260 INTESTINAL DIGESTION. 



merit is found in the descending colon. The transverse colon is almost com- 

 pletely invested with peritoneum ; and the two folds forming the transverse 

 mesocolon separate to pass over the tube above and below, uniting again in 

 front, to form the great omentum. The transverse colon is consequently 

 quite movable. In the course of the colon and the upper part of the rectum, 

 particularly on the transverse colon, are found a number of little, sacculated 

 pouches filled with fat, called the appendices epiploicae. The sigmoid flexure 

 of the colon is covered by peritoneum, except at the attachment of the iliac 

 mesocolon. This division of the intestine is quite movable. The upper por- 

 tion of the rectum is almost completely covered by peritoneum and is but 

 loosely held in place. The middle portion is closely bound down, and is 

 covered by peritoneum only anteriorly and laterally. The lowest portion of 

 the rectum has no peritoneal covering. 



Muscular Coat. The muscular fibres of the large intestine have an 

 arrangement quite different from that which exists in the small intestine. 

 The external, longitudinal layer, instead of extending over the whole tube, 

 is arranged in three distinct bands, which begin in the caecum at the vermi- 

 form appendix. Passing along the ascending colon, one of the bands is sit- 

 uated anteriorly, and the others, latero-posteriorly. In the transverse colon 

 the anterior band becomes inferior and the two latero-posterior bands 

 become respectively postero-superior and postero-inferior. In the descend- 

 ing colon and the sigmoid flexure the muscular bands resume the relative 

 position which they had in the ascending colon. As these longitudinal 

 fibres pass to the rectum, the anterior and the external bands unite to pass 

 down on the anterior surface of the canal, while the posterior band passes 

 down on its posterior surface. Thus the three bands here become two. 

 These two bands as they pass downward, though remaining distinct, become 

 much wider ; and longitudinal muscular fibres beginning at the rectum are 

 situated between them, so that this part of the canal, especially in its lower 

 portion, is covered with longitudinal fibres in a nearly uniform layer. 



Mucous Coat. The mucous lining of the large intestine presents several 

 important points of difference from the corresponding membrane in the small 

 intestine. It is paler, somewhat thicker and firmer, and is more closely ad- 

 herent to the subjacent parts. In no part of this membrane are there any 

 folds, like those which form the valvulae conniventes of the small intestine ; 

 and the surface is smooth and free from villi. 



Throughout the entire mucous membrane, from the ileo-caecal valve to 

 the anus, are orifices which lead to simple follicular glands. These struct- 

 ures resemble in all respects the follicles of the small intestine, except that 

 they are a little longer, owing to the greater thickness of the membrane, are 

 wider and rather more abundant. Among these small follicular openings 

 are found, scattered irregularly throughout the membrane, larger openings 

 which lead to utricular glands, resembling the closed follicles, in general 

 structure, except that they have an orifice opening into the cavity of the in- 

 testine, which is sometimes so large as to be visible to the naked eye. The 

 number of these glands is very variable, and they exist throughout the intes- 



