-formed brains are thrown below the frontal part of the hemispheres, instead of originating at their anterior borders. But the internal arrangements of the brain also become more complicated. The fornix, already described, establishes, by its longitudinal commissural fibres, a connexion between the anterior and posterior lobes of the cerebrum. In the Monotremata and Marsupialia the mid-brain retains a bifid form, constituting the optic lobes, or corpora bigemina, but in all higher animals each is divided into two by a transverse groove, forming the corpora quadrigemina, of which the anterior pair is the largest. As we ascend also, we find the surface of the brain becoming more and more convoluted (see figs. 19 and 20). This is the general fact; but whilst the convolutions are most numerous and deepest in the highest orders there is no regular gradation, as in each group there are very great variations in the degree of convolution (Allen Thomson). Thus in the Monotremata the Echidna has a more convoluted cerebrum than the Omithorhynchus, whilst in the Primates the brains of the marmosets show a comparatively smooth non-convoluted surface, in striking contrast to the rich convolutions seen on the brains of the higher monkeys and of the apes. It is important to note that the cerebellum also becomes more and more complicated as we ascend from the lower to the higher groups. At first merely a lamina or band, as seen in fishes and amphibia, it is a centrally differentiated body in crocodiles. In birds there is an indication of a division into three portions, a central and two lateral, whilst the central is by far the larger, the two lateral being feebly developed. In Monotremata the central portion is larger than the lateral, but, whilst it is larger in Marsupialia, Edentata, and Cheiroptera (bats, &c.), it is clear that the lateral portions are increasing in size so as to make the disproportion less. But in Carnivora (felines, hyaena, otter, bear, &c.) and in Ungulata (sheep, ox, camel, rhinoceros, horse) the lateral lobes, or hemispheres, of the cerebellum develop to a much greater· size; and in most of the Primates they are much larger than the median portion, which is now called the worm or “vermiform process.” As regards the development of the spinal cord continuous with the medulla oblongata, it.need only be said that it does not show any marked peculiarities ’of structure in different animals. The grey matter from which nerve-fibres originate and in which they end is found in the centre of the cord, and it is most abundant in the regions associated with the development of limbs. The white matter is external, and, in the cords of the higher animals, can be differentiated by fissures into columns, the special functions of which will be hereafter considered. The size of the cord is influenced by the masses of nerves given off from it, so that it attains its greatest thickness and development in the four higher divisions of the vertebrates possessing limbs. Thus, too, are formed cervical, dorsal, and lumbar enlargements, contrasting with the more uniform and ribbon-like form of the cord in fishes, although even in these there are special enlargements corresponding to the points of exit of important spinal nerves.

Size and Weight of Brain.—The gradual increase in the size of the brain, as compared with that of the body, which is observed as we rise in the animal scale, has some intimate proportional relation to a corresponding increase of the nervous and mental endowments. Information as to the size of the brain may be obtained by direct measurement of dimensions and weight; but as this is often difficult recourse may be had to the measurement of the capacity of the cranium, which contains, however, not only the brain but its accessories, such as membranes and blood-vessels. Details will be found in vol. i. p. 879. After considering the measurements of several thousand skulls made by different observers, the late Dr Allen Thomson arrived at the conclusion that the cranial capacity is on the whole greater among the highly-civilized than among the savage races, and that there is even a very manifest difference to be found between persons of higher mental cultivation and acknowledged ability and those of the uneducated class and of inferior intellectual powers; and he states further that the amount of this difference may be from 5 to 7⅛ per cent, iΛ persons of the same race, and about double that range in those of different races. Thus, the average adult brain of men in Britain being taken at 3 lb, or, more precisely, at 49⅜ oz. avoir, (women, about 44 to 44⅜ oz.), at an average specific gravity of 1040, would give a bulk of 82 - 5 cubic inches of brain-substance; 10 per cent, being deducted for loss by membranes, fluid, &c., the cranial capacity will be about 90 inches. Conversely, the weight of the brain may be calculated from the known cranial capacity. If, therefore, the brain of the uneducated class falls 2’5 oz. below the average, whilst that of the more cultivated persons rises to the same amount above it, or to 52⅜ oz., we may regard these brain-sizes as corresponding with brain-bulks and cranial capacities of 78 and 87 cubic inches, and of 88 and 97 cubic inches respectively. The average brain-weight of an Australian aboriginal man is about 42 oz., corresponding to a brain-bulk of about 70 cubic inches, and a cranial capacity of about 78 cubic inches. There are, however, great variations in all races. Thus the brain of Cuvier, the great naturalist, weighed 65 oz. avoir., corresponding to a brainbulk of 108 cubic inches and a cranial capacity of 118 cubic inches; whilst, on the other hand, in Europeans the brain-weight has fallen as low as 32 oz., or a brain-bulk of 53 cubic inches and a cranial capacity of 63 cubic inches. The brains of the anthropoid apes—gorilla, chimpanzee, and orang—are all inferior to man in their dimensions. In the gorilla the brain does not attain more than a third of the weight of the average human brain, and in the chimpanzee and orang it does not reach a fourth, so that the ratio of brain-weight to bodyweight in these animals may be as 1 to 100, whilst in man it ranges from 1 to 40 to 1 to 50. It is remarkable that in general among the largest animals of any group the brain does not reach a size proportionate to the greater magnitude of the other organs or of the whole body, so that in the smaller members of the same order a considerably greater proportional size of the brain is observed. Thus in the small marmosets the proportion of the brain-weight to the body-weight may be 1 to 20, or more than double the proportion in man. Similar facts are brought out in comparing the brains of cetaceans, pachyderms, dogs, &c., as shown in the following table.

Table of comparative sizes of Brain and Body, [table]

Although the proportion of brain-weight to body-weight in a male child at birth is 1 to 10, yet so rapidly does the brain continue to grow during the early period of childhood that by the age of three years it has attained more than three-fourths of its full size, by the age of seven years it has reached the proportion of nine-tenths, and after this, only by slow and small gradations, it attains the full size between the ages of twenty and twenty-five years. See Phrenology.

From this survey of the comparative development of the brain the following general conclusions can be drawn.

1. The first and essential portion of the cerebro-spinal axis is the portion forming the spinal cord and medulla oblongata, inasmuch as it is found throughout the whole range of vertebrate existence, and is connected with the reflex or automatic movements on which locomotion, respiration, and the circulation more or less depend, and with the simple sense of contact, or touch, or pressure. This portion is necessary to mere existence.

2. When higher senses are added, such as those of taste, smell, hearing, vision, portions of the anterior part of the cerebro-spinal axis are differentiated so as to form centres. The earliest and most important of these senses (next to touch) is vision, hence the high degree of development of the optic lobes even in the lowest forms; to these are added the optic thalami, which may be regarded as the centres of tactile sensations involving appreciation of differences of touch as to softness, smoothness, hardness, &c., requiring in the periphery special terminal organs. Special centres for hearing, taste, and smell are not differentiated. It is remarkable that the organs relating to the sense of smell are most anterior and most closely related with the prosencephalon, indicating, apparently, that this sense is one of the earliest in appearance, and probably, along with vision and touch, one of the most necessary to existence.

It is equally striking that the origin of the auditory nerves should be placed so far back as in the medulla oblongata and cerebellum, indicating the primitive nature of simple auditory impressions and their relation to co-ordination of movement. The sense of taste originates in nerves springing from the medulla, and in close connexion with those regulating the movements of the tongue and swallowing.

8. When sensations of a simple character are elaborated into ideas and give rise to the physical changes in some way correlated to mental states, involving memory, emotions, volitions, and intellectual acts, a part of the cerebro-spinal axis is differentiated for these functions in proportion to the extent to which such mental phenomena are manifested by the animal. Judging from the facts obtained by comparing animal intelligences, so far as they can be appreciated by us, we have the right to infer that in proportion to the degree of development in size and complexity of structure so is the mental condition of the animal. Taking it broadly, there can be no question that the intelligence of a bird is higher than that of a reptile, amphibian, or fish, and that the intelligence of the higher mammals, such as one of the Primates, is superior to that of the lower, as one of the Insectivora (hedgehog), or of the Marsupialia (kangaroo); and along with the higher intelligence is the more complex brain. There are qualifications to this statement to be afterwards alluded to, but they arise from deficient knowledge and do not vitiate the main conclusion. In proportion, therefore, to the degree of development of the prosencephalon do we find the intelligence of the animal, and we may regard this portion as sιιperadded to the cerebro-spinal axis as the organic mechanism for such mental operations.

4. There is also a correspondence between the degree of development of the cerebellum and the faculty of co-ordination of movement. Movements of the members of the body may be of a very simple character, or they may be very complex. They may be due to the action only of flexor and extensor muscles, causing the limb to move almost in the same plane, or they may be associated with the action of adductor and abductor muscles, by which there may be many kinds of circular or rotatory movements. There is a great difference between the movements of a fish’s fin, of a bird’s wing, of a horse’s fore-leg, and of the arm of a monkey or a man. In the first three they are almost to-and-fro movements, unlike the delicate movements of flexion, extension, pronation, supination, and prehension seen in the latter. Delicacy of movement of the anterior limb reaches its highest condition in man. It may be put generally that simplicity of movement is associated with an imperfectly-developed cerebellum, whilst in animals having the power of complicated movements, involving especially the knowledge of how the limbs are acting at any moment, and of adjustment of movement in special circumstances, the cerebellum is highly developed. From this point of view, the degree of development of the cerebellum is as characteristic of man as the degree of development of the cerebrum. That this is no accidental correspondence will be shown in treating of the functions of the cerebellum.

Having reviewed the physiological anatomy of the cerebro-spinal system, an account will now be given of the more special physiology of the centres composing it,—namely, spinal cord, medulla oblongata, pons Varolii, basal ganglia (including corpora striata optic thalami, and corpora quadrigemina), cerebellum, and hemispheres of the cerebrum.

Spinal Cord.— The spinal cord is described at vol. i. p. 865 sq., but it is necessary here to allude to a few points of physiological importance. The cord consists externally of white and internally of grey matter. The white matter, composed of nerve-fibres, forms a series of strands or columns in each half of the cord. The grey matter in the central part of the cord is arranged in two crescentic masses, and shows under the microscope numerous multipolar cells connected with nerve - fibres and imbedded in neuroglia, or the special connective tissue of the nerve-centres. The neuroglia is composed of a kind of semi-fluid matrix, fibrils, and peculiar cells having numerous branches called “Deiter’s cells.” (See fig. 21.) These nerve-cells are arranged in definite groups and occupy the same relative position in successive sections, forming the ganglionic or vesicular columns of the grey matter, as follows. (1) Cells found along the whole of the anterior part of the anterior cornua, many of the processes of the nerve-cells being continuous with the nerve-fibres of the anterior roots of the spinal nerves. This column of nerve-cells has been called the “motor ganglionic column,” or the “vesicular column of the anterior cornua.” (2) A group or column of nerve-cells at the inner or mesial angle of the base of the posterior cornu, in the middle region of the cord from the third lumbar to the seventh cervical nerve. This is termed the “posterior vesicular column,” or “Clarke’s column,” after the late Mr Lockhart Clarke, who did much to unravel the intricate anatomy of the nerve-centres. The nerve-cell processes are continuous chiefly with nerve-fibres coming from the lateral column. This vesicular column is best developed where the column of the anterior cornu is least so. (3) The third column of nerve-cells is in the outermost portion of the grey matter, midway between the anterior and posterior cornua. Development has shown that at an early period the anterior horns are distinctly differentiated from the posterior, and that the grey matter between them is the last to be formed. The nuclei in the latter may be regarded, therefore, as accessory nuclei. It has also been observed by Flechsig and others that the white substance of the cord also makes its appearance first in the neighbourhood of the anterior and posterior roots. The cord at a very early period consists almost entirely of grey matter, and the columns are superadded in the anterior first, the posterior last. The posterior can also be traced to the cortex of the cerebellum (Flechsig).

The anterior and posterior roots of the spinal nerves are attached along the sides of the cord, opposite to the corresponding cornua of grey matter. Some of the fibres of the anterior roots end in nerve-cells in the anterior cornu. Others pass through the grey matter and cross to the other side of the cord through the anterior· commissure, a layer of white matter at the bottom of the anterior median fissure. A third set passes to the anterior part of the lateral column and to the posterior cornu. The course of these fibres is shown in fig. 22. A portion of the fibres of the posterior roots ends in the grey matter on the same side, but many cross to the grey matter on the opposite side. There is thus a decussation of fibres connected with both the anterior and the posterior roots. The arrangement of the white or fibrous columns of the cord is seen in the table under medulla oblongata below.

The spinal cord acts (1) as a transmitter of motor and sensory— or centrifugal and centripetal—impressions between the encephalon and the periphery, and (2) as a reflex centre.

1. Transmission of Motor and Sensory Impressions.— Each spinal nerve, as already mentioned, is connected with the spinal cord by two roots, an anterior and a posterior. Section of a number of anterior roots causes paralysis of motion of muscles on the same side of the body, whilst irritation of the distal or peripheral end of the divided roots causes twitchings or tetanus of the muscles. Neither section nor irritation has any effect on sensation. Hence the anterior roots contain motor fibres, carrying impressions from the cord outwards. Again, section of a number of posterior roots is followed by loss of sensation of a part of the body on the same side, and, if the proximal ends of the divided roots—those next the cord—be irritated, painful sensations are excited. The posterior roots, therefore, contain sensory fibres, carrying impressions into the cord from the periphery. As we have seen, these roots are connected with the grey and white matter of the cord, and it is practically impossible to trace all their ramifications. Recourse must therefore be had to the evidence supplied by experiment (cutting, or by the Wallerian method, p. 26) and by pathological observation. In tracing the path of fibres, what may be called the “developmental method” has been pursued. It has been shown by Flechsig that, “if the development of the cord be carefully observed, the medullary substance of the nerve-fibres is formed later along certain tracts of the white columns than in the rest of the white matter, so that in transverse sections of the cord these tracts are easily distinguishable by their more transparent grey appearance” (Quain, vol. ii. p. 277). If the anterior columns be cut by an incision extending into the grey matter, leaving the posterior columns intact, voluntary movements disappear in the parts below the section. Again, section of the posterior columns and grey matter, leaving the anterior uninjured, enfeebles but does not destroy the power of voluntary movement below the section. Finally, section of an antero-lateral column on one side paralyses voluntary motion on the same side. From these facts it is inferred (a) that the motor tracts passing from the brain to the periphery are in the anterolateral columns, and (b) that the fibres forming these tracts are chiefly distributed to the same side of the body. These inferences are supported by pathological observation. In diseases where the anterior horns of grey matter are affected paralysis ensues, with complete flaccidity of the limbs; and if, from haemorrhage, softening, or the pressure of tumours, the anterior portion of the cord be irritated there are spasmodic twitchings of muscles. Complete transverse section of the posterior columns does not abolish sensibility in the parts below; but there is a loss of the power of making co-ordinated movements. Section of the posterior columns and of the antero-lateral columns, leaving only the grey matter in the centre of the cord intact, does not abolish sensibility. Again, section of the antero-lateral columns and of the whole of the grey matter, leaving only the posterior columns uninjured, is followed by complete loss of sensibility in the parts beneath. The inference therefore is that sensory impressions pass through the grey matter. As already seen, many of the sensory fibres connected with the posterior roots decussate in the grey matter. This explains some of the results obtained by Brown-Séquard, that hemi-section of the cord, involving the grey matter, enfeebled sensibility on the opposite side more and more as the section cut deeply into the grey matter; that a vertical section in the bottom of the posterior median fissure caused loss of sensibility on both sides; and that a lateral section, whilst it caused loss of sensibility (anaesthesia) on the opposite side, was followed by increase of sensibility (hyperaesthesia) on the same side,—a curious fact, explained by Brown-Séquard as being due to irritation caused by paralysis of the vessels of the cord on the side of the section. It would appear also that tactile impressions travel, for a certain distance at all events, in the posterior columns. This has been inferred chiefly from the fact that in certain cases of paralysis involving the posterior columns, where the sensation of touch was absent, the patient could still feel a painful sensation, as when a needle was thrust into the skin; whilst in other cases, in which these columns were not affected, the converse held good. In the disease known as locomotor ataxia (see Ataxy and Pathology, vol. xviii. p. 392) the patient first passes through a period in which there are disorders of general sensibility, especially lancinating pains in the limbs and back. By and by there is unsteadiness of gait when the eyes are closed or in the dark, and to a large extent the patient loses the power of co-ordinating movement. Especially he is unable to judge of the position of the limbs without seeing them; in other words, the so-called muscular sense is enfeebled. At last there is a stage before death in which there is almost complete paralysis. A study of this disease has thrown much light on the physiology of the cord. It is known to be caused by a slow disorganization or sclerosis of the posterior root-zones, the posterior columns,—slowly passing on to affect the columns of Goll, the lateral columns, and the anterior grey horns, and ultimately involving the cord. The disordered sensations at an early stage, the staggering gait at a later, show that the posterior part of the cord has to do with the transmission of sensory impressions. The man staggers, not because he is paralysed as regards the power of movement, but because, in consequence of the sensory tracts being involved, he does not receive those peripheral impressions which excite or indirectly regulate all well-ordered movements of locomotion.

2. As a Reflex Centre. — The grey matter of the lower cervical, dorsal, and lumbar regions of the cord may be regarded as composed of reflex centres associated with the general movements of the body, whilst in the upper cervical region there are more differentiated centres corresponding to special actions. The initial excitation may commence in any sensory nerve; the effect passes to the cord, and sets up changes in the nerve-cells of the grey matter, involving time, and resulting in the transmission outwards along motor fibres of impulses which excite particular groups of muscles. There is an exact co-ordination, with a given strength of stimulus, between certain areas of skin and certain groups of muscles, and thus movements may be so purpose-like as to simulate those of a conscious or voluntary character. Thus irritation near the anus of a decapitated frog will invariably cause movements of the limbs towards the irritated point. The activity of reflex centres may be inhibited, as already shown, by higher centres, or possibly by certain kinds of sensory impressions reaching them directly from the periphery. Hence removal of these higher centres is followed by apparently increased reflex excitability. Strychnia and the alkaloids of opium increase it, whilst aconite, hydrocyanic acid, ether, chloral, and chloroform have an opposite effect. In certain pathological conditions also, as in tetanus, or in some slow progressive diseases of the cord, reflex excitability may be much increased. In tetanus the slightest touch, a movement of the bedclothes, the closing of a door, the vibration caused by a footstep, may throw the patient into severe and prolonged convulsions. The earlier formed ganglionic cells are those specially concerned in reflex acts.

Special, reflex centres have been clearly made out in the cord. (1) A ciliospinal centre, between the sixth cervical and third dorsal nerves, associated with the movements of the iris. The fibres controlling the radiating fibres of the iris, and found in the sympathetic, originate here (see Eye). Hence ' irritation of this region causes dilatation of the pupil, an effect not produced if the sympathetic fibres have been divided. (2) Accelerating centres, supplying fibres to the sympathetic which ultimately reach the heart, and irritation of these centres quickens the movements of that organ. (3) Respiratory centres. The movements of respiration, of a reflex character, involve the action of many thoracic and abdominal muscles. Section of the cord above the eighth dorsal paralyses the abdominal muscles; above the first dorsal, the intercostale; above the fifth cervical, the serratus magnus and the pectorals; and above the fourth cervical, by paralysing the phrenics, it arrests the action of the diaphragm. (4) Genito-spinal centre. This is in the lumbar region. Irritation causes erection, &c.; destruction or disease is followed by loss of virile power. (5) Ano-spinal and υesiculo-spinal centres. These, connected with the movements of the sphincter ani and of the bladder, exist in the lower portion of the dorsal and upper portion of the lumbar regions. Disease or injury involving these centres causes involuntary evacuation of the bowel and complete paralysis of the bladder, with non-retention of urine. The bladder may be full whilst the urine constantly escapes in small quantity.

3. As a Trophic Centre.— The ganglion-cells in the anterior cornua undoubtedly have a trophic or nutritive influence upon muscles. This has been determined chiefly on pathological evidence. If these cells undergo atrophy or degenerative changes, the muscles, even though they may be kept periodically in a state of activity by galvanism, become soft and fatty changes take place. There is thus a correlation between the nutritive condition of muscle and nerve-centre, and influences affecting the one affect the other also.

It has been supposed that the cells in Clarke’s vesicular column may form the centres in visceral innervation. They are bipolar, like those in the sympathetic, and not multipolar as in the rest of the cord, and the columns are absent in the lumbar and cervical enlargements. The cells are found where nerves come off that influence the viscera, and similar cells are found at the roots of the vagus in the medulla,—a nerve also much concerned in the innervation of viscera.

Inhibition of Reflex Actions.— The reflex actions of the spinal cord may be inhibited or restrained to a greater or less extent by the action of centres in the encephalon, so that pure reflex actions only occur after removal of the cerebrum, or during profound sleep, when the cerebrum is inactive. Thus a strong eff ort of the will may restrain from scratching an irritated part of the skin, whilst the same amount of irritation would certainly cause reflex movements if the will were in abeyance. Such power of voluntary control, however, is limited with respect to most reflex actions, whilst some reflex acts cannot be so influenced. Any movement that may be orginated by the will may be inhibited or restrained to a certain extent when the movement is of a reflex character; but, if the movement be invariably involuntary, it can never be inhibited. Thus the ejaculation of semen cannot be voluntarily induced, whilst the reflex act once provoked cannot be arrested (Hermann). That these inhibitions of reflex actions of the cord depend on mechanisms in the brain is proved by the fact that removal of the brain is followed by an increase in the reflex excitability of the cord, and that even section of the cord permits of increased reflex excitability below the plane of section (Setschenoff). Further, after section of the spinal cord in the cervical region, irritation of the lower end arrests reflex movements dependent on reflex centres in the lower cervical, dorsal, and lumbar regions (Μ‘Kendrick).

Medulla Oblongata.— This is the prolongation into the cranium of the spinal cord so as to unite it with the brain. Strictly speaking, the medulla spinalis and the medulla oblongata form one organ. The columns of white matter of the cord undergo changes in form, structure, and relative position when they pass into the medulla (see vol. i. p. 870). Without again detailing the minute anatomy, it is necessary to show, as in the following table, the connexions of the cord and of the medulla with the rest of the brain.

[table]

It is important to note the fact that each column of the cord, through the medulla, is thus connected both with the cerebrum and with the cerebellum. Development has shown that the fibres of the bundles which are first formed develop a medullary sheath at a time when the fibres of the later-formed bundles are non-medullated. “When the cord of a human embryo is examined at the end of the fifth month it will be found that the pyramidal fibres of the lateral columns, the fibres of the columns of Tiirck, and of the columns of Goll are non-medullated; while the fibres of the anterior and posterior root-zones and the cerebellar fibres of the lateral columns are medullated” (Ross). It would appear, therefore, that the latter are the more primitive structures, and that the former are superadded in the cords of the higher animals. The grey matter of the medulla is broken up by changes in the distribution of the white matter into nuclei or masses of nerve-cells, instead of having the crescentic form seen in the spinal cord. These nuclei are connected with the roots of important cranial nerves, and may be regarded as corresponding with the anterior horns of grey matter, with the posterior horns, and with the grey matter between these.

The following nuclei can be found :—(1) the hypoglossal nucleus, for the hypoglossal nerve, the motor nerve of the tongue; (2) a common nucleus, for a portion of the spinal accessory, vagus, and glosso-pharyngeal nerves; (3) the principal or lower auditory nucleus, for the auditory nerve; (4) nuclei for the sixth or abducent nerve, supplying the external rectus muscle of the eye; (5) nucleus for the fourth nerve, supplying the superior oblique muscle of the θye; (6) the facial nerve, the motor nerve of the face; (7) the corpus dentatum of the olivary body, not directly connected with the roots of nerves, but containing nerve-cells. Some fibres, both of the sensory and motor roots of the fifth nerve, originate also as far back as the medulla. The third, fourth, sixth, and hypoglossal nerves belong to the system of anterior motor nerves, related to the anterior cornua, whilst the spinal accessory, vagus, glossopharyngeal, and fifth belong to the “mixed lateral system,”—that is, they are related to the posterior cornua and intermediate grey matter.

Lake the spinal cord, the medulla may be regarded as containing tracts for sensory and motor transmission, and as constituting a series of reflex centres for special movements.

1. As a Conductor of Motor and Sensory Impressions.— Inasmuch also, as such movements as those of the circulation, respiration, and vaso-motor action are necessary to life, destruction of the medulla causes almost instant death. Motor fibres coming from the brain above decussate in the anterior pyramids and then run down the lateral columns of the cord, issuing to the muscles by the anterior roots of the spinal nerves. Hence, whilst section of an antero-lateral column of the cord will cause paralysis of motion on the same side, section of an anterior pyramid above the decussation causes paralysis of motion on the opposite side. But fibres carrying sensory impressions also decussate in the grey matter at the bottom of the posterior median fissure of the cord. It follows, therefore, that disease, such as rupture of a vessel causing a clot in the brain, say in the left corpus striatum and left optic thalamus, causes paralysis both of motion and of sensation on the opposite side,—that is, in the case supposed, there would be right hemiplegia. The path of sensory impressions is probably in the grey matter but the precise course of sensory fibres has not been traced.

2. As a Reflex Centre. —Numerous special centres have been referred to the medulla oblongata.

(1) Respiratory, centres, two in number, expiratory and inspiratory, connected with the roots of the pneumogastric nerves. Destruction at once causes cessation of respiratory movements. (2) Caso-motor centre, regulating the calibre of the smaller blood-vessels throughout the body (see p. 30). (3) Cardiac centres, probably two in number—one accelerating, associated with the sympathetic; the other inhibitory, connected with the pneumogastric (see p. 29). (4) Centres for deglutition, associated with the sensory and motor nerves involved in this process (see Nutrition, vol. xvii. p. 670). (5) Centre for voice, regulating to some extent, through the sterno-cleido-mastoid muscle, the emission of air through the glottis in expiration and phonation. (6) Centre influencing glycogenesis, probably by the action of the vaso-motor centre on the bloodvessels of the liver (see Nutrition, vol. xvii. p. 682). (7) Centre directly influencing salivary secretion, from which originate those fibres of the facial, forming the chorda tympani and lesser superficial petrosal, distributed to the salivary glands (see Nutrition, vol. xvii. p. 672). (8) Centre for the motor fibres supplying the face and muscles of mastication. These exist in the facial for the muscles of the face and in the motor portion of the fifth for the muscles of mastication. Further, the medulla receives nervous influences from the higher centres, by which all the centres above enumerated may be more or less influenced.

Rons Varolii.— The pons Varolii is above and in front of the medulla oblongata, and between the hemispheres of the cerebellum. It consists of fibres passing in two directions, viz., longitudinally, connecting the brain above with the medulla and cord below; and transversely, connecting the lateral hemispheres of the cerebellum, thus forming the middle peduncles of that organ. Its general position and appearance are seen in fig. 23. Mixed up with these fibres are various nuclei of grey matter connected with the roots of cranial nerves. The most important of these nuclei are—(1) the nucleus of the facial nerve; (2) the motor nucleus of the fifth nerve; (3) the upper sensory nucleus of the fifth nerve;

(4) the inner or chief nucleus of the auditory nerve; (5) the outer or superior nucleus of the auditory nerve; (6) the accessory nucleus of the auditory nerve; (7) the nucleus of the sixth nerve. It will be observed that several of these nerves are also connected with nuclei in the medulla oblongata. Like the cord and medulla, the pons is to be regarded as a conductor of impressions and probably also as a reflex centre. Motor transmission occurs chiefly in the anterior part. As the fibres of the facial nerve decussate in the pons, and then carry influences outwards, unilateral injury or disease of the pons may cause paralysis of the face on the same side as the disease, and paralysis of the limbs on the opposite side if the disease has affected the facial before its decussation in the pons. Usually in cases of paralysis of one side (hemiplegia) from a clot or disorganization in one corpus striatum the paralysis of the face is on the same side as that of the limbs. In diseases of the pons loss of sensibility is a much more rare result than loss of motion, and is always on the opposite side. According to Brown-Séquard, tactile, thermal, and painful impressions pass through the central part of the pons. The numerous centres in the pons are associated in complex reflex movements. Nothnagel has described it as a convulsive centre, because irritation caused severe cramps, but this was no doubt due to irritation of the motor strands passing through it.

