THE LARGE INTESTINE .301 



circumference of the organ the outer muscular layer is slightly 

 thinner than in the small intestine. 



The mucous membrane of the large intestine may be best de- 

 scribed by comparison with that of the small intestine. If the 

 mucosa of the latter organ contains two zones, a superficial layer 

 of villi and a deeper glandular layer, that of the large intestine 

 may be said to consist of only the deeper of these zones. It there- 

 fore possesses no villi, and its simple tubular crypts of Lieberkiihn 

 extend from the free surface almost to the muscularis mucosae. 



The lining epithelium of the large intestine is of the simple 

 columnar variety and has only an indistinct cuticular margin. 

 That of the crypts of Lieberkiihn contains both columnar and 

 goblet cells, the latter being far more numerous than in the small 

 intestine. 



The lymphoid tissue of the large intestine occurs in the form 

 of diffuse lymphoid tissue in the corium, and as solitary follicles, 

 which frequently break through the muscularis mucosae and lie in 

 the submucosa. The fundus of such crypts of Lieberkiihn as may 

 occasionally be inclosed within the lymphatic follicles are fre- 

 quently prolonged into the superficial portion of the submucosa 

 where they often possess alveolar dilatations. Elsewhere the mus- 

 cularis mucosae forms a complete membrane which is nowhere 

 penetrated by the simple tubular glands of Lieberktihn. 



Lymphatic nodules are especially abundant in the rectum and 

 in the vermiform appendix. In the latter the nodules are more or 

 less confluent, a condition which is not found elsewhere in the 

 large intestine. In the appendix the greater portion of the mucous 

 membrane is invaded by the lymphoid tissue and the crypts are 

 much diminished in both number and size (Fig. 246). 



The vascular and nerve supply of the large intestine is identical 

 in its arrangement with that of the small intestine. The mucous 

 membrane contains a capillary plexus of blood and lymphatic 

 vessels in the corium about the crypts. The nerves of the large 

 intestine supply its muscular coats and blood vessels and, in the 

 mucosa, end in naked varicose or knobbed fibrils beneath the epi- 

 thelium of the glands of Lieberkiihn. 



In the rectum the lining epithelium is continuous at the anus 

 with the stratified squamous epithelium of the skin. In this region, 

 also, the circular fibres of the inner layer of the muscular coat are 

 much thickened to form the internal rectal sphincter. Lymphoid 

 tissue abounds in the rectal mucous membrane. 



