Chap. XVIIL] PELVIS AND PERINEUM. 377 



junction of the skin and mucous membrane, indi- 

 cates the interval between the external and internal 

 sphincters (Hilton). 



The anus may be torn during defsecation, when 

 the stools are hard. A case is reported of a woman, 

 who, during violent efforts at defsecation, felt something 

 give way, and discovered faeces in her vagina. The 

 recto-vaginal wall had ruptured two inches from the 

 anus. During labour, the child's head has passed into 

 the rectum, and has been delivered per anum. 



As congenital defects, the anus and entire rectum 

 may be absent ; or the anus be not evident, and the 

 rectum be more or less complete ; or the anus and 

 lowest part of the rectum may be normal, and the 

 upper part of the bowel be quite wanting. 



Nerves of pelvis and perineum. The 

 pelvic viscera are supplied by the pelvic plexus of 

 the sympathetic. This plexus is joined by at least 

 three spinal nerves, the second, third, and fourth 

 sacral. 



It is well known that in certain affections of the 

 bladder, rectum, prostate, etc., pain is felt along the 

 perineum, in the penis, over the buttock, and down 

 the thigh. These parts are supplied by the pudic 

 and small sciatic nerves, and the reason for the pain 

 is explained by noticing that the very spinal nerves 

 that join the plexus for the viscera give off also the 

 pudic and small sciatic nerves, the former from the 

 third and fourth sacral, the latter from the second 

 and third. Thus, the pelvic viscera, and the skin of 

 the buttock, perineum, and external genitals are all 

 kept in association by the same spinal nerves. 

 The upper part of the rectum is provided with but 

 little sensation, as illustrated by the passage of instru- 

 ments, by the comparative painlessness of malignant 

 and other growths high up in the bowel, and by the 

 little inconvenience felt when the gut is distended 



