THE LARGE INTESTINE. 



927 



moid flexure, but is capable of considerable distention. In the lower part of the 

 second portion it becomes a transverse slit, its anterior and posterior walls lying 

 close together when the tube is empty, on account of the organs in the front part 

 of the pelvis pushing the rectum backward on the sacrum and coccyx. The third 

 part of the rectum, the anal canal, is also a slit, with, however, an antero-poste- 

 rior direction, so that its lateral walls are in apposition (Fig. 509). 



The first portion of the rectum is almost completely surrounded by peritoneum, 

 and is connected to the anterior surface of the sacrum by a double fold, called the 

 mesorectum, which is continuous above with the sigmoid mesocolon. The meso- 

 rectum is triangular in shape, the apex 

 of which ends below at the third sacral 

 vertebra; between its two layers is 

 the superior hemorrhoidal artery. The 

 second portion has no mesorectum, but 

 is covered in front and laterally by 

 peritoneum at its upper part ; gradu- 

 ally the peritoneum leaves its sides, 

 and about an inch above the prostate 

 is reflected from the anterior surface of 

 the bowel on to the posterior wall of the 

 bladder in the male, and the upper fifth 

 of the posterior wall of the vagina in 

 the female, forming the recto-vesical 

 and recto-vaginal pouches, respectively. 

 The third portion of the rectum has no 

 peritoneal covering. The level at which 

 the peritoneum leaves the anterior wall 

 of the rectum to be reflected on to the 

 viscus in front of it is of considerable 

 importance from a surgical point of 

 view, in connection with removal of 

 the lower part of the rectum. It is 

 higher in the male than in the female. 

 In the former the height of the recto- 

 vesical pouch is about three inches ; that 

 is to say, -the height to which an ordinary index finger can reach from the anus. 

 In the female the height of the recto-vaginal pouch is about 2^ inches from the 

 anal orifice. 



The first portion of the rectum is in relation, behind, with the mesorectum and 

 the superior hemorrhoidal artery, the left Pyriformis muscle, and left sacral plexus 

 of nerves, which separate it from the anterior surface of the upper sacral vertebrae ; 

 to its left side are the branches of the left internal iliac artery and the left ureter ; 

 in front it is separated, in the male, from the posterior surface of the bladder; in 

 the female, from the posterior surface of the uterus and its appendages, by some 

 convolutions of the small intestine, and frequently by the sigmoid flexure of the 

 colon. The second portion of the rectum is in relation, in front, in the male, with 

 the recto-vesical pouch, the triangular portion of the base of the bladder, the vesic- 

 ulae seminales, and vasa deferentia, and more anteriorly with the under surface of 

 the prostate. In the female, with the posterior wall of the vagina below, and the 

 recto-vaginal pouch above, in which are some convolutions of the small intestine. 

 The third portion or anal canal is invested by the Internal sphincter, supported 

 by the Levatores ani muscles, and surrounded at its termination by the External 

 sphincter ; in the empty condition it presents the appearance of a longitudinal 

 slit. In the male it is separated from the membranous portion and bulb of the 

 urethra by a triangular space ; and in the female it is separated from the lower 

 end of the vagina by the perineal body. Laterally is the fat in the ischio-rectal 

 fossae. 



FIG. 509. Coronal section through the anal canal. 

 (Symington.) B. Cavity of bladder. VD. Vas deferens. 

 sv. Seminal vesicle. R. Second part of rectum. AC. 

 Anal canal. LA. Levator ani. is. Internal sphincter. 

 ES. External sphincter. 



