3. The Œsophagus.
—The œsophagus is a straight tube, dorsoventrally flat when empty, which extends from the pharynx to the stomach. It has a uniform diameter when moderately dilated of about one centimeter. It lies dorsad of the trachea and against the longus colli muscles ([Fig. 72], g′) covering the centra of the cervical vertebræ, until it reaches the caudal end of the thyroid gland ([Fig. 96], 6); then it passes to the left and lies laterodorsad of the trachea until it reaches the bifurcation of the trachea. It there returns to the median line, passes gradually distad, separated from the vertebræ by the aorta, and finally pierces the diaphragm about two centimeters from the dorsal body wall, and enters the stomach. Its attachment to the diaphragm is loose enough to permit of longitudinal motion. In passing through the thoracic cavity it lies in the posterior mediastinum ventrad of the aorta. Its wall consists of a muscular coat, a submucosa, and a mucosa, and its inner surface presents many longitudinal folds. It has no serous covering, its side walls being merely in contact with the halves of the mediastinal septum.
4. The Stomach. Ventriculus
([Fig. 97]).—The stomach is the widest part of the alimentary canal. It is a pear-shaped sac, the long axis of which is curved nearly into a semicircle. The broad end of the sac lies to the left and dorsad; here the stomach communicates with the œsophagus (a). The narrowed end extends to the right and lies more ventrad than the other end; it passes here into the duodenum (g). That portion of the stomach which communicates with the œsophagus is known as the cardiac end (b); the opposite is the pyloric end. Owing to the curved form of the stomach above mentioned it is possible to distinguish a concave and a convex side. The concave side is directed craniad and dextrad; it is called the lesser curvature of the stomach (c). The longer convex border is directed caudad and to the left; it is called the greater curvature (d). The greater curvature extends to the left, next to the œsophagus, into a prominent convexity known as the fundus (e) of the stomach.
The stomach lies at the cranial end of the abdominal cavity, mostly to the left of the middle line. Its cardiac end is in contact by its dorsal surface with the dorsal, nearly horizontal, portion of the diaphragm. On its ventral side the cardiac end does not touch the diaphragm, so that a small part of the œsophagus passes here for a short distance into the abdominal cavity, to join the stomach. The communication of œsophagus and stomach is by a simple conical increase in size of the former. The pyloric end of the stomach extends to the right of the middle line, becoming constantly smaller; at its junction with the duodenum there is a constriction which marks the position of the pyloric valve (f). This valve is formed by a ring-like thickening of the circular muscle-fibres of the alimentary canal, forming a sphincter muscle at the junction of the stomach and duodenum and causing a projection of the mucosa into the lumen of the canal. The ventral surface of the stomach lies against the liver except when the stomach is much distended with food, when the ventral surface comes to lie against the ventral abdominal wall.
Fig. 97.—Stomach, Ventral View.
a, œsophagus; b, cardiac end of the stomach; c, lesser curvature; d, greater curvature; e, fundus; f, pyloric valve; g, part of duodenum.
The stomach is supported by the great omentum and the gastrohepatic ligament. It is connected with the duodenum by the gastroduodenal ligament; with the spleen by the gastrolienal ligament.
The inner surface of the stomach presents longitudinal folds at its pyloric end and along the greater curvature as far as the fundus. The prominence of these depends on the degree of distension. Its walls are composed of an external peritoneal layer, an internal mucous layer, and an intervening muscular layer. This may be seen with the unaided eye in sections of the wall.