THE LARGE INTESTINE 



1183 



length from the center toward the ends of the loop, where it disappears, so that the 

 loop is fixed at its junctions with the iliac colon and rectum, but enjoys a consider- 

 able range of movement in its central portion. Behind the sigmoid colon are the 

 external iliac vessels, the left Piriformis, and left sacral plexus of nerves; in front, 

 it is separated from the bladder in the male, and the uterus in the female, by some 

 coils of the small intestine. 



The Rectum (ititestinum rectum) (Fig. 1077) is continuous above with the sigmoid 

 colon, while below it ends in the anal canal. From its origin at the level of the 

 third sacral vertebra it passes downward, lying in the sacrococcygeal curve, and 

 extends for about 2.5 cm. in front of, and a little below, the tip of the coccyx, 

 as far as the apex of the prostate. It then bends sharply backward into the anal 

 canal. It therefore presents two antero-posterior curves: an upper, with its con- 

 vexity backward, and a lower, with its convexity forward. Two lateral curves are 

 also described, one to the right opposite the junction of the third and fourth sacral 

 vertebrae, and the other to the left, opposite the left sacrococcygeal articulation ; 

 they are, however, of little importance. The rectum is about 12 cm. long, and at 

 its commencement its caliber is similar to that of the sigmoid colon, but near its 

 termination it is dilated to form the rectal ampulla. The rectum has no sacculations 

 comparable to those of the colon, but when the lower part of the rectum is con- 

 tracted, its mucous membrane is thrown into a number of folds, which are longitudi- 

 nal in direction and are effaced by the distension of the gut. Besides these there 

 are certain permanent transverse folds, of a semilunar shape, known as Houston's 

 valves (Fig. 1078). They are usually three in number; sometimes a fourth is found, 

 and occasionally only two are present. One is 

 situated near the commencement of the rectum, 

 on the right side; a second extends inward from 

 the left side of the tube, opposite the middle of 

 the sacrum; a third, the largest and most con- 

 stant, projects backward from the forepart of 

 the rectum, opposite the fundus of the urinary 

 bladder. When a fourth is present, it is situated 

 nearly 2.5 cm. above the anus on the left and 

 posterior wall of the tube. These folds are about 

 12 mm. in width, and contain some of the 

 circular fibers of the gut. In the empty state of 

 the intestine they overlap each other, as Houston 

 remarks, so effectually as to require considerable 

 maneuvering to conduct a bougie or the finger 

 along the canal. Their use seems to be, "to 

 support the weight of fecal matter, and prevent 

 its urging toward the anus, where its presence 

 always excites a sensation demanding its dis- 

 charge. 1 



The peritoneum is related to the upper two- 

 thirds of the rectum, covering at first its front 

 and sides, but lower down its front only; from 

 the latter it is reflected onto the seminal vesicles 

 in the male and the posterior vaginal wall in the 

 female. 



The level at which the peritoneum leaves the anterior wall of the rectum to be 

 reflected ofi to the viscus in front of it is of considerable importance from a surgical 



Rectal columns 



Sphincter ani 



internus 

 Sphincter ani 



externun 



FIG. 1078. Coronal section of rectum and 

 anal canal. 



1 Paterson ("The Form of the Rectum," Journal of Anatomy and Physiology, vol. xliii) utilizes the third fold for 

 the purpose of dividing the rectum into an upper and a lower portion; he considers the latter "to be just as much a 

 duct as the narrower anal canal below," and maintains that, under normal conditions, it does not contain feces except 

 during the act of defecation. 





