stomach, saclike dilation in the gastrointestinal tract between the esophagus and the intestines, forming an organ of digestion. The stomach is present in virtually all vertebrate animals and in many invertebrates. In ruminants such as the cow, the stomach is divided into four separate chambers. One of these, called the rumen, breaks down complex plant materials, particularly cellulose. In birds, the stomach forms a thick-walled gizzard that is capable of grinding food. The human stomach is a muscular, elastic, pear-shaped bag, lying crosswise in the abdominal cavity beneath the diaphragm. It is capable of gross alterations in size and shape, depending on the position of the body and the amount of food inside. The stomach is about 12 in. (30.5 cm) long and is 6 in. (15.2 cm) wide at its widest point. Its capacity is about 1 qt (0.94 liters) in the adult. Food enters the stomach from the esophagus, through a ring of muscles known as the cardiac sphincter that normally prevents food from passing back to the esophagus. The other end of the stomach empties into the first section of the small intestine, or duodenum; the pyloric sphincter, which separates the two, remains closed until the food in the stomach has been modified and is in suitable condition to pass into the small intestine. The wall of the stomach is composed of four layers, or tunics: an outer fibrous membrane called the serosa, a three-ply layer of muscle, a submucous layer, and, forming the stomach lining, a mucous layer called the gastric mucosa. The surface of the mucosa is honeycombed with over 35,000 gastric glands and is folded into numerous ridges that almost disappear when the stomach is distended with food. The muscular action of the stomach and the digestive action of the gastric juice convert food in the stomach into a semiliquid state (chyme). The stomach comprises complex interconnections of neurons formed into intrinsic nerve plexuses, including the submucosal, subserous, or myenteric plexuses. The stomach is believed to be independent of the central nervous system. See also digestive system.