LARGE INTESTINE Iig 



which is easily 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 rectum. The 

 middle third of the rectum has no serous coat behind, being 

 firmly held in place, while the lower third lacks this coat en- 

 tirely. 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 intestine 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 adher- 

 ent 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 digestive fluids 

 and are on their way to be discharged in defecation. 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 digestive enzyme, and the changes going 

 on in the alimentary mass (now feces) are chiefly due to absorp- 

 tion. By some unknown process, however, rectal aliments 

 of an easily digestible nature are absorbed, and that in a nutritive 