Cerebral Peduncles.— These contain both sensory and motor fibres, and they establish a connexion between the cerebellum and the cerebrum, and also between the ganglia at the base,—corpora striata, optic thalami, and corpora quadrigemina on the one hand, and the pons and medulla on the other. Little is known of their functions except that they are conductors. Destruction of one peduncle causes the animal to move to the side opposite the lesion, describing a circle somewhat in the manner of a horse in a circus. Irritation may cause pain or movements of various groups of muscles.

Basal Ganglia.— As already shown in tracing the development of the cerebro-spinal system, the brain consists of a series of ganglia, in pairs, more or less overlapped by the cerebral hemispheres. These ganglia, termed the “basal ganglia,” are usually held to include, from behind forwards, the corpora quadrigemina, the optic thalami, and the corpora striata; but in addition there are bodies meriting an equal amount of attention, inasmuch as they cannot be regarded as belonging to the cerebral hemispheres. These are the locus niger, the red nucleus of the tegmentum, and the corpora geniculata; but we have no knowledge of their functions. No doubt all these ganglia act along with the cerebral hemispheres, so that practically the whole mass forms one organ.

Corpora Quadrigemina. — These are two pairs of rounded bodies found above the Sylvian aqueduct, which passes between the third and fourth ventricles. They are situated behind the optic thalami, and are intimately related to the crura, and through these to the pons, medulla, and cord. Homologous with the optic lobes seen in the brain of the fish, frog, and bird (see figs. 17 and 18), and in marsupials and monotremes, their relative size to the mass of the encephalon is much less in the brain of man and of the higher animals. These bodies contain grey matter, covered by a thin stratum of white matter. The two posterior bodies are probably connected with the cerebellum by the superior peduncles of that organ; at all events these peduncles disappear under the base of the corpora quadrigemina. The two posterior bodies are also related to the crura cerebri by the prominences on the sides of the crura known as the inner geniculate bodies. Both anterior and posterior bodies, more especially the anterior, are connected with the optic tracts, and finally, the two anterior bodies unite with the optic thalami. (See fig. 24 below; also, plate XVIII., vol. i., fig. 1, g, g.) As shown by their anatomical connexions, the corpora quadrigemina are part of the mechanism of vision. Destruction causes immediate blindness. If, in a pigeon, the encephalon be removed with the exception of these bodies, the iris will still continue to contract on the influence of light. On then destroying one of these bodies, the iris is immobile, and the power of accommodation is lost. As the third cranial nerve (which is known to contain fibres controlling the circular fibres of the iris by which the pupil contracts, and the fibres governing the ciliary muscle by which the eye is accommodated or focused to varying distances) originates in the grey matter of the floor of the Sylvian aqueduct, close to the corpora quadrigemina, it is held that these bodies are the centres of the reflex movements of the iris and of the ciliary muscle. The corpora quadrigemina are also the first recipients of visual impressions. When light falls on the retina changes are there induced which stimulate the optic nerve-fibres, and these fibres carry impressions through the optic tracts to the corpora quadrigemina. What then occurs is matter of conjecture. Whether sensation is there excited, or whether to produce sensation it is necessary that the impulses be sent onwards to the cerebrum, or whether the impressions directly received from the retina may excite, through the corpora quadrigemina and adjacent ganglia, reflex movements (like those of the somnambulist, who may see so that his steps are taken rightly, but who may at the same time not see consciously), are all speculative questions. We know that these bodies are concerned in the movements of the iris and of the ciliary muscle, but their great proportionate size in lowly-formed brains indicates that this is probably a secondary function, and that they are largely concerned in the phenomena of consciousness of light and colour.

Optic Thalami. — These are two ganglionic masses placed behind the corpora striata and in front of the corpora quadrigemina. The internal surfaces are seen chiefly in the third ventricle, the upper surfaces in the same ventricle and the lateral ventricles (see vol. i. pp. 875, 876, figs. 74 and 75), whilst the external and under surface of each thalamus is united with other parts of the brain. The under surface receives fibres from the crus cerebri, whilst the upper surface is covered by fibres which diverge and pass between the thalamus and a mass of grey matter in the extra-ventricular portion of the corpus striatum (called lenticular nucleus), to form a white layer called the “internal capsule.” From the internal capsule, which thus contains fibres from the optic thalamus, fibres radiate outwards to the surface of the cerebral hemispheres. The under surface of the thalamus is connected with the tegmentum,— that is, with the layer of fibres forming the upper surface of the crus cerebri. They also receive fibres from the corpora quadrigemina, and according to some authorities from the superior peduncles of the cerebellum. The substance of the thalamus contains nervecells, scattered and also aggregated into two nuclear masses, but the relations of these to nerve - tracts have not been ascertained; It is important physiologically to notice that the thalami receive fibres from the back of the crura, and therefore are probably related to the posterior or sensory portion of the spinal cord.

There is still much uncertainty as to the functions of the optic thalami. The most commonly received opinion is that they are centres for the reception of peripheral impulses, which they may elaborate and transmit forwards to the corpora striata, or directly to the cerebral hemispheres. If the sensory impulses received by the optic thalami are sent to the corpora striata, and by these transmitted downwards and outwards through the crura cerebri, then reflex actions may occur in which the basal ganglia are the centres; but, if the impulses are sent up, in the first place, to the cerebral hemispheres, and by these transmitted down to the corpora striata, then the action must include the higher mechanism of the grey matter of the hemispheres. In the first case it is supposed by those who hold that consciousness is specially connected with the grey matter of the hemispheres that the action would be purely reflex and unconscious. Experiment has not thrown much light on this problem, owing to the deep-seated situation of these bodies rendering the results of operative interference untrustworthy. The little that has been done shows that injury to them does not cause paralysis of motion. Nor can it be said that such injuries cause loss of sensation, the only phenomenon observed being that the animal places its limbs in anomalous positions, and does not seem to be aware of having done so. Meynert is of opinion that the optic thalami fulfil the same functions as to tactile impressions—that is, impressions on the periphery of the body—that the corpora quadrigemina do for visual impressions—that is, impressions on the retina. In cases of apoplexy in which these bodies are involved there are always sensory disturbances on the side opposite the lesion. This would lead to the inference that the optic thalami are the sensory ganglia of the opposite sides of the body. They are not, however, the first ganglionic apparatus through which sensory impressions pass, but they probably co-ordinate in some way centripetal impulses before these are sent to the cerebral hemispheres, where they are correlated with feeling. Further, as the old name “optic thalami” indicates, these ganglia are concerned in some way in vision, because, if seriously injured, blindness, or at all events disturbance of vision, is one of the constant results. This favours the view that they are the “middlemen” between special sensory centres and the higher centres of the cerebrum.

Corpora Striata.— These ganglia, sometimes termed the “ganglia of the cerebral hemispheres,” situated in front and on the outer side of the optic thalami, are seen in the lateral ventricles. (See vol. i. pp. 875, 876, figs. 74 and 75.) The greater part of each is imbedded in the white substance of the hemisphere (extra-ventricular portion), whilst the part seen in the floor of the lateral ventricle is called the intra-ventricular portion. Each of these contains a nucleus of grey matter, the nucleus caudatus in the intra-ventricular and the nucleus lenticularis in the extra-ventricular. The latter is separated internally from the intra-ventricular portion by a layer of white matter called the “internal capsule,” whilst on the outer side there is another layer of white matter called the “external capsule,” beyond which, again, is a lamina or web of grey matter, called the “claustrum,” which separates the external capsule from the island of Reil. The internal capsule is of great importance inasmuch as it is continuous with the crusta, a portion of the crus cerebri, which, in turn, is a continuation of the pyramidal fibres of the medulla oblongata and the pons. Multipolar nervecells are found in the nucleus caudatus; in the claustrum the cells are small and spindle-shaped. Posteriorly, therefore, the corpus striatum is related by fibres with the optic thalamus; inferiorly, through the internal capsule, with the pyramidal portion of the medulla and cord; and externally and superiorly with the grey matter of the cerebrum. The corpus striatum is a centre for the co-ordination of centrifugal or motor impulses. It may be roused into activity by impressions reaching it directly from the optic thalamus, but probably it usually acts in obedience to impulses coming from the cerebral hemispheres. When a clot of blood is formed in, say, the right corpus striatum there is motor paralysis of the opposite side of the body, and, according to the size of the clot, the paralysis may affect more or less completely the different groups of muscles. Destruction of the two bodies destroys voluntary movement, but the animal may move forwards as in running. Destruction of the nucleus caudatus renders movements of progression impossible, and the animal performs movements of rotation. Nothnagel by injecting a minute drop of a solution of chromic acid destroyed the nucleus lenticularis of a rabbit, with the result of throwing the animal into complete unconsciousness. He also states that in the corpus striatum of the same animal there is a point, the nodus cursorius, the excitation of which caused the rabbit to rush forwards. This observation agrees with the statement of Magendie that, when he injured the corpora striata, the animal seemed to have an irresistible propulsion forwards. Ferrier states that when the corpora striata were stimulated by an interrupted current convulsive movements of the opposite side of the body took place; and when the current was powerful the side of the body opposite to the side of the brain stimulated was forcibly drawn into an arch.

Cerebellum.— In connexion with the physiology of this organ it is important to note its connexions with the rest of the cerebrospinal axis. It has three peduncles: (1) the superior peduncles (see fig. 24)—crura ad cerebrum, or processes ad testes—together with the valve of Vieussens, connect the cerebellum to the cerebrum; (2) the inferior peduncles, or crura ad medullam, are the superior extremities of the restiform bodies; (3) the middle peduncles, or çrura ad pontem, much the largest, are the lateral extremities of the transverse fibres of the pons Varolii. They act as commissural fibres for the hemispheres of the cerebellum. All these peduncles pass into the interior of the cerebellum at its forepart. In the interior of the organ, where the peduncles enter, we find a nucleus of grey matter, the corpus dentatum. The cortical substance consists of two layers,—an outer molecular layer, consisting of a delicate matrix containing a few round cells and fibres, and an inner or granule layer, containing granules or nucleated corpuscles closely packed together. The corpuscles are from 17 ⅛ τ th to ∙2√0th of an inch in diameter, and are mixed, with a network of delicate nerve-fibres. At the junction of the granular layer with the molecular layer there are peculiar large cells called “Purkinje’s cells.” They are flask-shaped and about ι ⅞ σ th to T5¼th of an inch in diameter, and the long process is directed towards the surface of the cerebellum (see fig. 25). The white centre of each lamina consists of delicate nerve-fibres, the terminations of which have not been satisfactorily made out. Probably they end in the plexus of nerve-fibres in the granule layer, or in the processes of Purkinje’s cells. On comparing the section of cerebrum (fig. 28) with that of cerebellum (fig. 25) the contrast is striking. The structure of cerebellum is more like that of the retina (vol. i. p. 888, fig. 78) than of any other nerve-centre.

Results of Experiments. — The cerebellum is insensible to mechanical excitations. Puncture causes no indications of pain, but there may be twisting of the head to the side. Ferrier states that Faradaic irritation causes movements of the eyeballs and other movements indicative of vertigo. Section of the middle peduncle on one side causes the animal to roll rapidly round its longitudinal axis, the rotation being towards the side operated on.

If the cerebellum be removed gradually by successive slices—an operation easily done in a pigeon—there is a progressive effect on locomotive actions. On taking away only the upper layer there is some weakness and a hesitation in gait. When the sections have reached the middle of the organ the animal staggers much, and assists itself by its wings in walking. The sections being continued further, it is no longer able to preserve its equilibrium without the assistance of its wings and tail; its attempts to fly or walk resemble the fruitless efforts of a nestling, and the slightest touch knocks it over. At last, when the whole cerebellum is removed, it cannot support itself even with the aid of its wings and tail; it makes violent efforts to rise, but only rolls up and down; then, fatigued with struggling, it remains for a few seconds at rest on its back or abdomen, and then again commences its vain struggles to rise and walk. Yet all the while sight and hearing are perfect. See fig. 26. It attempts to escape, and appears to have all its sensations perfect. The results contrast very strongly with those of removing the cerebral lobes. “Take two pigeons,” says Longet; “from one remove completely the cerebral lobes, and from the other only half the cerebellum; the next day the first will be firm on its feet, the second will exhibit the unsteady and uncertain gait of drunkenness.”

There is thus a loss of the power of co-ordination, or of regulation of movement, without the loss of sensibility, and hence it has been assumed that in some way or other the cerebellum acts as the co-ordinator of movements.

Co-ordination of Movement.— The nervous mechanisms by which movements are co-ordinated—that is, adapted to specific ends—are not thoroughly understood, but a short description of what is known may be here given. Muscular movements may be either simple or complex. In winking, the movement of the eyelid is effected by two muscles, one bringing the lid down, the other raising it. But picking up a pen from the table, taking a dip of ink, and writing a few words involve a complicated set of movements of the muscles of the trunk, shoulder, arm, forearm, fingers, and thumb. To perform the movements with precision each muscle or group of muscles must act at the right time and to the proper amount. It is also clear that all this is accomplished automatically. We are not conscious of the requisite combinations; but it must be noted that many of these complicated movements are first acquired by conscious efforts, and that they become automatic only by repetition. Again, in walking, equilibrium is maintained by a delicate series of muscular adjustments. When we swing forward one leg and balance the body on the other many muscular movements occur, and with every change in the position of the centre of gravity in the body there are corresponding adjustments. It would appear that in all mechanisms of co-ordination the first part of the process is the transmission of sensory impressions from the periphery. These sensory impressions may be derived from the skin or muscles, and may be caused by variations of pressure arising in them. Thus, if we lift a heavy weight, as a large stone, by the right hand and raise it to the bend of the elbow we throw the body to the other side by the action of the muscles of that side, thus maintaining the equilibrium. We judge of the amount of force necessary to overcome an obstruction by th⅛ feeling of resistance we encounter. All the movements of the body, therefore, give rise to feelings of varying pressures, and these feelings regulate the amount or degree of muscular action necessary to maintain equilibrium, or to perform a requisite movement. This is at first a conscious experience, and a child has to pass through an education, often involving pain, before the nervous mechanisms become automatic and the movement is done without effort. But the ordinary sensory nerves, coming from skin and muscle, are not the only channels by which such guiding mechanisms are set in action. As one would expect, sensory impressions, such as those associated with sight and hearing, may be brought into play.

1. Peripheral Impressions from Semicircular Canals.— If the membranous portion of the horizontal semicircular canal in the internal ear of a pigeon be cut, the bird moves its head from side to side, and if one of the vertical canals be divided it moves the head up and down. The effects may pass off in a few days if only one canal has been cut. If the canals on both sides be divided the movements are exaggerated and the condition becomes permanent. It will then be observed that the animal has lost the power of coordinating its movements. It can rest with only a twitching, perhaps, of the head, but if it attempt to fly or walk its movements are indefinite and irregular, like those of a dizzy person, or like those described as following injury to the cerebellum. The irregular movements do not arise from deafness, or noises in the ears, or partial paralysis, or from an uncontrollable impulse. Any strong sensory impression, such as holding the bird, supporting its beak, or holding a brilliant light before its eyes, will steady it, and it has been noticed that “it can, even without assistance, clean its feathers and scratch its head, its beak and foot being in these operations guided by contact with its own body.’’ It has been supposed that the semicircular canals are concerned in the mechanism of equibration, —a view urged chiefly by Mach and Crum-Brown. If a blindfolded man is seated on a horizontal rotating table, such as that used in a lighthouse for rotating or eclipsing the light, and the table is turned round, at first there is a sensation of movement in the same direction as that of the table; then this sensation fades away, until he has no sensation of movement, although the table may be rapidly rotating; finally, if the table be stopped without a jerk there is first a very short period in which there is no definite sensation, which is succeeded by a sense of rapid movement in the opposite direction, often accompanied by a feeling of nausea. Now it is evident that neither sight nor touch nor muscular sensations can give a sense of rotation in these circumstances, and yet it is possible to form a fairly accurate judgment of the angle through which the body has moved. It has been suggested that this is effected by the action of the semicircular canals. The membranous portions are surrounded by a fluid called the “endolymph,” and are free to move through a short distance. Hence it has been supposed that rotations more or less rapid must cause variations of tension of the membranous portion. Thus, if the membranous part, especially the ampulla or dilated ends of the canals, lag behind when rotation in one direction takes place, the nerves ending in them will be subjected to a strain; by and by both membranous portions and surrounding parts will be moving with the same velocity, when there will be no strain and no sense of movement; and at last, when the rotatory movement is arrested, there will be a tendency on the part of the membranous portions, as they are floating in a fluid, to move on a little farther than the rest, and thus again produce a strain, causing a sensation of movement in the opposite direction. By similar· reasoning it can be shown that if we take the peculiar position of the three canals into consideration any movement in space might be thus appreciated, and these appreciations enter into the judgment we form of the movements. According to this theory, the sense of equilibrium may be largely due to impressions derived from the position of the head, and, as muscular movements required for placing the body in definite positions are determined, as we have seen, by peripheral impressions, the irregular movements of the pigeon, after injury to these canals, may be accounted for. It is not improbable that in the bird, which from the structure of its extremities—feet and wings—can have no peripheral impressions so delicate as those derived from the papillae of the skin on the extremities of other animals, the sense of equilibrium is maintained chiefly by impressions from these canals, and this may account for the comparatively large size of these organs in birds and fishes. This is in correspondence also with the requirements of birds in the balancings of flight and of fishes in swimming. It is well known that disease or injury of these canals in the human being produces symptoms of vertigo and a diminution of the power of co-ordinated action, as in Menière’s disease, showing that the canals, even in man, have similar functions to those in the bird.

2. Peripheral Impressions from the Eye.— Many movements are guided and controlled by the sense of vision. Simply blindfolding a bird usually makes it passive, and it will not attempt either to walk or to fly; the same effects to a less degree may be seen in a mammal; and a blindfolded man will stagger in his gait. The wonderfully accurate movements of the blind in walking are acquired by long and laborious effort, and are guided by the sensations of hearing, of touch, and of resistance. If the optic lobes of a frog be destroyed, its power of balancing itself is lost. There are thus at least three channels by which peripheral impressions pass to the centres and seem to guide or co-ordinate movement: (1) from the periphery, by nerves of ordinary sensibility arising in the skin, muscles, and viscera; (2) from the semicircular canals of the ear, by special nerve-fibres in the auditory nerve; (3) from the eye, by fibres of the optic nerve. How and where these skeins of sensitive impressions are gathered up and so arranged as to call forth the requisite movements can only be conjectured; but the cerebellum is the organ most likely to be concerned in such a mechanism. It is in organic connexion with many of the nerve-fibres conveying sensory impressions. By the restiform bodies it receives many of the sensory fibres of the spinal cord; the auditory nerve has roots intimately related to the cerebellum; and it is fair to assume that there are communications between the corpora quadrigemina and the cerebellum. Stimulation of the cerebellum causes movements of the eyeballs, and disease of the cerebellum is sometimes attended by blindness. How the cerebellum co-ordinates movement is quite unknown, and the difficulty in explaining its functions is not lessened by the clinical fact that extensive disease of this organ may exist without any appreciable sensory or motor disturbance. There is no evidence to support the view of the founders of phrenology that the cerebellum has to do with the sexual functions.

Cerebral Hemispheres.— As these have been fully described in vol. i. p. 873, it is only necessary here to point out the anatomical facts that assist in explaining the functions of the organ. It is important to observe, first, the general arrangements of the fibres, and, secondly, the arrangement and structure of the grey matter. The white matter of the cerebrum consists of ascending or peduncular fibres, longitudinal or collateral fibres, and of transverse or commissural fibres.

(1.) Peduncular Fibres.— The crusta of the cerebral peduncles consists of bundles of longitudinal fibres derived mainly from the anterior pyramid of the medulla. The crust is quadrilateral in form, but in ascending to the hemispheres it becomes flattened from above downwards, so that the fibres spread out like a fan. The fan formed by these fibres is bent into the form of an incomplete hollow cone, the convex surface of which is directed upwards and inwards. Thus the fibres pass between the optic thalamus and the lenticular nucleus, forming the internal capsule. Higher up the fibres pursue their course beneath and to the outside of the thalamus and the caudate nucleus, and over the lenticular nucleus. “Still higher up the internal capsule has spread out' from before backwards, while the anterior half forms an obtuse angle with the posterior. The angle where the halves meet is called the knee (fig. 27, K), while the divisions themselves are called the anterior (fig. 27, IK , ) and posterior segments of the internal capsule” (Ross). On emerging from the basal ganglia the fibres of the internal capsule radiate in all directions to reach the cortex of the brain, giving rise to the appearance called the “corona radiata.” The following sets of fibres have been traced into connexion with the cerebrum.

(a) Sensory peduncular fibres, derived from the posterior root-zones and the columns of Goll. These are in connexion with the cerebellum; but, as shown by Meynert, Flechsig, and others, many pass up through the pons to reach the crus cerebri, occupying the posterior and external portion of the pyramidal tract. They do not appear to be connected with the optic thalamus and the lenticular nucleus, but pass between them to the cortex. (6) Fibres from the roots of the optic nerves, reaching the brain by what have been called the “optic radiations of Gratiolet.” This bundle of fibres issues from the posterior and external border of the optic thalamus and is closely applied to the peduncular sensory tract in its passage through the internal capsule, and the fibres seem to be connected with the convolutions of the occipital lobe. It is important to note that at least one of the roots of the optic nerve (the internal) passes into the external geniculate body and thence into the anterior bodies of the corpora quadrigemina. Thus there is a path for the passage of impressions from the retina to the cortex of the brain, (c) Fibres from the olfactory lobes have been traced to a junction with the optic radiations of Gratiolet, and pass with them to the convolutions of the cortex of the occipital or temporo-sphenoidal lobe, (d) The pyramidal tract has already been traced through the spinal cord, medulla, and pons. It then goes on, as already seen, to form the internal capsule and the corona radiata, the fibres ending in the parietal lobule, the paracentral lobule, the superior extremities of the ascending frontal and parietal convolutions, probably the posterior extremity of the first frontal convolution, the posterior extremity of the third frontal, and the inferior extremities of the ascending frontal and parietal convolutions,—in short, in the convolutions forming the middle and parietal region of the surface of the cortex, (e) Fibres issuing from the external surface of the optic thalamus to join the internal capsule. These are distributed to the convolutions of the frontal and parietal lobes. (∕) Fibres issuing from the external surface of the caudate nucleus, passing also into the corona radiata, (g) Fibres issuing from the superior and internal surface of the lenticular nucleus to join the ascending fibres of the internal capsule. (⅛) Fibres ascending from the superior peduncle of the cerebellum. Flechsig says that some of the fibres of the superior peduncle of the cerebellum of the opposite side pass uninterruptedly through the red nucleus and along the internal surface of the fibres of the pyramidal tract to be distributed to the central convolutions of the cerebrum, (i) Fibres issuing from the corpus callosum and descending into the internal capsule, (λ) Fibres of the external capsule which ascend from the crusta and ultimately reach the cortex through the corona radiata.

In addition to the peduncular fibres above enumerated, all of which belong to what may be termed the system of the internal capsule and corona radiata, fibres from the fornix, tenia semicircularis, outer layer of septum lucidum, and the fillet of the crus also pass from below upwards to the cortex of the hemispheres (Ross).

(2.) Longitudinal or Collateral Fibres.— (a) Fibres running immediately below the surface of the cortex? and connecting the grey matter of adjacent convolutions, (ά) Fibres in the gyrus fornicatus, a convolution immediately above the corpus callosum. It is said that bands of these fibres arise in the anterior perforated space and pass completely round the corpus callosum to end in the same perforated space, and that offsets of these fibres pass upwards and backwards to reach the summits of the secondary convolutions derived from the gyτus fornicatus near the longitudinal fissure, (c) Longitudinal fibres of the corpus callosum (nerves of Lancisi), connecting the anterior and posterior ends of the callosal convolution. (<i) Longitudinal septal fibres, lying on the inner surface of the septum lucidum, and entering into the gyrus fornicatus, (e) The fasciculus uncinatus, passing across the bottom of the Sylvian fissure and connecting the convolutions of the frontal and temporo-sphenoidal lobes. (∕) The longitudinal inferior fasciculus, connecting the convolutions of the occipital with those of the temporal lobe.

(3.) Transverse or Commissural Fibres.— (a) Many, if not all, of the fibres of the corpus callosum pass transversely from one side to the other and connect corresponding convolutions in the hemispheres. This is the generally accepted view; but Professor Hamilton of Aberdeen has recently stated that his preparations show that there is no such commissural system between convolutions, and that the fibres decussating in the corpus callosum are not continued to convolutions on the other side, but pass downwards. (⅛) The fibres of the anterior commissure wind backwards through the lenticular nuclei to reach the convolutions round the Sylvian fissure, (c) The fibres of the posterior commissure run through the optic thalami. Arrangement and Structure of Grey Matter.— The grey matter in the medulla and basal ganglia has been already considered. A web or sheet of it is also thrown over the surface of the cerebrum, and forms the outer portion of all the convolutions. The cortical substance consists of cells and fibres imbedded in a matrix similar to the neuroglia of the spinal cord. It may be divided into five layers, which merge into each other by almost insensible gradations. The most external layer consists of delicate nerve-fibres, neuroglia, and a few small round cells destitute of processes (see fig. 28). Going deeper we find cells of a characteristic pyramidal form, the largest being in the deepest layer. Their bases are turned inwards, and their apices towards the surface of the convolution. Cleland states that fibres passing from the apices are continuous with the delicate fibres found on the very surface of the cortex. In the ascending frontal convolutions Betz and Mierzejewski have found pyramidal cells two or three times larger than those of other regions of the cortex, and these have been termed “giant-cells.” All the pyramidal cells, no doubt, anastomose by their processes, and give origin to the nerve-fibres of the white substance, but it is rarely possible to trace the fibres from cell to cell. A consideration of these anatomical facts, along with those mentioned in connexion with the comparative anatomy of the brain, shows that the cerebral hemispheres are in intimate connexion by fibres with all the other portions of the cerebrospinal system. Further, they are not only intricate in structure themselves, but the commissural sets of fibres indicate that there is harmony of function between one part and another. In determining the function of so complicated an apparatus recourse must be had to the evidence (1) of development, (2) of comparative anatomy, (3) of human anatomy, (4) of the observed effects of disease before and after death, and (5) of experiment. Facts have already been collected from the first three of these fields of inquiry, all tending to show that the grey matter of the hemisphere is associated with the manifestation of intelligence in its various forms. The phenomena of disease support the same conclusion. Diseases producing slow changes in the layer of grey matter on the cortex are invariably associated with mental disturbance, such as melancholia, mania, or dementia. If the grey matter be suddenly injured or submitted to compression, as by a blow causing fracture and depression of a portion of the skull, or the effusion of fluid consequent on inflammation, unconsciousness is a certain result. So long as the pressure continues there is no consciousness; if it be removed, consciousness may soon return. On the other hand, if the disease affect the white matter of the central portions or the ganglia at the base, there may be paralysis or convulsions without consciousness being affected. All the facts, therefore, of pathology relating to the brain indicate that the grey matter on the surface of the hemispheres is the organ of consciousness and of all mental operations. This statement is now an axiom of medical science, and the basis of the rational treatment of the insane and of all maladies of the central nervous organs?

