SOME POINTS IN THE ANATOMY OF THE DIGESTIVE SYSTEM. 5? 



erect attitude. In quadrupeds the contents of the rectum do not 

 press unduly on the sphincter, owing to the horizontal position 

 of the body. With the assumption of the erect attitude, on the 

 other hand, the whole weight of the contents would be thrown 

 on the sphincters were it not for the lateral foldings and the 

 resulting shelves which break up the contained mass into seg- 

 ments, each supported by the corresponding rectal valve. 



As pointed out by W. Otis, this folding — or alternate 

 sacculation of the sides, as he considers it to be — is maintained 

 by the arrangement of the longitudinal muscular fibres, the 

 majority of which are accumulated in the form of two wide 

 bands, one in front, the other on the back of the bowel. At 

 the same time it should be pointed out that longitudinal fibres 

 are found in abundance at the sides also, where they dip into 

 the angles formed by the Hexures, in this differing from the 

 condition present in the colon. 



Another purpose served by these foldings is to greatly increase 

 the capacity of the rectum without unduly dilating the tube. 

 When the rectum is empty its course is comparatively straight, 

 its lateral flexures being but slightly marked, and its whole 

 calibre very much reduced. In this condition it occupies only 

 a small portion of the posterior division of the pelvic cavity 

 near the mesial plane. At each side, between it and the lateral 

 wall of the pelvis, is a large fossa of the peritoneum (the ' para- 

 rectal fossa '), which when the bowel is empty contains a 

 mass of small intestine or pelvic colon. When the rectum is 

 distended, the lateral flexures become much more marked, and 

 projecting alternately to each side, they pass out beneath the 

 peritoneum, obliterating the pararectal fossa, and fill the greater 

 part of the posterior division of the pelvis, — a condition which 

 could not be brought about with a straight rectum without an 

 enormous increase in all the diameters of the tube. 



In length, the rectum measures about 5 or 6 inches. Its 

 diameter — which is least above — is about 1 inch when the 

 bowel is empty, but it may be as much as 3 inches in extreme 

 distension. 



It is of interest to notice that the connection of the peritoneum 

 to the rectum varies in its different parts ; above and in front 

 it is closely adherent, and can be removed only with the greatest 



