848 A MANUAL OF ANATOMY 



The nerve to the quadratus femoris and gemellus inferior arises 

 by three roots from the ventral divisions of the descending branch 

 of the fourth lumbar, fifth lumbar, and first sacral nerves. It 

 leaves the pelvis through the lower compartment of the great 

 sacro-sciatic foramen, and then lies between the ischium and the 

 great sciatic nerve. It subsequently descends over the back of the 

 capsular ligament of the hip- joint, to which it usually gives a branch, 

 and beneath the gemelli and obturator internus to the deep surface 

 of the quadratus femoris, in which it ends, having previously given 

 a branch to the gemellus inferior. 



Cutaneous Branches. — The small sciatic nerve arises by three 

 roots from the posterior aspects of the first, second, and third sacral 

 nerves. It leaves the pelvis through the lower compartment of the 

 great sacro-sciatic foramen, and is distributed to (i) the lower and 

 outer part of the gluteal region by its gluteal cutaneous branches, 

 (2)] the integument of the scrotum by the long pudendal nerve of 

 Soemmering, (3) the integument of the back of the thigh, and 

 (4) the integument of the back of the leg as low as about the centre 

 of the calf. 



The perforating cutaneous nerve arises by two roots from the 

 posterior aspects of the second and third sacral nerves. It passes 

 backwards through the great sacro-sciatic ligament, after which it 

 turns round the lower border of the gluteus maximus near the 

 coccyx to be distributed to the integument over the lower and 

 inner part of that muscle. 



Terminal Group. — ^This group is comprised of two nerves, namely, 

 the great sciatic and the pudic. 



The great sciatic nerve is the continuation of the upper or sciatic 

 band of the sacral plexus. In reality it is made up of the two 

 divisions in which it ultimately ends, namely, the external and 

 internal popliteal, which lie close together within the same sheath. 

 Sometimes, however, these two divisions arise separately from the 

 sacral plexus, in which cases the external popliteal nerve may pass 

 through the pyriformis. Again, though the great sciatic nerve 

 may be apparently single, a careful dissection of it, after the removal 

 of its sheath, will reveal the external and internal popliteal nerves, 

 which can be shown to have independent origins. The external 

 popliteal nerve derives its fibres from the dorsal divisions of the 

 descending branch of the fourth lumbar, fifth lumbar, and first 

 and second sacral nerves;; and the internal popliteal nerve derives 

 its fibres from the ventral divisions of the foregoing nerves, and in 

 addition from the upper branch of the third sacral. 



The great sciatic nerve leaves the pelvis through the lower com- 

 partment of the great sacro-sciatic foramen below the pyriformis. 



The pudic nerve is the continuation of the lower or pudic band 

 of the sacral plexus. It arises by three roots from the ventral 

 division of the second, the lower branch of the third, and the upper 

 branch of the fourth sacral, the root from the third being the 

 largest. The nerve leaves the pelvis through the lower compart- 



