Here it is that the true business of digestion commences. For as soon as any substance except water enters the stomach, this organ, with involuntary movements, that seem almost like instinct, commences the secretion of the gastric juice, and by long-continued contractions of its muscular coat, succeeds in effecting a most perfect mixture of the food with this juice, by which the contents of the stomach are reduced to the softest pulp.
The gastric juice, in its pure state, is a colorless, transparent fluid; “inodorous, a little saltish, and perceptibly acid. It possesses the property of coagulating albumen, and separating the whey of milk from its curd, and afterwards completely dissolving the curd. Its taste, when applied to the 446 tongue, is similar to that of mucilaginous water, slightly acidulated with muriatic acid.” The organs of its secretion are an immense number of tubes or glands, of a diameter varying from one five hundredth to one three hundredth of an inch, situated in the mucous coat of the stomach, and receiving their blood from the gastric arteries. A chemical analysis shows it to consist of water, mucilage, and the several free acids—muriatic, acetic, lactic, and butyric, together with a peculiar organic matter called pepsin, which acts after the manner of ferments between the temperature of 50° and 104° F.
The true process of digestion is probably owing to the action of pepsin and the acids, especially if the presence of the chloro-hydric or muriatic be admitted; since we know, by experiments out of the body, that chlorine, one of its elements, is a powerful solvent of all organic substances.
The antiseptic properties of the gastric juice, as discovered by experiments made on Alexis St. Martin, doubtless have much influence on digestion, although their true uses are probably not yet known.
As soon as the food is reduced to a state of fluidity, the pyloric orifice of the stomach is unclosed, and it is thrust onwards through the alimentary canal, receiving in the duodenum the secretions of the liver and pancreas, after which it yields to the lacteals its nutrient portion, and the residuum is expelled from the body.
There have been many hypotheses in regard to the nature of the digestive process. Some have supposed that digestion is a mere mechanical process, produced by the motion of the walls of the stomach; while others, in later times, have considered it as under the influence of a spirit separate from the individual, who took up his residence in the stomach and regulated the whole affair; while others still would make it out to be a chemical operation, and thus constitute the stomach a sort of laboratory. But to all these ridiculous hypotheses Sir John Hunter has applied the following playful language: “Some will have it that the stomach is a mill; others that it is a fermenting vat; and others that it is a stewpan; but in my view of the matter, it is neither a mill, a fermenting vat, nor a stewpan, but a stomach, a stomach!”
At the present day this process is regarded as a complex, and not a simple operation. It seems to be a process in which the mechanical, chemical, and vital agencies must all act in harmony and order; for if one of these be withdrawn, the function cannot be sustained for any considerable length of time; and of the chemical and mechanical parts of the process, since the former is much more important, and, as a matter of course, the vital powers are indispensable, therefore digestion may be considered as a chemical operation, directly dependent on the laws of vitality, or of life; since the proper consistency of the food depends, in a great measure, upon the character of the solvents, while the secretion of these fluids, their proper amount, 447 together with the peculiar instinct—as it almost seems to be—necessary to direct the stomach in its many functions, are exclusively and entirely dependent on the laws and conditions of life.
G.
As food is necessary to supply the waste and promote the growth of the body, it follows that that will be the best adapted to the system which contains the same chemical elements of which the body is composed; viz., oxygen, hydrogen, carbon, and nitrogen. These elements are found in greater or less quantity in all animal food, and in many vegetable products. Hence, that article of food which contains all these elements in a proper proportion will tend much more to the growth and strength of the body than those kinds which are deficient in one or more of them. Much experience on this point, and scientific research, seem to show that a reasonable amount of animal food in health tends to give greater strength of muscle, and a more general sense of fulness, than in ordinary cases a vegetable diet is able to do, owing to the presence of nitrogen in animal tissues. Yet there are examples of the healthiest and strongest men, who live years without a morsel of animal food; and the fact can only be accounted for, by supposing that the system has the power to make the most economical use of the little nitrogen offered to it in the food; or else that it has by some means the power to abstract it from the atmosphere, and transform it to the living animal substance.