LARGE INTESTINE 119 



opened by pressure from the direction of the ileum but 

 which pressure from the cecum tends to close more firmly. 



Structure. The large intestine has the usual three coats. 

 The peritoneal, however, is lacking on the posterior part of 

 the cecum, ascending and descending colons, these parts 

 being bound down closely and having no meso-colon. The 

 sigmoid is entirely covered as is the upper third of the rec- 

 tum. The middle third of the rectum has no serous coat be- 

 hind, being firmly held in place, while the lower third lacks 

 this coat entirely. The muscular coat is peculiar, in that its 

 longitudinal fibers are collected into three quite strong bands, 

 evident to the eye. When the rectum is reached they spread 

 out over the whole circumference of that part of the canal. 

 These bands are shorter, as it were, than the wall proper, and 

 the consequence is that the whole length of the large intes- 

 tine is gathered up into a number of pouches. The mucous 

 coat is paler than that of the small intestine, presents no villi 

 and is rather closely adherent to the subjacent parts. In it 

 are found glands corresponding in appearance to the crypts 

 of Lieberkuhn, and they are so classed; but they probably 

 secrete mucus only. Some solitary lymphoid follicles also 

 usually exist here. 



Changes Taking Place in the Alimentary Mass in the 

 Large Intestine. Most of the substances which enter the 

 large intestine have resisted the action of the various diges- 

 tive fluids and are on their way to be discharged in defeca- 

 tion. Doubtless, though, some materials undergo digestive 

 changes in the colon, not under the influence of any secretion 

 there formed, but of the intestinal juice with which they are 

 incorporated on leaving the ileum. The secretion of the 

 mucous membrane of the large intestine furnishes no diges- 

 tive enzyme, and the changes going on in the alimentary 

 mass (now feces) are chiefly due to absorption. By some 

 unknown process, however, rectal aliments of an easily di- 

 gestible nature are absorbed, and that in a nutritive form. 

 The consistence of the fecal matter increases in its passage 



