VII THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 139 



thicker than that of the esophagus or the intestine. The 

 inner surface is thrown into several longitudinal folds, which 

 become less prominent posteriorly, and near the pyloric end 

 entirely disappear. 



In a cross section of the stomach one may observe a very 

 thin outer layer composed of much flattened cells ; this is 

 the serous coat or serosa, and it is formed by the peritoneum. 

 Within the serosa is a thicker layer, the siibserosa, zox\%\'i'img 

 mainly of connective tissue. This layer has been frequently 

 described as a layer of longitudinal muscles, and it has the 

 appearance of such ; but if treated with the proper stains, 

 it can readily be shown to be mainly connective tissue. 

 Some writers (Valatour, P. Schultze), on the other hand, 

 have been disposed to deny the existence of longitudinal 

 muscles in the frog's stomach. In sections across the cardiac 

 end of the stomach, however, one may detect a few muscle 

 fibers among the connective tissue, and in the pyloric end, 

 according to Gaupp, there are a few longitudinal fibers 

 which are continuous with those of the intestine. 



Within the subserosa is a thick layer of circular muscles 

 which becomes thicker toward the pylorus. Internal to the 

 circular muscles is a layer of connective tissue, the sub- 

 mucosa, in which there are numerous blood vessels. The 

 tissue of the submucosa extends into the folds of the inner 

 coat. Between the mucosa and submucosa there is a thin 

 muscular layer, the muscularis mucosce, composed of an 

 inner layer of circular fibers and an outer stratum of longi- 

 tudinal ones. 



The mucosa of the stomach is a thick layer composed of 

 glands embedded in a supporting matrix of connective tissue. 

 These glands represent invagination of the epithelium lining 

 the inner surface of the stomach. They are elongated 

 tubular structures set very closely together, and frequently 



