160 ABDOMEN 



The weakest wall of the fossa is the medial wall, which is 

 formed by muscles and fasciae only ; therefore if an abscess 

 forms in the fossa the contents of the abscess are apt to 

 force their way into the anal canal, unless an exit is made 

 for them through the skin of the base of the fossa. 



Arteria Pudenda Interna et Nervus Pudendus. If the 

 dissector passes his index finger upwards and downwards over 

 the obturator fascia, which forms the lateral wall of the ischio- 

 rectal fossa, he will feel a ridge which runs from behind for- 

 wards. It lies about 38 mm. (one and a half inches) above 

 the inferior border of the ischial tuberosity, and is caused by 

 the internal pudendal artery and the accompanying perineal 

 nerve and the dorsal nerve of the penis, which are branches 

 of the pudendal nerve. At the posterior end of the ridge the 

 inferior haemorrhoidal artery and nerve will be found piercing 

 the wall of the fascial canal and passing into the fossa, and 

 at the anterior end the superficial and deep terminal branches 

 of the perineal nerve and the perineal and transverse perineal 

 branches of the internal pudendal artery pierce the medial 

 wall of the canal as they enter the fossa. For the present 

 the dissector must be satisfied with palpating the structures 

 which lie in the canal. To expose them it would be neces- 

 sary to divide the obturator fascia, and that must be kept 

 intact until the pelvic fascia can be studied as a whole. In 

 the meantime, however, the dissector should recall to mind 

 the fact that he displayed the pudendal nerve and the 

 internal pudendal artery during the dissection of the buttock, 

 when he cleaned the structures exposed by the reflection of 

 the glutseus maximus (see Vol. I. p. 287). There they 

 rested on the spine of the ischium or the sacro-spinous 

 ligament and disappeared into the perineum through the 

 lesser sciatic foramen. Now they are met with again 

 as they lie in a canal in the obturator fascia in the 

 lateral wall of the ischio-rectal fossa, where the nerve 

 divides into the inferior haemorrhoidal nerve, the perineal 

 nerve, and the dorsal nerve of the penis, and the artery gives 

 off inferior haemorrhoidal, perineal, and transverse perineal 

 branches. At the anterior end of the ischio-rectal fossa the 

 canal in the obturator fascia opens into the space between 

 the two layers of fascia of the urogenital diaphragm, and the 

 internal pudendal artery and the dorsal nerve of the penis 

 pass into that space, where they will be' dissected later. 



