The _stomach_ may be felt at the lower extremity of the breast bone, in the triangular space formed by the divergence of the ribs. It is a large hollow, compound gland, the walls of which contain muscular fibers in addition to the tubules which elaborate the special secretions. Its cavity is lined with a thick mucous membrane, packed with tubular glands, into which is poured out a complex secretion called the gastric juice. =The Chemical Changes which Food Undergoes in the Stomach.=—There are two chief phases or periods of gastric secretion: (_a_) The psychic or appetite juice, and (_b_) the chemical juice. Gastric juice is not constantly poured into the stomach to accumulate there, but is secreted only as it is needed under the influence of certain stimuli. These stimuli may be classified as psychic and chemical. In the second phase, or period of gastric digestion, the exciting agent is the presence of food in the stomach. It has been shown that a correlation of the different organs of the body is brought about by _hormones_ or chemical messengers. The hormones of gastric digestion are produced from proteins in the early stage of gastric digestion, and this shows the importance of the psychic or appetite juice. A similar effect is produced by meat extracts or infusions, which are well-known provocatives of gastric secretion. _Gastric Juice._—The principal active agents of the gastric juice are hydrochloric acid and the enzymes; pepsin, the proteolytic enzyme; rennin, the milk-curdling ferment; and lipase, the fat-splitting ferment. The uses of the gastric juice are digestive, activating, and disinfecting. Gastric juice does not dissolve and chemically change all food that enters the stomach, but acts only on that class of foods called proteins, converting them into peptones. The changes which food undergoes in the stomach are as follows: food is disintegrated; meat is broken up, gelatin dissolved; the muscle-fibers fall apart, and are split into disks and sarcous elements; the framework of fatty tissue undergoes similar dissolution of cellular membranes; milk is curdled; the caseinogen is transformed into casein, and the latter broken down by pepsin. Bread and other starchy foods are disintegrated, some of the starch being formed into sugar by ptyalin. The other vegetable foods are but little digested by this viscus. The whole is broken into irregular fragments by the disintegrating power of the stomach and gradually converted into _chyme_, the creamy emulsion which passes through the pylorus into the intestines. The passage from the stomach into the duodenum is aided by the motor activity of this organ.