946 THE NERVOUS SYSTEM 



(fig. 691). At the lower part of the popliteal region it perforates the deep fascia, and 

 it terminates in branches which are distributed to the skin of the calf. 



Branches. 1. Perineal branches are distributed in part to the skin of the upper 

 and inner sides of the thigh on its dorsal aspect. One of the branches, known as the 

 long pudendal nerve, runs forwards and inwards in front of the tuberosity of the 

 ischium to the lateral margin of the anterior part of the perineum, where it perforates 

 the fascia lata and Colles' fascia and enters the anterior compartment of the perineum. 

 It communicates in the perineum with the superficial perineal nerves, and its terminal 

 filaments are distributed to the skin of the scrotum in the male, and to the labium 

 majus in the female. 



2. Inferior clunial (gluteal) branches, two or three in number, are given off 

 beneath the gluteus maximus; they turn around the lower border of this muscle and 

 are distributed to the skin of the lower and outer part of the gluteal region. 



3. Femoral cutaneous branches are given off as the nerve descends through the 

 thigh. They perforate the deep fascia and are distributed to the skin of the back of 

 the thigh, especially on the inner side. 



In case of the separate origin of the tibial (internal popliteal) and common peroneal 

 (external popliteal) nerves, the posterior femoral cutaneous also arises from the sacral 

 plexus in two parts. The ventral portion descends with the tibial nerve below the 

 piriformis and gives off the perineal branches and inner femoral branches, while the 

 dorsal portion passses through that muscle with the common peroneal nerve, and 

 furnishes the gluteal and outer femoral branches. 



(b) The inferior medial clunial (perforating cutaneous) nerve arises from the 

 posterior portion of the second and third sacral nerves (figs. 692, 693). It perforates 

 the lower part of the sacro-tuberous (great sciatic) ligament, turns around the in- 

 ferior border of the gluteus maximus, and is distributed to the skin over the lower 

 and inner part of that muscle. It is sometimes associated at its origin with the pudic 

 nerve. It is not always present. Its place is sometimes taken by a small nerve 

 (the greater coccygeal perforating of Eisler) , arising from the third and fourth or fourth 

 and fifth sacral nerves, and sometimes it is represented by a branch of the pos- 

 terior femoral cutaneous (small sciatic) . 



Muscular Branches. (a) One or two small nerves to the piriformis pass from 

 the posterior divisions of the first and second sacral nerves. 



(6) The superior gluteal nerve receives fibres from the posterior branches of the 

 fourth and fifth lumbar, and the first sacral nerves. It passes out of the pelvis through 

 the great sciatic foramen, above the upper border of the piriformis, and it is accom- 

 panied by the superior gluteal artery. As soon as it enters the buttock it divides into 

 two branches, an upper and a lower. 



1 . The upper branch is the smaller. It accompanies the upper branch of the 

 deep division of the superior gluteal artery below the middle curved line of the ilium, 

 and it ends entirely in the gluteus medius (fig. 694) . 



2. The lower branch, larger than the upper, passes forwards across the middle 

 of the gluteus minimus, with the lower branch of the gluteal artery; it supplies the 

 gluteus medius and the gluteus minimus, and it ends in the inner and posterior part 

 of the tensor fascia? latse. 



(c) The inferior gluteal nerve is formed by fibres from the posterior branches 

 of the fifth lumbar, and the first and second sacral nerves. It passes through the 

 great sciatic foramen, below the piriformis, and divides into a number of branches 

 which end in the gluteus maximus (figs. 692, 693). 



(d) The nerve to the quadratus femoris is formed by the anterior branches of 

 the fourth and fifth lumbar and the first and second sacral nerves. It lies on the 

 front of the plexus and issues from the pelvis below the piriformis. In the buttock 

 it lies at first between the sciatic nerve and the back of the ischium, and, at a lower 

 level, between the obturator internus with the gemelli and the ischium. It terminates 

 in the anterior surface of the quadratus femoris, having previously given off a branch 

 to the hip-joint and another to the inferior gemellus. 



(e) The nerve to the obturator internus is formed by the anterior branches 

 of the fifth lumbar, and the first and second thoracic nerves (figs. 692 and 693) . It 

 leaves the pelvis below the piriformis, and crosses the spine of the ischium on the outer 

 side of the internal pudic artery and on the inner side of the sciatic nerve. It gives a 

 branch to the gemellus superior, and turns forwards through the small sciatic fora- 

 men into the perineum, where it terminates in the inner surface of the obturator 

 internus. 



