which it takes place. It is, in fact, a chemical transformation of an alimentary substance. This transformation may be realized outside the organism, in vitro, just as it can in the living being without masticating organs, without an intestinal apparatus, without glands, in a vessel placed in a stove, simply by means of a few soluble ferments—pepsine, trypsine, amylolytic diastases.
All alimentary substances, whether taken from without or borrowed from the reserves accumulated in the internal stores of the organism, must undergo preparation. This preparation is digestion. Digestion is the prologue of nutrition. It is over when the reparative substance, whether food or reserve-stuff, is brought into a state enabling it to pass into the blood, and to be utilized by the organism.
The Identity of Categories of Foods in the Two Kingdoms.—Now the alimentary substances are the same in the two kingdoms, and so is their digestive preparation. Alimentary materials are of four kinds: albuminoid, starchy, fatty, and sugary substances. The animal takes them from without (food properly so-called), or from within (reserve-stuff). Man obtains starch, for instance, from different farinaceous dishes. It may, however, equally well be borrowed from the reserve of flour that we carry within us in our liver, which is a veritable granary, full of floury substance, glycogen. And so it is with vegetables. The potato has its store of flour in its tuber just as the animal has in its liver. The grain which is about to germinate has it in reserve-stuff in its cotyledons, or in its albumen. The bud which is about to develop into a tree or a flower carries it at its base.
