611. The food is delivered by the pharynx to the esophagus (fig. [CLIII]. 12), a tube composed partly of membrane and partly of muscle. Its muscular fibres consist of a double layer, an external and an internal layer; the external has a longitudinal direction; the internal describes portions of a circle around the tube. By the contraction of the longitudinal fibres the length, and by the contraction of the circular fibres, the diameter of the tube is diminished. Cellular membrane envelops these layers of fibres externally, and mucous membrane covers them internally. When the tube is contracted, the mucous membrane is disposed in folds, which disappear when it is dilated, and these folds allow of the expansion of the tube without injury to the delicate tissue that lines it. The food passes slowly along the esophagus urged towards the stomach, not by its own gravity, but by a force exerted upon it by the tube itself, chiefly by the contraction of its circular fibres. Delivered at length to the stomach, the food is incapable of returning into the esophagus in consequence of the oblique direction in which the esophagus enters the stomach, the obliquity of its entrance serving the office of a valve.
Fig. CLXVI.—View of the Stomach with its Muscular Coats displayed.
1. The esophagus terminating in the stomach. 2. The cardiac orifice. 3. The pylorus. 4. The commencement of the duodenum. 5. The large curvature of the stomach. 6. The small curvature. 7. The large extremity. 8. The small extremity. 9. The longitudinal muscular fibres. 10. The circular muscular fibres.
612. The stomach is a bag of an irregular oval shape (fig [CLXVI].), capable, in the adult, of containing about three pints. It is placed transversely across the upper part of the abdomen (fig. LX. 7). It occupies the whole epigastric (fig. CV. 3), and the greater part of the left hypochondriac regions (fig. CVII. 3). Above, it is in contact with the diaphragm, the arch of which extends over it (fig. LX. 7, b); below with the intestines (fig. LX. 8, 9), on the right side with the liver (fig. LX. 6), and on the left side with the spleen (fig. [CLXVIII]. 5).
Fig. CLXVII. Internal View of the Stomach and Duodenum.
1. Mucous membrane, forming the rugæ. 2. Pyloric orifice opening into the duodenum. 3. Duodenum. 4. Interior of the duodenum, showing the valvulæ conniventes. 5. Termination of, 6. The biliary or choledoch duct. 7. Pancreatic duct, terminating at the same point as the choledoch duct. 8. Gall-bladder removed from the liver. 9. Hepatic duct proceeding from the liver. 10. Cystic duct proceeding from the gall-bladder, forming by its union with the hepatic, a common trunk, the choledoch.
613. Into the left extremity, which is much larger and considerably higher than the right (fig. [CLXVI]. 7), the esophagus opens by an aperture called the cardiac orifice (fig. [CLXVI]. 2). At the right extremity, a second aperture called the pyloric orifice (fig. [CLXVII]. 2), leads into the first intestine.
614. Between the cardiac and the pyloric orifices are two curvatures, one above, called the smaller (fig. [CLXVI]. 6), the other below, termed the larger curvature (fig. [CLXVI]. 5).