16 PHYSIOLOGY Vascular minute hydra the nutritive material can permeate the system, 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 ac tivity, 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 endoderin 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 arrange ment, 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 disintegra tion, 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. Neverthe less 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 Avails 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 VASCULAK SYSTEM) be inter preted 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 de termined, 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 cell ular 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 dissimi lar 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 con stitution, 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 domi nant 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 struc ture and function, of nervous protoplasm, differ most widely from each other. These several tissues of diverse consti tution 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 constitu ent tissue-elements are brought to bear in special direc tions. These organs range from those in which the mechan ical 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 mechan ical provisions are insignificant, as in the central nervous system, Avhere 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 in- Natur quiry of any large scope is, or sooner or later becomes, r 5 ysi< of a mixed nature. On the one hand, investigation has s a to be directed to the processes taking place in the actual i ems> tissue-elements, in the protoplasmic celk 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 con tinually 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