THE LARGE INTESTINE 435 



The Relations of the Descending Colon. Passes to the iliac 

 fossa, to the left of the left kidney and psoas muscle; in front 

 are the coils of the jejunum and ileum; behind, is the quadratus 

 lumborum muscle and the twelfth thoracic nerve. 



The sigmoid flexure ends in the rectum. From the end of 

 the descending colon it forms an S-shaped curve, ending oppo- 

 site the left sacroiliac joint. In front of it are the belly wall 

 and some coils of small intestine. The peritoneum forms a 

 loose mesocolon for it. It is the narrowest part of the colon. 



The Relations of the Sigmoid Flexure of the Colon. Anteriorly, 

 it is covered by the coils of the jejunum and ileum; behind, 

 in the iliac fossa, it passes over the psoas, and iliacus, muscles, 

 the branches of the lumbar nerves, except the obturator, the 

 ureter, the spermatic artery and vein. At the brim of the 

 pelvis it curves over the internal and external iliac vessels, 

 and behind and below, are the left common iliac artery and 

 vein. 



The rectum is the lowest part of the large intestine, and 

 extends from the sigmoid flexure to the anus. It has been 

 divided into three parts: The first part extends from the 

 left sacroiliac joint to the centre of the third piece of the sacrum; 

 the second part, to the tip of the coccyx; and the third part, 

 to the anus. 



The rectum is about 8 inches long and somewhat cylindrical 

 in form, narrower above than the sigmoid flexure, but it en- 

 larges as it descends, and just above the anus is remarkably 

 dilated, forming the ampulla. The first part has a mesorectum; 

 the second part is covered by peritoneum in front and laterally; 

 the third part has no peritoneal covering. 



The Relations of the Rectum. Behind, the upper part of the 

 rectum is in relation with the superior hemorrhoidal vessels, 

 the left pyriformis muscle, and left half of the sacral plexus 

 of nerves, which lie between it and the sacrum; its lower part 

 rests on the sacrum, coccyx, and levatores ani muscle, a dense 

 fascia intervening; in front, it is in relation with the posterior 

 surface of the bladder, in the male, and the posterior aspect 

 of the uterus and its left appendages, the coils of the intestines 

 intervening. The extremity of the rectum is in relation in 

 front with the triangular portion of the bladder, the seminal 

 vesicles, the vas deferens, and more anteriorly with the prostate 

 in the male; in the female, with the posterior wall of the vagina. 



