1056 



THE NERVE SYSTEM 



while from the third, and frequently from the second and fourth anterior sacral 

 divisions, white rami communicantes are given to the pelvic plexuses of the sym- 

 pathetic. 



THE SACRAL PLEXUS (PLEXUS SACRALIS) (Fig. 774). 



The sacral plexus is formed by the lumbosacral cord, the anterior primary 

 division of the first and portions of the anterior primary divisions of the second 

 and third sacral nerves. 



The lumbosacral cord comprises the whole of the anterior primary division of 

 the fifth and a part of the anterior primary division of the fourth lumbar nerves 

 it appears at the inner margin of the Psoas magnus and runs downward (caudad) 

 over the pelvic brim to join the first sacral nerve. The third sacral nerve divides 

 into an upper and a lower branch, the former entering the sacral and the latter 

 the pudenda! plexus 



INFERIOR 

 GLUTEAL 



FOURTH LUMBAR 



FIFTH LUMBAR 



FIRST SACRAL 



SECOND SACRAL 



THIRD SACRAL 



FOURTH SACRAL 



PERINEAL BR. TO SPHINCTER 

 4. LEVATOR ANI 



S, FIFTH SACRAL 



N.TOCOCCYCEUS 







' 



S VISCERAL v .V--", COCCYGEAL 



FIG. 774. Plan of sacral plexus with the pudenda! plexus. (Gerrish.) 



the perforating cutaneous nerve.) 



(See text for variations in origin of 



The branches or divisions forming the sacral plexus converge toward the 

 lower part of the great sacrosciatic foramen, and unite to form a flattened band, 

 from the anterior and posterior surfaces of which several branches arise. The 

 band itself is continued as the great sciatic nerve, which splits on the back of the 

 thigh into the internal and external popliteal nerves; these two nerves sometimes 

 arise separately from the plexus, and in all cases their independence can be shown 

 by dissection. 



Relations. The sacral plexus lies on the anterior or ventral surface of the pelvic part of 

 the Pyriformis and is covered in front by the pelvic fascia, which separates it from the internal 

 iliac vessels, the ureter and the pelvic colon. The gluteal vessels run between the lumbosacral 

 cord and the first sacral nerve, and the sciatic vessels between the second and third sacral nerves. 



