THE PUDENDAL PLEXUS 183 



sacral plexus and enters the buttock through the great 

 sciatic foramen. Beyond the lower border of the glutaeus 

 maximus, the nerve descends immediately under the deep 

 fascia. It supplies branches to the skin on the posterior 

 aspect of the thigh, over the popliteal fossa, and over the 

 proximal part of the posterior aspect of the leg. This nerve 

 is derived from segments of the spinal medulla which are 

 accustomed to receive impulses via the sympathetic from the 

 lower part of the rectum and the upper part of the anal canal, 

 and from the internal trigone of the bladder. In most cases 

 in which a " focus of irritation" (p. 195) is established in the 

 mid-sacral region of the spinal medulla, the referred pains are 

 experienced in the perineum, but in some cases the pain is 

 referred to the posterior aspect of the thigh, and the condition 

 may be mistaken for sciatica, unless the sciatic nerve is 

 subjected to local examination by deep pressure. 



THE PUDENDAL PLEXUS 



The Pudendal Plexus is formed by portions of the second, 

 third and fourth sacral nerves. Like the sacral plexus, it lies 

 in front of the sacrum and behind the rectum, and it is 

 therefore subject to the same varieties of intra-pelvic pres- 

 sure (p. 171). 



The Pudendal (Internal Pudic) Nerve (S. 2, 3 and 4) is the 

 most important branch of the pudendal plexus. It arises in 

 the pelvis and enters the lateral wall of the ischio-rectal fossa, 

 where it gives off the inferior hcemorrhoidal nerve^ and divides 

 into the perineal nerve and the dorsal nerve of the penis (or 

 clitoris). 



The Inferior Hsemorrhoidal Nerve is distributed to the 

 external sphincter muscle and to the skin around the anus. 

 In the condition of anal fissure, the external sphincter 

 becomes reflexly contracted owing to the tearing of the 

 mucous membrane, which receives its nerve-supply from the 

 same source (p. 284). 