Two methods of experiment upon the cerebrum have usually been followed, and both have yielded important results.

(a.) Removal.— Flourens and the older observers were aware of the fact that as successive slices of grey matter are removed from the surface of the cerebrum an animal becomes more dull and stupid, until at last all indications of perception and volition disappear. A pigeon in this condition (see fig. 29), if carefully fed, may live for many months; to quote from Dalton —

“The effect of this mutilation is simply to plunge the animal into a state of profound stupor, in which it is almost entirely inattentive to surrounding objects. The bird remains sitting motionless upon his perch or standing upon the ground, with the eyes closed and the head sunk between the shoulders. The plumage is smooth and glossy, but is uniformly expanded by a kind of erection of the feathers, so that the body appears somewhat puffed out, and larger than natural. Occasionally the bird opens its eyes with a vacant stare, stretches its neck, perhaps shakes its bill once or twice, or smooths down the feathers upon its shoulders, and then relapses into its former apathetic condition.”

Similar observations have also been made on reptiles and mammals, but the latter survive the operation for a comparatively short time. In watching such an animal it is difficult to divest one’s mind of the belief that it still feels and sees and hears. It may be observed that it rarely makes movements unless stimulated from without Thus it may remain motionless for many hours; but if pushed, or gently touched, it moves. As remarked by Prof. Μ. Foster—

“No image, either pleasant or terrible, whether of food or of an enemy, produces any effect on it, other than that of an object reflecting more or less light. And, though the plaintive character of the cry which it gives forth when pinched suggests to the observer the existence of passion, it is probable that is a wrong interpretation of a vocal action; the cry appears plaintive, simply because, in consequence of the completeness of the reflex nervous machinery and the absence of the usual restraints, it is prolonged. The animal is able to execute all its ordinary bodily movements, but in its performance nothing is ever seen to indicate the retention of an educated intelligence.”

(b.) Electrical Stimulation of Surface of Brain.— It is remarkable that, although many of the early workers in cerebral physiology stimulated the surface of the brain by electric currents, they observed no effect, and therefore Magendie, Matteucci, Longet, Weber, Budge, Schiff, and others taught that irritation of the surface of the hemispheres called forth no muscular movements; and it was generally accepted that the grey matter on the cortex of the brain was entirely concerned in the phenomena of sensation, volition, and intellectual action. During the Franco-German war in 1870 Hitzig had occasion to apply galvanism to a portion of the exposed brain of a wounded soldier, and he observed contractions of the muscles of the eyeball. When peace was restored, experiments were made on the lower animals by Hitzig and Fritsch, in which a portion of exposed brain was irritated by a continuous current, and it was observed that the phenomena took place on opening and closing the current. By these experiments the German observers discovered that, when certain areas of grey matter were stimulated, contractions of certain muscles occurred, and they were thus able to map out areas for grotιps of muscles. Immediately afterwards the research was taken up by Professor David Ferrier of King’s College, London, who, using a Faradaic instead of a continuous current, greatly extended the field of inquiry, and obtained many important results, which are not only of value in cerebral physiology but have been successfully applied to the diagnosis of various diseases of the nervous system. The motor areas as determined by Ferrier in the monkey are shown in fig. 30. Dr Ferrier has also indicated the corresponding motor areas in man by carefully comparing the convolutions with those of the monkey. An inspection of the figures shows that the areas which, when stimulated, give rise to definite movements are distributed only over a part of the cortex. As stimulation gives rise to no movements over other regions of the brain, these have been assumed to be connected with psychical states, such as sensatiori, volition, &c. Much controversy has arisen as to the real nature of these so-called “motor areas.” It has been clearly ascertained that the effects are not due to diffusion of the electric currents influencing other parts of the brain. That there is to some extent such diffusion between the electrodes there can be no doubt, but the exact correspondence between the area stimulated and the movements produced, and the fact that shifting the electrodes a very short distance to one side or another is followed by different results, show that the effect is somehow owing to changes excited by the electric current in that particular area of grey matter. Hitzig, Ferrier, and others have also found that removal of the layer of grey matter of a “motor centre” is followed by enfeeblement of the movements assigned to the area, but in the course of a few days the paralytic symptoms disappear. The latter effect cannot be due to the corresponding centre on the opposite side taking up the work “as subsequent destruction of the latter produced the usual paralysis on the side opposite to the lesion, but did not cause a repetition of the paralysis on the side opposed to the first lesion” (Carville and Duret). It would appear, therefore, that after destruction of a centre on one side some other part of the same hemisphere may take up the functions of the destroyed part. Goltz of Strasburg has removed large portions of the grey cortex (even to the extent of almost the whole of one hemisphere) by a jet of water so as to avoid haemorrhage, and still recovery of motor power took place after a time, although there remained “clumsiness in the execution of certain movements.” His opinion is that the paralytic phenomena are caused by the injury exciting an inhibitory action on lower centres. This view, substantially that advocated for many years by Brown-Séquard does not explain why it is that gentle irritation of the centre by a weak Faradaic current calls forth movements of a definite character. The evidence, therefore, is strongly in favour of the view that there are definite motor areas of grey matter on the cortex,—that is, in ordinary circumstances these areas are intimately related to specific muscles or groups of muscles. It is quite possible, however, that each group of muscles does not depend on one area alone, but on several, whilst it is more intimately related to one than to the others. This would account also for the fact that movements of a group of muscles may be excited by stimulation of other areas than those mapped out by Ferrier and Hitzig. Recently areas associated with definite movements of the thorax, abdomen, and pelvis have been discovered by Horsley and Schafer, and thus almost all the muscular mechanisms have been connected with some of the cerebral convolutions.

Ferrier has also attempted to differentiate sensory centres. On stimulating the angular gyrus he obtained movements of the eye and associated movements of the head, and he regarded the phenomena as being “merely reflex movements on the excitation of subjective visual sensation.” He then found that, “when the angular gyrus of the left hemisphere was destroyed, the animal was blind on the right eye soon after the operation, but recovered sight completely on the following day.” On destroying the angular gyri of both hemispheres, an animal became permanently blind in both eyes. In neither case was there motor paralysis. By similar processes of thought and experiment he placed the auditory centre in the superior temporo-sphenoidal convolution, the centres of taste and smell at the extremity of the temporo-sphenoidal lobe, and that of touch in the gyrus uncinatus and hippocampus major. On the other hand, Goltz asserts that even after removal of a considerable part of the cortex the animal is not actually blind, but suffers from an imperfection of sight; and he states that he “can no more obtain distinct evidence of localization in reference to vision or other sensations than in reference to movements.” Ferrier’s view is supported by the observations of Munk, who finds that destruction of a considerable portion of the occipital lobes causes blindness. Munk has put forth the important distinction that there may be blindness in the sense of total deprivation of vision, and “psychical blindness,” or the “inability to form an intelligent comprehension of the visual impressions received”; and he supposes that the grey matter of the cortex over the occipital lobes has to do with the elaboration of simple visual impressions into perceptions. In like manner he concludes that other parts of the cortex may have to do with the elaboration of tactile, olfactory, gustatory, and auditory sensations. This is a likely hypothesis, and not very dissimilar to what has been held for many years, the only novelty being that there is localization in these actions. At present the question cannot be regarded as settled; but it may be stated generally that the posterior portion of the brain has to do chiefly with the reception of sensory impressions, and the middle and lateral regions with the transmission outwards of motor impulses. But there still remains the anterior portion. Electrical irritation of the prae-frontal region of the cortex in the monkey causes no -motor reaction. Complete destruction causes no paralysis of motion and no sensory disturbance. Dr Ferrier states :—

“Removal or destruction by the cautery of the antero-frontal lobes is not followed by any definite physiological results. The animals retain their appetites and instincts, and are capable of exhibiting emotional feeling. The sensory faculties—sight, hearing, touch, taste, and smell—remain unimpaired. The powers of voluntary motion are retained in their integrity, and there is little to indicate the presence of such an extensive lesion or a removal of so large a part of the brain. And yet, notwithstanding this apparent absence of physiological symptoms, I could perceive a very decided alteration in the animal’s character and behaviour, though it is difficult to state in precise terms the nature of the change. The animals operated on were selected on account of their intelligent character. After the operation, though they might seem to one who had not compared their present with the past fairly up to the average of monkey intelligence, they had undergone a considerable psychological alteration. Instead of, as before, being actively interested in their surroundings, and curiously prying into all that came within the field of their observation, they remained apathetic or dull, or dozed off to sleep, responding only to sensations or impressions of the moment, or varying their listlessness with restless and purposeless wanderings to and fro. While not actually deprived of intelligence, they had lost to all appearance the faculty of attentive and intelligent observation” {Functions of the Brain, 1st ed., p. 231).

Thus the frontal lobes appear to have to do with cognition and intellectual action. If so, the grey matter on the surface of the brain may be mapped out into three great areas—an area concerned in cognitions and volitions in front, a motor or ideo-motor area in the middle, and a sensory area behind. These distinctions are no doubt arbitrary to a considerable extent; but, if they are retained as the expressions of a working hypothesis, they are of service. Long ago, and prior to the researches above alluded to, Dr Hughlings Jackson pointed out that disease of certain areas of grey matter on the cortex of the hemispheres may occasion epileptiform convulsions, localized to particular groups of muscles. The theory of the localization of motor functions has been of great service in the diagnosis and prognosis of such diseases. As to the localization of the faculty of language in the third left frontal convolution, founded on pathological evidence, see Aphasia, vol. ii. p. 171.

The functions of the nervous system have now been described; but they are so complicated and so closely related to each other as to make it no easy matter to form a conception of the system working as a whole. The progress of discovery naturally tends to differentiation, and probably to attach too much importance to one organ as compared with the others, so that we are in danger of losing sight of the solidarity of the whole nervous system. Probably every nervous action, however minute and evanescent, affects more or less the entire system, and thus there may be an under-current of nervous action streaming into and out of the nerve-centres, along with a perpetual series of interactions in the centres themselves, contributing to and accounting for the apparent continuity of conscious experience. Certain relations of one nerve-centre to the others are indicated in fig. 31. No one now doubts that consciousness has an anatomical substratum, but the great problem of the relation between the two is as far from solution as in the days when little or nothing was known of the physiology of th∣ nervous system. Consciousness has been driven step by ste] upwards until now it takes refuge in a few thousand nerve-cells in a portion of the grey matter of the cortex of the brain. The ancients believed that the body participated in the feelings of the mind, and that, in a real sense, the heart might be torn by contending emotions. As science advanced, consciousness took refuge in the brain, first in the medulla and lastly in the cortex. But even supposing we are ultimately able to understand all the phenomena —chemical, physical, physiologi cal—of this intricate ganglionic mechanism we shall be no neareι a solution of the problem of the connexion between the objective and subjective aspects of the phenomena. It is no solution to resolve a statement of the phenomena into mental terms or expressions and to be content with pure idealism; nor is it any better to resolve all the phenomena of mind into terms describing physical conditions, as in pure materialism. A philosophy that recognizes both sets of phenomena, mutually adjusted and ever interacting, may be no explanation; but at all events it is unpretentious, recognizes facts, and does not delude the mind by offering a solution which is no solution at all. But apart from the ultimate question there is the important one of whether physiologists are on the whole right in relegating sensation or consciousness entirely to the grey matter of the brain. The facts of comparative physiology are against such an exclusive notion, because we cannot deny consciousness to many animals having rudimentary nervous systems. As already said, research in anatomy and physiology and the observation of disease have driven physiologists to adopt the view that the brain is the organ of sensation. This is no doubt true in the sense that it ultimately receives all those nervous impressions that result in consciousness; but the parts transmitting the nervous impressions are in another sense as much concerned in the production of conscious states as the brain. This view of the matter, put forward by Professor John Cleland in 1870, has not received from psychologists the attention it deserves. His thesis is—

“that the consciousness extends from its special seat so far as there is continuity of the impressed condition; that when an irritation is applied to a nerve-extremity in a finger or elsewhere the impression (or rather impressed condition) travels, as is generally understood, but exists for at least a moment along the whole length of the nerve, and that as soon as there is continuity of the impressed condition from finger to brain the consciousness is in connexion with the nerve and is directly aware of the irritation at the nerve-extremity” (Evolution., Expression, and Sensation, Glasgow, 1881, p. 106).

This view is quite consistent with all the facts of nervous physiology and presents fewer difficulties than the one generally held, which drives consciousness into the recesses of the nerve-cells in the cortex of the cerebral hemispheres. It appears to keep clear of the prevailing error in the philosophy of modern physiology,—that of regarding the body and even the nervous system as a vast series of almost independent organs, losing sight of the community of function and interdependence of parts, characteristic of the body of one of the higher animals.

Circulation in the Brain.

A due supply of healthy arterial blood and the removal of venous blood are essential to cerebral activity. The brain is contained in an osseous case of which the total capacity is variable. The cerebral substance undergoes almost insignificant changes of volume even under a pressure of 180 mm. of mercury. The quantity of blood in the cranium may vary. In the rabbit not more than 1 per cent, of the total quantity of blood of the body (equal to about 5 per cent, of the total weight of the organ) is present at any one time in the brain, whereas in the kidney, by weight, the blood may amount to nearly 12 per cent., and in the liver to as much as nearly 30 per cent. (Foster). If a small round window be made in the cranium and a suitable piece of glass fitted into it, the veins of the pia mater may be observed to dilate or contract if intermediate pressure be made on the veins of the neck. There is evidently, then, within the cranium some arrangement by which such variations become possible. This is probably accomplished by the anatomical arrangements of the sub-arachnoid spaces. These spaces, containing fluid, communicate freely with each other and with the space surrounding the spinal cord, so that when the quantity of blood increases in the cranium a corresponding quantity of fluid escapes into the spinal space, the walls of which are not inextensible like those of the cranium. In young children, before the fontanelles are closed, the variations of circulation and bloodpressure cause pulsations, of which there are two kinds—those coinciding with the ventricular systole, produced by the pulsation of the arteries at the base of the brain, and those coinciding with expiration. Pressure on the brain-substance beyond a limit leads to paralysis, unconsciousness, and death. The large sinuses probably assist in equalizing internal pressure, and, as inspiration favours the flow of blood from the sinuses, too great distension of these is also avoided. Vaso-motor nerves regulate the calibre of the arterioles of the brain, but we know nothing of the conditions affecting the nerves. Nor do we know how the waste-products of the brain are got rid of. There are no lymphatic vessels, but there are spaces around many of the vessels. These probably communicate with the cavities in the membrane containing the cerebro-spinal fluid, the value of which, as suggested by Foster, “depends in all probability more on its physiological properties as lymph than on its mechanical properties as a mere fluid.” The grey matter is much more richly supplied with capillaries than the white matter, as seen in fig. 32.

Cranial Nerves.

The general anatomy of these nerves is described under Anatomy, vol. i. p. 880 sq., and it remains only to enumerate their functions. Their deep roots have also been alluded to in treating of the medulla oblongata and the pons Varolii above.

1. The olfactory nerve. The nerve of smell (see Smell).

2. The optic nerve. The nerve of sight (see Eye).

3. The oculo-motoτ or third nerve, —motor, supplying all the muscles of the eyeball except the superior oblique and external rectus; it also supplies the circular fibres of the iris and the ciliary muscle (see Eye).

4. The pathetic or fourth nerve,— :motor, supplying the superior oblique muscle.

5. The trigeminal or fifth nerve. It has three branches: (A) The ophthalmic division of the fifth, or nerve of Willis, is sensory and supplies (a) the skin of the forehead, the eyebrow, the upper eyelid, the root and lobule of the nose: (ft) the palpebral and ocular conjunctiva, the mucous membrane of the lacrymal passages, the frontal sinuses, the upper part of the nasal mucous membrane; (c) the cornea, the iris, the choroid, and the sclerotic; (d) the periosteum and bones of the frontal, orbital, and nasal regions; and (e) muscular sensibility to the intra-orbital muscles. It also influences the secretion of the lacrymal gland. It contains the fibres from the sympathetic governing the radiating fibres of the iris (see Eye), and also the vaso-motor fibres for the iris, choroid, and retina. It is associated with the otic ganglion. (B) The superior maxillary division of the fifth furnishes sensory branches to (a) the skin of the lower eyelid, alae of the nose, upper lip, and skin covering the malar bone; (&) the mucous membrane of the nasal, pharyngeal, and palatine regions, the maxillary sinuses, the gums, the upper lip, and the Eustachian tube; (c) the periosteum of the bones corresponding to its distribution; and (d) the teeth of the upper jaw. It furnishes filaments to the nasal and palatine glands, and probably to the glands of the velum palati. It contains vaso-motor fibres from the sympathetic for the vessels, and is associated with the spheno-palatine ganglion. (C) The inferior maxillary division of the fifth contains sensory branches to (a) the skin of the cheeks, temples, lower lip, chin, front part of the ear, and external auditory canal; (ft) the mucous membrane of the cheeks, lips, gums, front part of the tongue, the mucous membrane of the tympanum, and the mastoid cells; (c) the periosteum of the lower jaw and temporal bones; (d) the teeth of the lower jaw; (e) the temporo-maxillary articulation; and (∕) the muscles in the neighbourhood (muscular sensibility). It thus exercises an influence on taste, hearing, and secretion (see Nutrition). It contains vaso-motor fibres for the blood-vessels. Its motor branch is distributed to the muscles of mastication; and it is related to two ganglia, the otic and the sub-maxillary.

6. The sixth nerve is motor, and supplies the external rectus muscle of the eyeball only.

7. The facial or seventh nerve is purely motor, and supplies all the muscles of expression. It also contains secretory fibres influencing the action of the salivary glands (see Nutrition). Some assert that the chorda tympani contains gustatory fibres, but the question has not been settled. Claude Bernard found that the facial contains vaso-motor fibres.

8. The auditory or eighth nerve is the nerve of hearing (see Ear). In addition to purely auditory filaments it contains fibres from the semicircular canals having to do with impressions of movement in space (see pp. 38, 39).

9. The glosso-pharyngeal nerve is sensory to (a) the mucous membrane of the posterior part of the tongue (nerve of taste), the pillars of the fauces, the anterior face of the epiglottis, and the tonsils; and (ö) the mucous membrane of the tympanum, the fenestra ovalis and fenestra rotunda (see Ear), the mastoid cells, and the Eustachian tube along with the fifth. It probably supplies motor fibres to the muscles of the pharynx, but this is doubtful. Vulpian states that it contains vaso-dilator fibres for the vessels of the posterior third of the tongue.

10. The pneumogastric or vagus nerve has many complicated actions. (A) It is sensitive to (a) the mucous membrane of all the respiratory passages, including specially the larynx; (⅛) the heart; (c) a portion of the digestive tube, namely, the base of the tongue, the velum palati, the pharynx, oesophagus, stomach, and probably the duodenum; (<Z) it confers muscular sensibility on the muscles to which it is distributed; (e) the mucous membrane of the biliary passages; (∕) a part of the dura mater corresponding to the transverse and occipital sinuses; (g) the posterior part of the auditory canal. By the laryngeal branches it specially stimulates expiratory movements. (B) It is motor to (a) many of the muscles of the palate; (ft) the constrictors of the pharynx; (c) the oesophagus; (cl) the larynx by (a) the superior laryngeal to the cricothyroid muscle and a portion of the arytenoid, and by (∕3) the inferior or recurrent laryngeal to the rest of the muscles of the larynx; and (e) to the muscular fibres of the bronchial tubes. (C) It contains vaso-inhibitory fibres for the heart. In connexion with this organ the vagus also contains sensory fibres and fibres belonging to the depressor system. (D) It influences secretion in the stomach (see Nutrition) ; but it is doubtful whether it has any action on the renal secretion. (E) It influences the production of glycogen in the liver (see Nutrition).

11. The spinal accessory is a motor nerve supplying the sterno-cleido-mastoid muscle and the trapezius. It gives an important branch to the vagus (internal branch), which supplies all the motor fibres in that nerve distributed to the larynx, except those in the superior laryngeal supplying the crico-thyroid muscle, and also the motor fibres of the vagus sent to the pharynx. According to Heidenhain, it also supplies the vagus with the inhibitory cardiac fibres.

12. The hypoglossal is exclusively a motor nerve supplying the muscles of the tongue and also the sub-hyoid muscles (see Anatomy). It also contains vaso-motor fllaments.

Spinal Nerves.

The spinal cord gives origin in its course to thirty-one pairs of spinal nerves, each nerve having two roots, anterior and posterior, the latter being distinguished by its greater thickness and by the presence of an enlargement called a ganglion, in which are found numerous bi-polar cells. The anterior root is motor, the posterior sensory. The mixed nerve after junction of the roots contains (a) sensory fibres passing to the posterior roots; (δ) motor fibres coming from the anterior roots; (c) sympathetic fibres, either vasomotor or vaso-dilator.

Sympathetic System.

The fibres of the sympathetic system consist of two kinds—(1) of grey or gelatinous fibres, destitute of the white substance of Schwann; and (2) of medullated fibres similar to those met with in the cerebro - spinal centres. The grey fibres originate in the ganglia so prevalent in the sympathetic system, whilst the medullated fibres are believed to come from the cerebro-spinal system. The trunk of the great sympathetic nerve consists of a chain of swellings or ganglia, connected by intermediate cords of grey nerve-fibres, and extending nearly symmetrically on each side of the vertebral column, from the base of the cranium to the coccyx. On this part of the nerve twenty-four ganglia are placed on each side. This great trunk, as it passes along the spine, is connected with the spinal nerves, the connecting fibres being of the two kinds already described. The grey fibres dominate in the sympathetic nerves, and the medullated in the cerebro-spinal; and these two elements are mixed in various proportions in both of the great divisions of the nervous system. At their lower extremities the main trunks of opposite sides generally unite in the middle line; and at the upper ends each trunk, after being connected with the eighth and ninth cranial nerves, extends to the cranium, passes into that cavity along with the internal carotid artery, and there, as well as in other situations, comes into connexion with all the remaining cranial nerves, except the olfactory, auditory, and optic. This conjunction may be effected directly, as with the fourth, sixth, and ninth nerves; or through a ganglion, as the ophthalmic, with the third and fifth; the spheno-palatine, otic, and sub-maxillary, with the fifth and seventh, or facial; the geniculate, with the seventh or facial; the jugular, with the glosso-pharyngeal; and with the vagus, through one of its own ganglia. On the fibres of the sympathetic distributed to the viscera numerous ganglia, or plexuses in which ganglia exist, are met with, and frequently there is a plexus following the course of each vessel.

As to the functions of the sympathetic, experiment has led to the following conclusions.

(a. ) The vaso-motor fibres of the head are supplied by the cervical portion of the sympathetic, and originate in the cervical region of the cord, proceeding from it by the anterior roots of the lower cervical and upper dorsal nerves. The fibres supplying the radiating fibres of the iris also come from that region (see Eye). {b. ) The vaso-motors of the upper limbs and of the thorax come (a) from the inferior cervical and superior thoracic ganglia, and (∕3) from the cord, by communicating branches between the third and seventh dorsal vertebrae.

(c.) The vaso-motor fibres of the lower limbs come from the cord through the sciatic and crural nerves, whilst those of the pelvic organs are derived from the abdominal ganglia of the sympathetic.

(d. ) The vaso-motors of the abdominal viscera exist chiefly in the splanchnic nerves; some fibres supplying the stomach appear to be derived - from the pneumogastric.

(e.) The splanchnic nerves all arise in man from the thoracic ganglia of the sympathetic—the greater splanchnic from the fifth to the tenth ganglia, the lesser splanchnic from the tenth and eleventh, and the smallest splanchnic from the twelfth ganglion. The splanchnics supply the stomach, liver, spleen, pancreas, intestines, and kidneys. Division causes dilatation of vessels; irritation causes contraction of vessels, and appears also to arrest or inhibit peristaltic motions of the stomach and intestines. Probably they also contain secretory filaments. The functions of vaso-motor nerves have been already described.

A very complete bibliography of works relating to the nervous system will be found in the Dictionnaire Encyclopédique des Sciences Médicales, 2d ser., xii. p. 619. For the comparative anatomy of invertebrates consult Gegenbauer, Elements of Comparative Anatomy (trans, and rev. by F. Jeffrey Bell and E. Ray Lankester, London, 1878); for the comparative anatomy of vertebrates, Owen, Anatomy of Vertebrates (3 vols., London, 1866); for details in human anatomy, Quain, Elements of Anatomy (9th ed., ed. by Allen Thomson, E. A. Schäfer, and G. D. Thane, London, 1882); for general physiology, Foster, Text-Book of Physiology (4th ed., London, 1883); for special details in physiology, H. Beaunis, Nouveaux Eléments de Physiologie Humaine (2d ed., Paris, 1881); (specially for the functions of the cranial nerves) Flint, Physiology of Man (vol. v., New York, 1872); Feιτier, On the Functions of the Brain (London, 1876, also new ed.); Meynert, “On the Brain of Mammals,” in Stricker’s Manual of Human and Comparative Histology (vol. ii., the New Syd. Soc., London, 1872); Flechsig, Die Leitungsbahnen im Gehirn und Rückenmark des Menschen (Leipsic, 1876), also “Zur Anatomie und Entwickelungsgeschichte der Leitungsbahnen im Grosshirn des Menschen,” in Du Bois-Reymond’s Archiv für Anatomie (Leipsic, 1881); Gudden, “Experimentaluntersuchungen über das peripherische und centrale Nervensystem,” in Arch. f. Psychiatrie (vol. ii., 1869, p. 713); Hitzig, Untersuchungen über das Gehirn (new series, 1874); Goltz, “Ueber die Verrichtungen des Grosshirns,” in Pflüger’s Archiv (1876), also “The discussion on the localization of function in the cortex cerebri,” in Trans, of Inter. Med. Cong. (vol. i., 1881, p. 218); Munk, Ueber die Functionen der Grosshirnrinde (Berlin, 1881); Ross, Treatise on the Diseases of the Nervous System (2d ed., London, 1883). (J. G. Μ.)

PART III.—PHYSIOLOGY OF PLANTS.

The body of a plant, like that of an animal, consists of one or more structural units which are termed “cells,” and in plants, as in animals, the cell consists essentially of an individualized mass of protoplasm.

The probable structure and chemical composition of protoplasm have been already considered. It need only be stated here that the protoplasmic cell-contents do not consist of pure protoplasm, but that the protoplasm contains imbedded in it particles of various substances which may be of the nature of food, or which may have been formed from food, or which are products of the metabolism of protoplasm: it is to the presence of these particles that the granular appearance of protoplasm is largely due. Moreover, there is present in the protoplasm of the cell, in the vast majority of cases at least, a well-defined, highly-refractive, usually somewhat spherical body, also protoplasmic in nature, the nucleus.

The cell or cells constituting the body of a plant present, in most cases, the important peculiarity that the protoplasm is enclosed in a membrane termed the “cellwall.” This membrane does not consist of protoplasm, but of a substance, cellulose, belonging to the group of the carbohydrates, and having the formula zrC 6 H 10 O 5 . All cell-walls do not, however, consist exclusively of this substance, though this is probably always the case at their first formation; but the cell-wall may undergo considerable modification during the life of the cell. It may, for example, undergo lignification; it then comes to consist largely of a substance termed “lignin,” which is much richer in carbon than is cellulose; this takes place typically in those cells which form woody or sclerenchymatous tissue. Or it may undergo cuticularization, when it comes to consist largely of a substance termed “suberin” or “cutin,” which, like lignin, is richer in carbon than cellulose. Or, again, it may become gummy or mucilaginous. These chemical differences are accompanied by differences in the physical properties of the cell-wall. A cellulose cell-wall is extensible, capable of swelling from taking up water into itself by imbibition, and is readily traversed by water. A lignified or cuticularized cell-wall is more rigid and less capable of swelling by imbibition; moreover, a cuticularized cell-wall is almost impermeable to water. A gummy or mucilaginous cell-wall·is more extensible and more capable of swelling by imbibition.

