THE SACRAL PLEXUS 173 



together with the thick layer of superficial fascia which covers 

 it, forms the normal prominence of the buttock. It has a wide 

 origin from the ilium, sacrum, coccyx and sacro-tuberous (great 

 sacro-sciatic) ligament and its fibres run downwards and laterally 

 to be inserted into the^ilio-tibial tract and the dorsal aspect of the 

 proximal part of the femur. The principal action of the muscle 

 is to extend the thigh, but it is also a powerful lateral rotator 

 and an abductor. When the lower limbs are fixed, the glutaei 

 maximi help to extend the trunk on the hip-joints. 



The Qnadratus Femoris is a small muscle, which extends from the lateral 

 border of the ischial tuberosity to the posterior aspect of the proximal part 

 of the femur. It acts as a lateral rotator of the thigh, and is supplied by 

 a special branch from the sacral plexus (L. 4 and 5, and S. i). The same 

 nerve supplies the inferior gemellus. 



The Obturator Internus arises from the pelvic surface of the obturator 

 membrane and from the adjoining area of bone. Its tendon leaves the 

 pelvis by passing through the lesser sciatic foramen. In the buttock, 

 the tendon is joined by the two small gemelli muscles, which arise from 

 the lesser sciatic notch, and all three have a common insertion into the 

 apex of the greater trochanter. When the thigh is flexed, they help the 

 movement of abduction, but, when the thigh is extended, they assist in 

 lateral rotation. The obturator internus and the superior gemellus are 

 both supplied by a special branch from the sacral plexus (L. 5 and S. I 

 and 2), while the inferior gemellus receives its supply from the nerve to 

 the quadratus femoris. 



The Sciatic Nerve is the largest and most important branch 

 of the sacral plexus. It receives fibres from the fourth and 

 fifth lumbar nerves and also from the first, second and third 

 sacral nerves. Entering the buttock through the greater 

 sciatic foramen, the sciatic nerve descends vertically into the 

 thigh. In its proximal part it lies under cover of the glutseus 

 maximus and, just before it emerges, it is placed mid-way 

 between the ischial tuberosity and the greater trochanter. The 

 nerve is crossed by the long head of the biceps and, in the 

 rest of its course, it is overlapped by the hamstring muscles. 

 The projections formed by the ischial tuberosity and the greater 

 trochanter of the femur protect the nerve from violence, but it 

 may be injured by falls in which the edge of some hard substance 



