THE LOWER LIMB 439 



company, the biceps femoris passing downwards and outwards, and 

 the semitendinosus and semimembranosus downwards and inwards ; 

 and so the popUteal space begins to open out. 



Great Sciatic Nerve. — ^This nerve arises from the sacral plexus, 

 more particularly from the descending branch of the fourth lumbar, 

 the fifth lumbar, and the first, second, and upper branch of the 

 third sacral nerves. It emerges from the pelvis through the lower 

 compartment of the great sacro-sciatic foramen, below the pyri- 

 formis, and descends between the great trochanter and tuber ischii, 

 being somewhat nearer the latter than the former. It then passes 

 down the middle line of the back of the thigh, under cover of 

 the long or ischial head of the biceps femoris, and, about the 

 junction of the middle and lower thirds, it terminates by dividing 

 mto internal and external popliteal nerves. 



The great sciatic nerve is about f inch broad. Its course may be 

 indicated as follows : draw a Une from the centre of the back of the 

 knee-joint to a point between the great trochanter and tuber ischii, 

 rather nearer the latter than the former. The upper two-thirds 

 of this line correspond with the great sciatic nerve, and the lower 

 third with the internal popliteal nerve. 



Chief Relations. Superficial or Posterior. — Gluteus maximus, small 

 sciatic nerve, and long or ischial head of the biceps femoris. Deep 

 or Anterior. — From above downwards the nerve is in contact with 

 the following structures : the ischium, the ner^'e to the quadratus 

 femoris, gemellus superior, obturator internus, gemellus inferior, 

 quadratus femoris, and posterior surface of adductor magnus. 

 Internal. — Semimembranosus. 



Branches. — The branches are muscular and terminal. 



The muscular branches arise in the upper part of the thigh, 

 with the exception of the branch to the short head of the biceps 

 femoris, which arises about the centre. They supply the ham- 

 string muscles, and also that part of the adductor magnus which 

 descends from the tuber ischii to the adductor tubercle of the 

 lemur. The branch to this part of the adductor magnus arises 

 in common with the nerv'e to the semimembranosus. The branch 

 to the ehort head of the biceps femoris is derived from the external 

 popliteal part of the great sciatic nerve, but all the other muscular 

 branches come from the internal popliteal part. 



The terminal branches are the internal and external popliteal 

 nerves. They arise about the junction of the middle and lower 

 thirds of the thigh, and will be described in connection with the 

 popliteal space. 



The great sciatic nerve supplies an articular branch to the hip- 

 joint in those cases where the nerve to the quadratus femoris faUs 

 to do so. 



Blood-supply. — ^The nerve is supphed by the arteria comes 

 nervi ischiadici from the sciatic, and the perforating branches of 

 the arteria profunda femoris. 



The great sciatic nerve is not infrequently replaced by the external 



