THE RECTUM, ANAL CANAL, AND ANUS. 



1673 



the rectum, situated in the thickness of the pelvic floor, and directed downward 

 and backward, making a sharp angle with the rectum proper. 



The rectum proper, having passed the tip of the coccyx, rests on the levator ani 

 muscle, although separated from it, as well as from the sacrum and coccyx, by the 

 dense rectal fascia. The rectum, although not exhibiting the pouching seen in 

 the colon, is sacculated, presenting, when distended, usually three dilatations, of 

 which the lowest and largest, called the a?npiilla, may measure 25 cm. {gyi in.), or 

 even more, in circumference. The saccules are separated by deep creases, passing 

 about two-thirds around the gut, caused by a folding in of all the coats internal to the 

 two bands of longitudinal muscular fibres. The folds form the valves of the rectum, 

 to be described with its interior. In the male the ampulla extends against the back 

 of the prostate and the lower part of the seminal vesicles and the terminal parts of 

 the vasa deferentia, to all of which it is connected by areolar tissue. A pocket of 

 peritoneum intervenes higher up, the walls of which, however, come in contact when 

 the hollow organs are distended. In the female the end of the ampulla lies against 

 the posterior wall of the vagina from about opposite the level of the os uteri to the 

 junction of the middle and lower thirds. There is above this a fold of peritoneum 

 corresponding to that of the male. 



Glands of mucosa 



Levator am 



Internal sphinctei^ 

 Longitudinal muscle 



External sphincter 



Levator ani 



External sphincter 



Skin Anal glands Anal glands '^■^^' 

 Frontal section through anal canal. 



The Anal Canal. — This part of the large intestine (pars analis recti) is situ- 

 ated in the thickness of the pelvic floor and extends downward and backward. It 

 differs from the rest of the intestinal canal in having no lumen under ordinary cir- 

 cumstances, when the sphincters surrounding it are contracted. The anus is the 

 very vaguely used name of the termination of the anal canal. It is deeply situated 

 between the nates, especially in the female ; its distance from the tip of the coccyx, 

 variously stated by different observers, may be said to be about 5 cm. (2 in.). 

 Much confusion has arisen from the difflculty of defining the lower end of the anal 

 canal, since the skin, which is puckered up by the external sphincter and the cor- 

 rugator cutis ani, somewhat resembles mucous membrane, so that the canal appears 

 longer than it really is. The anatomical boundary, the ayio-rectal groove, the so- 

 called white line of Hilton, is a slight zigzag furrow, usually to be seen on the living 

 and not on the dead. It lies a little above the lower limit of the internal sphincter, 

 which, covered by dilated veins, projects towards the potential lumen above the 

 external sphincter, and is i cm. or more within what, on a superficial examination, 

 would be called the anus. When the dissected rectum is laid open, much is evidently 

 a part of the skin which during life is drawn into the canal by the contraction of the 

 muscles ; hence the length of the canal is very variously stated. Seldom does it 



