Chap, xviii.] THE PERINEUM. 347 



lateral wall is exposed for a distance of nearly three 

 inches, its posterior wall for about an inch (Quain). 

 The fossa is occupied by a mass of fat which affords 

 to the rectum the support of an elastic cushion. This 

 fatty tissue is badly supplied with blood, and this fact, 

 in addition to the dependent situation of the part, and 

 its exposure when the patient sits upon damp, cold 

 seats, etc., leads to abscess being very frequent in the 

 space (ischio-rectal abscess). These abscesses are hem- 

 med in on all sides, soon fill the fossa, and then tend 

 to discharge themselves in the two directions where 

 the resistance is least, viz., through the skin and 

 through the wall of the rectum. When this double 

 discharge of the abscess has taken place, a complete 

 fistula in ano is established. It is well to note that in 

 all fistulse in" ano the opening into the rectum is nearly 

 always within half an inch of the anus. An opening 

 into the bowel higher up is resisted by the attachment 

 of the levator ani, by the anal fascia and by the recto- 

 vesical fascia. 



Crossing the space obliquely from its hinder part 

 to the anus are the external hsemorrhoidal vessels 

 and nerves ; crossing the anterior and outer corner 

 of the fossa are the perineal vessels and nerves, 

 and about the posterior border of the space are the 

 fourth sacral nerve and some branches of the small 

 sciatic nerve. It will be readily understood, therefore, 

 that ischio-rectal abscesses are associated usually with 

 extreme suffering until they are relieved. The most 

 severe pain is probably due to the stretching of the 

 external hsemorrhoidal nerve by the abscess as it 

 progresses towards the surface. In opening an abscess 

 in the fossa the main structures to avoid are the 

 rectum, the pudic and external hsemorrhoidal vessels. 

 Anus. (See paragraph on the Rectum.) 

 The urethra! triangle. The skin of the peri- 

 neum between the anus and the scrotum is thin, and 



