TEIE FOOD. - The function of digestion has for its chief object the replacement of that loss of substance which the body is con stantly undergoing.
Even the hardest materials of the globe we inhabit experience a gradual disintegration ; as the result of the various physical processes to which they are exposed. Such processes rnay be instanced in the attrition and solution of solids, the evaporation of liquids, and the diffusion of gases. And hence, when we turn from these inorganic substances to the animal fabric ; and consider its slight cohesion, the friction which its locomotion implies, its large watery constituent, and the feeble chemical affinities which enchain its elemen tary atoms — we shall scarcely be surprised to find, that the rapidity of its waste far ex ceeds that of the inanimate solids around us.
But the rate of waste, and the consequent need of replacement, both depend, far less on simple physical causes of this kind, than on certain actions which are specific to the or ganized body. These actions, which, in the aggregate, make up what we term LIFE, do not so much imply, as actually consist in, a per petual process of flux and metamorphosis. This multiform change engages the whole of the corporeal tissues ; and conducts their various ingredients, through a number of suc cessive phases of composition, to an effete and useless state, in which they are finally' ejected from the organism.
And hence, whatever the share taken by the physical actions of diffusion, solution, fric tion, and evaporation, in the removal of the substance of the body, they are not in any sense the true causes of its process of waste ; or the real sources of its egesta or losses. They are but, as it were, the janitors of the animal fortress ; the nature and amount of the matters which pass out by them being controlled and regulated by the higher life that rules within.
The ingesta, which replace these egesta, and thus form the opposite extreme of nu tritional life, are equally influenced by the general requiretnents of ihe animal. Ex cluding, for the present, all consideration of that preponderance of absorption which de termines the growth of a young animal, or the converse excess of excretion which results in the decrease and decay of an old one ; and limiting our attention to the mere maintenance of the adult body 1—we shall find that it is the composition of its structures, and the rate of their wear and tear, that chiefly determine the kind of food it makes use of, and the quantity it consumes within a given space of time.
While as regards the exact degree of this dependence, we shall further find that, here as elsewhere, the operations of organized nature are only limited by wide general prin ciples ; within which are apparently conceded great variety and fluctuation. The laws of nu trition are, so to speak, universal in their range, but elastic in their application.
In respect to the nature of the food, we may first notice, that by far the larger part of it is always derived from the organic, and never from the inorganic, world. In other words, the chemistry of the organism has little power of construction or synthesis. So that, although a proximate analysis of the tissues of the animal body presents us with compounds, which may be shown to consist chiefly of a few elementary substances united to each other in varying proportions, still the carbon, oxygen, hydrogen, and nitrogen, which surround or penetrate the living animal, are never directly built up into these tissues. On the contrary, the various substances which form the proximate principles of the several structures of the organism are theinselves produced by the metamorphosis of kindred compounds introduced in the food : — com pounds which have been in their turn derived from the vegetable kingdom ; either directly, in the shape of plants, or indirectly, from sub stances constructed out of vegetable tissues by the organism of another animal. And the inorganic substances introduced into the body seem to be ahnost restricted to the subordinate (though equally indispensable) office, of com bining with these products of vegetable life, and modifying their actions in obedience to the necessities of the existing individual.