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...