172 



NORMAL HISTOLOGY. 



border. It differs, however, in having neither villi nor plicae circulares, and, 

 in consequence, lacks the velvety appearance of the small intestine, the mucous 

 surface being smooth, although thrown into folds and pouches by modifications 

 in other coats. The muscularis mucosaa is less regular in its development, 

 being feebly represented in the colon and exceptionally thick in the rectum. 

 The crypts of Lieberkuhn resemble those of the small intestine, but are 

 larger (.4 .5 mm. long) and less interrupted. Within the rectum they may 

 attain a length of . 7 mm. The lining of the crypts is conspicuous on account 

 of the abundance of goblet-cells, which in the middle and upper parts of the 

 tubules almost replace the ordinary epithelial elements. As in the small 

 intestine so here, mitotic figures are often seen in the cells lining the crypts, 

 A the new elements so arising being eventually 



pushed to the surface to replace the old ones 

 that are disappearing. 



The lymphoid tissue occurs as solitary 

 nodules only, Peyer's patches being absent in 

 the large intestine. The lymph-nodules are 

 largest and most plentiful in the caecum and in 

 the vermiform appendix; in the latter situation 

 they are so numerous 

 that in places they form 

 an almost continuous 

 mass of lymphoid tis- 

 sue. In the colon the 

 nodules are less abun- 

 dant, but in the rectum 

 they are again numer- 

 ous. They are gener- 

 ally of larger size (1.5 

 3 mm.) than in the 

 small intestine. 



The submucous 

 coat closely resembles 

 the similar fibro-elastic connective tissue tunic of the small intestine, and 

 allows fairly free play of the mucous membrane. In addition to the blood- 

 vessels, lymphatics and nerve-plexus of Meissner, it contains the deeper and 

 more expanded portions of the solitary lymph-nodules. 



The muscular coat includes a thicker internal layer of circular fibres 

 and an external one of longitudinal fibres; the latter, however, are not 

 uniformly distributed, but, in most places, are collected into three bands, 

 the tcenia, between which the longitudinal muscular coat is extremely thin 

 or imperfect. These bands are shorter than the layers of the intestinal wall 

 internal to them, and are responsible for the characteristic sacculation of the 

 large intestine. The circular muscle increases markedly towards the lower 

 end of the rectum, and in the anal canal becomes augmented into a sheet of 

 involuntary muscle, some 4 mm. thick, known as the internal sphincter. 



The serous coat is incomplete in certain parts of the large intestine 

 owing to secondary changes during development and growth. In structure 

 it corresponds to the peritoneal investment of other parts of the alimentary 

 tract (page 175). The characteristic little fringes or bags, the appendices 

 epiploicce, that are attached, particularly along the median aspects of the 

 ascending and descending colon and on the lower side of the transverse 

 colon, consist of pouches of peritoneum filled with adipose tissue (Fig. 218). 



FIG. 214. Portion of mucosa of large intestine, showing crypts of 

 Lieberkuhn cut lengthwise (A) and crosswise (); epithelial elements 

 contain mucus and are "goblet-cells." X 160. 



