STRUCTURE OF THE COLON. 463 



The size of the opening depends upon the distension of the intestine ; 

 for when the retinacula of the valve are stretched the margins of the 

 aperture are approximated, and may be made to touch. 



Each piece of the valve is formed by circular muscular fibres of the in- 

 testinal tube, covered by mucous membrane ; as if the ileum was thrust 

 obliquely through the wall of the caecum, after being deprived of its peri- 

 toneal coat and layer of longitudinal fibres. This construction is easily 

 seen on a fresh specimen by dividing the peritoneum and the longitudinal 

 fibres, and gently drawing out the ileum from the caecum. 



The opening of the appendix into the caecum (d) is placed below that 

 of the ileum. A piece of mucous membrane partly closes the aperture, 

 and acts as a valve. 



Folds or ridges are directed transversely in the interior of the gut, and 

 correspond with depressions on the outer surface : these folds result from 

 the doubling of the wall of the intestine, and the largest inclose vessels. 



STRUCTURE OF THE COLON. The coats of the large are similar to 

 those of the small intestine, viz., serous, muscular, fibrous, and mucous. 



Serous coat. The peritoneum does not clothe the large intestine, 

 throughout, in the same degree. It covers the front of the caecum, and 

 the front and sides of the ascending and descending colon ; but in neither 

 does it reach commonly the posterior aspect (p. 438). The transverse 

 colon is incased like the stomach, and has intervals along the borders, 

 where the transverse meso-colon and the great omentum are attached. 



The muscular coat is formed by longitudinal and circular fibres, as in 

 the small intestine. 



The longitudinal fibres may be traced as a thin layer over the surface, 

 but most are collected into three longitudinal bands, about a quarter of an 

 inch in width. On the vermiform appendix the fibres form a uniform 

 layer ; but they are continued thence into the bands on the caecum and 

 colon : on the rectum they are diffused over the surface. When the 

 bands are divided the intestine elongates the sacculi, and the ridges in 

 the interior of the gut, disappearing at the same time. 



The circular fibres are spread over the whole surface, but are most 

 marked in the folds projecting into the intestine. In the rectum (to be 

 afterwards seen) they form the band of the internal sphincter muscle. 



The fibrous coat resembles that of the small intestine. It will be ex- 

 posed by removing the peritoneal and muscular coverings. 



The mucous coat, which may be examined on opening the intestine, is 

 smooth, and of a pale yellow color ; and it is not thrown into special folds, 

 except in the rectum. The surface is free from villi ; and by this circum- 

 stance the mucous membrane of the large, can be distinguished from that 

 of the small intestine. This difference in the two portions of the alimen- 

 tary tube is well marked on the ilio-csecal valve ; for the surface looking 

 to the small intestine is studded with villi, whilst the lower surface, covered 

 by the lining membrane of the caecum, is free from those small eminences. 



Microscopic appearances. In a piece of fresh intestine the microscope 

 will show the mucous membrane to possess small tubes or crypts, and 

 some larger solitary follicles; with an epithelial covering on the free sur- 

 face, and a thin muscular layer (^muscularis mucosce) on the other, whose 

 arrangement is similar to that of the small intestine. 



The tubules (fig. 156, B ) occupy the whole length of the large gut, and 

 resemble those of the small intestine, but are more numerous and closer 

 together. Their orifices on the surface are circular ('), and are more uni- 



