an expanded section of the digestive tract in which food is treated chemically and mechanically.

Structure in animals. In animals, one may distinguish the glandular, or digestive, stomach, whose walls contain digestive glands, and the muscular, or masticatory, stomach, whose walls are usually lined with cuticle. In vertebrates (and some invertebrates) the muscular stomach forms as part of the glandular; in the majority of invertebrates it originates independently. Among the invertebrates, certain coelenterates and a number of flatworms and annelids already have a stomach as a differentiated part of the intestinal system. The stomach is well developed in rotifers, brachiopods, and bryozoans. In mollusks the stomach is usually horseshoe-shaped. In many gastropods, bivalves, and cephalopods a blind, sometimes spiral process protrudes from the posterior end of the stomach and, in cephalopods, the liver ducts open into that process. In some gastropods the stomach is divided into a masticatory foregut and a stomach proper. The masticatory stomach plays an important role in the mechanical processing of food in the digestive system of arthropods. Among crustaceans, only the Entomostraca have a glandular stomach; the Malacostraca have a masticatory stomach equipped with chitinous “teeth.” In arachnids the midgut is usually divided into two sections, one of which is located in the cephalothorax and the other in the region of the abdomen. The first section, with its blind pouchlike appendages, is sometimes called the stomach. In insects, the masticatory stomach is well developed, the glandular stomach, as an independent section of the midgut, does not develop in all species. Among echinoderms, the stomach is well developed in sea lilies, starfishes, and Ophiuroidea. Among the lower chordates, some hemichordates and tunicates have a distinctly separate stomach.

In vertebrates the stomach is an expanded part of the anterior gut, located posterior to the esophagus. In cyclostomes and some fish the stomach is not differentiated. The stomach of fish is usually horseshoe-shaped. Its descending bend, starting from the esophagus, is called the cardiac part, while the ascending bend, which connects to the duodenum, is called the pyloric section. The pouchlike part of the stomach that lies between the two bends forms its fundus. The concave part of the stomach is called the lesser curvature; the convex part, the greater curvature. In bony fish, pyloric appendages (caeca) usually develop in the region of the stomach. The stomach is lined with a single layer of cylindrical epithelium, from which tubular glands are formed. In many fish, amphibians, reptiles, and birds, the glands of the fundus and those of the pylorus are distinct. In the majority of mammals there are, in addition, cardial glands (these are absent in predators and primates). The glands of the stomach secrete mucus and gastric juice. The smooth musculature of the stomach walls usually forms the powerful pyloric sphincter at the place where the stomach enters the intestine. The stomach of birds consists of a glandular region, proventriculus, and a distal region, the muscular gizzard. In many birds the cuticle of the gizzard forms processes that (in view of the absence of teeth in birds), together with swallowed pebbles or grains of sand (so-called gastroliths), promote the mechanical processing of food. In carnivorous birds, the gizzard is thin-walled; in granivorous, insectivorous, and omnivorous birds, it is thick-walled; in fish-eating birds that swallow fish whole, the gizzard is small, while the proventriculus forms a roomy pouch. In mammals the stomach attains its most complex differentiation and is divided into the esophageal, cardiac, fundic, and pyloric sections. In herbivorous mammals (rodents, ruminants) the esophageal section of the stomach is very highly developed, lined with multilayered epithelium, and lacks glands. It is often separated into two or three sections, which serve simultaneously as repositories for bulky fodder and as “fermentation tanks” in which, under the influence of bacteria and symbiotic infusoria living in the stomach, the plant cellulose ferments. The stomach of some ruminants is the most complex of all, divided into four sections: the rumen, the reticulum, the psalterium, and the abomasum. Only the abomasum contains glands; they are absent in the first three sections, which develop from the esophageal part of the stomach. A groove, whose edges usually adjoin one another and form a tubule, passes from the esophagus along the upper edge of the stomach to the psalterium. The camel has numerous pits in the wall of the rumen (so-called water cells) in which water is stored.

