The mucous membrane of the stomach is thick and highly developed. It contains great numbers of minute tube-shaped bodies, known as the gastric glands (Fig. 68). These are of two general kinds and secrete large quantities of a liquid called the gastric juice. When the stomach is empty, the mucous membrane is thrown into folds which run lengthwise over the inner surface. These disappear, however, when the walls of the stomach are distended with food.
[pg 148]The muscular coat consists of three separate layers which are named, from the direction of the fibers, the circular layer, the longitudinal layer, and the oblique layer (Fig. 69). The circular layer becomes quite thick at the pyloric orifice, forming a distinct band which serves as a valve.
Fig. 69—Muscles of the stomach (from Morris' Human Anatomy). The layer of Longitudinal fibers removed.
The outer coat of the stomach, called the serous coat, is a continuation of the peritoneum, the membrane lining the abdominal cavity.
Stomach Digestion.—In the stomach begins the definite work of dissolving those foods which are insoluble in water. This, as already stated, is a double process. There is first a chemical action in which the insoluble are changed into soluble substances, and this is followed immediately by the dissolving action of water. The chief substances digested in the stomach are the proteids. These, in dissolving, are changed into two soluble substances, known as[pg 149] peptones and proteoses. The digestion of the proteids is, of course, due to the
Gastric Juice.—The gastric juice is a thin, colorless liquid composed of about 99 per cent of water and about 1 per cent of other substances. The latter are dissolved in the water and include, besides several salts, three active chemical agents—hydrochloric acid, pepsin, and rennin. Pepsin is the enzyme which acts upon proteids, but it is able to act only in an acid medium—a condition which is supplied by the hydrochloric acid. Mixed with the hydrochloric acid it converts the proteids into peptones and proteoses.
Other Effects of the Gastric Juice.—In addition to digesting proteids, the gastric juice brings about several minor effects, as follows:
1. It checks, after a time, the digestion of the starch which was begun in the mouth by the saliva.[58] This is due to the presence of the hydrochloric acid, the ptyalin being unable to act in an acid medium.
2. While there is no appreciable action on the fat itself, the proteid layers that inclose the fat particles are dissolved away (Fig. 79), and the fat is set free. By this means the fat is broken up and prepared for a special digestive action in the small intestine.