THE PERINEUM, MALE. 407 



ward and inward to the scrotum, which it supplies. It also 

 supplies the integument of the inner and upper part of the 

 thigh. 



In the female it is distributed to the labium majus. 



Sphincter Ani Externus. Figs. 82, 83. 



Origin. From the tip of the coccyx. 



Insertion. Into the tendinous centre of the perineum. 



It is a broad, though thin, muscle which encircles the 

 anus, and by its contraction firmly closes it. 



Nerve Supply. The inferior hemorrhoidal, from pudic, 

 and the fourth sacral. 



Action. To close the anus. 



DISSECTION. 



Carefully pick out the fat filling the posterior half of the perineum, 

 demonstrate the boundaries of the fossa exposed and the vessels and nerves 

 crossing it. 



The Ischiorectal Fossa. Fig. 82. 



The posterior portion of the perineum. 



This fossa is of an irregular prismatic shape. Its outer 

 wall is (approximately) vertical and is formed by the lower 

 portion of the obturator fascia. Its inner wall is formed by 

 the anal fascia backed by the levator ani and coccygeus 

 muscles and the external sphincter of the anus, with the 

 tendinous centre of the perineum in front and the tip of the 

 coccyx behind. In front, the fossa is limited by the junc- 

 tion of the perineal fascia and the superficial and deep 

 layers of the triangular ligament and the anal fascia ; be- 

 hind, by the margin of the gluteus maximus muscle, the 

 great sacrosciatic ligament, and the junction of the obtura- 

 tor and anal fasciae. The contents of the fossa : Adipose 

 tissue, inferior hemorrhoidal arteries, vessels, and nerves. 