The structure of the plant-cell is not the same at all periods of its life. When a cell is young the protoplasm occupies the whole of the cavity enclosed by the cell-wall. But in the course of growth the increase in bulk of the protoplasm is not nearly so great as the increase in surface of the cell-wall, so that in the mature cell the protoplasmic contents form merely a rather thin layer known as the primordial utricle, which lies in close contact with the internal surface of the cell-wall at all points. There thus comes to be a relatively large cavity in the cell, the vacuole, which is filled with a liquid, the cell-sap, consisting of water holding various substances, organic and inorganic, in solution. The structure of a mature living cell is then this: it consists of a cell-wall, lined with a layer of protoplasm, which encloses the vacuole, filled with cell-sap.

The protoplasm of plants is endowed with all those fundamental properties w*hich are possessed by that of animals. When a plant is unicellular these properties are all exhibited, so far as they are necessary to the maintenance of the organism, by its protoplasm; in other words, all the necessary vital functions are performed by the protoplasm of the single cell of which the plant consists. The performance of all the necessary vital functions by the protoplasm of one cell obtains also in the case of not a few multicellular plants,—in those, namely, in which all the cells are similar to each other in structure and contents. In the great majority of multicellular plants, however, the functions are distributed to a greater or a less extent; there is more or less complete physiological division of labour. In these plants the cells are not all similar in appearance, and their diversity is to be ascribed to their adaptation in different ways to the performance of particular functions. Further, the cells which have undergone modification in some particular direction for the performance of some particular function are grouped together in certain parts of the plant, and these parts are spoken of as “organs.” Thus the roots of one of the higher plants are the organs for the absorption from the soil of water and substances in solution; the leaves are the organs for the absorption of gases from the air, and, in virtue of the green colouring-matter chlorophyll, which their cells contain, they are also the organs in which certain important constructive processes are carried on. But the extent to which physiological division of labour is carried out in plants is not nearly so considerable as it is in animals, and accordingly the protoplasm of the different cells of plants exhibits only in a very slight degree that specialization of structure which is so conspicuous in animals.

Absorption.

1. Absorption of Water and Substances in Solution.— The bodies of plants, unlike those of the great majority of animals, do not contain any internal cavity into which the food may be taken as a preliminary to its being absorbed by the tissues. The materials of the food of plants are therefore taken up directly from without into the cells of the absorbent organs. The cells which are especially concerned in absorption are, in the higher and subaerial plants, the root-hairs,— thin-walled, unicellular, unbranched filaments which are developed from the epidermal cells some way behind the growing-point of the root; in the lower plants, and even in those of the higher plants which lie submerged, all the cells of the plant may take part in absorption. Since the food is directly absorbed by the cells, and since the cells all possess a cell-wall, the materials of the food must be taken up in solution. Salts and other substances are, as a matter of fact, taken up by the absorbent cells in the form of watery solutions. Substances which are soluble in water are dissolved in the water which is present in a greater or smaller proportion in all soils, and of those which are not soluble in water many are brought into solution by the acid sap which saturates the walls of the root-hairs. The actual process of absorption is an instance of diffusion through a membrane,—that is, of osmosis. Only such substances can be absorbed by a root-hair, for instance, as are capable of diffusing not only through the cell-wall but also through the protoplasmic primordial utricle. Further, only such substances can be absorbed by the root-hair as are present in larger proportion in the water to be absorbed than they are in the cell-sap of the root-hair; this inequality between the proportion of any substance in solution in the liquid on the one side and in that of a membrane on the other is a necessary condition of osmosis. Hence, in order that the absorption of any particular substance by the root-hairs may be continuous, it is necessary that the substance in question should not accumulate in the cell-sap; this accumulation is prevented either by the actual consumption (i.e., chemical decomposition) of the substance in the cell or by the withdrawal of it to supply the needs of adjacent cells. In fact, so far as the process of absorption is concerned, the cell-sap of the internal cells of the root stands in the same relation to the cell-sap of the root-hairs as the cell-sap of the root-hairs does to the external liquid; and, as this relation exists between the successive internal layers of cells, there is set up a current of absorbed substances which travels from the surface towards the centre.

It appears from the foregoing considerations that the amount of any particular salt absorbed in a given time depends upon (1) its diffusibility and (2) its consumption in the plant. Of these two conditions the second is the one which is of real physiological importance, and, if only the given time is sufficiently long, the first condition may be neglected. For instance, let us suppose that a plant is absorbing by its roots two salts—the one (A) being very diffusible, the other (B) much less diffusible—and that, whilst the former undergoes no change in the plant after absorption, the latter is at once decomposed. Now, if the time of observation is short, it may happen that the amount absorbed of the salt A will be found to be greater than that of the salt B ; but, if the time be extended, the amount absorbed of the salt B will certainly be found to be greater than that of the salt A. The explanation is that the salt A would at first be absorbed very rapidly, on account of its high diffusibility; but the absorption of it would gradually diminish, in consequence of the accumulation of it in the cell-sap of the plant, until it ceased altogether. The absorption of the salt B, on the other hand, if less active at first than that of A, would be continuous, and thus, over a relatively long period of time, the amount of it absorbed would come to be much greater than that of A.

As a matter of fact, it has been ascertained that when different salts or other substances are presented to the root of a plant they are absorbed in different quantities. And further, it has been ascertained that the different salts are absorbed in different proportions by the roots of different plants; it is, in fact, upon this that the necessity for the “rotation of crops” depends. A striking illustration of this is afforded by a comparison of the amount of silica present in the ash of equal dry weights of gramineous and leguminous plants. According to Wolff— [table]

* The absorption of salts in certain proportions by a plant is the expression of what may be conveniently termed its ‘ ‘ specific absorbent capacity.” It must not be supposed that this term suggests that the roots possess any selective power by which they absorb this salt and reject that one, or by which this one is absorbed in larger proportion than that one. The question as to whether or not a particular substance will be absorbed is a purely physical one, dependent upon the relation between the molecules of the substance and the cell-wall and primordial utricle which they have to traverse, and in no degree dependent upon the usefulness or hurtfulness of the substance to the plant. The amount absorbed of any particular substance depends ultimately upon the activity with which the plant chemically alters the substance after absorption. To return to the illustration just given. The great difference between the amounts of silica present in the ash of gramineous and of leguminous plants respectively is the expression of the fact that the former are capable of withdrawing relatively large quantities of absorbed silica from the sphere of osmotic activity, and depositing it in the insoluble form in the tissues, whereas the latter can only do so to a comparatively small extent. The specific absorbent capacity of a plant is simply a manifestation of its specific metabolic properties.

The amount of the various salts absorbed is not, however, exclusively dependent upon the specific absorbent capacity of the plant, for it is materially affected by the composition of the soil. The larger the quantity of any substance presented to the roots, the greater, other things being equal, will be the amount of it absorbed. This does not mean that substances can be absorbed by the roots in solutions of any degree of concentration. It appears that the root-hairs can only absorb very dilute solutions; but for the watery solution of any salt capable of being absorbed there is a certain degree of concentration at which the proportion of the amount of the salt absorbed to that of the water absorbed is the same as that of the solution. If the solution be more concentrated the proportion of water absorbed will be greater, if the solution be more dilute the proportion of salt absorbed will be greater. This is the general “law of absorption” determined by the experiments of De Saussure and of Wolff. It must, however, be borne in mind that, though the proportion of salt absorbed is larger in the case of a dilute than of a more concentrated solution, yet the absolute quantity of it absorbed from a more concentrated solution in a given time is greater than that absorbed from a dilute solution.

2. Absorption of Gases.— An interchange of gases is constantly taking place between the plant and the medium in which it lives—in the case of terrestrial plants, between the plant and the air; in the case of aquatic plants, between the plant and the water. "When the plant is a simple one each of its cells is in direct relation with the external medium; when it is of complex structure there is usually some means provided by which the more internal cells are brought into relation with it, namely, a continuous system of intercellular spaces which communicate with the exterior in terrestrial plants by certain apertures termed “stomata,” in the epidermis of the leaves and young stems, and by others termed “lenticels,” in the cortical tissue of older stems and of roots.

The gases principally absorbed by plants are oxygen and carbon dioxide. The former is absorbed by every living cell, and at all times; the latter is absorbed exclusively by cells which contain chlorophyll, and by them only when exposed to light. In the more highly-organized plants the cells which contain chlorophyll are confined almost entirely to the leaves, so that the leaves may be regarded as the organs by which these plants absorb carbon dioxide. It has been held that the stomata are of great importance in promoting the absorption of this gas by the leaves, but the experiments of Boussingault prove that this view is not well founded. He discovered, namely, that the upper surface of the leaves of various plants with which he experimented absorbed carbon dioxide more actively than the lower surface, although the upper surface had scarcely any stomata, whereas they were very numerous on the lower. The absorption of carbon dioxide by the leaves is directly effected by the superficial cells.

Gases, like solid substances, are only absorbed in solution by the cells of plants. They may be brought to the surface of the cell-wall already dissolved in water, as in the case of submerged plants, or they may be dissolved from the atmosphere by the sap which saturates the cellwall, as in the case of land-plants; in either case they reach the interior of the cell in solution. When a gas has been taken up at the surface it diffuses throughout the cellsap; and in the case of a gas like nitrogen, for instance, which is not chemically altered in the cell, the absorption of it will cease when the cell-sap has become saturated with it. If, however, the metabolism of the cell changes the chemical condition of a gas its absorption will be continuous. This accords with what has been said with regard to substances absorbed by the roots.

Another analogy exists between the absorption of gases and the absorption of substances in solution, namely, that, just as the root can only absorb a solution below a certain degree of concentration, so the leaf can only absorb a gas below a certain degree of pressure. Let us take in illustration the case of carbon dioxide. The pressure of the carbon dioxide in the air is very slight (0Ό4 per cent, by volume). It was first observed by Percival that an increase in the quantity of carbon dioxide in the air is favourable to the nutrition of green plants; De Saussure found that a considerable increase is prejudicial; and subsequently Godlewski showed that the optimum proportion is from 8 to 10 per cent.,—that is, that carbon dioxide is most readily absorbed by the plant when its pressure is about 200 times greater than in ordinary air. Boussingault found that when leaves are exposed to sunlight in an atmosphere of pure carbon dioxide at the ordinary pressure they cannot decompose it, but if the gas is at a low pressure (in his experiment 0*17 mm. of mercury) they can do so.

Besides oxygen and carbon dioxide other gases are also absorbed by plants, but to a small extent only. Nitrogen is absorbed in small quantities merely in virtue of its solubility and diffusibility; as mentioned above, it is not in any way acted upon by the cells after its absorption. It appears that ammonia may be absorbed from the air in the form of gas by the leaves, and that, when thus absorbed, it contributes to the nutrition of the plant. Other gases, such as sulphur dioxide, sulphuretted hydrogen, and hydrochloric acid, which are occasionally present in the air as impurities, are absorbed by the leaves, as is shown by the pernicious effects which they produce.

Circulation.

It is obviously necessary, in multicellular plants in which certain cells only are in a position to absorb food-materials from without, that these food-materials should be conveyed from the absorbent cells to the remainder of the plant. In no plant is there any organ comparable to the heart of animals by means of which a distribution throughout the tissues of absorbed food-materials is effected. The distribution is accomplished by purely physical means, principally by osmosis. When the cell-sap of a cell becomes charged, by absorption from without or from neighbouring cells, with any substance, diffusion-currents are at once set up between this cell and any adjacent cells the cell-sap of which may contain the substance in question in smaller proportion, and these currents will persist until osmotic equilibrium, as far as this substance is concerned, is established. The diffusion-currents do not flow in any definite direction, but their course is determined simply by inequalities in the chemical composition of the cell-sap of the cells in different parts of the plant. Since in subaerial plants the roots are as a rule the only organs which absorb substances from the soil, and since the cell-sap of their cells is therefore relatively rich in absorbed food-materials, the general direction of the diffusion-currents is from the roots upwards into the stem and leaves.

In cellular plants—that is, in plants which possess no vascular tissue—the distribution of absorbed food-materials is effected solely by osmosis. Many of these plants are small, so that the distribution is effected from cell to cell with sufficient rapidity by this means. Those of them that are large have a very considerable absorbent surface, many of them being aquatic in habit, so that the absorbed substances have no great distance to travel. In vascular plants, more particularly in those which are subaerial in habit, the distribution of the water, holding substances in solution, which is absorbed by the roots is effected to a considerable extent by means of the vascular system. The forces by which the flow of liquid through the vascular tissue is maintained are the following. The first is the root-pressure. It is a matter of common observation that, when the stems of vascular plants are cut across, particularly in the spring, an escape of water takes place from the surface of that portion of the stem which still remains connected with the root, an escape which may persist for some considerable time. It has been ascertained that this outflow of water takes place under considerable pressure; for instance, Hales observed, in the case of a Vine, that the pressure was sufficiently great to support a column of mercury 32⅜ inches in height. But the rootpressure not only manifests itself by causing a flow of water from the cut surfaces of stems, it also causes in many plants the exudation of drops of water at the free surface. Drops may commonly be seen on the surface of certain Fungi (Pilobolus crystallinus, Pénicillium glaucum, Merulius lacrimans'), which are exuded in consequence of the hydrostatic pressure set up in the plant by the active absorption effected by the organs (rhizoids) which here perform the functions of roots. Again, drops are frequently to be found on the margins and at the apices of the leaves, especially the younger ones, of many plants, such as Grasses, Aroids, Alchemillas, Saxifrages, &c. That the formation of these drops depends upon the forcing of water upwards through the vessels by the root-pressure is proved by the fact that, if the stem be cut off from the root and then placed with its cut end in water, no more drops will appear on the leaves. The water thus forced into the vascular system is not pure water, but a watery solution of various substances, principally salts absorbed by the roots. It is therefore obvious that the root-pressure assists in the distribution of these substances throughout the plant.

In order to understand how the root-pressure is set up it will be necessary to give a brief description of the general structure of the root. It consists of a central fibro-vascular cylinder which is surrounded by several layers of parenchymatous cells, the most external of these layers being in contact with the epidermal layer, certain cells of which are developed into root-hairs. Water is absorbed by the root-hairs and passes from them by osmosis into the subjacent parenchymatous cells. It is obvious, however, that osmosis cannot take place between the cells of the innermost layer and the vessels, for the conditions of osmosis are not fulfilled, inasmuch as the vessels at first contain no liquid. The passage of water from the cells into the vessels can only take place by filtration. For this a certain pressure is necessary, and this pressure is set up by the absorbent activity of the root-hairs and of the parenchymatous cells. The system of cells absorbs large quantities of water, more indeed than the cells can contain, so that at length the resistance of the cell-walls is overcome at what is presumably the weakest point, and water filters into the cavities of the vessels of the wood. There it collects, and it may, under certain circumstances, fill the whole vascular system; then, since absorption is still going on at the surface of the roots, sufficient pressure is set up to cause that exudation of drops on the leaves to which allusion has been made, and, if the stem be cut across, to cause “bleeding” at the cut surface. From the foregoing account it is apparent that the root-pressure is the expression of the absorbent activity of the root-hairs.

But the vessels of the wood do not always contain water. Hales observed that, whereas a Vine will bleed freely if its stem be cut across in the month of April, no bleeding is observed if it be cut in July. And yet it cannot be doubted that the plant is absorbing water by its roots more actively in July than in April. The explanation of these facts is that, although in July the plant is absorbing water actively by its roots, yet it is losing so much in the form of vapour from its leaves that water does not accumulate in the cavities of the vessels. This loss of water in the form of vapour from the general surface of the plant exposed to the air is termed “transpiration.” The parts of the plant which are more especially concerned in transpiration are the leaves. By their structure they are peculiarly adapted for this purpose. The tissue of a leaf is penetrated in all directions by intercellular spaces, which communicate directly with the external air by means of the stomata in the epidermis. In this way a very large surface of moist and thin cell-wall is brought into contact with the air, a condition most favourable to evaporation. Some idea of the activity of transpiration in a plant is afforded by the following determinations made by Hales. In the case of a Sunflower with a leaf-surface of 5616 square inches the amount of water transpired during twelve hours of daylight was 30 fluid oz. (a pint and a half); in the case of a Cabbage with 2736 square inches of leaf-surface the amount of water transpired in the same time was 25 fluid oz. The activity of transpiration is very much affected by external conditions,—the moister the air, the smaller will be the transpiration; and conversely, the drier the air and the higher the temperature, the greater will be the amount of water transpired. Light, too, has a remarkable influence: it has been ascertained by a great number of observers that transpiration is more active in light than in darkness. It seems probable that this is to be attributed largely to the influence of light upon the stomata. Each stoma is usually bounded by two cells, termed “guard-cells,” which are capable of so altering their form as to close or to open the aperture between them. The form of the guard-cells is dependent upon the amount of water which they contain. When they hold comparatively little water, and are flaccid, their adjacent free surfaces are straight and in contact with each other; the stoma is then closed. When, however, they contain so much water that their cell-walls are under considerable pressure from within—in a word, when the guard-cells are turgid—they curve so that their adjacent free surfaces are no longer in contact, but a space is left between them; the stoma is then open. It appears that the guard-cells become turgid under the influence of light; and it is probably to this open condition of the stomata that the greater transpiration of leaves when exposed to light is to be ascribed.

It is obvious that the effect of transpiration upon the dis- tribution of water through the plant is very great. It sets up a rapid current, known as the “transpiration-current,” which travels from the roots upwards towards the leaves. Sachs has made some observations as to its rate by means of the lithium-method, which consists in supplying the root of a plant with a solution of a salt of lithium, and determining by means of the spectroscope the length of stem in which lithium could be detected after the lapse of a given time. He estimates the rate per hour to be in Nicotiana Tabacum 118 centimètres (46'458 inches), in Helianthus annuus 63 (24'793 inches), and in Vitis vinifera 98 (38'583 inches). But the water of the transpiration-current holds salts and other substances in solution. It is clear, therefore, that transpiration promotes the distribution not only of water but also of the substances which the water holds in solution. Sachs’s experiments go to prove that salts travel in solution in the current; hence it affords a ready means of transport of substances from the roots, where they are absorbed, to the leaves where (as will be shown below) the food undergoes certain changes which fit it for the nutrition of the plant. There is yet another important point to be noted with regard to the physiological significance of transpiration. It has been mentioned that the roots absorb from the soil only very dilute solutions of salts and other substances, so that for any given quantity of a salt absorbed an excessive quantity of water has to be absorbed likewise. It is obvious that the absorption of salts from the soil by the roots can only go on provided that the plant is able to get rid of the excess of absorbed water, and this is effected chiefly by transpiration, though, as mentioned above, an actual excretion of water in the form of drops not unfrequently takes place.

It has been conclusively proved that the channel along which the transpiration-current travels is the fibro-vascular tissue, and that it is the xylem or woody portion of a fibro-vascular bundle which is the conducting tissue. In the case of plants like Conifers and Dicotyledons, in which there is a formation of secondary xylem or wood from a cambium-layer, it is the younger wood, the alburnum, along which the transpiration-current passes. The older wood, the duramen, it is true, usually contains water, but it does not serve as a conducting channel, only as a reservoir. The question now arises as to the mode in which the transpiration-current travels through the wood. Since the vessels contain no water in their cavities at the time when transpiration is most active, it is clear that it is not in the cavities of the vessels that the water of the current travels. Sachs is of opinion that it moves in the substance of the lignified cell-walls. Others, amongst whom Hartig may be especially named, consider that it travels from the cavity of one wood-cell to that of the next by filtration under pressure. The mechanism of conduction would, in the latter case, be this: the conducting cells contain air and water; when water is withdrawn from one of them the contained air becomes rarefied, and the water in that cell is then subject to a lower pressure than that in neighbouring cells; as a consequence water is forced into the former cell through the thin membranes of the pits in its walls until equilibrium is re-established. Inasmuch, then, as the air in the conducting wood-cells in the leaves is constantly undergoing rarefaction in consequence of transpiration, a current is set up towards the leaves from the stem and the root.

There can be no doubt, however, that, as Hales first pointed out, transpiration has the éffect of diminishing the pressure of the gases contained in the cells and vessels. Von Höhnel has found that, if the stem of a transpiring plant be cut through under mercury, the mercury will at once rise to a height of several centimètres in the vessels, the greatest height being reached in the younger vessels. This rise can only be accounted for by ascribing it to the difference between the atmospheric pressure and the pressure of the gases in the vessels, the lower pressure of the latter being due to the removal of water by transpiration, which necessarily involves an expansion, and therefore also a diminished pressure of the gases. The effect of this so-called “negative pressure” is to set up diffusion-currents of gases from the surrounding tissues into the cells and vessels of the fibro-vascular bundles. It must not be assumed, however, that the vessels are the principal channels in which gases circulate throughout the plant. They circulate principally in the intercellular spaces which communicate with the external air by means of the stomata.

Stating’the foregoing facts in the most general terms, it appears that in a plant the food-materials travel by osmosis from the absorbent organs to the organs in which the processes of constructive metabolism are carried on— in one of the higher plants, for instance, from the roots to the leaves—and that the distribution of the food-materials is assisted and accelerated by root-pressure and by transpiration, the fullest expression of this being the transpiration-current in terrestrial vascular plants. And just as there is a current of food-materials tending towards the organs in which the processes of constructive metabolism are carried on, so also there is a current of the organic nutrient substances formed in these organs travelling from them to the other parts of the plant. The final cause of the current is the same in both cases. A given salt, for instance, which has been absorbed by the root travels towards the leaves because it is in some way undergoing chemical alteration in those organs; similarly, a given organic substance formed in the leaves travels from them towards any part of the plant in which that substance is being chemically altered, or, to use a somewhat different expression, is being consumed. The cause of the diffusion in either case is the disturbance of osmotic equilibrium by the chemical alteration of the substance, and the result is a current of the substance from those parts which are relatively rich in it to those which are relatively poor.

Distribution of Organic Nutrient Substances.— In vascular plants the distribution of the organic nutrient substances is, like the conduction of substances absorbed by the roots, assisted by the vascular tissue; but, whereas it is the wood which is the conducting tissue in the latter case, in the former it is the bast or phloem, and more especially the bast-vessels or sieve-tubes. These vessels consist of elongated cells placed end to end, the septa between the adjacent cells being perforated so as to admit of a direct continuity between their protoplasmic contents.

The importance of the wood and of the bast respectively as conducting tissues is well illustrated by the “ringing” experiments which have been repeatedly made on plants, such as Dicotyledons and Conifers, which have the fibro-vascular bundles arranged in a ring in the stem. When a ring of tissue, extending inwards as far as the cambiumlayer, is removed from the stem of a dicotyledonous plant the following facts are to be observed: (1) that the leaves which are borne on branches arising from the stem above the level at which the ring of tissue has been removed will not exhibit any signs of withering; (2) that the part of the stem below the incision will not increase in thickness to nearly the same extent as the part above the incision. From these facts it is clear (1) that the operation in question has not materially affected the conduction of water and foodmaterials in solution upwards to the leaves, and, since the wood is the only unimpaired tissue, it is obviously in the wood that the upward current travels; and (2) that the operation has materially affected the conduction of organic nutrient substances to the parts below the incision, the diminished growth of these parts being the result of inadequate nutrition; this effect of the operation is to be ascribed, principally at least, to the destruction of the continuity of the bastλtissue.

In various families of vascular plants, and in some cellular plants also (certain Fungi), there are to be found cells, forming what is known as “laticiferous tissue,” which probably assist in distributing both food-materials and organic nutrient substances throughout the plant. In some plants (Euphorbiaceae, Asdepiadacex, Moreae, &c.) the cells are quite distinct from each other, and extend from one end of the plant to the other, growing with its growth, so that they attain a very considerable size, and are much branched; these are spoken of as “laticiferous cells.” In other plants (Cichoriaceae, Papaveraceae, &c.) the cells are comparatively small, and fuse together to form an intricate network; these are spoken of as “laticiferous vessels.” The cells of the laticiferous tissue contain a milky liquid, termed “latex,” which consists of water holding inorganic salts, sugar, gum, extractives and proteids, in solution, and holding in suspension resinous and fatty bodies. The cells contain protoplasm in addition, and not uncommonly starch-granules.

Food of Plants.

A rough idea of the nature of its food can be obtained by analysing a plant. It is found that, in the process of incineration, a considerable weight of its dry solid is burned up and given off in the form of gas; this represents the combustible or organic portion of the plant. The incombustible residue, the ash, is found to be of a mineral or inorganic nature. The gases given off are carbon dioxide, watery vapour, and nitrogen, showing that the combustible portion of the plant contained the elements carbon, hydrogen, and nitrogen. In the ash occurs a number of elements, of which the principal are sulphur, phosphorus, potassium, calcium, magnesium, iron, sodium, chlorine, and silicon. But it does not necessarily follow that, because any given chemical element can be detected in a plant, that element is to be regarded as part of the food of the plant, for, as has been already pointed out, plants may absorb substances which in no way contribute to their nutrition, or are even injurious. When an element enters into the chemical composition of the substances of which the organized structure of the plant consists (as C, Η, O in starch and cellulose, C, H, O, N, S, P in proteids), then it is clear that this element must form part of the food; but, when, as in the case of the rest of the elements mentioned above, an element does not thus contribute to the building up of the organized substance of the plant, its admission to the rank of a food-material must be the subject of direct experiment. It has been ascertained that many of the elements enumerated above, though, so far as is known, they are not essential constituents of the organized structure of the plant, are nevertheless essential to the maintenance of its life; they may not, indeed, go to build up the plant-substance, but in some way or other they promote the metabolic processes.

The method which has afforded the most valuable results bearing upon the relative physiological importance of various food-materials is that which is known as “waterculture.” It consists in growing plants with their roots immersed in water holding certain salts in known quantities in solution. The mixture of salts can, of course, be varied at pleasure, and the effect upon the plant of the absence of certain elements, as of their presence in smaller or larger quantities, can be observed. Further, by an analysis of that portion of the solution which remains unabsorbed at the close of the experiment, the proportion in which the various salts have been absorbed can be ascertained.

The elements of the food of plants may be conveniently classified into two groups, the first consisting of those which enter into the composition of organized plant-substance, the second consisting of those which, without actually entering into the structure of the plant, are essential to the proper performance of the metabolic processes. To the first group belong the elements C, H, O, N, S, P; to the second, K, Ca, Mg, Fe, Cl (?).

We will now briefly discuss the form in which the various chemical elements are absorbed, and their use in the economy of the plant, beginning with those which enter into the composition of organized plant-substance. A few words will also be said about those elements, such as sodium and silicon, which, though always present in the ash of plants, appear to have no real physiological significance as far as nutrition is concerned.

Carbon.— This element constitutes a large percentage of the total dry weight of plants. It enters into the composition of all the organic substances, such as starch, cellulose, and other carbohydrates, fats and other hydrocarbons, proteids, organic acids, alkaloids, &c., which may be present in plants. The form in which carbon is absorbed depends upon the nature of the plant. It may be broadly stated that all those which contain chlorophyll absorb their carbon in the form of carbon dioxide, whereas those which do not contain chlorophyll absorb their carbon in the form of more complex carbon compounds which contain C, H, and 0, and in which the C is directly combined with H. Moreover, in green plants it is only those cells which contain chlorophyll that can absorb carbon dioxide, and this only under the influence of light. It must not be assumed, however, that plants containing chlorophyll are incapable of absorbing complex carbon compounds. It is known from the researches of Darwin and others that the “insectivorous” plants absorb such compounds by their modified leaves, and it is known also that a number of green plants, such as the Mistletoe, the Rattle, and others, live parasitically on other plants. It has indeed been proved by direct experiment that green plants can absorb substances such as urea, glycocoll, asparagin, leucin, tyrosin, which are all highly complex carbon compounds. The physiological distinction to be drawn between plants which do and those which do not contain chlorophyll is really that the former are capable of assimilating carbon in a simple compound, such as C0 2 , whilst the latter are incapable of doing this, and require, therefore, compounds of more complex constitution. Plants which do not contain chlorophyll are either parasites (that is, they live upon other living organisms) or saprophytes (that is, they live upon the products of the waste and decay of other living organisms). The plants which do not contain chlorophyll are the Fungi and a few Phanerogams,— Epipogium Gmelini, Cuscuta, Monotropa, Lathraea, Corallorhiza. Of these the Fungi include both parasites and saprophytes; Epipogium Gmelini is a saprophyte, Cuscuta a parasite, and Monotropa may apparently be either the one or the other. The Orobancheae, which are parasitic, and Neottia, which is saprophytic, have not a green colour, but small quantities of chlorophyll have nevertheless been detected in them.