Structure in man. The stomach is located in the abdominal cavity. Its long axis runs up and down, left to right, and back to front; the greatest portion (five-sixths) occupies the left upper quadrant of the abdomen. In form, the stomach is reminiscent of a flattened retort. The stomach is distinct anterior and posterior walls. The place at which the esophagus enters the stomach, near the diaphragm, is called the cardia. The upper part of the body of the stomach, the fundus, is expanded and turned toward the diaphragm. The place of exit from the stomach, or the pylorus, lies a little to the right of the abdominal midline; it is fixed to the posterior wall of the abdomen at the level of the first and second lumbar vertebrae. The concave border of the stomach, the lesser curvature, faces right and upward; the convex border, the greater curvature, faces left and downward. The spleen lies to the left of the stomach; the pancreas, below and behind the stomach. The entire stomach is covered by the peritoneum, which passes to the lesser curvature from the liver and the diaphragm to form the hepatogastric and diaphragmogastric ligaments; these, together with the hepatoduodenal ligament, make up the lesser omentum. The anterior and posterior sheets of the peritoneum merge along the greater curvature and extend to the transverse colon (the gastrocolic ligament), giving rise to the greater omentum. A peritoneal fold, the gastrosplenic ligament, extends from the fundus of the stomach to the spleen. The capacity of the stomach varies individually and with age: in the newborn it is between 20 and 30 cu cm; in an adult, up to 2,500 cu cm.

The wall of the stomach consists of three coats. Under the peritoneal, or serous, coat there is a muscular coat, which consists of outer longitudinal, middle circular, and inner oblique layers. Constructed of smooth muscle, the stomach contracts involuntarily; this changes the contours and the lumen of the stomach. The inner surface of the stomach is lined with a mucous coat, separated from the muscular coat by a submucous layer of loose (areolar) connective tissue. The mucous coat has its own muscle bundles, during whose contraction (because of the presence of the loose submucous tissue) the mucous coat gathers into folds that are characteristic of the inner topography of the stomach. A single layer of cylindrical epithelium lines the mucous coat. Numerous glands are enclosed within the mucosa. Glands in the area of the cardia (cardiac glands) produce mucus; in addition to this, glands in the area of the pylorus (pyloric glands) secrete enzymes that decompose proteins. The secretion of the glands in the region of the fundus (the fundic glands) contains pepsin and hydrochloric acid. The effluent ducts of the gastric glands open into fossae in the stomach margins, rounded outgrowths 1–6 mm in diameter. The pyloric sphincter, constructed of several circular layers of muscle and regulating the periodic evacuation of the stomach, is located at the boundary between the stomach and the duodenum.

Blood is supplied to the stomach by the celiac trunk system, an unpaired branch of the abdominal aorta. The left gastric artery, leaving the celiac trunk, unites (anastomoses) along the lesser curvature with the right gastric artery (a branch of the common hepatic artery). Branches of the gastroepiploic arteries run along the greater curvature. The veins of the stomach enter the portal vein system (with the exception of the left gastric vein, which passes into the venous plexuses of the esophagus). The stomach is innervated by branches of the vagus nerves and of the sympathetic celiac plexus, which form three nerve plexuses in the stomach wall.

Functions. The principal functions of the stomach are the deposit of food, its mechanical and chemical processing, and its evacuation into the intestine. The mechanical processing and evacuation of food are the result of the motor activity of the stomach; the chemical processing is carried out predominantly by the enzymes and hydrochloric acid of the gastric juice. The stomach also has protective, incretory, absorptive, and excretory functions.

Digestive processes in the stomach of invertebrates are extremely diverse. In some invertebrates, such as the river crayfish, the masticatory stomach grinds the food and filters it. The food is processed in the glandular stomach by enzymes secreted by the mucous coat of the stomach and by enzymes that enter the stomach through ducts from digestive glands outside the stomach.

