474 SACRAL NERVES. SACRAL PLEXUS. 



muscle, accompanies the popliteal artery, and is distributed to the 

 synovial membrane of the knee-joint on its posterior aspect. 



The LUMBOSACRAL NERVE. The anterior division of the fifth 

 lumbar nerve, conjoined with a branch from the fourth, constitutes the 

 lumbo-sacral nerve which descends over the base of the sacrum into 

 the pelvis, and assists in forming the sacral plexus. 



SACRAL NERVES. 



There are six pairs of sacral nerves ; the first escape from the ver- 

 tebral canal through the first sacral foramina, and the two last between 

 the sacrum and coccyx. The posterior sacral nerves are very small 

 and diminish in size from above downwards ; they communicate with 

 each other immediately after their escape from the posterior sacral 

 foramina, and divide into external and internal branches. The 

 external branches pierce the gluteus maximus, to which they give fila- 

 ments, and are distributed to the integument of the posterior part of 

 the gluteal region (n. cutanei clunium posteriores). The internal 

 supply the integument over the sacrum and coccyx. 



The anterior sacral nerves diminish in size from above downwards ; 

 the first is large and unites with the lumbo-sacral nerve ; the second, 

 of equal size, unites with the preceding ; the third, which is scarcely 

 one-fourth so large as the third, also joins with the preceding nerves in 

 the formation of the sacral plexus. The fourth anterior sacral nerve is 

 about one-third the size of the preceding sacral nerve, it divides into 

 several branches, one of which is sent to the sacral plexus, a second to 

 join the fifth sacral nerve, a third to the viscera of the pelvis commu- 

 nicating with the hypogastric plexus, and a fourth to the coccygeus 

 muscle, and to the integument around the anus. The fifth anterior 

 sacral nerve presents about half the size of the fourth ; it divides into 

 two branches, one of which communicates with the fourth, the other 

 with the sixth. The sixth sacral nerve (coccygeal) is exceedingly 

 small ; it gives off an ascending filament which is continuous with the 

 communicating branch of the fifth ; and a descending filament which 

 passes downwards by the side of the coccyx and traverses the fibres 

 of the great sacro-ischiatic ligament to be distributed to the gluteus 

 maximus and to the integument. All the anterior sacral nerves re- 

 ceive branches from the sacral ganglia of the sympathetic at their 

 emergence from the sacral foramina. 



SACRAL PLEXUS. 



The Sacral plemis is formed by the lumbo-sacral, and by the ante- 

 rior branches of the four upper sacral nerves. The plexus is trian- 

 gular in form, the base corresponding with the whole length of the 

 sacrum, and the apex with the lower part of the great ischiatic fora- 



