From the mouth the alimentary mass descends through the pharynx and œsophagus to the stomach, where it mingles with the gastric juice, one of the most powerful agents of digestion. The gastric juice is secreted by glandular tubes situated in the mucous membrane of the stomach. It is a colourless fluid, saltish as well as acid to the taste. It contains, among other elements, alkaline chlorides, lactic acid, and an organic substance called pepsine, which is peculiar to it. The gastric juice pours into the stomach in considerable quantities when food is introduced into it, mingles there with the mass, softens it, and induces a fermentation which results in their ultimate liquefaction. During digestion a characteristic movement takes place in the stomach and intestinal canal—the circular fibres of the muscular membrane contract successively from above downwards, and push the alimentary substances in the same direction. Gradually as the lower fibres contract the upper ones relax in order to contract anew. This is called the peristaltic movement. Under its influence the contents of the stomach are kept incessantly in motion, mixed with the gastric juice, and directed toward the pylorus. This orifice is so named because it is like a door-keeper to the stomach, allowing the aliments which have been sufficiently elaborated to pass out while the others are retained in the organ. After a time, which varies from three to five hours in different individuals and at different ages, the alimentary mass is converted into a grayish paste, acid, and almost fluid; it then takes the name of chyme, and the function of the stomach, chymification, is accomplished.