BRANCHES OF THE SACRAL PLEXUS 



813 



ligaments, and tht-n, perforating' tlie former ligament, runs between it and the 

 gluteus maximus. It then winds round the border of that muscle, ])ehind the 

 ischio-rectal fossa, and supplies the integument covering the lower and inner part 

 of the muscle (figs. 464 and 466). 



TERMINAL BRANCHES. — 1. The pudic nerve arises b}' three roots from the 

 anterior divisions of the second, third, and fourth sacral nerves, and escapes from 

 the pelvis by passing through the great sacro-sciatic foramen, below the pyrifomiis. 

 It crosses the posterior surface of the sniall sacro-sciatic ligament, near the attach- 

 ment of that ligament to the spine of the ischium, and to the inner side of the 

 pudic vessels. It then accompanies the pudic vessels tln-ough the small sacro- 

 sciatic foramen, and enters a canal formed by a delamination of the parietal pelvic 

 fascia (Alcock's canal), and, having given off the inferior ha-morrhoidal nerve, 

 terminates by dividing into the perinaeal ner\'e and the dorsal nerve of the penis. 

 At its origin, the pudic nerve is often connected in a plexiform manner with the 

 nerve to the obturator internus. 



Fig. 467. — A Dissection of the Xerves in the Gluteal Region. 



(The gluteus maximus and gluteus medius have been divided near their insertions, and 



thrown upwards.) 



Sciatic artery 



INFERIOR GLUTEAL NERVE 



Gluteal artery 



Gluteus maximus 



BRANCH TO GLU- 

 TEUS MAXIMUS 



Tendon of obturator extemus 



Vastus esternus 



Gluteus maximus 



(a) The inferior haemorrhoidal nerve pierces the wall of Alcock's canal, and 

 pa.sses inwards through the fatty tissue which occupies the ischio-rectal fossa. It 

 is deejjly placed at the outer part of the fossa, but becomes superficial as it 

 approaches the anus. It divides into terminal twigs, some of which sup])ly the 

 external sphincter ani, Avhile others are distril»uted to the adjacent integument. 



(b) The perinseal nerve runs for a short distance in Alcock's canal at a lower 

 level than the ])udic vessels, and then divides into cutaneous and muscular divisions. 

 The cutaneous division takes the form of two nerv'es, which are termed }>osterior 

 and anterior superficial perinteal nerves. The posterior or external superficial 

 perinaeal nerve escapes from Alcock's canal at the anterior part of the ischio-rectal 

 fossa, ] tierces the base of the triangular ligament, winds round the transverse 

 perinseal muscle, and passes forwards under cover of Colles's fascia. It then 

 divides into several long slender twigs, which communicate with the anterior sujjcr- 

 ficial perina\il and long pudendal nerves, and end in the integuments of the scrotum. 

 In the female they are distributed in a similar manner to the labium majus. The 



