The structure of the stomach consists of four coats—a serous, a muscular, an areolar, and a mucous coat. The external or serous coat is derived from the peritoneum. There are three kinds of muscular fibers—longitudinal, circular, and oblique, and the internal mucous lining is a rather thicker, soft, smooth, pulpy membrane, lying in ridges or rugæ, and containing a large number of glands—tubular or gastric glands, and another variety of gland called peptic, besides others.
While the stomach contains no food, and is inactive, no gastric fluid is secreted; and mucus, which is either neutral or slightly alkaline, covers its surface. But immediately on the introduction of food into the stomach, the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a large quantity of blood; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradually run together and flow down the walls of the stomach, or soak into the substance introduced. The quantity of this fluid secreted daily has been variously estimated; but the average for a healthy adult has been assumed to range from ten to twenty pints in twenty-four hours.
The specific gravity of gastric juice has been found to differ little from that of water, varying from 1.001 to 1.010, and the amounts of solid present to be very small, viz., about 56 per cent.
The chemical composition of gastric juice is:
| Water, | 994.40 | |||
| Solids, | 5.59 | |||
| Solids, |  | Ferment, pepsin, and a trace of ammonia, | 3.19 | |
| Hydrochloric acid, | 0.20 | |||
| Chloride of calcium, | 0.06 | |||
| Chloride,, of,, sodium | 1.46 | |||
| Chloride,, of,, potassium, | 0.55 | |||
| Phosphate of lime, magnesia, and iron, | 0.12 | |||
On starch gastric juice per se has no effect whatever, nor has healthy gastric juice any effect on grape sugar or cane sugar. On fats gastric juice is powerless.
The essential property of gastric juice is the power of dissolving proteid matters (meats, albumens, nitrogenous substances), and converting them into a substance called peptones. Gastric juice thus readily dissolves coagulated proteids which otherwise are insoluble, or soluble only with difficulty in very strong acids.
Certain conditions are required for the perfection of the process, which are all found in the stomach. The first is a temperature of 100° F. Second, minute division and constant movement favor digestion. Third, the greater the surface presented to the action of the juice, the more rapid the solution.
Neutralization of the juice wholly arrests digestion.
The digestive action of gastric juice on proteids, like that of saliva on starch, is a ferment action; in other words, the solvent action of gastric juice is essentially due to the presence in it of a ferment body called pepsin.