A characteristic feature of digestion in the vertebrate stomach (with the exception of certain groups of fish) is the presence of proteases and an acid medium. The processing of food in the multichambered stomach of ruminants is most complex. The structure and functions of the stomach of omnivorous mammals are very similar to the structure and functions of the stomach of carnivorous mammals. Stomach activity has been most fully studied in dogs and man. A mixture of solid and liquid matter, previously processed in the oral cavity, enters the stomach. Because of the hydrochloric acid in the stomach, the cellular structures of the food undergo denaturation and saturation, and an optimal medium is created for the action of the hydrolytic enzymes of the gastric juice. Food entering the stomach through the esophagus is wedged into the food already there, thus occupying an interior position, so that proteins are digested at the surface of the food bolus and the decomposition of carbohydrates, already begun in the mouth cavity or by the salivary enzymes, continues. Gastric digestion proper amounts basically to the initial hydrolysis of proteins by the proteases of gastric juice. Fats are digested to a small extent in the stomach, predominantly by enzymes discharged from the duodenum. The secretion of enzymes and hydrochloric acid by the gastric cells corresponds to the nature and quantity of the food ingested; the secretion is regulated by nervous and humoral factors. In the first (compound-reflex) phase, gastric secretion is stimulated by the usual external conditions associated with taking food; that is, its appearance and odor, its effect on the receptors of the mouth and pharynx, and by the acts of chewing and swallowing. In the second (neurohumoral) phase, secretion is stimulated by the direct action of the food on the mucosa of the stomach. In the third (intestinal) phase, secretion is determined by reflex influences that arise upon stimulation of the duodenal receptors and by humoral influences brought about by the products of food decomposition absorbed in the intestine. The mucosa of the pylorus contains gastrin, a histohormone that stimulates the secretion of hydrochloric acid by the parietal cells of the stomach. The formation of gastrin is suppressed by enterogastrone, a hormone produced in the upper regions of the intestine. Hormones of the pituitary, adrenals, thyroid, parathyroid, and gonads also influence the secretory activity of the stomach. An important role in gastric activity is played by mucus that is secreted in the stomach and protects the gastric mucosa from autodigestion by adsorbing bicarbonates and phosphates.

Food is processed mechanically through the stomach’s motor activity. Peristaltic, tonic, and possibly systolic contractions are characteristic of the stomach when filled with food. Only the surface layers of the food bolus in the region of the fundus are subject to processing by the peristaltic activity of the stomach against a background of tonic contractions and waves. The greater mass of the stomach contents remains unmixed, while the ground and liquefied surface layers of the food are shifted by peristaltic waves into the pyloric section of the stomach. Here, the contents are mixed and squeezed into the duodenum. The nature of the stomach’s activity depends on the consistency and chemical composition of the food. This motor activity is regulated by both nervous and humoral factors. The vagus nerves (predominantly) stimulate, and the celiac nerves inhibit, the gastric motor system. Gastrin, choline, histamine, and potassium ions stimulate the stomach’s motor activity; enterogastrone, epinephrine, and calcium ions inhibit it.

Food is evacuated from the stomach through the coordinated activity of the pyloric sphincter and the peristaltic waves of the stomach; this is a complex dynamic process, which depends on the physicochemical properties of the food, the speed of processing by the gastric juice, the functional condition of the feeding and thirst centers, the general emotional state of the body, the body’s needs for certain substances, and the reflex effects arising from the influence of osmotically active substances on the receptors of the upper portions of the intestinal tract. The average portion of food taken in a three- or four-meal-a-day regime is evacuated from the human stomach in 3½ to 4½ hours, although fatty food may be held in the stomach as long as ten hours. Periodic motor activity (10–30 minutes every 1–1½ hours) is characteristic of the empty stomach. The contractions of an empty stomach are usually accompanied by sensations of hunger.

The protective function of the stomach consists in the bactericidal and bacteriostatic action (on microorganisms entering the stomach with the food) of hydrochloric acid and a lysozyme-type substance manufactured in the stomach. The degree of absorption in the stomach is insignificant. The excretory role of the stomach consists in the discharge into its cavity of intermediate metabolic products. The stomach is associated with the manufacture of blood corpuscles, since its glands secrete an intrinsic factor, or Castle’s factor, which is necessary for that process. Gastric activity is closely associated with the maintenance of homeostasis in the body and the maintenance of water-salt metabolism, kidney and endocrine function, and blood circulation. Signals entering the central nervous system upon stimulation of the gastric receptors participate in the formation of behavioral reactions by influencing both general alimentary excitation and the more specialized phenomena of appetite and thirst.