THE DIGESTIVE SYSTEM 



251 



nal. Folds of considerable extent occur in the mucous membrane. 

 Those of the stomach are known as rugce, and are not constant, 

 depending upon the degree of distention of the organ. Those of the 

 small intestine are much more definite, and are known as valvules 

 conniventes. 



ffi^-}}* 



Fig. 152. — Diagram of Structure of Wall of Gastro-intestinal Canal (longitudinal 

 section). A, Mucous membrane; a, glands; b, surface epithelium; c, goblet cells; rf, 

 stroma; e, inner circular,/, outer longitudinal layers of g, muscularis mucosae. B, Sub- 

 mucosa. C, Muscular coat; h, ;its inner circular layer; j, its outer longitudinal layer; 

 i, intermuscular connective- tissue septum. D, serous coat; k, its connective-tissue laj-er; 

 /, its endothelial layer. 



2. The submucosa (Fig. 152) is a loose connective- tissue structure. 

 It contains the larger blood-vessels, lymphatics, and nerves. 



3. The muscular coat (Fig. 152) consists of two layers of smooth 

 muscle, which in the intestine are sharply differentiated into an inner 

 circular and an outer lo?igitudinal. In the stomach the direction of 

 the layers of the muscular coat is less definite. A thin layer of 



