390 DISSECTION OF THE PERINJEUM. 



muscle, and its edge will be seen by removing the corrugator and the 

 mucous membrane. The fibres of the muscle are pale, fine in texture, 

 quite separate from the surrounding external sphincter, and encircle the 

 lower part of the rectum in the form of a ring about half an inch in depth. 

 The muscle is a thickened band of the involuntary circular fibres of the 

 large intestines, which is not attached to the bone. 



Action. This sphincter assists the external in closing the anus ; and its 

 contraction is altogether involuntary. 



The LEVATOR ANI muscle (fig. 129, D ) can be seen only in part; and 

 the external sphincter may be detached from the coccyx, in order that its 

 insertion may be more apparent. The muscle descends from its origin at 

 the inner aspect of the hip bone, and is inserted along the middle line from 

 the coccyx to the central point of the perinaeum : The most posterior 

 fibres are attached to the side of the coccyx ; and between that bone and 

 the rectum the muscles of opposite sides are united in a median tendinous 

 line. The middle fibres are blended with the side of the intestine (rec- 

 tum). And the anterior are joined with the opposite muscle, in front of 

 the rectum, in the central point of the perinaeum. 



This muscle bounds the ischio-rectal fossa on the inner side, and unites 

 with its fellow to form a fleshy layer, convex downwards, through which 

 the rectum is transmitted. Deeper than the muscle is the recto-vesical 

 fascia. Along the hinder border is placed the coccygeus. 



Action. Its action on the rectum is to elevate and invert the lower end 

 of the gut after this has been protruded and everted in the passage of the 

 feces. 



With the description of the muscle in the pelvis its action on the urethra 

 will be referred to. 



ARTERIES (fig. 129). The pudic artery, with its inferior hoemorrhoidal 

 branch, and other small offsets of it and the sciatic, are now visible. 



The pudic artery (a) is derived from the internal iliac in the pelvis, and 

 ascending to the genital organs, distributes offsets to the perinaeum ; one 

 portion will be laid bare in the hinder, and the other in the anterior half 

 of the perinaeum. 



As now seen, the vessel enters the posterior part of the ischio-rectal 

 fossa, and courses forwards along the outer wall at the depth of one inch 

 and a half behind, but of only half an inch at the fore part. It is con- 

 tained in an aponeurotic sheath which attaches it to the obturator fascia. 

 The usual companion veins lie by its side ; and two nerves accompany it, 

 viz., the trunk of the pudic, and the perinaeal branch of the same nerve 

 which is nearer the surface. Its offsets in the posterior half of its course 

 are the following : 



The inferior hcemorrhoidal branch (>) arises internal to the ischial 

 tuberosity ; it sends offsets inwards across the ischio-rectal fossa to the 

 teguments, and the sphincter and levator ani muscles. On the rectum it 

 anastomoses with the upper ha^morrhoidal branch, and with the artery of 

 the opposite side. In a well injected body cutaneous branches may be 

 seen to run forwards to the anterior part of the perinaeum, and to commu- 

 nicate with the superficial perinaeal branch. 



Small muscular branches cross the front of the ischio-rectal fossa, and 

 supply the anterior part of the levator ani muscle. 



The branches of the sciatic artery (c) appear on the inner aspect of the 

 gluteus maximus at the back of the fossa; some end in that muscle, and 

 others are continued round its border to the fat. 