Hydrogen.— This element is absorbed by all plants in the form of water and of ammonia and its compounds; it may also be absorbed in the form of organic compounds.

Oxygen.— Oxygen is taken up either in the free state, or in combination in the form of water or of salts; it may also be absorbed in the form of organic compounds. The free oxygen absorbed is especially concerned in the processes of destructive metabolism, the combined oxygen in those of constructive metabolism.

Nitrogen.— Nitrogen is absorbed in the forni of ammonia and its compounds and of nitrates; it may also be absorbed in the form of organic nitrogenous compounds. The researches of Lawes, Gilbert, and Pugh, as also those of Boussingault, have proved that plants are incapable of assimilating free nitrogen. It appears that, on the whole, nitrogen absorbed in the form of ammonia compounds is more readily assimilated by plants than nitrogen absorbed in the form of nitrates. Pasteur has shown, for instance, that the Yeast plant cannot assimilate nitrates.

Sulphur.— Sulphur is absorbed from the soil as sulphates, those of ammonium, potassium, magnesium, and calcium being the most advantageous. It may also be absorbed to some extent in the form of organic compounds.

Phosphorus.— Phosphorus is absorbed from the soil in the form of phosphates. Besides being a constituent of certain substances allied to the proteids, such as nuclein and plastin, phosphorus seems to bear an important relation to certain of the metabolic processes. Phosphates are to be found especially in those parts of plants which are rich in protoplasmic cell-contents. It appears that a supply of phosphates promotes considerably the assimilation of nitrogen by the plant.

Potassium.— Potassium is absorbed in the form of a variety of salts, of which the chloride is the most advantageous form, according to Nobbe. Like phosphorus, it is to be found in largest quantity in those parts of plants which are rich in protoplasmic cell-contents. It appears to have an important influence on the constructive metabolic processes of plants which contain chlorophyll. Nobbe found, in the case of a Buckwheat plant, that in the absence of a supply of potassium its growth was diminutive, and that the amount of starch in the plant was very small. On the addition of potassium chloride to the water-culture the starch-grains became more numerous in the chlorophyll-corpuscles, and made their appearance also in the tissues of the stem. The precise significance of potassium in relation to these processes is not known. Liebig was of opinion that it played an important part in the distribution of carbohydrates throughout the plant, but this view has not been confirmed. It appears rather that the facts upon which this view was based point to an effect due not to the potassium itself but to the particular salt of it which was absorbed (see “chlorine” below). There can be no doubt, however, that potassium bears some important relation to the formation and storing up of carbohydrates, for it is always present in large quantity in organs, such as leaves, tubers, seeds, &c., in which these processes especially take place.

Calcium.— The compounds in which calcium is usually absorbed are the sulphate, phosphate, nitrate, and carbonate, the last-named salt undergoing decomposition in the process. It appears that the chloride is injurious to plants. The precise use of calcium is unknown. It very commonly occurs in the cells of plants in the form of crystals of the carbonate or the oxalate, and possibly one of its important functions is to form insoluble salts with acids which are of no further use in the plant, and are even injurious to it.

Magnesium. — Like calcium, this may be advantageously absorbed in the form of all its salts, except the chloride. Nothing definite is known as to its use.

Iron.— It appears that iron may be absorbed in the form of any of its salts. It is known to be essential only to those plants which contain chlorophyll. If a seedling be cultivated by the method of water-culture, with its roots in a solution which contains no iron, the leaves formed will be successively paler in colour until at length they are nearly white; in this state the plant is said to be “chlorotic.” If a small quantity of a salt of iron be then added to the solution in which the roots are, or if the pale leaves be painted over with a dilute solution of iron, they will soon become green. Iron, therefore, plays an important part in connexion with the formation of the green colouring-matter chlorophyll. It is still a debated question whether or not iron enters into the composition of the chlorophyll-molecule.

Chlorine.— Chlorine is absorbed from the soil in the form of chlorides. The evidence as to its significance in the nutrition of plants is conflicting. Nobbe, Leydhecker, Beyer, and more recently Farsky have observed that water-cultures of Buckwheat, Barley, and Oats do not flourish when grown in solutions containing no chlorides, and since the chlorophyll-corpuscles of the plants become crowded with starch-grains it was thought that chlorine had some importance in connexion with the translocation of carbohydrates. Knop and Dworzak have observed, on the other hand, that Maize plants will grow well in solutions containing no chlorine, and further, that the accumulation of starch in the chlorophyll-corpuscles may be induced by various abnormal external conditions.

i Sodium.— This element is never absent from the ash of plants, and in some cases, especially in maritime plants, it is present in considerable quantity. It might be inferred from its constant occurrence in the ash that sodium is of some importance as a foodmaterial; it was thought, in fact, that it might serve as a substitute for potassium, but this has not been found to be the case. Its constant presence in the ash is due merely to its universal distribution in the soil.

Silicon.— Silicon is absorbed in the form of soluble silicates, and possibly as soluble silicic acid. The silicates are brought into solution to some extent by the carbon dioxide present in the soil, and also by the acid sap of the root-hairs. It is always present in the ash of plants, sometimes in large quantity; in wheat-straw, for instance, it constitutes 67 , 50 per cent, of the ash (Wolff). It was thought that silicon must be essential to nutrition. Sachs found, however, that a Maize plant will grow well in a water-culture from which it can obtain no silicon. On the other hand, Wolff has ascertained that in the case of Oats the number of perfect seeds formed is greater when the plant is abundantly supplied with silicon.

Constructive Metabolism (Anabolism).

When a plant is adequately supplied with appropriate food-materials, the external conditions to which it is exposed being favourable, it increases in weight, owing to an accumulation of the substances which constitute its organized structure. But this gain in weight is only relative; for side by side with the constructive processes by which the food is converted into the substance of the plant—processes, that is, which have as their result the formation of relatively complex from relatively simple chemical compounds—there are going on destructive processes—processes, that is, which have as their result the formation of relatively simple from relatively complex chemical compounds—which are attended by a loss of weight. The gain in weight by the plant represents the difference between the activity of the constructive and of the destructive metabolic processes respectively. The end of constructive metabolism is the formation of protoplasm. Protoplasm is certainly a very complex substance, though its precise constitution is unknown, and the food-materials of plants are much simpler substances; there must, therefore, be a considerable number of processes to be gone through before protoplasm can be produced from the foodmaterials. We will now study these processes, and, in the first instance, confine our attention to those which have been ascertained to take place in plants which possess chlorophyll.

It has been already mentioned that a green plant absorbs carbon dioxide when it is exposed to light. Under these circumstances it also increases in weight; it does not increase in weight when kept in the dark, nor when it is kept in an atmosphere from which all carbon dioxide has been removed. The absorption of carbon dioxide is then an indication that the plant is performing certain constructive processes,—that it is assimilating carbon. The absorption of carbon dioxide is accompanied by an evolution of oxygen gas, the volume of the latter exhaled being approximately equivalent to that of the carbon dioxide absorbed. This is an indication that the absorbed carbon dioxide is undergoing chemical change. It seems probable that the change is of the nature expressed by the following equation—

arCO 2 +rrH 2 O=x(CH 2 0) + xθ 2

—that is, that from carbon dioxide and water a substance allied to formic aldehyde, or a polymer of it, is formed, free oxygen being evolved. It may be stated generally, with some considerable probability, that the first step in the constructive metabolism of a plant containing chlorophyll is the formation of a non-nitrogenous organic compound. It is just this formation of non-nitrogenous organic substance from carbon dioxide and water that the plant which is destitute of chlorophyll is unable to perform; and it is on account of this inability that the carbon of its food must be supplied to it in the form of organic compounds, as pointed out above. The further processes of constructive metabolism appear to be much the same in all plants, whether they contain chlorophyll or not. The next step is probably the formation of some relatively simple nitrogenous organic substances from the nitrogen of the food and the non-nitrogenous organic substance which has been either formed in the plant or absorbed as food from without. The nitrogenous substances thus formed are probably crystallizable bodies, such as asparagin and leucin, which all contain nitrogen in the form of the group NH 2 , The derivation of these substances from the nitrogenous food when it contains nitrogen in the form of ammonia (NH 3 ) is sufficiently obvious. When, however, it consists of nitrates it appears probable that the nitrogen of the nitric acid has to be transformed into the nitrogen of ammonia,—that is, to be combined directly with hydrogen; it is probably owing to their inability to effect this transformation that some plants, as mentioned above, cannot be supplied with nitrogen in the form of nitrates. The first step in this transformation is probably, as Emmerling has pointed out, the decomposition of the absorbed nitrates by the organic acids, especially the oxalic, of the plant; the liberated nitric acid then undergoes chemical change, resulting in the formation of ammonia. It is impossible to say with precision how this is effected, but there can be little doubt that it does take place; some direct evidence is afforded by Hosaeus’s observation that ammonia salts were to be found on analysis in a number of plants which had been supplied with manure containing no ammonia. The next process is an increase in the size and complexity of the molecule, attended in certain cases by the introduction of new elements (S and P), the product being one of those substances which are known as “proteids.” The last stage is the formation of living protoplasm from the proteid and other organic substances.

The formation of nitrogenous organic substance may take place in any living cell, and, unlike the formation of non-nitrogenous organic substance, it goes on quite independently of the presence of chlorophyll and of the action of light. But there is evidence to show that in green plants it is especially in the cells which contain the chlorophyll that the process goes on. The experiments by which this evidence has been obtained were made on plants with distinctly differentiated leaves. Emmerling observed in the Bean that, whereas in the root a relatively large quantity of nitric acid could be detected, there was much less in the stem, and in the leaves none at all, and he inferred that as the nitrates are supplied to the leaves they are used up in the formation of organic nitrogenous substance. Further, from the researches of Kellner, Emmerling, Borodin, and others it appears that the leaves contain the above-mentioned crystallizable organic substances, asparagin, leucin, Ac., in considerable quantity; and it is quite possible that these substances may be formed synthetically in the leaves, though it is true that they may be formed in other ways as well. Finally, Pott has found that the proportion of proteid in the plant increases from the roots upwards towards the leaves, the proportion in the latter being about twice as great as that in the former of many of the plants which he analysed.

The formation of living protoplasm from the organic substances elaborated from the food necessarily goes on in every living cell. It lias been already mentioned that destructive metabolism—that is, processes of decomposition— is active in living cells, and it is especially the protoplasm which is the seat of these processes. The maintenance of the life of the cell is therefore an indication of the fact that the activity of the destructive metabolism is at least equalled by the activity of the constructive metabolism. In a young cell the latter exceeds the former, so that the protoplasm is increased in quantity; then for a time the two are approximately equivalent, until at length the destructive gradually gains the upper hand, and eventually the death of the cell is the result.

Destructive Metabolism (Katabolism).

Just as all the processes by which increasingly complex organic substances are formed in the plant, and which intervene between the food-materials on the one hand and the protoplasm on the other, are designated collectively “constructive metabolism,” so all the processes of decomposition by which relatively simple substances are produced from relative complex ones, and which intervene between the protoplasm on the one hand and the excreta and other waste-products on the other, are designated collectively “destructive metabolism.” Of all the various processes of destructive metabolism the most fundamental is the decomposition of the protoplasm. It appears that this decomposition is spontaneous—that it is, as Pflüger terms it, a “self-decomposition”; and it is, in fact, only so long as this self-decomposition is proceeding that protoplasm can be said to be living. The destructive metabolism of an organism is not, however, confined to the self-decomposition of its protoplasm; the various complex organic substances which the cells contain may undergo chemical change quite independently of their entering into the metabolism of the protoplasm. The most active agents in producing chemical changes of this kind are certain bodies which are termed “ferments,” and are distinguished as “unorganized” ferments from the so-called “organized” ferments, such as Yeast and Bacteria. But little is known as to their chemical composition, and nothing as to the peculiarity of chemical constitution upon which their characteristic properties depend.

The unorganized ferments which have hitherto been detected in plants may be classified, according to the nature of the chemical changes which they induce, in the following four groups.

1. Ferments which convert starch into sugar (diastatic ferments). These have been found to be very widely distributed in plants, and in fact it seems probable that a ferment of this kind is present in all living plant-cells. Their mode of action is generally indicated by the following equation—

Starch. Maltose.

2(C 6 H 10 O b ) + H 2 O = C 1 ⅛O π .

2. Ferments which convert cane-sugar into glucose (inverting ferιħents). A ferment of this kind, termed “invertin,” has been obtained from Yeast; it is probable that a similar ferment is present in succulent fruits, for they commonly contain a mixture of cane-sugar and glucose. The following equatio , h will indicate the nature of the process—

Cane-sugar. Dextrose. Laevulose.

C 12 H 22 O 11 + H 2 O = C β H 12 O 6 + CθH ] 2 θβ∙

3. Ferments which decompose glucosides. The most familiar members of this group are emulsin or synaptase, found in the Bitter Almond; myrosin, in the seed of the Black Mustard; eryth-rozym, in the root of the Madder. The following equation represents the decomposition of the glucoside amygdalin by emulsin—

Λ., re a.ι 11 .. ¾≡' p ≡i e θ'™“'·

⅛H β NO 11 + 2H s 0=C,H,0 + HCN + 2(C β H 1 ,O,).

4. Ferments which convert proteids that are indiffusible and may be insoluble in water into others (peptones) which are both soluble and diffusible. These, which are only active in the presence of free acid, are termed “peptic” ferments. They have been found in quantity in the latex of certain plants (Carica Papaya and Ficus Carica) and in the liquid excretion of carnivorous plants. It is, of course, impossible to represent by an equation the nature of the chemical change which these ferments induce.

It is probable that other ferments than these may be present in plants, but they have not yet been actually obtained. There is probably one which decomposes fats (glycerides) into glycerin and the corresponding fatty acid, thus—

Olein. Oleic Acid. Glycerin.

Qs7B104O 6 + 3H 2 O = 3C 18 H 34 O 2 + C 3 H 8 O 3 .

Müntz and Von Rechenberg have pointed out that the quantity of free fatty acids in oily seeds increases very much during germination, and the only satisfactory explanation of this fact which can at present be offered is that it is the result of the decomposition of the fats, in the manner indicated above, by an unorganized ferment. Again, it was mentioned above that crystallizable nitrogenous organic substances, such as leucin, asparagin, and tyrosin, occur in plants, and it was pointed out that they may be formed synthetically. But there can be no doubt that they may be, and frequently are, formed analytically,—that is, by the decomposition of more complex substances. For instance, when the seeds of leguminous plants, such as the Pea or the Bean, germinate, the quantity of proteid substance diminishes, and the quantity of amides, notably asparagin, increases; there can be no doubt that the latter are derived from the former. It is well known that similar changes take place in the pancreatic digestion of animals, that leucin and tyrosin are formed from proteids, and that this is effected by an unorganized ferment termed ‘ ‘ trypsin”; it is quite possible that a ferment of this kind may be present in plants. Finally, there is probably, in certain plants at least, a ferment which converts cellulose into sugar. For instance, the Date seed contains a quantity of non-nitrogenous reserve material stored up as cellulose in its very thick cell-walls; on germination this undergoes absorption and is conveyed to the embryo; it is extremely probable that the conversion of the insoluble cellulose into some soluble substance (doubtless sugar) is effected by the action of a ferment. The penetration of the absorbent organs of parasites into the tissues of their hosts is probably effected by the action of a ferment of this kind which is excreted by the parasite.

But there are still other chemical changes to be accounted for as the result of which substances relatively rich in oxygen are produced from others which are relatively poor in that element. Some of these are, so far as is known, processes of simple oxidation, which go on as readily outside the organism as within it; for instance, chlorophyll is oxidized quite as readily in alcoholic solution as when it exists in the chlorophyll-corpuscles of a plant; these processes of simple oxidation may then be regarded as going on independently of the vital activity of the organism. But there are other and more complex oxidations which may be termed “oxidative decompositions”; these involve something more than mere oxidation, and appear to depend upon the vital activity of the organism.

The following instances may be given to illustrate the nature of these changes. Ethyl-alcohol becomes oxidized, under the influence of a Fungus known as the My coderma Aceti, as follows—

Acetic acid.

C 2 H 6 O + O 2 =C 2 H 4 O 2 + H 2 O.

Another similar Fungus, the Mycoderma Vini, induces a more complete oxidative decomposition of alcohol,—carbon dioxide and water being the products of its action. Again, a substance termed “pyrocatechin” and various organic acids occur in plants, and there seems reason, from the researches of Hoppe-Seyler and of Carl Kraus, to believe that they are derived from carbohydrates in some such way as the following—

Glucose. Pyτocatechin. Succinic acid.

3C 6 H 12 O 6 + 4O 2 = C 6 H 6 O 2 + 3C 4 H 6 O 4 + 3H 2 O, it being understood that this suggests only one of the ways in which the vegetable acids are formed.

There are yet other processes of decomposition which, like the oxidative decompositions, are effected under the influence of living protoplasm, but which, unlike them, do not depend upon the presence of oxygen; on the contrary, these decompositions, which may be generally termed “fermentations,” depend upon the absence of free oxygen, for their activity is the greater the more limited the supply of this element. A characteristic example of this kind is afforded by the decomposition of sugar into alcohol and carbon dioxide, which is effected by Yeast, and is known as the “alcoholic fermentation.” Its nature is indicated by the following equation—

C 6 H 12 O β =2C 2 H β O + 2CO 2 ∙

Again, various forms of Bacteria effect decompositions of this kind. Of these the putrefaction of organic matter, the lactic and butyric fermentations, are examples. It must not be supposed, however, that the property of exciting fermentation is confined to the protoplasm of lowly plants such as Yeast and Bacteria. It has been found that various fermentations are set up when living plant-organs of any kind—leaves, flowers, fruits, seeds—are kept in an atmosphere which contains no free oxygen.

The characteristic accompaniment of the destructive metabolism of plants, as of all living organisms, is, under normal conditions, that interchange of gases between the plant and the atmosphere which is known as “respiration,” and which consists in the absorption of oxygen and the evolution of carbon dioxide. It may be stated generally that the continual absorption of free oxygen is essential to the existence of at least the more highly-organized plants, and that in the absence of a supply of free oxygen they die. Death under these circumstances is to be attributed to the arrest of those metabolic processes which are accompanied by an evolution of kinetic energy in the organism—that is, of the destructively metabolic processes; and of these by far the most important is the self-decomposition of the protoplasm. It would appear that the absorption of oxygen is essential to the self-decomposition of the protoplasm-molecule. It is impossible to say anything definite as to the mode in which oxygen affects this process. Pflüger has, however, suggested that the absorbed oxygen enters into the protoplasm-molecule as “intramolecular” oxygen, that the molecule is thereby rendered unstable, and that it then readily undergoes decomposition.

In contrast to the plants which continue to live only when supplied with free oxygen (the aerobia, as Pasteur has termed them) stand the anaerobia —those, namely, which thrive best in the absence of free oxygen, and to which, in certain cases, the access of free oxygen is fatal; of the latter, certain Schizomycetes and Saccharomycetes may be taken as examples. It is remarkable that it is just the anaerobiotic plants which are most highly endowed with the property of exciting fermentation; and this, taken in conjunction with the fact that the activity of fermentation stands in an inverse relation to the supply of free oxygen, indicates the existence of some sort of correlation between the normal respiratory and the fermentative processes. It appears that in aerobiotic plants the normal processes of destructive metabolism, of which the absorption of oxygen and the evolution of carbon dioxide are the ©ut-λvard expression, may be replaced for a longer or shorter time by those abnormal processes of which fermentation is the outward expression; in completely anaerobiotic plants the fermentative are the normal processes. It is difficult to explain the physiological significance of fermentation, and to determine the manner in which it contributes to the maintenance of the life of the organism. Pasteur has suggested that it is the expression of an effort of the organism to obtain oxygen from substances which contain it in combination. Another possible view is that the organism obtains, by the fermentative decomposition of the substances upon which it acts, the supply of energy which, in the case of an aerobiotic plant, is afforded by the normal decomposition of its own protoplasm-molecules.

The products of destructive metabolism are extremely numerous and of very different chemical nature. They may be roughly classified into two groups: ( 1 ) the waste-products, substances which cannot be used in the constructive metabolism of the plant, and which may be excreted; and (2) the plastic products, substances which can enter into the constructive metabolism.

1. Waste-Products.— Among the waste-products the most constant are carbon dioxide and water, which are exhaled in respiration; it may, in fact, be stated generally that all living plants and parts of plants exhale carbon dioxide and watery vapour at all times. There is, however, no constant relation between the volumes of carbon dioxide exhaled and of oxygen absorbed in respiration, and the processes of destructive metabolism, of which the respiratory interchange of gases is the external expression, are so complex that the relation, whatever it may be, between the volumes of these gases in any particular case cannot be accounted for. The degree of independence between these processes is well illustrated by the fact that the absorption of oxygen is relatively greater at low temperatures, and that the exhalation of carbon dioxide is relatively greater at high temperatures. This seems to indicate that at a low temperature the storing-up of intramolecular oxygen is relatively more active than the decomposition of the protoplasm-molecules, whereas at a high temperature the converse is the case. At medium temperatures these processes are about equally active, for it has been ascertained in various cases that the volumes of oxygen absorbed and of carbon dioxide exhaled are under these circumstances approximately equal. It must not, however, be concluded that the exhalation of carbon dioxide is entirely independent of the absorption of oxygen, for the observations of Broughton, Wilson, and Wortmann all show that when plants are deprived of a supply of free oxygen the activity of the exhalation of carbon dioxide rapidly diminishes.

Among the other waste-products the following are those which are of most common occurrence,—organic acids, aromatic substances, colouring matters, bitter principles, certain fatty bodies, alkaloids.

(1.) Organic Acids.— The organic acids are very generally present in plants, either free or in combination with organic or inorganic bases, and it is to the presence of these acids or of their acid-salts that the acid reaction of plant-tissues is due. Those most commonly occurring are the malic, tartaric, citric, oxalic, and fatty acids, the last-named being generally in combination with glycerin, forming fats (glycerides). There can be little doubt that they are to be regarded as products of destructive metabolic processes, though Liebig regarded some of the more highly-oxidized acids as the first products of constructive metabolism, and as being formed from carbon dioxide and water in the cells which contain chlorophyll. It is not so clear that they are all to be regarded as waste-products; it appears possible that some of the less highly-oxidized may undergo reduction with the formation in carbohydrates, for it has been observed, especially by Beyer, that in ripening fruits the acids diminish and the sugar increases in quantity. Again, there can be no doubt that fats enter into constructive metabolism, and hence the fatty acids must be regarded as plastic products. The more highly - oxidized acids are almost certainly waste-products. Oxalic acid, for instance, is commonly found as crystals of calcium oxalate which, in most cases at any rate, undergo no alteration. It appears that the oxalic acid is withdrawn in this way from the sphere of metabolism, and, inasmuch as these crystals are deposited especially in the deciduous parts of the plant, it is also ultimately got rid of. It is probable that the organic acids are largely produced as the result of oxidative decompositions (see supra). There can be no doubt that the selfdecomposition of protoplasm is attended by a formation of acids, especially of nitrogenous acids, such as the aspartic and gluta-minic, and of fatty acids.

In addition to their significance in the constructive metabolism of plants the organic acids are of use in other ways. Their presence in the living cells contributes to the maintenance of the turgid condition; the presence of acid-sap in the root-hairs renders possible the solution and absorption of mineral substances which are insoluble in water; oxalic acid, at least, decomposes the salts absorbed by the roots; and finally it appears that the organic acids are capable of inducing the conversion of one carbohydrate into another—cane-sugar into glucose, for instance—and they may in this way play an important, though hitherto undetermined, part in the general metabolism of plants.

(2.) Aromatic Substances.— These occur generally in the form of glucosides, the most common of which is tannin. The glucosides are bodies, for the most part non-nitrogenous, which yield sugar on decomposition amongst other substances. In so far as they yield sugar they may be regarded as plastic products; but the aromatic substances to which they give rise on decomposition are waste-products, for it appears from the observations which have been made on this point that the higher plants, at least, cannot avail themselves of carbon when combined in an aromatic molecule for the purposes of their constructive metabolism. Probably the resins which are so commonly present in plants are derived from tannin. The first step is the formation of a terpene (C 10 H lβ ) in the secreting cells; this is then excreted into the ducts and undergoes partial oxidation with the formation of resin. In connexion with the terpenes two hydrocarbons, caoutchouc and gutta-percha (C 5 H 8 )ae, may be mentioned, which occur in the latex of certain plants.

It is not possible to make any definitive statement as to the mode of origin of the aromatic substances in the plant, but the fact that tannin is constantly present in the cells of parts in which destructive metabolism is active—growing points, mobile organs of leaves, galls, for example—tends to prove that this glucoside at least may be derived from protoplasm. It must not be overlooked, too, that substances like tyrosin, which contain an aromatic radical, occur in plants, and that they are derived more or less directly from protoplasm.

(3.) Colouring Matters. — The principal colouring matters of plants are—(«) those which occur in the walls of the bark-cells of trees and shrubs (phlobaphenes); {b) those of woods, such as logwood; (c) those which occur in solution in the cell-sap, as in most flowers; (d) those which occur in connexion with protoplasmic corpuscles, as in the Algae and in the leaves and other green parts of the higher plants. With regard to the three first-named groups it appears probable that they are derived in various ways from tannin.

Of the colouring matters which occur in connexion with protoplasmic corpuscles by far the most important is chlorophyll, the substance to which plants owe their green colour. The corpuscle has a spongy structure, the interstices of which are occupied by the chlorophyll in solution in some fatty substance. The other colouring matters which may be present in corpuscles are— ctiolin, yellow, which is apparently present in all chlorophyll-corpuscles, conspicuously so in those in parts of normally green plants which have been growing in darkness, and is apparently an antecedent of chlorophyll; xanthophyll, also yellow, and commonly present in chlorophyll - corpuscles, especially in those of fading leaves, probably a derivative of chlorophyll; anthoxanthin, also yellow, the colouring matter of yellow flowers, and a derivative of chlorophyll; phycoxanthin, brownish, present in the chlorophyll-corpuscles of the brown Algae (Phaeophyceae or Mclanophyceae) ; phycocrythin, red, present in the chlorophyll-corpuscles of the red Algae (llhodophyceae, Florideae).

