THE SACRAL PLEXUS 1007 



Cutaneous branches. — (a) The posterior femoral cutaneous (small sciatic) 

 nerve arises parti}' from the anterior and partly from the posterior branches of 

 the anterior primary divisions of the first, second, and third sacral nerves. It 

 lies on the back of the plexus (figs. 765, 769), leaves the pelvis at the lower 

 border of the piriformis, and descends in the buttock between the gluteus maxi- 

 mus and the posterior surface of the sciatic nerve (fig. 770). At the lower 

 border of the gluteus maximus it passes behind the long head of the biceps femoris, 

 and descends, immediately beneath the deep fascia, through the thigh and the 

 upper part of the popliteal space (fig. 740). At the lower part of the popliteal 

 region it perforates the deep fascia, and it terminates in branches which are dis- 

 tributed to the skin of the calf. 



Branches of the small sciatic. — 1. Perineal branches are distributed in part to the skin of 

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

 long pudendal nerve, runs forward and medialward 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. In the perineum twigs 

 join it 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 lateral 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 medial side. 



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

 pophteal) nerves, the posterior femoral cutaneous (small sciatic) 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 medial femoral branches, while the dorsal portion passes 

 through that muscle with the common peroneal nerve, and furnishes the gluteal and lateral 

 femoral branches. 



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

 portion of the second and third sacral nerves (figs. 765, 769). It perforates the lower part of the 

 sacro-tuberous (great sciatic) Mgament, turns around the inferior border of the gluteus maximus, 

 and is distribtued to the skin over the lower and medial 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 nerve of Eisler), arising from the third 

 and fourth or fourth and fifth sacral nerves, and sometimes it is represented by a branch of the 

 posterior femoral cutaneous. 



Muscular branches of the sacral plexus.— (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 accompanied 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 ihum, and it ends entirely 

 in the gluteus medius (fig. 770). 



2. The lower branch, larger than the upper, passes forward 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 medial and posterior part of the tensor fasciae 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. 765, 769). 



(d) The nerve of 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 of the obturator internus is formed by the anterior branches 

 of the fifth lumbar, and the first and second sacral nerves (figs. 765, 769). It 

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



