THE SACRAL PLEXUS 371 



The perineal runs in a sheath of the obturator fascia along 

 the outer wall of the ischiorectal fossa, and divides into super- 

 ficial and deep branches. The latter supply the external sphincter 

 (in part, see Fourth Sacral) and the muscles of the perineum, 

 sending a branch to the mucous membrane of the urethra, 

 which pierces the corpus spongiosum. 



The superficial branches are external and internal. The 

 former supplies the scrotum and inner side of the thigh, and 

 the latter runs nearer to the middle line and supplies the skin 

 of the scrotum. Both the superficial perineal nerves communi- 

 cate with the pudendal and hemorrhoidal branches, and in 

 the female end in the labia majora. 



The dorsal nerve of the penis (in the female, of the clitoris) 

 accompanies the pudic artery, and runs along the dorsum to 

 the glans. It supplies branches to the constrictor urethra; 

 to the integument of the penis, and to the corpus cavernosum. 

 On the penis it receives branches from the sympathetic. In 

 the female the analogue of this nerve is smaller, with a like 

 distribution. 



The great sciatic nerve is the largest nerve in the body, and 

 includes fibers from the greater part of the sacral plexus. From 

 the lower border of the pyriformis it descends on the gemelli, 

 obturator internus, and quadratus, then on the adductor 

 magnus, being covered by the gluteus maximus and long head 

 of the biceps, and accompanied by the small sciatic nerve and 

 the sciatic artery. It divides at the lower third of the thigh 

 into the external and 'internal popliteal nerves. It supplies the 

 biceps, semitendinosus and semimembranosus, adductor magnus 

 (in part), and hip-joint (in part). 



The internal popliteal nerve is the larger branch of bifurca- 

 tion of the great sciatic. It runs along the middle of the popliteal 

 space to the lower border of the popliteus, where it becomes 

 the posterior tibial. It is at first external to, then behind, 

 and lastly internal to, the popliteal artery. 



Branches. Three articular, one accompanying the azygos 

 articular artery, and one each to the upper and lower articular 

 arteries on the inner side of the knee-joint. 



Muscular. One to each head of the gastrocnemius, to the 

 plantaris, to the soleus, and to the popliteus. The latter 

 gives filaments to the tibia and interosseous membrane, and 

 turns beneath the lower border of the muscle. 



