The Stomach

The stomach (Ventriculus) is the large dilatation of the alimentary canal between the œsophagus and the small intestine. It is a sharply curved, U-shaped sac, the right branch being, however, much shorter than the left one. The convexity is directed ventrally. When moderately distended, there is often a slight constriction which indicates the division into right and left sacs. It is relatively small, and is situated in the dorsal part of the abdominal cavity behind the diaphragm and liver, mainly to the left of the median plane.

It presents for description two surfaces, two curvatures, and two extremities. The parietal surface (Facies parietalis) is convex and is directed forward, upward, and toward the left; it lies against the diaphragm and liver. The visceral surface (Facies visceralis), also convex, faces in the opposite direction; it is related to the terminal part of the large colon, the pancreas, the small colon, and the small intestine. The lesser curvature (Curvatura minor) is very short, extending from the termination of the œsophagus to the junction with the small intestine. When the stomach is in situ, its walls are here in contact, and the cardia and pylorus close together. The greater curvature (Curvatura major) is very extensive. From the cardia it is first directed upward and curves over the left extremity; it then descends, passes to the right, crosses the median plane, and curves upward to end at the pylorus. Its left portion is related to the spleen, while its ventral portion rests on the left divisions of the great colon. The left extremity or saccus cæcus is a rounded cul-de-sac which lies under the upper ends of the fourteenth, fifteenth, and sixteenth ribs and the diaphragm. It is related to the pancreas behind and the base of the spleen externally. The right or pyloric extremity is much smaller and is continuous with the duodenum, the junction being indicated by a marked constriction. It lies on the liver, a little to the right of the median plane, and a little lower than the cardiac opening. About two or three inches (ca. 5 to 8 cm.) from the pylorus there is a constriction which marks off the antrum pylori from the rest of the right sac. The œsophageal orifice or cardia is situated at the left extremity of the lesser curvature, but about eight to ten inches (ca. 20 to 25 cm.) from the left extremity. The œsophagus joins the stomach very obliquely. The opening is closed by the sphincter cardiæ and numerous folds of mucous membrane. The pyloric orifice communicates with the duodenum. Its position is indicated externally by a distinct constriction. Internally it presents a circular ridge produced by a ring of muscular tissue—the sphincter pylori.

The stomach is held in position mainly by the pressure of the surrounding viscera and by the œsophagus. The following peritoneal folds connect it with the adjacent parts:

1. The gastro-phrenic ligament (Lig. gastrophrenicum) connects the great curvature, from the cardia to the left extremity, with the crura of the diaphragm. This leaves a narrow area uncovered with peritoneum, and here the stomach is attached to the diaphragm by loose connective tissue.

2. The small or gastro-hepatic omentum (Omentum minus) connects the lesser curvature and the first part of the duodenum with the liver below the œsophageal notch and the portal fissure.

3. The gastro-splenic omentum (Lig. gastrolienale) passes from the left part of the great curvature to the hilus of the spleen.

4. The great or gastro-colic omentum (Omentum majus) connects the ventral part of the great curvature and the first curve of the duodenum with the terminal part of the great colon and the initial part of the small colon.

5. The gastro-pancreatic fold (Plica gastro-pancreatica) extends from the left sac above the cardia to the duodenum. It is attached dorsally to the liver and vena cava, ventrally to the pancreas.

The stomach of the equidæ is relatively small, its capacity varying from two to four gallons (ca. 8 to 15 liters).

The size, form, and position of the stomach are subject to considerable variation. When the stomach is nearly empty the saccus cæcus contains only gas and is strongly contracted; the middle portion (physiological fundus) contains the ingesta and preserves its rounded character, while the pyloric portion is contracted. When distended the middle portion settles down some four or five inches, pushing back coils of the small colon and small intestine which may lie between the great curvature and the large colon, and also pushing to the left or right the left dorsal part of the great colon; the spleen, small colon, and small intestines are pushed back by the distention of the left sac.

Structure.—The wall is composed of four coats—serous, muscular, submucous, and mucous.

The serous coat (Tunica serosa) covers the greater part of the organ and is closely adherent to the muscular coat except at the curvatures. It partially bridges over the lesser curvature, and covers here elastic tissue which assists in retaining the bent form of the stomach. The peritoneal folds have been described.

The muscular coat consists of three incomplete layers, an external of longitudinal, a middle of circular, and an internal of oblique fibers. The layer of longitudinal fibers (Stratum longitudinale) is very thin and exists only along the curvatures and at the antrum. At the lesser curvature it is continuous with the longitudinal fibers of the œsophagus. On the antrum pylori it forms a well developed complete layer. The layer of circular fibers (Stratum circulare) exists only on the right sac. At the pyloric orifice it forms a thick ring—the pyloric sphincter. Another ring, the antral sphincter, is found at the left end of the antrum pylori. The oblique fibers (Fibræ obliquæ) are arranged in two layers; the external stratum covers the left sac and is a continuation (in part) of the longitudinal fibers of the œsophagus; the internal stratum is found also on the left sac, and exchanges fibers with the circular and external oblique layers. It forms a remarkable loop around the cardiac orifice, constituting a powerful cardiac sphincter (Sphincter cardiæ).

The submucous coat is a layer of loose connective tissue which connects the muscular and mucous coats; in it the vessels and nerves ramify before entering the mucosa.

The mucous coat is clearly divided into two parts. That which lines the left sac resembles the œsophageal mucous membrane, and is termed œsophageal or cuticular. It is white in color, destitute of glands, and covered with a thick, squamous, stratified epithelium. At the cardiac orifice it presents numerous folds which occlude the opening. It terminates abruptly at an elevated, denticulated, sinuous line, termed the cuticular ridge (Margo plicatus). Below and to the right of this line the mucous membrane has a totally different character, being soft and velvety to the touch, and covered by a mucoid secretion. It is glandular, and three zones may be recognized, although no sharp line of demarcation exists. A narrow zone next to the cuticular ridge has a yellowish-gray color, and contains short tubular cardiac glands (Cardiac gland region). Next to this is a large area which has a mottled reddish-brown color, and contains fundus glands (fundus gland region). This part of the mucous membrane is thick and very vascular, and corresponds to the fundus of the stomach in man and the dog. The remainder of the mucous membrane has a reddish-gray color and contains branched, tubular, pyloric glands (pyloric gland region); it corresponds to the pyloric portion of man and the dog.

The folding of the stomach wall at the lesser curvature produces a prominent ridge which projects into the cavity of the stomach. Circular ridges occur at the antral and pyloric sphincters.

Blood-vessels and Nerves.—The stomach receives blood from all the branches of the cœliac artery. The gastric veins drain into the portal vein. The nerves are derived from the vagus and sympathetic nerves.