Chap, xviii.] THE RECTUM. 373 



The rectum in the adult is situated entirely 

 within the true pelvis, and presents three marked 

 curves, one in the lateral and two in the antero- 

 posterior direction. In the infant, however, a good 

 deal of the rectum is in the abdominal rather than the 

 pelvic cavity, the gut is nearly straight, and occupies 

 a more or less vertical position. For these reasons 

 prolapsus ani is much more common in children than 

 in adults ; children being, besides, especially liable to 

 such exciting causes of prolapse as worms and rectal 

 polypi. 



The rectum is about eight inches in length. Its 

 upper part, for some three inches, is entirely in- 

 vested by peritoneum. The serous membrane gra- 

 dually leaves its posterior surface, then its sides, 

 and lastly its anterior surface. Anteriorly the peri- 

 toneum, in the form of the recto- vesical pouch, extends 

 in the male to within three and a half or four inches 

 of the anus, while on the posterior aspect of the gut 

 there is no peritoneum below a spot five inches from 

 the anus. Thus, in excision of the rectum, more of 

 the bowel can be removed on the posterior than on the 

 anterior part of the tube. It will be seen, also, that car- 

 cinomatous and other spreading ulcers are more apt to 

 invade the peritoneal cavity when they are situated in 

 the anterior wall of the intestine. 



By inserting the finger into the rectum the pro- 

 state and seminal vesicles can be readily felt and 

 examined, and that triangular surface of the bladder 

 explored through which puncture per rectum is made 

 (page 355). 



It will be understood that the prostate, when en- 

 larged, may encroach upon the cavity of the rectum 

 and greatly narrow its lumen. The position of the 

 seminal vesicles with regard to the bowel is such that 

 in violent attempts at defsecation they may be pressed 

 upon by the rectal contents, and so in part emptied, 



