THE LARGE INTESTINE. 



173 



Mucosa 



Submucosa 



The blood-vessels, lymphatics and nerves of the large intestine follow, 

 in the details of their distribution, the general plan described in connection 

 with the small intestine (page 170). 



The ileo-colic valve, guarding the entrance of the ileum into the 

 large intestine, results from the thrusting of the small gut into the large, 

 during foetal life, in such a way that originally all layers of the intestinal wall 

 are involved. Where the two serous coats come into contact, the meso- 

 thelium disappears and the permanent union is effected by fibre-elastic tissue 

 and secondarily developed longitudinal muscle. Although both layers of 

 the original muscular coat are carried into the folds of the valve, it is the cir- 

 cular muscle that undergoes marked 

 thickening and forms the efficient 

 sphincter guarding the opening. The 

 mucosae covering the two sides of the 

 crescentic valve-folds differ, that con- 

 tinued from the ileum possessing villi 

 which, as rudimentary elevations, con- 

 tinue almost to the margin of the folds. 



The vermiform appendix, 

 the slender worm-like appendage at- 

 tached to the caecum, about 8.4 cm. 

 (3 1^ in. ) long and 6 mm. in diameter, 

 contains all the coats of the large in- 

 testine. The mucous coat is thrown 

 into longitudinal folds and encloses 

 a narrow irregular lumen. In its gen- 

 eral structure it' corresponds to the 

 mucosa of the large intestine, but is 

 infiltrated to an unusual degree with 

 lymphocytes. These are collected 

 into many lymph-nodules, with germ- 

 centres, the lymphoid tissue being so 

 abundant that it often almost encircles 

 the appendix as a continuous mass. 

 The crypts of Lieberkuhn contain an 

 unusually large number of goblet- 

 cells; these are, however, few on the 

 free surface. The inner circular mus- 

 cle is about twice as thick as the ex- 

 ternal longitudinal layer. The lym- 

 phoid tissue of the vermiform appendix is, as elsewhere, most developed in 

 childhood and tends to atrophy in middle life. Along with such atrophy, the 

 walls of the appendix manifest a disposition to adhere, more or less obliterating 

 the lumen of the tube. In consequence of these changes, after the thirty- 

 fifth year the appendix often exhibits variations from the normal condition. 



The rectum, including the anal canal, presents modifications calling 

 for passing notice. The crypts of Lieberkuhn are especially large (.7 mm. 

 in length), although less numerous, and do not entirely disappear until the 

 transition of the columnar to the stratified squamous epithelium has been 

 reached or, sometimes, even slightly passed. This transformation begins at 

 the upper ends of the vertical mucosa-folds, the rectal columns or columns of 

 Morgagni, that surround the anal canal and contain strands of muscle; at 

 the level of the crescentic folds, the anal valves, connecting the bases of the 



Y 



Longitudinal 

 muscle 



Serosa 



FIG. 215. Transverse section of injected large 

 intestine, showing distribution of arteries to the 

 coats. X 20. 



