64 THE SURGICAL ANATOMY OF THE HORSE 



THE GREAT SCIATIC NERVE 



This is sometimes termed the great femoro-popliteal nerve. It is an 

 enormous nerve — in fact, at its origin it is the largest nerve in the body. 

 Leaving the lumbo-sacral plexus, it emerges from the pelvis through 

 the greater sacro-sciatic foramen as a broad flat band, which is closely 

 applied to the outer surface of the great sacro-sciatic ligament, where it 

 is covered by the middle gluteus muscle. The nerve now passes down- 

 wards and backwards across the outer aspect of the deep gluteus muscle, 

 and between it and the middle gluteus. Leaving the deep gluteus, it 

 passes across the gemelli and the common tendon of insertion of the 

 obturator internus and pyriformis, and subsequently is related to the 

 quadratus femoris. It then takes its course down the thigh in the groove 

 which is bounded externally by the biceps femoris and semitendinosus 

 muscles and internally by the semimembranosus and adductor magnus. 

 It leaves the region of the thigh bypassing downwards between the two 

 heads of the gastrocnemius, where it is directly continued as the internal 

 popliteal nerve. 



During its course the great sciatic nerve gives off the following 

 branches : 



1. Shortly after leaving the greater sacro-sciatic foramen it detaches 

 a slender filament, which most frequently runs down behind the parent 

 nerve, but occasionally is found between it and the surface of the great 

 sacro-sciatic ligament. This branch is distributed to the obturator 

 internus, the pyriformis, the; gemelli, and the quadratus femoris muscles. 

 Occasionally the branches which supply the two last-named muscles arise 

 separately from the parent nerve, when the branch for the quadratus 

 femoris passes beneath the gemelli and the common tendon of the 

 obturator internus and pyriformis to reach the muscle which it 

 supplies. 



2. The external popliteal nerve. This large branch will be found in 



