NEUROLOGY. 137 



wanting. The four superior, with the branch from the last lumbar, form 

 the sacral plexus^ placed on the sacrum behind the pelvic fasciae ; its form 

 is somewhat triangular or palmate. Each of the sacral nerves, immediately 

 after its emergence from the foramen, is joined by a short branch from one 

 of the ganglions of the sympathetic. The sacral plexus sends off the follow- 

 ing branches, both internally and externally : the internal or pelvic are the 

 Jicemoirhoidal, vesical, and muscular, and in the female the uterine and the 

 vaginal ; the external branches are the inferior or lesser sciatic, the inferior 

 glutceal, posterior cutaneous, pudic, and great sciatic or posterior crural. All 

 these escape hy the lower part of the great sciatic notch below the pyriform 

 muscle. The visceral branches arise from the fourth and fifth sacral, and are 

 directed forwards into the cavity, there to be joined by numerous filaments 

 from the sympathetic nerve, forming the hypogastric plexus, a complex net- 

 work of nervous filaments entangled around the branches of the internal 

 iliac artery, and accompanying them to their terminations. 



The internal or pelvic muscular branches supply the levator ani, obturator 

 internus, pyriformis, and sphincter ani. The lesser sciatic, inferior glutoial, 

 and posterior cutaneous nerves, may all be regarded as one large, loose, and 

 flat fasciculus, escaping from the pelvis below the pyriform muscle ; branches 

 are distributed to the perinaeal regions, and to the upper part of the leg. 



The pubic nerve arises from the third and fourth sacral, and, passing 

 through the great sciatic notch or foramen, re-enters the pelvis by the lesser 

 sciatic notch, and i^i'oceeds towards the pubes ; it then divides into two 

 branches, an inferior and a posterior. The inferior or perinmal nerve, the 

 larger of the two, is distributed to the muscles and integuments of the peri- 

 naeum. The superior branch continues along the pubes to the symphyis, 

 and is finally distributed to the subcutaneous cellular tissue of the glans 

 penis. 



The great sciatic, or posterior crural nerve, is the principal branch of the 

 sacral plexus, and the largest nerve in the body. It proceeds from the four 

 superior sacral nerves, forms a flat broad band, which escapes from the pel- 

 vis below the pyriform muscle, and descends close to the outer side of the 

 tuber ischii, along the back part of the thigh, as far as the ham, where it 

 divides into the external and internal poplitaeal nerves, having given off 

 numerous muscular and cutaneous branches. About the middle of the 

 thigh it often gives off a large nerve, the externcd articular, to the outer side 

 of the knee joint. 



The external poplitceal, or the peronceal nerve, is destined to supply the 

 muscles on the external and anterior aspect of the leg, as also the integu- 

 ments of the leg and dorsum of the foot. Descending, it gives off several 

 long branches, termed the external cutaneous nerves of the leg ; one of these, 

 the communicans peroncei, passes backwards over the outer part of the gas- 

 trocnemius, and communicates with the external saphenous nerve. Passing 

 round the joint of the fibula, the perontpal nerve divides into two branches, 

 the musculo-cutaneous and the anterior tibial. The muscido-cutaneous passes 

 down the leg, and a little above the external malleolus, divides into the 

 internal and external tarsal nerves, or the dorsal nerves of the foot. The an- 



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