266 THE DIGESTIVE ORGANS. 



External to the submucosa is the muscular coat, which generally 

 consists of two layers of unstriped muscle-tissue. The inner layer 

 is composed of circular fibers (stratum circulare) ; the outer layer, of 

 longitudinal fibers (stratum longitudinale). In the colon the longi- 

 tudinal layer forms definite bands, the tcenice coli. In some regions 

 the circular fibers are also considerably reinforced, particularly in 

 the plica sigmoidea: which lie between the taeniae coli. At these 

 points the longitudinal layer also is thickened. In the rectum the 

 circular fibers form the internal sphincter ani muscle. In the 

 stomach a third layer is added to the two already mentioned, with 

 fibers running obliquely. It lies internal to the circular fibers, but 

 does not form a continuous layer. 



According to Legge, elastic fibers are present throughout the 

 entire digestive tract of all adult mammalia and vary only in minor 

 details in the different species. In regions in which the tunica mus- 

 cularis is prominent the elastic fibers attain a considerable size. 

 There is also a difference in their development in carnivora and 

 herbivora. In general, they form a dense network, present not only 

 in the serous layer, but also in the submucosa and mucosa. These 

 fibers preserve the elasticity of the intestinal walls and resist any 

 hyperextension of the glands and follicles. 



The intestine is covered externally by the peritoneum, forming 

 the serous coat, which consists of an inner, very thin connective - 

 tissue layer (subserosa) and an outer layer of mesothelial cells. 



2. THE STOMACH. 



The general structure of the gastric mucous membrane is essen 

 tially the same as that of the intestinal canal. It consists of a 



relatively coarse adenoid reticu- 

 lum, the spaces of which contain 

 lymphocytes and leucocytes, and 

 E itheiiai plasma cells. Thin strands or 

 ceil, bundles of nonstriated muscle 



^,2\ cells may be traced from the 



^B^ muscularis mucosae to various 



levels in the mucosa. It pre- 

 sents depressions or infoldings 

 known as crypts (foveolae, stom- 

 ach-pits, gland ducts) into which 

 the glands open. In the fundus 

 the crypts attain a depth of from 



one-fifth to one-sixth the thick- 

 Fig. 207. Epithelium of human stom- , 

 ach, covering the fold of mucosa between ness of the mucous membrane, 

 two gastric crypts ; x 7- In the pylorus they are deeper, 



many of them here extending 



through half the mucous membrane and some even reaching to 

 near the muscularis mucosae. The epithelium of the crypts and 



