We may best review the various stages of digestion generally, by supposing that we could track a mass of mixed food through the whole extent of the alimentary canal, and could observe the changes which it gradually under went in this course. Such a food rnust of course be assumed to consist of proper pro portions of all the alimentary principles, in the states in which they are ordinarily found in any diet suitable for the maintenance of health in the human subject.
The entry of such a food into the mouth would mark the end of IA hat is generally de scribed by systematic writers as the first stage of digestion :— namely, the act of prehension.
The food having arrived in the cavity of the mouth, is next subjected to the operations of mastication and insalivation.
Of these two processes, the first effects the mechanical division of the food :— reducing it to small particles ; increasing, therefore, its relative surface; and hence preparing it for the action of all those secretions to which it is exposed in its further course through the alinientary canal. The mechanism of this act is greatly aided by the simultaneous admixture of the saliva. This liquid is added to the food in quantities that vary, according to its dryness and the consequent need of such an addition, from 4 (apples) to SO (bread) per cent. of the alimentary substance that is undergoing mastication.
But insalivation also adds an important chemical influence to the preceding mecha nical advantage. The united secretions of the parotid, sublingual, and submaxillary glands, and the mucous membrane of the mouth, together furnish a liquid mixture, which converts starch into grape sugar with the greatest rapidity and energy. The perfect and instantaneous character of this change, which is only paralleled by the similar efficacy of the pancreatic juice, quite distinguishes it from that slower and less perfect metamorphosis which other animal secretions and substances are able to produce. The absence of the high temperature, and the evident putrefaction, which are generally associated with the action of these latter, still further distinguish the specific metamorphosis due to these secretions. But the substance and secretion of any one of the salivary structures, seems insufficient for the production of this agent. The sub maxillary glands, and the mucous membrane of the mouth can, hovvever, together furnish it without any aid from the parotid.* The mastication and insalivation of the food is immediately followed by its deglutition, which propels the pulpy or semifluid mass it now forms into the stomach.
On entering this organ, it is subjected to a special act of gastric digestion.
The energetic action of the mixed saliva is not affected by the gastric juice secreted by the stomach. Much of the starch of the food is probably converted into sugar during the short sojourn of the aliment in this cavity.. The sugar thus produced would seem to be absorbed by the vessels of the gastric mucous membrane with extraordinary rapidity. The water, salts, and soluble organic compounds of the food are similarly taken up. And the gastric juice attacks and dissolves the pro tinous element of the food. The perfect ness of this process of solution depends on the mechanical state of the substances con cerned, and the quantity and efficiency of the active liquid. Of the resulting solution or peptone, part is immediately absorbed by the gastric vessels, while part passes on into the duodenum, in company with portions of pro tein, which have not yet yielded to the solvent process. Many of these portions ultimately become dissolved, and with the peptone that accompanies them, are taken up by the veins of the intestine.
The intestinal digestion of the food is a still more complex act. The chyme that enters the duodenum probably contains all the alimentary principles originally present in the food. But it includes them in very different proportions compared with their original quantities. And these proportions have very diverse destinies in connection with the di gestive process.
The watery ingredient of the food, and the salts it introduces, probably have but to complete their absorption. The soluble cal careous compounds appear, however, to de compose the bile ; and to combine with and precipitate some of its acids in the shape of insoluble salts of lime.
The protein-compounds probably continue their course through the intestine, still un dergoing (or rather completing) a gradual process of solution under the action of the gastric juice which accompanies them. Whether any special intestinal juice* aids this process, may at present be looked on as doubtful. And whether the quantity of bile usually added in the duodenum can really hinder it, in the way in which Bidder and Schmidt have found that it suspends the power of the gastric juice out of the body, remains equally uncertain.