CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 451 



intestine. The circular muscular layer is thicker in the middle or 

 bellies of the sacculi than at the puckers, where it is very thin. 



The villi as we have just said are wholly absent. In the lower 

 part of the small intestine they become fewer and smaller, and 

 none at all are found beyond the ileocaecal valve. An increase of 

 surface is provided by longitudinal ridges, but these like the corre- 

 sponding ruga; of the stomach involve the whole mucous mem- 

 brane, including part of the submucous tissue. 



The glands of Lieberkiihn in the large intestine are in the 

 main like those of the small intestine but larger and better 

 developed, being both deeper and broader, and owing to the 

 absence of villi are more easily studied. The cells of the glands 

 have the same characters as in the small intestine except that 

 the hyaline border is rarely present ; goblet cells are perhaps 

 more abundant than even in the small intestine, especially in 

 some animals. On the ridges between the glands the cells become 

 longer and thinner, and the hyaline border, frequently striated, 

 makes its appearance. . The marked development of these glands 

 in the large intestine is noteworthy since, as we shall see, absorp- 

 tion of material and not the secretion of digestive juice is the 

 characteristic work of the large intestine. It can scarcely be 

 imagined that absorption takes place only at the ridges between 

 the glands and not by the immensely larger amount of surface 

 which is presented by the interiors of all the glands together; 

 but if these glands absorb in the large intestine, they probably 

 act in the same way in the small intestine. 



Lymphatic follicles are abundant in the large intestine, the 

 caecum and especially the appendix vermiformis being crowded 

 with solitary follicles. The patches of Peyer are absent. 



266. The Rectum. As the sigmoid flexure passes into the 

 rectum the three bands of the longitudinal muscular coat spread 

 out and become once more a uniform layer ; and with this change 

 the sacculation disappears. This longitudinal coat is continued to 

 the anus, where it ends abruptly. The circular coat at its termi- 

 nation at the anus is developed into a distinct ring, the internal 

 sphincter. 



The mucous membrane is thrown into numerous folds or ridges 

 which below are longitudinal but higher up oblique or even trans- 

 verse in direction ; to permit the formation of these folds, which 

 are obliterated when the rectum is fully distended, the submucous 

 tissue is more abundant and more loosely developed than in the 

 rest of the intestine. 



Down to the margin of the anus the mucous membrane retains 

 the characters of the large intestine, glands being still present ; 

 it then abruptly puts on the epiblastic characters of the epi- 

 dermis. The rectum has a special nervous supply, but of this we 

 shall speak in connection with the movements of the alimentary 

 canal. 



292 