Chlorophyll is a substance of such great physiological importance that the conditions of its formation and its properties must be treated of in some detail. The general conditions upon which its formation depends are (a) exposure to light, (δ) a sufficiently high temperature, (c) a supply of iron. Plants which are normally green are not green if they have been grown in the dark, or if the temperature has been too low, or if they have not been supplied with iron; they are usually yellow*, and in the last case especially they may be quite colourless. Normally green plants w*hich have been kept in the dark or at too low a temperature are said to be “etiolated,” since they form etiolin; plants which have grown in absence of a supply of iron are said to be “chlorotic.” There are good grounds for regarding etiolin as an antecedent to chlorophyll. It is formed in the corpuscle in darkness and at a temperature lower than that which is necessary for the formation of chlorophyll. It appears from the researches of Gris, Mikosch, and others that when the corpuscle is about to form etiolin it contains a starch-granule, and that as it assumes a yellow colour the included starch-granule diminishes in size and may disappear. It must not be inferred from this observation that the etiolin is directly formed from the starch. It is more probable that it is derived from the protoplasm, and that, as the protoplasm is consumed in the formation of the etiolin, the starch is used in the construction of fresh protoplasm. Under the influence of light and of a sufficiently high temperature the yellow etiolin is converted into the green chlorophyll, but nothing is known as to the nature of the process by which the conversion is effected.

With regard to the physical properties of chlorophyll it has long been know*n that it is soluble in alcohol, ether, benzol, chloroform, carbon disulphide, and various oils. Hansen has obtained, by a process of saponification, from the alcoholic extract of leaves a green crystalline substance, probably the purest form of chlorophyll yet obtained, which is readily soluble in water. All solutions of chlorophyll in the above-mentioned media are fluorescent,—that is, when they are viewed by reflected light they appear opaque and of a deep lake-red colour, but when thin layers are viewed by transmitted light they appear green. If the light which has passed through a layer of a moderately strong solution be examined w*ith the spectroscope a characteristic absorption-spectrum will be observed. Beginning at the red end of the spectrum, a well-marked dark band will be seen between Fraunhofer’s lines B and C, extending rather beyond C, a second dark band in the orange between C and D, a third very faint band at the junction of the yellow and the green, and a fourth more distinct band in the green near F. When an alcoholic extract of leaves is used, as is ordinarily the case, the whole of the blue end of the spectrum beyond F is absorbed, in consequence of the coalescence of three broad bands, which can be seen separately when a very dilute solution is used, two of the bands being in the blue between F and G, and one at the end of the violet. The spectrum of the alcoholic extract presents then seven bands in all. According to Hansen, the spectrum of solutions of his crystallized chlorophyll possesses only the first four of the above-mentioned bands, and it is only when very thick layers are used that the blue end of the spectrum is absorbed; this is true also of the spectrum of the green colouring matter obtained by Tschirch.

Little is as yet known as to the chemical composition of chlorophyll. Gautier and Hoppe-Seyler have both obtained a crystalline green substance from the alcoholic extracts of leaves, termed by the latter “chlorophyllan,” which is not to be regarded as pure chlorophyll. The following are their analyses of this substance, to which is added for comparison Hansen’s analysis of the green crystalline substance which he obtained— [table]

Hansen states that the ash found by him is due to the previous processes of preparation, and that the only normal ash-constituent is iron, which neither Gautier nor Hoppe-Seyler had discovered. Tschirch on reducing chlorophyllan by means of zinc-dust has obtained a green substance which does not crystallize, and is soluble in alcohol, ether, and oils, but not in water. This he believes to be pure chlorophyll. From the percentage composition of the crystals of chlorophyllan Gautier deduces the formula C 19 H 22 N 2 O 3 , and draws attention to the similarity between this and the formula of bilirubin (C 16 H 11 N 2 O 3 ). Hoppe-Seyler concludes that chlorophyllan contains phosphorus in its molecule, and is either a lecithin or a lecithin compound. Schuhck has found that the residue of an ethereal solution of chlorophyll when treated with sulphuric or hydrochloric acid yields glucose amongst other products; he therefore regards chlorophyll as a glucoside.

(4.) Bitter Principles.— It has been ascertained that some of these are glucosides, and some alkaloids, but the chemical nature of many of them is still undetermined. Such are santonin (Cj 5 H 18 O 3 ), aloin (Cj 5 H lβ O 7 ), quasiin (C 10 H 12 O 3 ). It is impossible at present to say anything as to the possible mode of their origin or as to their physiological significance in the plant.

(5.) Certain Fatty Bodies.— The ordinary fats (glycerides) are to be regarded as plastic products, and they will be subsequently treated of under that head. But there are certain fatty bodies of which this statement cannot be made; these are cholesterin, lecithin, and wax. It is not known how these substances are formed, but probably they, like the ordinary fats, are derived from protoplasm. This view is especially probable with regard to lecithin, which is a nitrogenous and phosphorized fat. Wax occurs especially in the external cell-walls or on the surface of those parts of plants which have a cuticularized epidermis; the “bloom” on fruits, for example, is a layer of wax.

(6.) Alkaloids.— The alkaloids are regarded as waste-products, because, as the observations of Knop and Wolff show, the demand for combined nitrogen cannot be met by supplying the plant with it in the form of alkaloids, though the plant can avail itself of such organic nitrogenous substances as urea, uric acid, leucin, tyrosin, or glycocoll. The alkaloids are compound ammonias which are not volatile at ordinary temperatures. With regard to their mode of origin in the plant, there can be little doubt that they are derived more or less directly from protoplasm, or at least from proteid, as are urea and uric acid in the animal body. But, although these nitrogenous waste-products are formed in the destructive metabolism of plants, their formation is not accompanied by a loss of nitrogen, for they are not excreted, as is the case in animals, but are deposited in the cells.

2. Excretion.— Of the waste-products, some, such as oxygen, water, and carbon dioxide, are excreted in the gaseous form—the oxygen and the carbon dioxide through the superficial cell-walls of the plant, the watery vapour through the stomata. Some of the carbon dioxide may combine with earthy bases to form carbonates, which are either retained in the plant or excreted in solution. The resins and ethereal oils, as well as wax, are frequently excreted. The mechanism of excretion is widely different in different cases. The resins and ethereal oils are usually excreted by means of special glandular organs. The gland may be a hair on the surface, and it is then commonly the terminal cell at the free end which is secretory; or it may be a group of epidermal cells between which large intercellular spaces are formed, which serve as receptacles for the excreted substance; or it may be formed by the absorption of the adjoining walls of a group of cells belonging partly to the epidermis and partly to the underlying ground-tissue, a cavity being thus constructed, which contains the excreted substance; or again, longitudinal strands of cells may become separated so as to enclose an elongated intercellular space into which they excrete (resin-ducts). In many cases the substance to be excreted may be detected in the glandular cells; not unfrequently, however, and always in the case of wax, no trace of it can be discovered in the cells themselves; it is first to be found in the cellwalls between the cuticular and the deeper layers. The actual excretion is usually effected, in the case of superficial glands, by the rupture of the cuticle which is continuous over the gland, and by the consequent escape of the contents; in some cases the gland remains closed, and any volatile substances (ethereal oil) which may be present escape by evaporation.

The excretion of the earthy carbonates in solution is most commonly effected by means of a well-developed gland. Such a gland consists of a group of modified parenchymatous cells in connexion with the termination of a fibro-vascular bundle; and one or two openings, termed “water-pores,” and somewhat resembling stomata, are present in the epidermis immediately over it. Under the action of the root-pressure the gland excretes water which holds the carbonates in solution. Glands of this kind are present in the leaves of various Saxifragaceous and Crassulaceous plants. In other cases these salts appear to be excreted by ordinary epidermal cells. In certain Ferns (various species of Polypodium and Aspidium), for instance, scales of calcium carbonate are found on depressions in the surface of the leaves which are situated immediately over the terminations of the fibro-vascular bundles.

It not unfrequently happens that plants excrete substances other than waste-products, but this has the effect of securing indirect advantages. In the great majority of flowers there are glandular organs which excrete a watery fluid holding principally sugar in solution; these organs are termed “nectaries,’’and the excretion “nectar.” The nectary has essentially the same structure as the water-gland described above, the only important difference being that, whereas the gland is sunk in the tissue and is covered by the epidermis, the nectary has a large free surface, so that the nectar is at once poured out on to the exterior. But there is an important functional difference between them, namely, that, whereas excretion by the gland can only take place under the influence of the root-pressure, excretion by the nectary is independent of the root-pressure, for it will continue when the flower has been removed from the plant. Another instance of an excretion of this kind is afforded by the carnivorous plants. The glands of their leaves excrete a watery liquid which holds in solution a peptic ferment and one or more organic acids. The use of the nectar is to attract insects, and thus to ensure crossfertilization. The use of the excretions of the carnivorous plants is to dissolve the organic matter (usually insects) which has been deposited on the leaves, so as to bring it into a form in which it can be absorbed.

Many of the waste-products are not excreted, but remain in the plant. Thus the terpenes are indeed excreted by the cells which line the resin-ducts, but these ducts have no aperture on the surface of the plant. Similarly the caoutchouc and gutta-percha which are contained in the laticiferous tissue of certain plants have no means of egress. This is also true of the tannic acid, of the calcium carbonate (usually) and oxalate, of the alkaloids, and of silica. These substances are usually deposited in the cells. Calcium carbonate and oxalate are deposited in the form of crystals either in the cell-wall or in the cell-cavity, and silica in the cell-wall. In some cases calcium carbonate is deposited on cellulose processes which extend into the cell from its wall, the whole body being termed a “cystolith.”

3. Plastic Products.— The principal non-nitrogenous plastic products are the carbohydrates and the fats (glycerides); to these, as suggested above, some of the organic acids are perhaps to be added.

Of the carbohydrates the substance which most demands attention is starch. Starch makes its appearance in the form of minute granules in chlorophyll-corpuscles in which constructive metabolism is being actively carried on. It was thought that it was the immediate product of the decomposition and recombination of carbon dioxide and of water in the corpuscle under the influence of light, the process being represented by some such equation as the following—

6CO 2 + 5H 2 O = CθHjθOg + 6O 2 ∙

Increased knowledge of the conditions under which starch makes its appearance in plants has, however, made it evident that this is not the case. Starch may be regarded as the first visible product of the constructive metabolism going on in a chlorophyll-corpuscle, but it is not the first non-nitrogenous substance formed. That, as has been pointed out, is probably a body allied to formic aldehyde. The starch is formed as one of the products of the decomposition of the protoplasm of the corpuscle. That this is so is made evident when the formation of starch in parts of plants which are not green and which are not exposed to light is considered. In the cells of these parts there are certain small colourless protoplasmic bodies which are termed “starch-forming corpuscles” or “leukoplasts,”and it is by these that the starch-granules are formed. It appears that the starch-granule is formed in the first instance by the decomposition of a portion of the protoplasm of the corpuscle, successive layers of starch being deposited upon the primitive granule by the decomposition of successive layers of protoplasm. It is in this way that the stratified structure of these starch-granules is produced. There is no reason for assuming that the process of starch-formation in chlorophyll-corpuscles is essentially different from that in leukoplasts; on the contrary, in view of the close relationship of these bodies, the one being convertible into the other, there is every reason for believing it to be the same. There is, however, this functional difference between chlorophyll-corpuscle and leukoplast, that in the former the synthetic processes, i.e., the construction of protoplasm, begin with such simple substances as carbon dioxide, water, and mineral salts, whereas in the latter they begin (see below) with tolerably complex substances, such, for example, as glucose and asparagin. Starch, then, is the immediate product, not of constructive, but of destructive metabolism.

Various kinds of sugar, notably glucose and cane-sugar, are also commonly to be found in plants. Glucose may be regarded as having been derived by the action of an unorganized ferment from one or other of the other carbohydrates, except in certain plants, the Onion for example, in which it appears to be formed in the chlorophyll-corpuscles in the first instance. Nothing is known at present as to the mode of origin of cane-sugar, which exists in such large quantities in certain plants, as the Beet and the Sugar-cane.

With regard to the fats, it is commonly assumed that they are formed directly from the carbohydrates, because in oily seeds, for example, as the starch which they contain when young diminishes in quantity it is replaced by fats. There is, however, sufficient evidence to prove that the fats are the products of the decomposition of protoplasm. The disappearance of the starch in ripening oily seeds is due to its being used up in the construction of protoplasm, as the protoplasm undergoes decomposition in connexion with the formation of fat.

The nitrogenous plastic products are proteids and amides. There can be little doubt that the proteids may be derived from protoplasm. If the molecule of living protoplasm be regarded as an extremely complex one, there is no difficulty in inferring that proteid may be one of the products of the decomposition of the protoplasm-molecule. The amides may also be products of the decomposition of protoplasm, or they may be formed from proteids by the fermentative action of living protoplasm or by the action of some as yet undiscovered unorganized ferment, as pointed out above.

It has been stated that the plastic products are so called because they are substances which can be used in the constructive metabolism of plants. But it must not be inferred that they are so used immediately. The very fact that it is possible to detect their presence in considerable quantity is a proof that this is not the case. They are largely stored up either for the use of the plant itself at some future time, or for the benefit of the progeny of the plant. In a perennial plant, for example, plastic products are stored in the persistent parts for the use of the plant when it recommences its active growth; they are also stored up in seeds and spores to be used by the young plant during the early stages of germination. Plastic products thus stored up are termed “reserve materials,” and the organs in which they are deposited are termed “depositories for reserve materials.” The non-nitrogenous reserve materials are stored up in the form of carbohydrates or of fats. The starch which is formed in the green parts of the plant (which is, be it observed, a temporary reserve material) is converted into a soluble substance, probably glucose, and is conveyed in solution to the depository; and from it, directly or indirectly, the non-nitrogenous reserve materials are formed. The reserve carbohydrates are stored up either in the insoluble or the soluble form. In the former case they are deposited as starch-granules, or as cellulose (as in the endosperm of the Date) in thick cellwalls; in the latter they exist as various forms of sugar in solution in the cell-sap. The starch-granules are formed in these depositories by the leukoplasts. From the soluble non-nitrogenous substance, probably glucose, together with nitrogenous substances conveyed to the cells, the leukoplasts construct protoplasm; and it is as the result of the decomposition of this protoplasm in a certain way that starch is formed. This is true also of the reserve cellulose. From the plastic materials, both nitrogenous and non-nitrogenous, which are supplied to the cells protoplasm is constructed, and the external layers of protoplasm undergo decomposition in such a way that cellulose is formed and deposited in successive layers upon the internal surface of the cell-wall. The various kinds of sugar (cane-sugar in the Beet root, glucose in the Onion, inulin in the Dahlia root, mannite in the unripe fruits of the Olive and in some Agarics, trehalose in many Agarics) are probably formed more or less directly from the glucose conveyed from other parts to the depository in each case. The fats occur as reserve materials characteristically in seeds and sometimes in fruits; they are not stored up in any considerable quantity in any other kind of depository. They too are formed by the decomposition of protoplasm which has been constructed from plastic materials, nitrogenous and non-nitrogenous, which have been conveyed to the cells.

The nitrogenous reserve materials are stored either in solution or as solid granules. In the former case they are amides, such as asparagin and glutamin, leucin and tyrosin, and are held in solution in the cell-sap; they are present characteristically in roots and tubers, but they have also been found, though in small quantity, in seeds. In the latter case these materials are stored in the form of proteids, chiefly globulins and peptones, and the granules in which they are deposited are termed “aleurone grains.” The aleurone grain may consist simply of an amorphous mass of proteid, or a portion of the proteid may have crystallized out so as to form a crystalloid; in most cases the grain contains a small mass of mineral matter which consists, according to Pfeffer, of double phosphate of lime and magnesia. Aleurone grains occur characteristically in seeds, and they are especially well developed in oily seeds.

When once deposited, the reserve materials suffer no further change, or at most the proteids may slowly undergo some alteration (globulin being converted into albuminate), ' so long as the organ in which they are deposited remains in an inactive condition. But when the external conditions become favourable the quiescent organ resumes its active life—in a word, it germinates—and the reserve materials which it contains then undergo chemical changes of such a nature as to convert them into substances which can readily travel to the seat of growth and can be used as plastic material by the growing cells. In a germinating seed, for instance, as the embryo grows the reserve materials of the seed diminish in quantity; they are evidently conveyed to the seedling, and are used by it in the construction of new protoplasm. Beginning with the non-nitrogenous reserve materials, the starch in starchy seeds and the cellulose in such seeds as the Date are converted into sugar; this is proved by the detection of sugar as well in the seed as in the seedling, and by the detection in the seed of an unorganized ferment which possesses the property of converting starch into sugar. In oily seeds the fats are replaced by starch, formed through the intermediation of protoplasm, and the starch so formed is converted into sugar. With regard to the reserve proteids, they are converted into amides, for it has been ascertained that, as they diminish in quantity, the amides, and notably asparagin, increase.

The effect of the absorption of these plastic substances by the embryo is that the cell-sap of its cells becomes charged with them, for the supply is more rapid than the consumption in the formation of protoplasm. If the seedling is growing under favourable conditions these substances gradually diminish in quantity. Some light has been thrown upon the nature of these conditions by the researches of Pfeffer. He found that Lupin seedlings grown in the dark contained a very large quantity of asparagin so long as they continued to live, but that if they were exposed to light the asparagin gradually diminished. But he ascertained further that mere exposure to light is not the cause of this, since the asparagin did not diminish in seedlings exposed to light in an atmosphere which contained no carbon dioxide. The disappearance of the asparagin depended, therefore, upon conditions which were essential to the formation of non-nitrogenous organic substance by the seedlings. Now the Lupin seed is one which is particularly rich in nitrogenous reserve materials, the quantity of non-nitrogenous reserve materials being relatively small. The accumulation of the asparagin in the seedlings grown in the dark is then to be ascribed to the absence of an adequate supply of non-nitrogenous substance with which it could combine to form proteid. When the seedlings were exposed to light this supply was forthcoming, and then the asparagin disappeared.

Supply of Energy.

It is evident that the various chemical processes which make up the metabolism of plants involve an expenditure of energy; hence the maintenance of the life of the plant is dependent upon a supply of energy.

In the case of animals the food affords the principal supply of energy. It consists for the most part of complex organic substances which represent a considerable amount of potential energy, and when these substances are decomposed in the body the potential energy appears in the kinetic form. This holds good also with reference to plants which are destitute of chlorophyll, for their food necessarily includes, like that of animals, complex organic substances. But with plants which possess chlorophyll the case is entirely different. Their food consists of inorganic substances which do not represent any considerable amount of potential energy; from these simple substances green plants build up complex organic substances which do represent a considerable amount of potential energy; it is evident, therefore, that green plants must be largely supplied from without with kinetic energy in some form or other. It has been already mentioned that the metabolic processes of plants are materially affected by external conditions, especially by the presence or absence of light, and by variations in the temperature of the surrounding medium. A somewhat elevated temperature is essential to the active life of all plants, but light is essential only to the life of those which contain chlorophyll. This naturally suggests that the energy requisite for the maintenance of the life of plants is obtained by them in the form either of light or of heat.

Light.— In discussing the constructive metabolism of green plants it was pointed out that such can only assimilate their food—that is, can only construct protoplasm from it—when exposed to light, whereas plants which do not possess chlorophyll can assimilate their food in the absence of light. It is true that a green seedling can live for a time in continuous darkness and increase in light, but it does so, not by assimilating its food, but at the expense of the organic reserve materials which may be present in it. The fact of the dependence of green plants upon exposure to light suggests that the energy necessary for the processes of their constructive metabolism is obtained in the form of light, and that their chlorophyll enables them to avail themselves of this form of kinetic energy. The function of chlorophyll has been made clear by the researches of Timiriaseff and of Engelmann. They have shown that the evolution of oxygen by a plant containing chlorophyll, which is the expression of the first stages of constructive metabolism, is most active when the plant is exposed to those rays of the solar spectrum which correspond to the absorption-bands of the chlorophyll-spectrum; the more conspicuous the absorption - band, the greater is the degree of activity, so that the evolution of oxygen is most considerable in the rays between the lines B and C of the solar spectrum, at the junction of the red and the orange, which correspond to the absorption-band I in the chlorophyll-spectrum. It is, then, in consequence of this absorption by the chlorophyll that the kinetic energy of the solar rays is made available for the work of constructive metabolism in the plant. The whole of the kinetic energy absorbed by the chlorophyll is not converted into potential energy; still the chlorophyll-corpuscle appears to be a very perfect machine in this respect, for, according to Timiriaseffs calculations, it converts into the potential form as much as 40 percent. of the absorbed energy. Inasmuch as light exercises so great an influence upon the constructive metabolism of green plants, it may be inferred that it must indirectly affect the absorption of food-materials by the roots. Rudolph Weber has, in fact, ascertained that the greatest absorption of the essential ash-constituents takes place when the plant is kept exposed to those rays of light which are most efficacious in promoting its constructive metabolism.

The effect of light upon the destructive metabolism of plants appears to be unimportant. This subject has been investigated by means of observations upon the respiration of plants; and such a method is calculated to afford the necessary information, inasmuch as the activity of respiration may be taken as a measure of the activity of destructive metabolism.

It has been generally stated that chlorophyll is not formed in the absence of light. There are, nevertheless, certain cases in which its formation in complete darkness has been observed, provided that the temperature has been sufficiently high, namely, in the cotyledons of some Conifers and in the leaves of Ferns. The colouring matter etiolin is formed in the corpuscles in darkness, but the conversion of this into chlorophyll can only take place, as a rule, under the influence of light. The formation of chlorophyll will take place in light of very low intensity, but, as Wiesner’s experiments show, there is a lower limit of intensity below which light is inactive. With regard to the relative efficacy of the different rays of the spectrum in promoting the formation of chlorophyll, it appeal’s from Wiesner’s researches that all the rays between Fraunhofer’s lines B and H promote it in different degrees, and further, in confirmation of older observations, that seedlings turn green more rapidly in the yellow than in any other part of the spectrum. This last statement is true only for light of moderate intensity; when the light is very intense the formation of chlorophyll takes place more rapidly in blue than in yellow light. The reason of this is that in intense light chlorophyll undergoes decomposition, or at least chemical alteration of the nature of oxidation, which goes on most actively in yellow light.

Heat.— Plants behave in relation to temperature like the coldblooded animals. When they are maintained at a low temperature they cease to exhibit any signs of life. The meaning of this is that at a low temperature the activity of the metabolic processes is so reduced that they appear to be altogether arrested. But the importance of a moderately high temperature for the maintenance of the active life of the plant is not, as might be supposed, that it affords a continuous supply of energy to be converted into work; it is rather that it determines the initiation of chemical processes which are earned on by means of energy obtained from other sources. Hence the supply of energy in the form of heat is relatively small as compared, on the one hand, with the supply of potential energy afforded by their food to the plants which do not possess chlorophyll, and, on the otheι∙ hand, with the supply obtained in the form of light by plants which do possess chlorophyll.

It is not possible within the limits of this article to enter fully into the relations existing between plant-life and temperature. The following statements will at least indicate their general nature. In the first place, the tolerance of extreme temperatures is different for different plants, as determined in the case of any particular organ, such as the seeds for instance. Secondly, for each of the processes which can be studied separately, such as germination, growth, respiration, the formation of chlorophyll, the action of unorganized ferments, the evolution of oxygen by green plants in light, &c., there are three cardinal points of temperature to be noted—the minimum, or zero point, at which the performance of the process is just possible; the optimum point, at which it is earned on with the greatest activity; the maximum point, at which it is arrested. But these different phenomena do not all stand in precisely the same relation to temperature,—that is, the cardinal points for the exhibition of any two or more of these phenomena by one and the same plant do not necessarily coincide. Thirdly, the larger the proportion of water in an organ, the more liable it is to be injured by exposure to extreme temperatures.

Expenditure of Energy.

We have now to ascertain what becomes of energy supplied to the plant. The matter may be briefly stated thus: a portion of it is stored up in the plant in the form of potential energy; the remainder is lost to the plant, being either spent in the performance of mechanical work in connexion with growth or movement, or given off, most generally in the form of heat, occasionally in the form of light, and possibly in the form of electricity. The storing-up of energy in the potential form may be termed the “accumulation of energy,” the loss as the “dissipation of energy.”

1. Accumulation of Energy.— The accumulation of energy is the necessary accompaniment of constructive metabolism; the formation of more and more complex organic substances involves the conversion of kinetic into potential energy. By taking into consideration the amount of organic substance formed by a plant from its first development to its death, it is possible to arrive at some idea of the amount of kinetic energy which the plant has stored up in the potential form. For the heat which is given out by burning the organic substance is but the conversion into kinetic energy of the potential energy stored up in the substance; it is but the reappearance of the kinetic energy which was used in producing the substance. The heat, for instance, which is given out by burning wood or coal represents the kinetic energy, derived principally from the sun’s rays, by which were effected the processes of constructive metabolism of which the wood or the coal was the product. The amount of energy thus stored up by plants in the potential form is very large, because they produce relatively large quantities of organic substance.

2. Dissipation of Energy.— The expenditure of energy in connexion with growth and movement, and with the evolution of heat, light, and electricity, is dependent upon destructive metabolism, for the conditions which are essential to destructive metabolism are also those which are essential to the exhibition of these phenomena. Taking growth, for example—that is, continuous change of form accompanied usually by increase in bulk—it appeal’s that in an aerobiotic plant it is dependent upon the following external conditions, namely, a supply of free oxygen and an adequate temperature, conditions which are precisely those upon which the destructive metabolic processes of such a plant also depend. This is true in such plants of the other above-mentioned phenomena also. Anaerobiotic plants can grow when the conditions are such that they can induce active fermentation,—that is, when their destructive metabolism is active. After what has been said in the section on the “Nervous System” above (p. 38 sq.) about animal movement it is hardly necessary to prove that the movements of plants, which are of essentially the same nature as those of animals, depend uρou destructive metabolism and involve a dissipation of energy.

An evolution of energy in the form of heat is the inseparable result of destructive metabolism. With regard to plants, it may be stated generally that the evolution of heat is not sufficiently active to raise the temperature of the plant-body above that of the surrounding medium, it being remembered that plants are constantly losing heat, principally by radiation and in connexion with transpiration. In organs, however, in which destructive metabolism is very active it is easy to detect a rise of temperature, especially when a large number of them are collected together. A good instance of this is afforded by germinating seeds; for example, a rise of temperature is a familiar fact in the process of the malting of Barley. It can also be readily observed in the case of opening flowers in dense inflorescences; Warming observed, for example, that, at the time of the opening of the flowers, the inflorescence of an Aroid {Philodendron bipinnatifidum) attained a temperature of 18 o- 5 C. above that of the air.

The evolution of light by plants is a phenomenon which has been known from the times of Aristotle and of Pliny, and is commonly spoken of as “phosphorescence.” All the well-authenticated instances of luminosity are confined to the Fungi, to various Agarics, and to Schizomycetes {Bacteria'). The so-called “phosphorescence” of decaying wood is due to the presence of the mycelium of Agaricus melleus {Rhizomorpha), and that of putrefying meat and vegetables to micrococci. See Phosphorescence. The evolution of light is essentially dependent upon the life of the organism, and further, it is dependent upon the destructive metabolism; for it ceases when the organism is killed (as by dipping it into hot water), or deprived of its supply of free oxygen, which is essential to the metabolic processes.

In view of the changes, both chemical ami physical, which are going on with greater or less activity in the various parts of a living plant it has not been unnaturally inferred that the electrical equilibrium is being constantly disturbed, and that differences of electrical potential energy may exist in different parts. Many experimenters have investigated this subject, and such differences have been apparently observed. It is impossible to enter here into a detailed consideration of the results obtained; it may suffice to state that in the majority of cases the electrical currents detected do not indicate a dissipation of the energy of the plant, but are due to physical causes, and in some cases even to the effect upon the organism of the apparatus employed for the puιq>ose of detecting them. It has been clearly made out in certain instances that the currents persist in organs which have been suddenly killed in such a way as not to destroy their gross organization.

There is, however, one instance in which an electrical current has been detected which seems to be connected with the destructive metabolism of the plant. Burdon-Sanderson and Munk have both observed that, when the two electrodes are placed upon a mobile leaf of Dionsea muscipula (Venus’s Fly-trap) when at rest, a certain electrical current is indicated by the galvanometer. When the leaf is stimulated, whether the stimulation be or be not followed by a movement, the direction of the observed current is suddenly reversed. This change in the direction of the current—or “negative variation,” as it is termed—is, according to Burdon-Sanderson, the “visible sign of an unknown molecular process,” which he considers to be “an explosive molecular change,” of the same nature as the negative variation which follows upon the stimulation of the muscles and nerves of animals.

