158 ABDOMEN 



the anus, and secure the inferior hsemorrhoidal vessels and nerves. 

 Follow them to their temiination in the medial wall, in the 

 sphincter ani externus and the levator ani muscles. Next turn 

 to the anterior part of the lateral wall of the fossa, dissect care- 

 fully in the angle between it and the base of the urogenital 

 diaphragm and find the lateral and medial posterior scrotal 

 branches of the superficial division of the perineal nerve (posterior 

 labial nerves of female), and the scrotal branches (labial in 

 female) of the perineal artery which accompany the nerves. 

 Find also the deep division of the perineal nerve and the trans- 

 verse perineal artery. The posterior scrotal nerves and arteries 

 (posterior labial in female) pierce the fascia of Colles as it 

 blends with the base of the fascia of the urogenital diaphragm, 

 the deep branch of the perineal nerve pierces the base of the 

 fascia of the urogenital diaphragm, and the transverse perineal 

 artery runs along the posterior border of the diaphragm. 



The perineal branch of the fourth sacral nerve must be 

 sought about a finger's breadth from the tip of the coccyx. It 

 pierces the coccygeus muscle and runs downwards and forwards 

 to the external sphincter ani. After the nerves and vessels have 

 been found and cleaned the remains of the fat must be removed 

 from the fossa and its boundaries and contents must be studied. 



Ischio-rectal Fossae. There are two ischio-rectal fossae, 

 right and left. Position. Each fossa lies in the anal triangle 

 of the perineum between the anal canal of the rectum and 

 the ischium, but neither the anal canal nor the ischium enters 

 directly into the formation of the walls of the fossa, for both 

 are separated from the cavity of the space by fascial and 

 muscular layers. 



The anal canal is covered by the superior fascia of the 

 pelvic diaphragm (O.T. rectal layer of the visceral portion of 

 the pelvic fascia), the levator ani and the inferior fascia of 

 the pelvic diaphragm (O.T. anal fascia) in that order from 

 the medial to the lateral side. The medial aspect of the 

 ischium is separated from the cavity of the fossa by the 

 obturator internus muscle and by the obturator fascia, which 

 covers the muscle and binds it to the bone. 



Shape. Each ischio-rectal fossa is pyramidal in shape. 

 The apex is above, towards the cavity of the pelvis, the base 

 below, at the integument of the perineum. The medial wall 

 slopes upwards and laterally. The lateral wall ascends 

 vertically from the tuber ischii and meets the medial wall 

 at the apex of the fossa. 



Boundaries. Speaking strictly, the medial wall of the 

 fossa is formed by the inferior fascia of the pelvic diaphragm 

 which covers the lower surfaces of the levator ani and the 

 coccygeus muscles. The lateral wall is formed by the 



