PHYSIOLOGY 9 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 conscious ness, 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. Chemical The movements of man or of an animal are not, how- Ganges. ev er, 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 sub stances, 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 com bustion 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 pro ducts 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 reduc tion thereby to the above-named products, with a develop ment 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 reduc tion 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 sub stance 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. Source of This breaking down of complex substances, this con- bodily tinued partial decay, is indeed the source of the body s ergy 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 con sciousness, and of the beating heart and writhing visceral muscles, is sooner or later, by friction or otherwise, con verted 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, con stitutes a drain upon the body which must be met by con stantly-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 sub stance, 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 mole cular 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 mu&t 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 Prob- be spoken of as threefold. (1) On the one hand, we have lems of to search the laws according to which the complex unstable j*^ 810 " 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 sub stances, 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.
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