In concluding this part of the subject it may be well, for the sake of clearness, to draw up an account of the income and expenditure of a plant.

In the case of a plant possessing chlorophyll the income of matter consists of the food (salts, water, carbon dioxide, free oxygen), and the income of energy of kinetic energy in the form of light and heat, the former being the more important of the two items. The great bulk of the food absorbed is converted into organic matter, and is for the most part retained by the plant in the form of organized structures, of reserve materials, and of waste-products which are not excreted; but a certain proportion of it is lost in the form of the carbon dioxide and water exhaled in respiration, of oxygen exhaled by green parts in sunlight, and of excreted organic or inorganic matter. Besides these items of loss there are yet others. All plants lose a certain amount of matter in connexion with reproduction, for all plants throw off in the course of their lives certain portions of their structure in the form of seeds, spores, antherozoi<ls, &c. Again, plants which persist for more than one period of growth lose matter by the falling off of certain of their organs and of portions of their structure,—for example, by the falling of the leaves in autumn, and by the shedding of bark, fruits, &c. With reference to the expenditure of energy, a large proportion of the income of energy remains stored up in the potential form in the organic matter which the plant accumulates. A dissipation of energy as heat and in connexion with growth is common to all plants: in some there is dissipation of energy in the form of motion, in some in the form of light, in some, probably, in the form of electricity. A loss of energy—potential energy—occurs also when the plant loses organic matter in any of the ways mentioned above. These various items may be tabulated under the two heads of “income” and “expenditure.” The water lost in transpiration is not considered, for it simply traverses the plant; only that amount of water is considered which may be assumed to enter into the processes of constructive metabolism or to be produced in the processes of destructive metabolism. [table]

Movement of Plants.

It has been pointed out above that movement, including in the conception the slow movement of growth, is an item in the expenditure of energy by the plant. The phenomena connected with movement are of such physiological importance that it will be well to consider them rather fully.

In dealing with this large subject attention will be directed for the present simply to the external phenomena, leaving the internal causes and mechanisms till subsequently, and those presented by growing organs will be taken first.

1. Growth.— In commencing the study of growth it is important to have a perfectly clear idea of what the word means. It means the continual change in form of the body of the plant, or of any organ of it, the change being frequently accompanied by increase in bulk, though this is not necessarily the case. For the purposes of this article it will be convenient to use “growth” as meaning, unless expressly stated otherwise, growth in length, that is, the elongation of the organ along the line joining its base and its apex. The conditions upon which growth is dependent are—(1) a supply of plastic material for the formation of new protoplasm; (2) favourable external conditions, especially an adequate temperature; (3) a supply of free oxygen in the case of aerobiotic plants, or, in the case of anaerobiotic plants, of fermentable substance; (4) a supply of water to maintain the turgidity of the cells. Any variation in these essential conditions will lead to a variation in the rate of growth. The capacity for growth is limited, as a rule, to a certain period of the life of an organ and of its constituent cells; when this period is past growth ceases, however favourable the external conditions may continue to be.

The rate of growth of an organ is not uniform. At first the organ grows slowly, then more and more rapidly, until a maximum rapidity is reached, and then the rate diminishes until growth ceases altogether. This cycle of spontaneous variation in the rate of growth is known as the “grand period of growth.” It can be conveniently studied by marking on the growing point of an organ a series of transverse zones of known length, and observing their relative elongation in a given time. It will be found that the youngest (nearest the apex) have elongated slightly, that the elongation is greater the farther each successive zone is from the apex, until a zone of maximum elongation is reached; the elongation of the zones lying behind this will be found to be less and less, until at last zones will be found which have not elongated at all. In addition to the variations in the rate of growth in length of an organ which make up its grand period it is found, if its growth be watched from hour to hour, or at even shorter intervals, that it presents irregular variations, which are likewise to be regarded as spontaneous. Variations in the rate of growth may be induced by variations in the external conditions, especially by variations of temperature and of illumination. It will be of interest to inquire briefly into these relations between growth and temperature and growth and light.

Inasmuch as the decompositions which determine the evolution of energy in the plant are dependent upon temperature, their activity being promoted by a rise of temperature within certain limits, it will be readily understood that growth, which is one expression of the evolution of energy, should likewise be affected by variations in temperature. It has been found, in fact, that the growth of any given plant will only take place within certain limits of temperature, a lowest or minimum temperature on the one hand and a highest or maximum temperature on the other; and further, that between these two points there is one, the optimum temperature, at which the rate of growth is most rapid. Growth is more rapid at each degree as the temperature rises from the minimum to the optimum point; it is less rapid at each degree as the temperature continues to rise from the optimum to the maximum point; and conversely, growth is more rapid at each degree as the temperature falls from the maximum to the optimum, and less rapid at each degree as the temperature further falls from the optimum to the minimum. This dependence of growth on temperature, and this relation between different degrees of temperature and different rates of growth, may be conveniently spoken of as the “tonic influence” of temperature. The mere variation in temperature as such does not appear, as a rule, to affect the rate of growth. Roots exposed to rapid and considerable variations of temperature for some time are found by Pedersen to have grown to about the same extent as similar roots which had been grow ing for the same time at the mean temperature. The only case in which it appears that variation in temperature produces a distinct effect is afforded by Pfeffer’s observations upon the opening and closing of flowers. He found that a rise of temperature caused the flowers to open and a fall to close, the opening or closing being ah expression of the accelerated growth in length of the organ as a whole. This effect of variation of temperature is distinguished as the “stimulating” effect.

In considering the relation of light to growth we have principally to consider its influence as being an essential normal condition of growth, its “tonic” influence,—that is, it is a question whether or not light exercises any influence which can be regarded as “stimulating” on the rate of growth. Speaking generally, it may be stated that plant-organs, with the exception of ordinary flattened horizontally expanded leaves and other organs of similar organization, grow at least as well in darkness as in light,—that exposure to light is not an essential condition of their growth. With leaves and leaf-like organs the case is different. When plants are kept for some time in darkness one of the most striking features is the smallness of the leaves of the shoots which have been developed during that time. This is not to be ascribed to an absence of plastic material, for it is exhibited when plastic material is abun dantly present in the tissues; nor can it be attributed to the fact that in darkness the leaves are not able to carry on the formation of organic substance, for it is not all leaves which remain small in darkness, but only those which have the organization described above. The long tubular leaves of the Onion, for example, continue to grow in darkness, and so do the long flattened leaves of Irises. The arrest of the growth of flattened horizontally expanded leaves in darkness is due to some peculiar effect, which we must regard as of a “tonic” nature, exercised by light upon the growing cells. Intermittent exposure to light for brief periods suffices to enable the leaves to carry on their growth in darkness, and it is not necessary that the light should be intense. The intermittent exposure induces in the leaf a condition, though it cannot be precisely stated what, which permits of the continuance of growth,—a condition which is termed “phototonus.” Assuming that the organ is actually growing, we find that in all cases light retards the rate of growth, and this the more markedly the greater its intensity. Wiesner has, in fact, shown that growth may be altogether arrested by exposure of the growing organ to intense light. The effect of light in retarding growth has been ascertained by comparative measurements of similar organs growing, some in darkness others in light, and is proved negatively by the greater length usually attained by shoots which have grown in darkness for a given time as compared with that attained in an equal time by shoots growing in light. It appears that variations in the intensity of light, as such, affect the rate of growth. Pfeffer has found that exposure to light caused certain flowers to open, and to darkness to close. Probably this is accompanied, as in the parallel case of temperature, by an acceleration of the mean rate of growth.

The relation of the long axis of a growing organ to the line of action of gravity appeal's also to affect its rate of growth. Elfving has found that the sporangiferous hyphae of Phycomyces, which normally grow vertically upwards, grow somewhat less rapidly when they are maintained in the inverse position, that their growth in length is more rapid in opposition to the action of gravity than in the same direction, and this he thinks is true of all organs which normally grow vertically upwards. Gravity would appear, therefore, to exercise a tonic influence on growth. The substitution for gravity of a considerable centrifugal force produced no apparent effect on the rate of growth.

The direction of growth of an organ is determined partly by inherent and partly by external causés. Beginning with the inherent causes, we find that, when the action of external directive influences upon growing organs is as far as possible eliminated, the axis of growth—that is, the line joining the apex and the base—is approximately a straight line. Thus Vöchting has shown that, when growing shoots are caused to grow in darkness and to rotate slowly about a horizontal axis by means of an instrument termed a “clinostat,” their long axes become straight. This is the expression of an inherent tendency which he terms “rectipetality.” But the line of growth is not at all times straight; for instance, the apex of an organ growing vertically upwards does not travel upwards in a straight line, but oscillates from side to side of the vertical. This oscillation is termed “nutation,” and is due to the fact that growth in length is not uniformly rapid on all sides of the growing organ, but that during any given period of time one side grows more rapidly than the others. This unequal growth, which we may term “heterauxesis,” is apparently spontaneous. The particular path which the apex of an organ describes in the course of its growth depends upon the properties of the organ, a point which may now be conveniently dealt with.

Some information as to the properties of an organ may be obtained from observation of its anatomical structure. For instance, cylindrical organs, such as many shoots, are radially symmetrical in structure, and they are in most cases found to be also physiologically radial. Some organs, again, such as the leaves of some Irises, are bilaterally symmetrical in structure, and they are found to be physiologically bilateral. Other organs, finally, such as flattened expanded leaves, prothallia of Ferns, thalloid shoots of Liverworts, though, like the bilateral organs, they present two opposed surfaces, are not bilaterally symmetrical in structure, inasmuch as the tissues in relation with the two surfaces are differently constituted. Such organs are dorsiventral both anatomically and physiologically. The properties of an organ are not, however, always to be inferred from structure. In some cases radial symmetry of structure is accompanied by dorsiventral properties. This is the case, as Sachs has shown, with the shoots of Tropaeolum majus under certain circumstances.

The question now naturally arises, How are the peculiar properties induced? In some cases they can only be accounted for by regarding them as inherent; this is true of radial organs, of bilateral organs, and to a certain extent of dorsiventral organs. The lateral branches of dicotyledonous trees have, in many cases, been found to possess inherent dorsiventral properties. In other cases, however, dorsiventrality is induced. It has been found by Frank —and his observation has been corroborated by De Vries—that the dorsiventrality of the lateral shoots of Conifers is induced by their relation to gravity. When maintained in a horizontal position during their growth the side which is uppermost becomes the dorsal surface, the other the ventral surface. Again, Sachs has shown that the young shoot of Tropaeolum is radial, but that if it be exposed to strong unilateral illumination it exhibits dorsiventral properties, the more strongly illuminated side becoming the dorsal surface, and further, that the plane of dorsiventrality may be altered by causing the light to fall on a fresh side of the shoot. A similar case of the induction of dorsiventrality is offered by the thalloid shoot of Marchantia. Tracing the development of the shoot from a gemma, we find the gemma to be bilaterally symmetrical in structure and in physiological properties. It falls to the earth with one surface undermost, which becomes the ventral surface, whilst the upper surface becomes the dorsal. That the dorsiventrality is in this case induced by light is proved by the fact that, if an experiment be so arranged that the light falls on the under surface of the gemma, this becomes the dorsal surface, whilst the upper surface becomes the ventral.

The further question now arises, How can the nature of the properties of an organ be determined, if, as has been shown, it cannot always be determined by an examination of the structure? The answer is, that it can be determined by observing the mode of growth of the organ, and especially its response to the action of external directive influences. Thus, the spontaneous heterauxesis of a radial organ is such that each side in turn is the one which is growing with the greatest rapidity, so that, seen from above, the apex will appear to describe a somewhat circular orbit, and its path upwards will be spiral. This kind of nutation is termed “revolving nutation,” or “circumnutation” (Darwin). Similarly, in bilateral organs each of the two sides grows alternately the more rapidly, so that, seen from above, the apex appears to oscillate from side to side of the vertical, and its path will be a zigzag line. Also in dorsiventral, as in bilateral, organs each of the two sides grows alternately the more rapidly, but the period of alternation is much longer, and may occur only once during the whole period of growth. For instance, the young leaves of Ferns are rolled up upon their internal (dorsal) surfaces; they present what is known as “circinate vernation.” This is due to the fact that at first the lower (ventral) surface of the leaf grows more rapidly than the upper (dorsal); it is only towards the end of the period of growth that the upper surface grows the more rapidly, and then the leaf expands. Special terms have been applied to these phases of growth; when the upper surface of the organ is growing the more rapidly the growth is said to be “epinastic,” when the lower “hyponastic.”

The spontaneous variations in the direction of growth of an organ thus afford some indication of the nature of its organization, but this is more clearly shown by its response to the action of external directive influences. These will now be taken in order.

Radiant Energy.— It will be convenient to consider separately the phenomena exhibited by organs of different physiological properties.

Beginning, then, with radial shoot-organs, it is usually found that when light falls upon one side of such an organ the organ curves so as to direct its apex towards the source of light,—in other words, that it tends to place its long axis parallel to the direction of the incident rays. The effect of the unilateral illumination is to cause heterauxesis of the organ such that the side upon which the light directly falls is the one which grows the most slowly, and therefore becomes concave. Inasmuch as all curvatures induced by light are included under the term “heliotropism,” organs which exhibit the kind of curvature above described may be said to be “positively heliotropic.” As examples of positively heliotropic radial organs may be mentioned radial stems, the multicellular stipes of some Fungi {Coprinus, Claviceps), the sporangiferous hyphae of unicellular Fungi (Λfucor, Pilobolus), radial leaves, such as those of the Onion, and, as exceptional cases, some roots (Onion, Ranunculus aquatilis). In other cases the effect of unilateral illumination is the reverse of the above: the organ curves so as to direct its apex away from the source of light, though it still tends to place its long axis parallel to the direction of the incident rays. Organs curving in this manner are said to be “negatively heliotropic.” This condition has been frequently observed in roots, and among shoots it is characteristic of the hypocotyl of the Mistletoe. Many cases of negative heliotropism in shoots have been mentioned, particularly the tendrils of Vïtis and Ampélopsis, in which its existence was first detected by Knight; but the apparent negative heliotropism in these cases is probably the expression of something altogether different, as will be subsequently pointed out. There appear to be some well-authenticated cases of a reversal of heliotropic properties in the course of development of certain organs. Hofmeister states that the floral peduncles of Linaria Cymbalaria are positively heliotropic, but that when the fruit has replaced the flower the peduncle is negatively heliotropic; and Wiesner states that the peduncle of Helianthemτιm vulgare is negatively heliotropic after fertilization has taken place. The nature of the heliotropic properties appears thus to vary with the biological conditions of the organs.

When organs are exposed throughout the whole period of their growth to unilateral illumination they usually take up a certain position which is termed the “fixed-light position,” such that they curve towards the direction of incidence of the brightest light. In some cases this is not so. This may be explained by an absence of heliotropic sensitiveness, but in some cases it is due to the fact that the organs follow the daily course of the sun. Wiesner mentions Sonchus arvensis as a striking example of the latter condition. The activity of the curvature stands in a direct relation to the intensity of the incident light. The same botanist has found that for the organs of each plant there is an optimum intensity of light which produces the maximum of heliotropic effect, and tlιat any increase or diminution of this intensity is followed by a diminished heliotropic effect.

With regard to the relative heliotropic effect of rays of different wave-length, it has long been known that the rays of high refrangibility are much more powerful than those of low renangibility. Wiesner finds the distribution of heliotropic effect in the spectrum to be more exactly this: the greatest curvature is produced by the rays at the junction of the ultra-violet and violet; from this point the heliotropic effect diminishes until, in the yellow, it disappears; it begins to manifest itself again in the orange, and increases until it reaches a small secondary maximum in the ultra-red. Heliotropic effect is by no means confined to the luminous rays of the spectrum. Wiesner, and more recently Wortmann and Barthélemy, have shown that the dark heat-rays possess it. The curvatures produced by the dark rays are sometimes designated by the term “thermotropism.”

The effect of unilateral illumination is not immediately exhibited, nor does it cease immediately upon the withdrawal of the organ from light. If an organ be exposed for only a short time to unilateral illumination, it may not exhibit any curvature during the period of exposure, but will curve subsequently in darkness. The exposure had sufficed to induce heliotropic curvature. This Wiesner terms “photo-mechanical induction,” but it is simply due to the slow response of the organ to the directive influence of the incident rays,—to a long “latent period.”

Turning now to the part played by the growing organ in heliotropic curvature, it is clear that the curvature will largely depend upon the heliotropic sensitiveness of the organ, and it must be borne in mind that organs vary widely in this respect. The curvature is in all cases confined to the growing region of the organ, or, to put it more generally, to the region which is capable of growing. In connexion with this point, the further one naturally arises as to the seat of heliotropic sensitiveness. It is usually assumed that those zones which are growing most rapidly are those which are most sensitive, but Darwin found that in some cases (cotyledons of Phalaris and Avena, hypocotyls of Cabbage and Beet) illumination of the tip of the organ appeared to induce heliotropic curvature. This cannot at present be regarded as fully established.

With regard to the heliotropic phenomena presented by simply bilateral organs, such as the leaves of Irises, it will suffice to say that they are positively heliotropic.

The heliotropic phenomena presented by dorsiventral organs are more complicated. It has been observed that dorsiventral branches (including the tendrils of Vitis and Ampélopsis mentioned above), when exposed to vertical light, tend to assume a horizontal position, whereas in darkness they usually grow erect. Dorsiventral leaves usually remain horizontal in darkness. A full discussion of these facts cannot be entered upon here; a few remarks must suffice. It is argued by some, especially by De Vries, that the horizontal position of dorsiventral organs when exposed to light is due to negative heliotropism; but there is no adequate proof that this is really the case. Sachs says on the subject, ‘ ’ So far as I can apprehend the facts, the negative heliotropism of the Marchantia-shoot, and that of many other shoots which behave in the same way, is the same phenomenon as the epinasty of foliage-leaves described by De Vries.” Detmer has in fact found that exposure to light, quite independently of the direction of the incident rays, induces the rapid growth of the dorsal surface of dorsiventral leaves—in a word, induces photo-epinasty. Continued photo - epinasty would clearly cause the organ to curve downwards below the horizontal plane, and this is occasionally actually the case. But more frequently the organ remains in the horizontal plane when the light falls vertically upon it, or, to put the case more generally, the organ takes up such a fixed-light position that its long axis is at right angles to the direction of the incident rays. Moreover, it has been ascertained that it is always the dorsal surface of leaves which is directed towards the brightest incident light. It is clear that, in addition to the induction of photo-epinasty, light exercises a directive influence upon the growth of these organs. The attempt has been made to explain this by ascribing to the dorsiventral organs merely the heliotropic properties which belong to radial organs, but this explanation is quite insufficient. They can only be accounted for by attributing to dorsiventral organs, as Frank has done, a peculiar heliotropic sensitiveness, which he terms “transverse heliotropism” and Darwin “diaheliotropism,” which manifests itself in the assumption of such a position that the dorsal surface of the organ is placed at right angles to the direction of the incident rays.

Gravity.— The influence of gravity in determining the direction of growth of an organ manifests itself in phenomena which are designated by the term “geotropism.” We will again consider separately the phenomena which are presented by organs of different physiological properties.

Beginning with radial organs, we find that a great number of them normally grow either upwards or downwards; thus, primary shoots grow upwards and primary roots grow downwards. If any attempt is made to alter the direction of growth of these organs they at once curve so as to regain their normal direction. That the direction of growth is actually the result of the action of gravity was first demonstrated by Knight. He caused seeds to germinate on a wheel revolving with sufficient rapidity to set up a considerable centrifugal force, and he found that the roots and stems of the seedlings behaved with regard to the direction of the centrifugal force precisely as they do with regard to that of gravity: the roots grew radially outwards, and the stems radially inwards. He states his conclusion thus: “I conceive myself to have proved that the radicles of germinating seeds are made to descend, and their plumules to ascend, by some external cause, and not by any power inherent in vegetable life; and I see little reason to doubt that gravitation is the principal, if not the only, agent employed in this case by Nature.”

In conformity with the terminology used with reference to heliotropism, organs which grow towards the centre of the earth are said to be “positively geotropic” and those growing in the opposite direction “negatively geotropic.” As examples of positively geotropic radial organs may be mentioned, in addition to primary roots, the hyphae of Moulds which penetrate into the substratum, the root-like filaments of Voucher ia, Caulerpa, and other Algse, the rhizoids of Muscineae, the rhizomes of some plants, such as Yucca and Cordyline. As examples of negatively geotropic radial organs may be mentioned, in addition to primary shoots, the stipes of Mushrooms, the sporangiferous hyphae of Moulds, the stalks of the receptacles of Liverworts, the setae of Muscineae, the peduncles of many flowers, the climbing roots of various epiphytes. Cases of the absence of geotropic irritability are afforded by the hypocotyl of the Mistletoe, and by the aerial roots of various epiphytes. A reverse of its geotropic properties may take place in the course of the development of an organ. Vöchting has found, for instance, that the peduncle of the Poppy is negatively geotropic whilst the flower is in the bud, but positively geotropic during flowering and fruiting.

In dealing with these phenomena we have first to consider the effect of gravity acting at different angles. Sachs concludes, and his conclusion is generally accepted, that the geotropic influence of gravity is greatest when the long axis of the organ is at right angles to the vertical, and that it is zero when the long axis of the organ coincides with the vertical, whether the apex point upwards or downwards, or whether the organ be positively or negatively geotropic,—that is, if the force of gravity acting at any point of an organ be decomposed into two forces, the one acting at right angles to the long axis of the organ and the other along it, it is only the former which produces a geotropic effect. Sachs bases this view upon the fact that geotropic curvature is more rapidly produced when an organ is horizontal than when it is in any other position. Elfving has made observations which suggest a different view, namely, that the geotropic action of gravity upon an organ is greatest when that organ is removed as far as possible from its normal relation to the vertical.

The degree of geotropic sensitiveness is different in different organs. This is shown by the different directions of growth taken by different organs when grown under conditions which prevent to a sufficient extent other directive influences from producing any effect. For instance, primary roots grow vertically downwards, but lateral roots grow more or less nearly horizontally. It has been ascertained, by means of centrifugal force, that lateral roots can be induced to behave like primary roots if only the force is sufficient: lateral roots grow radially outwards on the wheel when the centrifugal force is ig (7=force of gravity). It may be inferred that their geotropic sensitiveness is one-fourth of that of primary roots. The response of an organ to the directive influence of gravity, as in the case of heliotropism, is not immediate, but is preceded by a long latent period. An organ placed horizontally will not begin to curve for some time, and if then placed vertically the curvature will proceed for some time. Geotropic curvature, like heliotropic curvature, is a phenomenon of induced heterauxesis, the result being the assumption by the organ of such a position that gravity ceases to exert any directive influence upon it. Primary shoots and roots, for instance, find the vertical to be their position of rest, whereas lateral branches take up a more or less inclined position. It appears that, generally at least, the most rapidly growing zones of organs are those in which geotropic influence is most active. It has been hitherto generally accepted that the seat of most active curvature was also the seat of geotropic sensitiveness. Darwin recently brought forward the view, based upon the behaviour of roots with their tips cut off, that, in the root at least, the tip is the seat of geotropic sensitiveness. This suggestion has given rise to a number of researches, the results of which are so conflicting that it is impossible at present to come to any definite conclusion on the subject.

With regard to the geotropic properties of simply bilateral organs (leaves of Irises, &c.) it need only be said that they are negatively geotropic.

Coming, finally, to the geotropic phenomena presented by dorsiventral organs, we find that many organs which when growing exposed to light have a more or less oblique direction of growth, grow erect in darkness. This is obviously an effect of gravity, and the organs are clearly negatively geotropic (examples are afforded by the runners of Polygonum aviculare, Atriplex latifolia, and others, by radical leaves, and by thalloid shoots of Marchantia}. But some dorsiventral organs do not grow erect in darkness. Frank mentions the runners of Fragaria lucida, lateral branches of Conifers, and many dicotyledonous shrubs and trees as examples. He finds, further, that when these organs are placed otherwise than horizontally they curve until they come to lie in that plane, and, if they are placed in an inverse position so that their normally inferior surface (ventral) is uppermost, they twist on their own axes until the normal relation of their surfaces with respect to the vertical is attained; many leaves also behave in this way. Frank accounts for these phenomena by ascribing to such dorsiventral organs a peculiar geotropic sensitiveness, which he terms “transverse geotropism,” and Darwin “diageotropism.” De Vries has severely criticized this assumption. He regards the curvatures of shoots and branches into the horizontal plane as being, to some extent, the expression of the negative geotropism of the stems interfered with by the weight of the leaves, and, to some extent, also the expression of those forms of spontaneous heterauxesis termed epinasty and hyponasty which were alluded to above. Similarly he accounts for the torsions observed by Frank by ascribing them to the unequal twisting moment of the leaves on the two sides of the shoot when in the inverse position. In view of the existence of diaheliotropism, it may be regarded as probable that diageotropism exists also. But at present the case for the latter is not sufficiently made out. More experimental evidence must be forthcoming before the assumption of diageotropism can be regarded as fully justified.

Moisture.— It has long been known that roots when brought into the neighbourhood of a moist surface curve towards it. For instance, when seeds are sown in a box of damp sawdust, the bottom of the box being perforated with sufficiently large holes, the roots of the seedlings grow downwards into the sawdust, and ultimately project through the holes. They then no longer grow vertically downwards, but curve so as to apply themselves to the moist surface offered by the bottom of the box. To these phenomena Darwin has applied the term “hydrotropism.” Organs which curve in this way are said to be “positively hydrotrqpic”; but there are also “negatively hydrotropic” organs. Wortmann has observed, for instance, that the sporangiferous hyphae of Phycomyces curve away from a moist surface. The phenomena are precisely similar to those of heliotropism and of geotropism. The curvature is in this case also the expression of induced heterauxesis. It takes place in this case also with the greatest activity in the region of most rapid growth. Darwin came to the conclusion that the hydrotropic sensitiveness of roots at least is localized in their tips, a conclusion which, though opposed by Detlefsen and Wiesner, is so far confirmed by Molisch’s results that it may be accepted as well founded.

Electricity.— Elfving found that when a root is placed vertically between two electrodes it curves towards the positive electrode,—that is, against the direction of the current. In one case (Cabbage) the curvature was towards the negative electrode. Miiller (Hettlingen), in repeating Elfving’s experiments, found that the curvature was in all cases such as to tend to place the long axis of the root in the plane of the current, the curvature being towards the negative pole. These phenomena are spoken of as “galvanotropism.” Miiller found that the curvature was induced when the current traversed only the tip of the root, thus affording apparently another instance of localization of sensitiveness in the tip.

The Substratum.— Dutrochet long ago observed that the hypocotyl of the Mistletoe, in whatever position the seed may have been placed, assumes such a direction of growth that its long axis is perpendicular to the surface on which the seed has germinated. Sachs has shown, by means of rotation on the clinostat, that this position is assumed both by shoots and roots. It is clear that the substratum exercises a directive influence upon the organs growing either outwards from its surface or inwards into its surface, but the nature of this influence has not yet been investigated. It is certainly not to be ascribed, as Van Tieghem suggested, to the mere mass of the substratum. The effect of a cube of turf or a pot of earth would vanish entirely in comparison with the attraction of the earth,—in other words, with the influence of gravity. The phenomena are designated generally by the term “somatotropism.”

