176 TEXT-BOOK OF PHYSIOLOGY. 



GASTRIC DIGESTION. 



After the food has passed through the esophagus it is received by 

 the stomach, where it is retained for a variable length of time, during 

 which important changes are induced in its physical and chemic com- 

 position. The disintegration of the food inaugurated by mastication 

 and insalivation is still further carried on in the stomach by the sol- 

 vent action of the acid fluid there present, until the entire mass is 

 reduced to a liquid or semi-liquid condition. 



The stomach is a dilated and highly specialized portion of the 

 alimentary canal intervening between the esophagus and small intes- 

 tine. When moderately distended with food, it is somewhat conical 

 or pyriform in shape and slightly curved on itself. It is situated 

 obliquely and in some individuals almost vertically in the upper part 

 of the abdominal cavity, extending from the left hypochondrium to 

 the right of the epigastrium. . The dimensions and capacity of the 

 stomach undergo considerable periodic variation according to the 

 extent to which it is distended by food. In the average condition it 

 measures in its long diameter from 25 to 35 centimeters, in its vertical 

 diameter at the cardia 1 5 centimeters, in its antero-posterior diameter 

 from ii to 12 centimeters. The capacity of the stomach varies from 

 1500 to 1700 c.c.l/ In the empty condition its walls are collapsed and 

 partly in contact, and the entire organ is drawn up into the upper 

 part of the abdominal cavity.^ The opening through which the 

 food passes into the stomach is known as the esophageal orifice or 

 the cardia. The opening through which it passes into the intestine 

 is known as the pylorus, the pyloric or gastro-duodenal orifice. 

 Between these two orifices the stomach along its upper border pre- 

 sents a curve and along its lower border a much larger curve, known 

 as the lesser and greater curvatures respectively. The left end of the 

 stomach is termed the fundus or cardiac end; the right, the pyloric 

 end. Passing from the fundus toward the pylorus, the stomach 

 gradually narrows, and at a point situated about 5 cm. from the pyloric 

 opening it frequently presents a constriction which divides the general 

 cavity into two portions: viz., the fundus and the antrum of the 

 pylorus. 



The walls of the stomach are formed by four distinct coats united 

 by areolar tissue and named, from without inward, as the serous, 

 muscle-, suianiucous, and mucous. 



The external or serous c0a/7slthin and transparent and formed by a 

 reduplication of the general peritoneal membrane. 



The middle or muscle-coat consists of three layers of non-striated 

 muscle-fibers, named from their direction theTTongitudinal, circular, 

 and oblique (Fig. 71). The longitudinal fibers are most abundant 

 along the lesser curvature and are a continuation of those of the 



