228 INTESTINAL DIGESTION 



folds, leaving a simple and unprotected communication between the 

 large and the small intestine. Some modern observers attribute closing 

 of the ileo-caecal opening to contraction of circular muscular fibres, which 

 constitute a sphincter (ileo-colic sphincter). These, indeed, have been 

 demonstrated in the cat, dog and rabbit. It is possible that such an 

 arrangement and action may exist in man. 



Peritoneal Coat. Like most of the other abdominal viscera, the 

 large intestine is covered by peritoneum. The caecum is covered by 

 this membrane only anteriorly and laterally. It usually is bound down 

 closely to the subjacent parts, and its posterior surface is without a serous 

 investment ; although sometimes it is completely covered and there may 

 be even a short mesocaecum. The ascending colon is likewise covered 

 with peritoneum only in front and is closely attached to the subjacent 

 parts. The same arrangement is found in the descending colon. The 

 transverse colon is almost completely 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 consequently is 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 epiploi'cae. 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 

 portion of the rectum is almost completely covered by peritoneum and 

 is 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 vermiform appendix. Passing along the ascending colon, one of the 

 bands is situated 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 descending 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 mus- 