Contact.— The effect of contact upon the direction of growth of an organ must be clearly distinguished from the effects of considerable pressure. The latter are of two kinds: in the one case the pressure arrests the growth of that side of the organ which is exposed to it; in the other it excites the organ to active growth, particularly in thickness. Examples of the former effect are so common that they need not be specified; examples of the latter are afforded by the thickening of tendrils and of climbing stems when they have firmly grasped some support. The phenomena now to be considered are such as are induced by slight pressure. Striking instances of this are afforded by tendrils. A very slight touch suffices, in the case of the very sensitive, such as those of Passiflora gracilis and of Sicyos angulatus, to induce very perceptible curvature, which can be detected, according to Darwin, in half a minute after the touch.

In order to illustrate the subject adequately a brief account will be given here of the more important phenomena connected with the twining of tendrils. Darwin has shown that tendrils are not sensitive during the whole of their existence; speaking generally, they are not sensitive when they are either very young or full grown, and are most sensitive when they are about three-fourths grown. Darwin has also found that their sensitiveness is localized. In most the lower or basal part is either not at all sensitive or is sensitive only to prolonged contact. Most tendrils have their tips slightly but permanently hooked, and the sensitiveness is localized in the concavity of the hook. In some cases (Cobxa scandent, Cissus discolor) they are sensitive on all sides; in Mutisia the inferior and lateral surfaces are sensitive. As a rule, when in its most sensitive condition the tendril is actively circumnutating, so that it travels over a large area, and there is considerable probability that it will come into contact with some body around which it can twine. When contact takes place the tendril begins to curve round the support. As it does so new points of the sensitive surface are stimulated, and the curvature increases and extends until the whole of the tendril lying between the original point of contact and the apex is wound in a spiral coil round the support. In some cas< s this is all that takes place. In the great majority of eases, however, the coiling of the apical portion of the tendril round a support is followed by the spiral coiling of more or less of that portion of the tendril which lies between the original point of contact and the insertion of the tendril upon the stem, provided that this is mechanically possible. The spiral coiling of the basal part of the tendril involves, namely, a considerable shortening, and, if both the stem and the support are immovably fixed, this shortening cannot take place. The turns of the coil are not all in the same direction; they are grouped into two or more spires, separated by short straighter portions, the turns of any two successive spires being in opposite directions. This is a mechanical necessity associated with the spiral coiling of a filament attached at both ends. The spiral coiling of the basal part of the tendril usually begins jiιst below the point of attachment to the support. The coiling of tendrils, like all the curvatures hitherto considered, is a phenomenon of induced heterauxesis. Hence it is that the possibility of the twining of a tendril round a support depends upon the thickness of the support and upon that of the tendril. Most tendrils, inasmuch as they are very thin, can twine round strings, but those which are relatively thick can only twine round a support of some thickness, for there is a mechanical limit to the excess of elongation of the convex over the concave side. The spiral coiling of the untouched portion of the tendril has an especial interest, as it offers a striking illustration of the transmission of a stimulus. It is true that the tendrils of many plants, if they fail to come into contact with a support, likewise coil spirally; but this is a much slower process, and only begins at the time when the tendrils are ceasing to grow and to be sensitive.

Tendrils are not, however, the only organs which are sensitive to contact. Other instances are afforded by the petioles of most leaf-climbers, by shoots, and apparently by roots.

In the case of sensitive shoots Dutrochet observed that the twining stem of Cuscuta is sensitive like a tendril· Von Mohl suggested that all climbing stems are sensitive, but both Darwin and De Vries were unable to detect the sensitiveness. This view of Von Mohl has been recently revived by Kohl, who finds that the internodes of climbing stems are sensitive to a long-continued pressure which is insufficient to produce any simply mechanical effect. Darwin found the young internodes of Lophospermum scandens, which is not a stem-climber, as also the peduncles of Maurandia semperflorens, to be sensitive to touch, and Kerner states that this is also the case with the peduncles of many flowers (Poppy, Anemone, Ranunculus, Tulip).

With regard to roots, Darwin was led to suspect, by observing the behaviour of the radicles of seedlings in their attempts to pass over obstacles in the soil, that the tip of the radicle is sensitive to contact, and that the stimulus is transmitted from this, the sensory organ, to the growing zones behind it, in which the necessary curvature is then effected. He made experiments by attaching small objects to one side of the tip of the radicle in various plants, by touching one side of the tip with caustic, and by cutting a thin slice off one side, and found in most cases that the radicle curved away from the touched or injured side, that the curvature is precisely the opposite of that performed by tendrils when touched. The peculiar curving of radicles has been termed the “Darwinian curvature.” Darwin’s conclusions as to the sensitiveness of the radicle have given rise to considerable discussion. It is clear, in the first place, as he himself showed, that radicles are not perceptibly affected by brief contact or by friction; the contact must be prolonged. Those who dissent from Darwin’s view, such as Wiesner, Burgerstein, and Detlefsen, urge that the curvatures induced in his experiments· are pathological. It seems probable that this objection is valid. It may be admitted at once in the case of the experiments made by means of slicing the root-tip or touching it with caustic. With regard to the effect of small objects, such as pieces of card, it appears that the curvature of the radicle is due mainly, if not entirely, to the substances used in attaching them. In some cases, for instance, they were attached by a drop of shellac. It has been shown that the mere presence of the drop of shellac is sufficient to induce the Darwinian curvature, and microscopical examination has proved that the part touched by the shellac had died away. Moreover, it is known that a radicle can grow downwards against considerable resistance: it can penetrate into mercury; it can perforate tinfoil without deflexion; Darwin, in fact, estimates the force of downward growth of the radicle at ⅛ lb, and its lateral pressure, in particular cases, at 8 and 3 lb respectively. The evidence leads to the conclusion that the Darwinian curvature of roots is not the expression of sensitiveness to contact, but that it is the result of injury of one side of the root.

Combined Effects.— Now that the influences which determine the direction of growth have been individually considered, it is possible to account for the characteristic positions taken up by organs in the course of their development. In dealing with this subject it is convenient, as Sachs suggests, to classify organs, according to their ultimate position, into two groups,—those which, under normal conditions, have their long axes vertical and those which have their long axes more or less inclined to the vertical; the former Sachs terms “orthotropic” organs, the latter “plagiotropic.”

The direction of growth of plant-organs under normal conditions is the expression of the resultant effect of various external directive influences. To illustrate this in the case of orthotropic organs, let us consider the primary shoot and the primary root of a seedling growing under conditions which may be taken as normal. In the case of a shoot growing upwards into the air when light falls vertically upon it, its vertical upward growth is chiefly due to the action of gravity,—that is, it is the expression of the particular degree and quality of the geotropic sensitiveness of the shoot. Since the light is equally intense on all sides of the shoot, it exerts no directive influence. Orthotropism is then mainly due to negative geotropism. That this is so can be readily proved in various ways. For instance, the hypocotyl of the Mistletoe, as mentioned above, is not geotropic at all; hence it cannot be included among either orthotropic or plagiotropic organs, for it may grow vertically or it may grow obliquely, its direction of growth being determined chiefly by its somatotropism. Again, when a normally orthotropic organ is grown in darkness on a clinostat its direction of growth is horizontal. Passing now to the case of primary roots growing in the earth, when the conditions are normal—that is, especially when the earth is uniformly moist around the root—their direction of growth is vertically downwards. This is chiefly due to their strong positive geotropism. Let us suppose, now, that the conditions of growth of these organs are somewhat different from those which we have regarded as normal; let us suppose that the shoot or the root is exposed to lateral light, or that the soil about the root is not equally moist on all sides. In the former case, the action of light will tend to induce heliotropic curvature, but it will depend upon the relative strength of heliotropic and of geotropic sensitiveness whether or not a curvature actually takes place. In the case of most orthotropic shoots a curvature (positive) would take place, thus showing the heliotropic sensitiveness of shoots to be greater than the geotropic, but in some instances it would not take place; in the case of most orthotropic roots no curvature would take place, but in some instances (Sinapis alba and others) a curvature (negative) would take place, showing that in most cases the heliotropic sensitiveness of roots is less than their geotropic sensitiveness. The unequal moisture in the soil around the root would cause hydrotropic curvature, inasmuch as the sensitiveness of roots to the influence of moist surfaces is greater than their sensitiveness to gravity.

We will deal with plagiotropic organs in a similar way. The majority of such are lateral members, as branches, leaves, Ac. The direction of growth of a lateral member, certainly of branches of stems and roots and probably also of leaves, is at first determined by its relation to the parent axis. It has been found—by Dutrochet, Sachs, and others—that at their first development the long axes of lateral organs make a definite angle—termed the “proper angle ”— with the long axis of the parent organ. Dutrochet thought that the proper angle was in all cases a right angle, that the relation of lateral organ to parent axis was of the nature of somatotropism; but this is a too general statement. The original direction of growth of a lateral organ determined by its proper angle would be maintained, in the absence of internal directive influences, by its rectipetality, but in nature it is affected by light, by gravity, Ac. Lateral shoot - branches, for example, are either inherently dorsiventral or they become dorsiventral under the influence of gravity or of unilateral illumination; they are then diaheliotropic, though the manifestation of their diaheliotropism may be interfered with by photo-epinasty; they are usually negatively geotropic. Their direction of growth—that is, the direction of their long axes when mature—is the resultant effect of diaheliotropism and of negative geotropism. In the case of lateral root-branches these are plagiotropic but radial; they grow outwards, slightly inclined downwards below the horizontal; as they grow in the dark,—assuming that the moisture of the soil around them is uniform,—their direction of growth is affected to some extent by their slight positive geotropism. Though their geotropic sensitiveness is slight, their hydrotropic sensitiveness is great, so that their direction of growth is often very much modified by their coming into relation with moist areas of soil.

A complicated case of the action of a number of directive influences is afforded by climbing stems, and it may be worth while to specially consider it. When the stem is young and extends-only a few inches above the ground it appears to be growing almost vertically upwards, but as it elongates the last-formed internodes exhibit well-marked circumnutation. It continues to grow upwards mainly in virtue of its negative geotropism, the direction of its growth being little, if at all, affected by light in consequence of its low degree of heliotropic sensitiveness. If now one of the young growing apical internodes comes into contact with a vertical support it begins to twine around it in virtue of the sensitiveness to permanent though slight pressure which, as mentioned above, these organs possess, the direction of the curvature round the support being also that of circumnutation. The coils formed are nearly horizontal when the support is thick and become more nearly vertical as the support glows thinner; in any case, the steepness of the spire always increases after it is first formed, its diameter is thus diminished, and the stem gains a firm grip of the support. As the stem twines round the support it undergoes torsion around its own axis, so that any one side maintains throughout the same position, whether it be directed inwards, towards the support, or outwards or laterally. The direction of torsion may be either the same as that of coiling or the reverse,—that is, either homodromous or antidromous. The direction of torsion appears to depend principally on the relation between the thickness of the climbing stem and that of the support, and on the smoothness or roughness of the surface of the support; when the support is relatively thin the torsion is homodromous, but when it is relatively thick the torsion is antidromous; with smooth supports, up to a certain limit of thickness, the torsion is homodromous, and with rough supports, down to a certain limit of thinness, the torsion is antidromous; in a word, the direction of torsion is determined by the degree of friction between the climbing stem and the support.

2. Movements.— We pass now to the consideration of movements other than those associated with growth, and we take first movements exhibited by protoplasm. 'These may be classified into two categories,—(1) those which are performed by naked protoplasm,— by protoplasm, that is, which is not enclosed in a cell-wall; (2) those exhibited by protoplasm enclosed in a cell-wall. The movements of naked protoplasm are effected in two ways,—either by the protrusion of portions of the protoplasm, termed “pseudopodia,” or by permanent flagelliform protoplasmic filaments, termed “cilia the first kind of movement is known as “amoeboid,” the second as “ciliary” movement. The amoeboid movement is exhibited, though rarely, by isolated cells—for instance, by the zoospores of the Myxomycètes — and characteristically by those large aggregates of cells which constitute tire plasmodia of this group of Fungi. The pseudopodia are thrown out at first as protrusions of the denser hyaline outer layer of the mass of protoplasm, the ectoplasm, and into this the more watery granular internal protoplasmic substance, the endoplasm, gradually flows. The repeated formation of pseudopodia in any given direction will result in locomotion taking place in that direction. The ciliary movement is characteristic of zoospores and of antherozoids. In some cases the organism, as in the case of Vblvox and Pandorina, passes a large part of its existence in the mobile condition, and then the protoplasm is enclosed within a cell-wall which is perforated by the cilia. The number of cilia may be only one; more commonly in zoospores it is two, and sometimes four; occasionally the cilia are numerous, as in the zoospores of Vaucheria and Oedogonium ; in antherozoids they are usually numerous. The cilia are constantly performing a lashing movement, which causes the organism to move forward and at the same time to rotate on its own axis.

In considering the movements of protoplasm when enclosed within a cell-wall, the typical structure of a plant-cell, as described at the beginning of this section (p. 44), must be borne in mind. In many cells the vacuole is found to be traversed by protoplasmic filaments which extend between one part of the primordial utricle and another. These filaments are continually varying in number, in position, and in size; they are formed and withdrawn in the same manner as the pseudopodia of naked masses of protoplasm. This kind of movement is, in fact, amoeboid movement exhibited by protoplasm enclosed within a cell-wall. In all actively living protoplasm, whether naked or enclosed in a cell-wall, a streaming of the more fluid endoplasm can be observed, the direction and rapidity of the current being clearly shown by the granules which are carried along in it. This is very conspicuous in closed cells (as in leaf-cells of Vallisneria spiralis and root-hairs of Hydrocharis Morsus Banae) when the whole of the endoplasm rotates in a constant direction.

Movements of Mobile Organs.— With regard now to the movements exhibited by mobile organs, to the “movements of variation” as they are sometimes termed,—sometimes they are spontaneous, like the protoplasmic movements just considered; in other cases they are only performed in consequence of stimulation: they are induced. Instances of spontaneous movements of variation are, for reasons to be given hereafter, comparatively rare. A case in point is afforded by the Telegraph Plant, Hedysarum (Desmodium) gyrans. Under favourable conditions, particularly of temperature, the two lateral leaflets of the trifoliolate leaf move upwards and downwards, their apices describing nearly a circle, a revolution taking from two to five minutes. A familiar example of an induced movement is afforded by the leaves of the Sensitive Plant (Mimosa pudica). When a leaf is touched the lateral leaflets close in pairs, folding upwards and forwards, and, if the stimulus be strong enough, the main petiole sinks downwards. The movement of this plant is of special interest, inasmuch as it affords an instance of the transmission of a stimulus. It suffices, namely, to touch the terminal leaflet to cause the closing of the successive pairs of lateral leaflets and the sinking down of the main petiole. Another example of induced movement is afforded by the so - called ‘ ‘ sleep” of plants; the leaves of many plants, namely, take up during the night a position different from that which they take up during the day. Taking the Sensitive Plant as an example, during the day its leaflets are widely expanded, and its main petioles are directed obliquely upwards; at night its leaflets are folded together and its main petioles are directed obliquely downwards; it takes up at night, in fact, a position similar to that which is induced by a touch. Excessive illumination tends to induce closing. Other examples are afforded by the mobile stamens of the Cynareae and of Berberis and Mdhonia.

The relation of these movements to external conditions is as follows :—

1. Temperature.— These movements, .like the slow movement of growth, only take place within certain limits of temperature, which, however, vary in different cases. Movements of protoplasm, speaking generally, will only go on at temperatures between 0° and 50° C., and between these limits there is an optimum temperature at which they are most rapid. In the case of movements of variation the lower limit lies considerably higher, from 15° to 20° C. The foregoing illustrates the tonic relation between temperature and movement. Sudden changes of temperature have a stimulating effect. For instance, Dutrochet observed that the protoplasm of the internodal cells of a Chara exhibited rotation in water at 7° C., which soon ceased when the plant was placed in water at 32° C.; after some time the movement returned, and was again arrested on replacing the plant in water at 7° C.

2. Light.— In most cases of protoplasmic movements light appears to exert no influence; in other cases it exerts a tonic influence. For instance, Engelmann has discovered a form of Bacterium, termed by him Bacterium photometricum, which is only mobile when exposed to light. Again, organs which exhibit spontaneous movements of variation, like the leaflets of Hedysarum, or induced movements, like the leaflets of Mimosa, lose their power of movement when kept in darkness for a day or two. Exposure to light, or, as it is termed, the state of “phototonus,” is an essential condition of their movement. Bright light tends to arrest movement. For instance, the protrusion of pseudopodia by the plasmodium of Aethalium syρticum is less active in light than in darkness. This is well shown in the sleep of plants, alluded to above. The “diurnal position” of the leaves is due to exposure to light, the “nocturnal position” to its absence. This is perhaps most clearly exhibited by the Sensitive Plant. Under the normal alternation of day and night the leaves assume alternately the diurnal and nocturnal positions. If a plant be kept for some time in darkness, at a suitable temperature, it will be found that the leaves exhibit periodic movements of opening and closing. They are, in fact, endowed, like those of the Telegraph Plant, with the power of spontaneous movement, which is arrested when the plant is exposed to light. Variations in the intensity of light act as stimuli. For instance, a sudden variation will cause the closing of the leaves of Mimosa. Light also exercises a directive influence on mobile protoplasm in some cases. For instance, when bright light falls obliquely on a plasmodium of Aethalium septicum, it causes it to creep away from the light. Again, the chlorophyll-corpuscles in the cells of leaves exposed to bright light are found to accumulate on those surfaces of the cells which are least exposed, to assume what is termed the position of “apostrophe,” a change of position which is due to movements in the protoplasm in which the corpuscles are embedded. Finally, the direction of movement of ciliated zoospores swimming in water is affected by light. When light falls obliquely upon a vessel of water containing zoospores, they place themselves so that their long axes are more or less nearly parallel to the direction of the incident rays, and it is along this line that they move. They may either move towards the incident light or away from it, the direction being apparently determined by the intensity of the light, by the age of the zoospores, and by the amount of oxygen in the water. Protoplasmic masses which respond to the directive action of light are said to be “phototactic.”

3. Other Stimuli.— It has been mentioned that movements may be induced in the Sensitive Plant by mechanical stimulation, by variations of temperature, and variations in the intensity of light. They may also be induced by electrical and chemical stimuli. The effect of an electrical stimulus on protoplasm exhibiting the amoeboid movement is to cause retraction of the pseudopodia. It aτrests also the rotating movement of the protoplasm for a time.

4. Oxygen.— The presence of oxygen is an essential condition of movement of any kind, in the case at least of aerobiotic plants. It appears that anaerobiotic plants (Schizomycctcs) are mobile in the absence of oxygen.

Nature and Mechanism of Movements.— On comparing the statements which have been made above as to the movements of growing organs and of mature mobile organs, their general similarity is at once apparent. The spontaneous movement of growth is comparable to the spontaneous movements of protoplasm and of mobile organs, and the performance of the former is dependent upon the same external conditions as the latter. The reaction to the influence of external agents is the same In many instances; for example, strong light arrests growth, and it arrests also the spontaneous movements of the leaves of Mimosa and other plants, and contact stimulates tendrils as it stimulates the leaves of the Sensitive Plant. Again, light exercises a directive influence on the growth of growing organs; it also exercises a directive influence on the movements of zoospores and plasmodia. These considerations lead to the conclusion that the causes of the movements must in all cases be the same.

It has been already pointed out that growth and movement are expressions of the expenditure of energy on the part of the organism, that they are dependent upon the decomposition of some complex substance forming part of, or at least present in, the protoplasm. The conditions which are essential to movement of any kind are, then, these: that the decomposable substance in question is formed and decomposed in sufficient quantity, in other words, that the protoplasm is irritable; that the protoplasm is capable of manifesting by a molecular change, which may be accompanied by a change in external form, the evolution of energy attending the decomposition; and, finally, in the case of protoplasm surrounded by a cell-wall, that the anatomical structure is such as to permit of a movement ensuing upon the change in the protoplasm. It is clear that, if the cell-wall is rigid, no change in the protoplasm can cause a change in form of the cell as a whole.

We may regard spontaneous movement as being due to the spontaneous decomposition of the decomposable substance whereby the protoplasm undergoes a molecular change. The automatic decomposition not unfrequently takes place, as in the case of the Telegraph Plant, at regular intervals, so that the movement is rhythmic or periodic. Spontaneous movement is most active when a certain favourable combination of external conditions is ensured; any variation in the combination leads to a diminution in the activity, or even to complete arrest, of the movement. External conditions may affect the process either of formation or of decomposition of the decomposable substance. For instance, movements are arrested at a low temperature, most probably because either the formation of the decomposable substance or the necessary explosive decomposition does not take place under such circumstances with sufficient activity. Again, when movement is arrested at a high temperature, or by continuous darkness, it is probably for similar reasons. In this way the tonic effect of external conditions may be accounted for. The stimulating effect of external agents may be accounted for in a similar manner.

Movement, whether spontaneous or induced, is regarded as a phenomenon of contractility (see above, p. 13), but we have at present no knowledge of the exact nature of the molecular changes which constitute a contraction. It must be borne in mind not only that protoplasm contracts, but that, after contraction, it returns to its condition of rest. Spontaneous movement is the expression of automatic contraction. External tonic conditions either promote or retard movement, by either promoting or retarding contraction and recovery. Stimuli induce contraction.

The mechanism of the movements of protoplasm-masses appears to be as follows. Taking first the case of the amoeboid movement, the protrusion of pseudopodia is due to a molecular change, of the nature of a contraction of the protoplasm, which takes place in the ectoplasm at the spot where the pseudopodium is to be formed, an elevation being gradually produced into which the more fluid endoplasm is, as it were, sucked. The rotating movement of protoplasm appears to depend upon a kind of amreboid movement taking place constantly in one direction,—to be, that is, a creeping movement. Ciliary movement appears to depend on the alternate contraction of each longitudinal half of the cilium. It is not possible at present to attempt any explanation of the directive influence of light on moving protoplasm, but the fact itself is of great physiological importance.

The mechanism of the movements of organs, whether unicellular or multicellular, in which the cell-wall has to be considered is more complicated. The cells possess the structure described above: they consist of a cell-wall lined by the protoplasmic primordial utricle enclosing the cell-sap. They are, moreover, in a state of turgidity,—that is, they are tensely filled with water. The state of turgidity in a cell depends upon three conditions,—(1) upon a tendency to absorb more water in consequence of the presence of osmotically active substances dissolved in the cell-sap; (2) upon the resistance offered by the primordial utricle to the escape of water from the cell; (3) upon the elasticity of the cell-wall. The elasticity of the primordial utricle is so small that it may be neglected. The growth in length of a filament of Vauchcrin maybe taken as a case illustrative of the importance of turgidity as a condition of growth in unicellular organs. This growth cannot be attributed to a greater hydrostatic pressure at the apex of the cell, for the pressure is necessarily the same at all points, and clearly it cannot be referred to a diminished resistance on the part of the primordial utricle to the passage outwards of water at that point. It must be referred to the cell-wall, and it can only be accounted for on the assumption that the elasticity of the cell-wall is less at the apex than at any other point of its surface.

Growth is not, however, to be regarded as the result merely of the mechanical expansion of the cell which is rendered permanent. There is every reason to believe that the protoplasm takes an active part in producing this expansion, and in determining the direction in which expansion shall more particularly take place. The arrest of growth by strong light is a case in point. There is no reason to believe that this is to be ascribed to an increased rigidity of the cell-wall, or to a diminution of the attraction of the cell-sap for water. It can only be ascribed to a molecular change in the protoplasm, which causes it to offer considerable resistance to any change of form, either spontaneous or such as is induced by the hydrostatic pressure. The arrest of growth which, as we have seen, usually occurs when leaves are kept in continuous darkness is another case in point. The arrest of growth of the cells under these circumstances cannot be referred to a change in the physical properties of either the cell-wall or the cell-sap, but must be attributed to a change in the molecular condition of the protoplasm.

The phenomena of heterauxesis, spontaneous and induced, have now to be considered. It will be convenient to deal with induced heterauxesis first, and we will begin with the case of a unicellular organ. It has been mentioned that heterauxesis, in the form of curvature, is induced by the action of light, gravity, &c. Inasmuch as the hydrostatic pressure is necessarily the same at all points of the internal surface of the cell, the curvature must depend upon a local variation of the properties either of the cellwall or of the protoplasm. In the case of the cell-wall either its rigidity is increased on one side, the concave, or its extensibility increased on the other, the convex. It is just conceivable, with regard to the action of light, that such a difference in properties might be induced by the more direct exposure of one side of a delicate filament to light, though the difference of intensity on the two sides would be very small. But it is not at all conceivable that such a difference could be induced by the action of gravity, and no explanation can be regarded as satisfactory which fails to meet all cases of curvature. The cause of the curvature is doubtless to be sought in the protoplasm. Unilateral illumination of the organ, or an abnormal relation to the line of action of gravity, acts as a stimulus on the organ and causes an alteration in the properties of its protoplasm, which is perhaps of such a nature that it becomes relatively rigid on the side which becomes concave. The induced heterauxesis of multicellular organs is certainly of essentially the same nature as that of unicellular organs. Applying the above explanation of the curvature of unicellular organs to multicellular organs, the conclusion to be drawn would be that the curvature of the latter is due to the induction of the same changes in the protoplasm in each of their growing cells.

The phenomenon of spontaneous heterauxesis, as exhibited in nutation, may be accounted for in precisely the same way, but it is possible to imagine that it may be due to some extent in the case of unicellular organs to local variations in the extensibility of the cell-wall, and in that of multicellular organs to variations in the extensibility of the cell-walls of groups of cells on different sides of the organ.

The phenomenon exhibited by mature mobile organs, such as the leaves of the Sensitive Plant, &c., remains finally to be considered. The movement of the leaf as a whole is effected by a group of cells, constituting a swelling, the pulvinus, at the insertion of the main petiole, and of each leaflet by a similar organ at its attachment to the main petiole. The structure of the pulvinus is briefly a mass of parenchymatous cells having the same structure as that described above, traversed by a strand of flexible fibro-vascular tissue. When the leaf is fully expanded, its position is maintained by an equality between the downward pressure of the portion of the pulvinus above the fibro-vascular strand and the upward pressure of the portion below it. The downward movement of the leaf as a whole is due to a sudden diminution of the upward pressure of the lower portion of the main pulvinus; similarly, the upward movement of a leaflet is due to the sudden diminution of the downward pressure of the upper portion of its pulvinus. In both cases the diminution of pressure is due to a loss of turgidity of the portion of the pulvinus concerned: the cells become flaccid. This loss of turgidity has been shown to be due to an escape of water from the cells, which can only be accounted for by ascribing it to a change in the molecular condition of their protoplasm. In spontaneous movements this change is induced automatically, in induced movements by the action of a stimulus. This molecular change is probably of such a kind that the protoplasm takes up water into itself, and at the same time allows it to pass through. The recovery of turgidity is slow. The arrest of movement which is induced by long-continued darkness or by exposure to light is probably due to the prevention of the occurrence of molecular change in the protoplasm. The conduction of a stimulus, which undoubtedly takes place in the leaves of the Sensitive Plant, and probably in many other plant-organs (see above on heliotropism, geotropism, hydrotropism, tendrils), is effected by means of the delicate filaments of protoplasm which, as Gardiner has clearly shown in the pulvinus, are continuous between the protoplasm-bodies of adjacent cells.

For the reproduction of plants, see Reproduction. Literature.— The following works on the physiology of plants may be consulted,—Sachs, lÀhrbuch (2d Eng. ed., Oxford, 1882) and Vorlesungen uber Pflanzenphysiologie (Leipsic, 1882); Pfeffer, Pflanzenphysiologie (Leipsic, 1881); Van Tieghem, Traité de Botanique (Paris, 1884); Darwin, Climbing Plants (1875) and The Power of Movement in Plants (1880). (S. H. V.)

Index to Physiology. [table] [table] [table] [table]

Index to Physiology (continued). [table] [table] [table] [table]