NUTRITION, or the acts by which the indi vidual is preserved.—Every thing in nature changes, and organized beings only con tinue their existence with their aptitudes to manifest the acts that draw so wide a line of demarcation between them and unor ganized bodies, by a perpetual renewal or re composition, and as incessant a rejection or decomposition of their elements. Nutrition is, therefore, at least, a two-fold act, implying absorption or appropriation of nutritive matter, and excretion or rejection of the old and worn out particles that have already served their office in the economy : it consists, in fact, as we have said, of an incessant decomposition and reconstruction of the fabric of the living organized being. Nutrition, however, is a very comprehensive term, and includes the whole of the vital acts by which the individual continues its existence,—namely, among the higher tribes of living things, the absorption or ingestion of food or alimentary matter ; the preparation of this food by the processes of digestion and respiration ;_ the distribution of the nutritive matter fitted for its ends, to every part of the system by means of a circulation ; the conver sion of the nutritive matter into the solids and fluids or proper substance of the indi vidual, and finally the depuration and rejection of the worn-out parts and particles by means of certain secreting organs. These various pro cesses in themselves will be particularly con sidered in the article NUTRITION, to which the reader is referred. Meantime let us contrast these different functions as they manifest them selves in each of the two grand divisions of the organized world.
Assumption of aliment. — The earth and the atmosphere, and the carbonic acid and water they contain, are the sources whence vegetables derive their food. Here they find aliment ready prepared for their use, or rather, as passive agents, they depend on the earth and the atmosphere for a supply of the elements required for their continuance. Those physiologists are now admitted to have been mistaken who supposed that the food of ve getables was furnished by the inorganic earth, air, and water, with which their roots and leaves are in relation ; more accurate experi ments have shown that plants are as dependent as animals on supplies of substances that have once had life for their support. When plants
are made to grow in pure earth and in distilled water, they appear to do so by a kind of de composition of themselves, one part perishing and affording food to that which continues to live. To base a distinction between animals and vegetables, consequently, on the presump tion that the one lived on organic, the other on inorganic substances, was incorrect : animals and vegetables are alike in this respect ; both feed upon organized matter, and this not al ways or necessarily in a state of decomposition, as we observe among parasitic tribes, which subsist on the living juices of the individuals they cling to. The food of animals, however, may be stated generally to be both more various and also more complex in its chemical com position than that of vegetables, and whilst vegetables take all their food in a liquid shape, animals much more commonly live on a mix ture of solids and fluids.
The assumption of food by vegetables and animals takes place under very different cir cumstances. In vegetables it is necessary and independent of the individual; it is also in cessant ; and, farther, it takes place from the external surface, inasmuch as it is with this that the materials which supply the nutriment are in contact.
Animals, however, have not generally their food prepared for their use brought into con tact with their bodies, neither are they passive in its assumption ; they have mostly to search for it abroad, and are provided with special organs for this purpose. The act by which they take it is not necessary, neither is it in cessant. They have also to select their food, and are, therefore, furnished with faculties" which guide them in their choice; namely, taste and smell. Lastly, the absorption of the truly nutritious matter is accomplished from their interior, the crude material assumed as food having been first prepared by elaboration in a cavity called a stomach.