THE ABDOMEN 847 



foramen below the pyriformis, without piercing the parietal pelvic 

 fascia. The upper band and its contributory nerves are sometimes 

 spoken of as the sciatic plexus. The lower or pudic band, the 

 smaller of the two, receives fibres from the second, lower branch 

 of the third, and upper branch of the fourth sacral nerves, and 

 is continued into the pudic nerve, which leaves the pelvis, like 

 the great sciatic, through the lower compartment of the great 

 sacro-sciatic foramen without piercing the parietal pelvic fascia. 

 The lower band and its contributory nerves are sometimes spoken 

 of as the pudic plexus. The great sciatic and pudic nerves are 

 thus the terminal branches of the sacral plexus. The reason of 

 the name nervus bigeminus, as applied to the third sacral nerve, 

 is because it enters into the sciatic band by its upper branch, and 

 into the pudic band by its lower branch. 



Branches. — ^The branches of the sacral plexus are arranged in 

 two groups — collateral and terminal. 



Collateral Group. — The branches of this group form three sets 

 — visceral, muscular, and cutaneous. 



Visceral Branches. — ^These are derived from the third sacral and 

 that part of the fourth which enters into the sacral plexus (some- 

 times also from the second). They are white rami communicantes, 

 which reinforce the pelvic plexus of the sympathetic, a few of them 

 being traceable independently to the pelvic viscera. They are 

 known as the pelvic splanchnics. 



Muscular Brandies. — The superior gluteal nerve arises by three 

 roots from the dorsal divisions of the descending branch of the 

 fourth lumbar, fifth lumbar, and first sacral nerves. It passes 

 outwards and backwards, and leaves the pelvis with the gluteal 

 artery through the upper compartment of the great sacro-sciatic 

 foramen, to be distributed to the gluteus medius, gluteus minimus, 

 and tensor fasciae femoris muscles. 



The inferior gluteal nerve arises by three roots from the dorsal 

 divisions of the fifth lumbar and first and second sacral nerves. 

 It leaves the pelvis through the lower compartment of the great 

 sacro-sciatic foramen, below the pyriformis and superficial to the 

 great sciatic nerve, to be distributed to the gluteus maximus. It is 

 usually intimately associated with the small sciatic nerve. 



The nerves to the pyriformis, which are usually two in number, 

 spring from the dorsal divisions of the first and second sacral nerves. 



The nerve to the obturator internus and gemellus superior arises 

 by three roots from the ventral divisions of the fifth lumbar and 

 first and second sacral nerves. It leaves the pelvis through the 

 lower compartment of the great sacro-sciatic foramen, crosses the 

 back of the ischial spine, where it lies external to the internal 

 pudic vessels, and then passes through the small sacro-sciatic 

 foramen to the outer wall of the ischio-rectal fossa, where it enters 

 the inner or pelvic surface of the obturator internus. In the gluteal 

 region it gives a branch to the gemellus superior when that muscle 

 is present. 



