326 DISSECTION OF THE ABDOMEN. 



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

 caecum?* that of tlie il eum - Tne mucous membrane partly closes the aperture 



and acts as a valve. 



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

 the caecum; and correS p On d w ith depressions on the outer surface: these folds 

 how formed, result from the doubling of the wall of the intestine, and the 

 largest enclose vessels. 



Dissection. Portions of the transverse colon and the pelvic colon 

 should be examined to show the disposition of their coats, in the same 

 way as the pieces of small intestine, after the whole piece of large 

 intestine has been slit open and washed clean. 



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

 of the gut 1 to those of the small intestine, viz., serous, muscular, fibrous, and 



mucous. 



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



tne 6 intes ng throughout, in the same degree. It usually surrounds the caecum, 



tine - but covers only the front and sides of the ascending and descending 



colon (p. 302). The transverse colon is encased like the stomach, 



and has intervals along the borders, where the transverse meso-colon 



and the great omentum are attached. 



Two layers The muscular coat is formed of longitudinal and circular fibres, 

 fitaf y as in tne sma11 intestine. 



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



bands 66 surface, but most are collected into three longitudinal bands, about 



posterior, a third of an inch in width. One of these bands is placed along 



internal';' tae posterior or attached margin of the bowel, the other two are on 



the anterior and inner sides respectively. On the vermiform 



appendix the fibres form a uniform layer, but they are continued 



thence into the bands on the caecum and colon, and on the rectum 



the anterior and internal bands become united. When the bands 



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



interior of the gut disappearing at the same time. 



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

 marked in the folds projecting into the intestine. At the end of 

 the rectum (to be afterwards seen) they form the band of the internal 

 sphincter muscle. 



coatas^n 118 The fibrous or submucous coat resembles that of the small intestine, 

 small gut. It will be exposed by removing the peritoneal and muscular 



coverings. 

 Mucous coat T ne mucous coat, which may be examined on opening the intestine, 



is without . J 



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



and villi. special folds. 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 alimentary tube is well marked on the ileo-colic valve ; for the 

 surface looking to the ileum is studded with villi, while the surface 

 continuous with the mucous lining of the large intestine is free from 

 those eminences, 

 glands" Glands. The mucous membrane is thickly beset with very small 



