290 DIGESTION. 



mence 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 trans- 

 verse colon, the anterior band becomes inferior and the two latero-posterior bands be- 

 come 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 are here 

 formed into two. These two bands as they pass downward, though remaining distinct, 

 become much wider; and longitudinal muscular fibres commencing 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 pretty uniform layer. 



The arrangement of the muscular fibres of the rectum has been closely studied by Sap- 

 pey. He has found that, as far as their terminations are concerned, the fibres may be 

 divided into an external, a middle, and an internal layer. The posterior fibres of the ex- 

 ternal layer pass away from the lower portion of the rectum, are reflected backward 

 along the concavity of the sacrum, and are attached to the promontory. These fibres, 

 which are generally pale, Sappey proposes to designate as retractors of the anus. A few 

 of the posterior fibres are attached to the aponeurosis and the parts between the coccyx 

 and the promontory. In front, the external fibres are attached to the aponeurosis which 

 covers the vesiculee seminales, and laterally they are inserted into the deep pelvic fascia. 

 The termination of the middle layer of the fibres is less clearly made out. Those situated at 

 the sides of the rectum are inserted into " a very dense cellulo-fibrous band, which, by its 

 opposite surface, gives insertion to a great number of fibres of the levator ani." The 

 others are many of them continuous with the fibres of the levator ani as they pass along 

 the floor of the pelvis. Some of the fibres of the deep layer are attached by little tendons, 

 which pass between the external and the internal sphincter, to the deep portions of the 

 skin encircling the anus. The importance of closely studying the attachments of these 

 fibres will be appreciated when we come to treat of defaecation. 



Over the cascum and the colon, the anterior band of muscular fibres is from one-third 

 to one-half an inch in width. The postero-external band is not more than half so wide, 

 and the postero-internal band is even narrower. The muscular bands are much shorter 

 than the canal itself, and their attachment to the walls gives the intestine a peculiar sac- 

 culated appearance. That this is produced by the arrangement of the muscular fibres, 

 may be demonstrated by dividing them in various places or by removing them entirely, 

 when the canal may be extended to double its original length. Between the bands there 

 are no longitudinal muscular fibres; but circular or transverse muscular fibres exist 

 throughout the whole length of the large intestine. In the csecum and the colon, the cir- 

 cular fibres are so pale and the layers are so thin that their presence is demonstrated with 

 great difficulty. In the rectum they are somewhat more numerous. About an inch 

 above the anus, the circular fibres are collected into a pretty well-marked muscular ring, 

 which has been called the internal sphincter. 



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

 points of difference from that which is found in the small intestine. It is paler, somewhat 

 thicker and firmer, and is more closely adherent to the subjacent parts. In no part of this 

 membrane are there any folds, like those which form the valvulse conniventes of the small 

 intestine ; and the surface is perfectly smooth and free from villosities. 



Throughout the entire mucous membrane, from the ileo-caacal valve to the anus, are 

 innumerable orifices which lead to simple follicular glands. These structures 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 are rather more numerous. 

 Among these small follicular openings are found, scattered irregularly throughout the 



