STOMACH, in comparative anatomy, a membranous sac, formed by a dilatation of the alimentary canal, in which food is received and subjected to the processes of digestion among the Vertebrata. The human stomach is an elongated, curved pouch, from 10 to 12 inches long, and 4 or 5 inches in diameter at its widest part, lying almost immediately below the diaphragm, nearly transversely across the upper and left portion of the abdominal cavity, and having the form of a bagpipe. It is very dilatable and contractile, and its average capacity is about 5 pints. The left and larger ex tremity is called the cardiac, great, or splenic extremity; the right and smaller, is known as the pyloric, from its prox imity to the pylorus. The food enters the stomach through the oesophagus by the cardia or cardiac orifice, and after having been acted on by the gastric juice, is passed on in a semi-fluid or pulpy state through the pylorus into the small in testines. The stomach has 4 coats, named from without inward: (1) the serous, (2) the muscular, (3) the areola or sub-mucous, and (4) the mucous coat. The last is a smooth, soft, rather thick and pulpy membrane, generally reddish in color from the blood in its capillary vessels; often ash-gray in old age. After death it becomes a dirty brown, and in acute inflammation, or from the action of strong acrid poisons, it becomes of a bright red, either continuously or in patches. Corrosive poisons also affect its coloration. The surface of the mu cous membrane is beset with secreting glands. The stomach is supplied with blood from the coeliac artery, which gives off arterial branches that ramify freely, and the veins return the residual blood into the splenic and superior mesenteric veins, and directly into the portal vein. The lymphatics of the stomach are very numerous, and arise in the mucous mem brane. The nerves are large, and con sist of the terminal branches of the two pneumogastric nerves belonging to the cerebrospinal system, and of offsets from the sympathetic system derived from the solar plexus. Their ending has not
been traced.
Medical electricians have devised a plan by which the interior of the human stomach may be illuminated for exami nation. The interior of the stomach is plainly lighted and all its parts are brought into view by a small movable mirror at the end of the tube. In the lower mammals three forms of stomach have been distinguished: (1) Simple, consisting of a single cavity, as in man; (2) Complex, in which there are two or more compartments communicating with each other, as in the kangaroo, the por cupine, and the squirrel; (3) Compound, in which the stomach is separated into a reservoir and a digestive portion. (See RUMINATION.) The family Camelids have a stomach divided into two com partments by a muscular band—one of the points of difference between them and the other ruminants. The lining of the second stomach, or honeycomb bag, and of a portion of the first stomach, or paunch, is provided with a great number of cells in which water is stored up and long retained for use in time of drought and of long journeys over the desert. (See CAMEL.) In birds there are three small, but dis tinct dilatations of the alimentary canal, called the crop, gizzard, and proventric ulus, and in most reptiles the simplicity of the oesophagus extends to the stomach. In fishes, two forms are found, the siphonal stomach and the meal, in which the upper portion gives off a long blind sac. In the higher invertebrata there is a digestive tract with functions anal ogous to those of the stomach of Verte brates; in the lower there may (Hydra) or may not (Amoeba) be a gastric cavity in which food is ingested and absorbed. In the latter case the living protoplasm closes over its prey, and after a time by a reversing process, the indigestible remains are ejected. To those tracts or cavities, the name stomach is often ap plied. See DIGESTION.