Chapter II: GASTRIC DIGESTION.--1. The Stomach is an irregular expansion of

the digestive tube. Its shape has been compared to that of a bagpipe. It holds about three pints, though it is susceptible of some distension. It is composed of an inner, mucous membrane, which secretes the digestive fluids; an outer, smooth, well-lubricated serous one, which prevents friction, and between them a stout, muscular coat. The last consists of two principal layers of longitudinal and circular fibers. When these contract, they produce a peculiar churning motion, called the _peristaltic_ (_peri_, round; _stallein_, to arrange) movement, which thoroughly mixes the contents of the stomach. At the farther end, the muscular fibers contract and form a gateway, the _pylorus_ (a gate), as it is called, which carefully guards the exit, and allows no food to pass from the stomach until properly prepared. [Footnote: With a wise discretion, however, it opens for buttons, coins, etc., swallowed by accident; and when we overload the stomach, it seems to become weary of constantly denying egress, and, finally, giving up in despair, lets everything through.]

FIG. 47.

2. _The Gastric Juice_.--The lining of the stomach is soft, velvety, and of a pinkish hue; but, as soon as food is admitted, the blood vessels fill, the surface becomes of a bright red, and soon there exudes from the gastric glands a thin, colorless fluid--the gastric juice. (See p. 319.) This is secreted to the amount of twelve pounds per day. [Footnote: The amount secreted by a healthy adult is variously estimated from five to thirty-seven pounds. As it is reabsorbed by the blood, there is no loss.] Its acidity is probably due to muriatic or lactic acid--the acid of sour milk. It contains a peculiar organic principle called _pepsin_ [Footnote: Pepsin is prepared and sold as an article of commerce. The best is said to be made from the stomachs of young, healthy pigs, which, just before being killed, are excited with savory food that they are not allowed to eat. One grain is sufficient to dissolve eight hundred grains of coagulated white of egg. A temperature of 130° renders pepsin inert.] (_peptein_, to digest), which acts as a ferment to produce changes in the food, without being itself modified.

The flow of gastric juice is influenced by various circumstances. Cold water checks it for a time, and ice for a longer period. Anger, fatigue, and anxiety delay and even suspend the secretion. The gastric juice has no effect on the fats or the sugars of the food; its influence being mainly confined to the albuminous bodies, which it so changes that they become soluble in water. [Footnote: The question is often asked why the stomach itself is not digested by the gastric juice, since it belongs to the albuminous substances. Some have assigned as the probable reason that life protects that organ, and assert that living tissues can not be digested; but the fallacy of this has been clearly shown by experiments that have been made with living tissues in the course of scientific research. The latest opinion is that the blood which circulates so freely through the vessels of the lining of the stomach, being alkaline, protects the tissue against the acidity of the gastric juice.]

The food, reduced by the action of the gastric juice to a grayish, soupy mass, called _chyme_ (kime), escapes through that jealously guarded door, the pylorus.

Fig. 48.