THE SACRAL PLEXUS 1057 



All the nerves entering the plexus, with the exception of the third sacral, split into anterior 

 and posterior divisions, and the nerves arising from these are as follows: 



Anterior or Ventral Posterior or Dorsal 

 Divisions Divisions. 



Nerve to Quadratus femoris and Gemellus inferior 4, 5, L. 1 , S. 



Nerve to Obturator internus and Gemellus superior 5, L. 1, 2, S. 



Nerve to Pyriformis ......... (1),2,S. 



Superior gluteal .......... 4, 5, L. 1 , S. 



Inferior gluteal .......... 5, L.I, 2, S. 



Small sciatic ........... 2, 3, S. 



4,5, L. 1. 2, 3, S. 



4, 5, L.I, 2, S. 



. . f Internal popliteal 

 Great sciatic | Externa l popliteal 



The nerve to the Quadratus femoris and Gemellus inferior arises from the anterior 

 divisions of the fourth and fifth lumbar and first sacral nerves; it leaves the pelvis 

 through the great sacrosciatic foramen, below the Pyriformis, and runs downward 

 beneath the great sciatic nerve, the Gemelli, and the tendon of the Obturator 

 internus, enters the anterior surface of these muscles; it gives off an articular 

 branch to the hip-joint. 



The nerve to the Obturator internus arises from the anterior divisions of the 

 fifth lumbar and first and second sacral nerves, it leaves the pelvis through the 

 great sacrosciatic foramen below the Pyriformis muscle, crosses the ischial spine, 

 reenters the pelvis through the small sacrosciatic foramen, and ends in the Obturator 

 internus, after entering the pelvic surface of that muscle. The branch to the 

 Gemellus superior enters the upper part of the posterior surface of that muscle. 



The nerve to the Pyriformis arises from the posterior division of the second, 

 or the posterior divisions of the first and second sacral nerves, and enters the 

 anterior surface of the muscle; this nerve may be double. 



The Superior Gluteal Nerve (n. glutaeus superior) (Figs. 774 and 776) arises 

 from the posterior divisions of the fourth and fifth lumbar and first sacral 

 nerves; it leaves the pelvis through the great sacrosciatic foramen above the Pyri- 

 formis, accompanied by the gluteal vessels, and divides into a superior and an 

 inferior branch. The superior branch accompanies the upper branch of the deep 

 division of the gluteal artery, and ends in the Gluteus minimus after giving off 

 branches to supply a part of the Glutens medius. The inferior branch accompanies 

 the lower branch of the gluteal artery across the Gluteus minimus; it gives fila- 

 ments to the Gluteus medius and Gluteus minimus, and ends in the Tensor 

 fasciae femoris. 



The Inferior Gluteal Nerve (n. glutaeus inferior) (Fig. 774) arises from the 

 posterior divisions of the fifth lumbar and first and second sacral nerves; it leaves 

 the pelvis through the great sacrosciatic foramen, below the Pyriformis muscle, 

 and divides into branches, which enter the deep surface of the Gluteus maximus. 



The Small Sciatic (n. cutaneus femoris posterior) (Figs. 774 and 776), or post- 

 femoral cutaneous nerve supplies the integument of the perineum and back part 

 of the thigh and leg. It arises partly from the anterior and partly from the pos- 

 terior divisions of the first, second, and third sacral nerves, and emerges from the 

 pelvis through the great sacrosciatic foramen below the Pyriformis. It then 

 descends beneath the Gluteus maximus with the sciatic artery, and passes down 

 the back part of the thigh beneath the fascia lata, and over the long head of the 

 Biceps to the lower part of the popliteal region, here it pierces the fascia and 

 accompanies the external saphenous vein (v. saphena parva) to about the middle 

 of the back of the leg, its terminal filaments communicating with the sural or 

 external saphenous nerve. 



The branches of the small sciatic nerve are all cutaneous, and are grouped as 

 follows: gluteal, perineal, and femoral. 



67 



