386 DIGESTION. 



the csecal side will approximate the lips of the valve, and 

 the greater the pressure the more firmly will the opening be 

 closed. The valve itself is composed of folds involving the 

 white fibrous tissue of the intestine (the cellular tunic of some 

 anatomists), and the circular muscular fibres from both the 

 small and the large intestine ; the whole being covered with 

 mucous membrane. The lips- of the valve unite at either 

 extremity of the slit and are prolonged on the inner surface 

 of the caecum, forming two raised bands or bridles ; and these 

 become gradually effaced and are thus continuous with the 

 general lining of the canal. The posterior bridle is a lit- 

 tle longer and more prominent than the anterior. These 

 assist somewhat in enabling the valve to resist pressure from 

 the caecal side. The longitudinal layer of muscular fibres 

 and the peritoneum pass directly over the attached edge of 

 the valve and are not involved in its folds. These give 

 strength to the part, and if they be divided over the valve, 

 gentle traction will suffice to draw out and obliterate the 

 folds, leaving a simple and unprotected communication be:- 

 tween the large and the small intestine. 



Peritoneal Coat. Like most of the other abdominal vis- 

 cera, the large intestine is covered by peritoneum. The 

 csecum is covered by this membrane only anteriorly and 

 laterally. It is usually bound down closely to the subjacent 

 parts, and its posterior surface is without a serous investment ; 

 though sometimes it is completely covered, and there may 

 be even a short mesocsecum. The ascending colon is like- 

 wise covered 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 trans- 

 verse mesocolon split to pass over the tube above and below, 

 uniting again in front to form the great omentum. The trans- 

 verse colon is consequently quite movable. In the course of 

 the colon and the upper part of the rectum, particularly on 



