gions are sharply separated by the horny boundary fold, which extends ventrodextrad, on the anterior and posterior surfaces, from the vicinity of the cardia. The wall of the cardiac sac is opaque in an empty stomach, and its inner surface is extensively folded. The esophagus opens into the cardiac sac, for it lies at the base of the tongue-like dextral extension of the sac's horny lining. The part of the boundary fold bordering the right end of this extension is highly sinuous.
Food passes to the small intestine through the pylorus, at the upper right hand extremity of the stomach. The pylorus contains a ring of muscle whose contraction closes the digestive tube, shutting off communication between the stomach and intestine. It regulates in this manner the passage of food from the stomach to intestine. The position of the pylorus is indicated externally by a groove around the alimentary canal. Internally the high ridges in the glandular part of the stomach are replaced at, or slightly to the right of, the pylorus by the velvety surface of the small intestine.
The stomach wall of the mouse (and rat also) are said to show the following structures. There are two muscle layers, a thick circular and a thinner longitudinal layer. The cardiac sac and boundary fold are lined internally by the statum corneum, or horny layer of epithelial cells. There are three zones in the glandular tissue at the right of the boundary fold. The cardial glands surround the tongue-like extension of the horny epithelium at the right of the cardia, and extend ventrally along the anterior and posterior walls. The rennet glands are on the ventral wall at the right of the boundary fold. The pyloric gland region lies between the pylorus, and the cardial and rennet glands. The glands of the mammalian stomach secrete the gastric juice which initiates the digestion of protein substances in
