584 ABDOMEN 



ani muscles. Below the pelvic floor in this region lies the 

 posterior part of the sphincter ani externus muscle. 



Peritoneal Relations of the Rectum. These relations are 

 of practical importance. In its upper third the gut is clothed 

 by the peritoneum both in front and on the sides ; then the 

 peritoneum passes away from the sides, so that in its middle 

 third the gut is covered merely in front; finally, about an 

 inch above the base of the prostate, at the bottom of the 

 recto -vesical excavation, the membrane quits the rectum 

 altogether, and is reflected on to the deferent ducts and the 

 seminal vesicles as they lie at the fundus of the bladder. 

 The lower third of the rectum is thus altogether devoid of 

 peritoneum. It is separated from the fundus of the bladder 

 and the posterior surface of the prostate by the recto-vesical 

 layer of pelvic fascia, and embedded in the fascia, behind 

 the bladder, are the lower parts of the deferent ducts and 

 the seminal vesicles. 



On each side of the upper part of the undistended rectum 

 is a pararectal fossa, and each lateral wall of the lower part 

 of the gut is supported by the corresponding levator ani 

 muscle (Figs. 229 and 230), 



Flexures of the Rectum. The rectum does not take a 

 straight course along the back wall and floor of the pelvis. 

 On the contrary, it presents three abrupt lateral bends or 

 flexures, of which, as a rule, two are convex to the right and 

 one to the left. The sharply marked infoldings of the wall 

 of the gut opposite to the flexures are the cause of the so- 

 called plicae transversales recti (O.T. rectal valves) in the 

 interior of the gut. The flexures are best marked when the 

 gut is distended, but even when it is empty they are usually 

 quite obvious. 



Lying between the bladder and prostate in front and 

 the sacrum and coccyx behind, the rectum, when empty, has 

 its anterior wall pressed against its posterior wall, and in this 

 condition its lumen appears, in transverse section, as a 

 transverse slit (Fig. 231). Behind the apex of the prostate, 

 where the gut bends to become the anal canal, its anterior 

 wall, in the distended condition, sometimes shows a slight 

 bulging cul-de-sac^ the ampulla recti, which gains a lower level 

 than the prostate. 



Anal Canal. This is the narrow slit-like passage about an 

 inch and a half in length (49 mm.) which leads from the rectum 



