THE SACRAL PLEXUS. 



1331 



LUMBO-&ACHAV. 

 CORD 



THE SACRAL PLEXUS. » 



The sacral or sciatic plexus (plexus sacralis) (Fig. 1112) is formed by a portion 

 of the fourth lumbar nerve, all of the fifth lumbar, the entire first sacral and parts of 

 the second and third sacral nerves. As previously stated (page 1320) the fourth 

 lumbar nerve or n. fiircalis splits into two portions, a larger upper and a smaller 

 lower, the former contributing to the lumbar plexus and the latter uniting with the 

 fifth lumbar nerve. The lower portion of the fourth lumbar having passed downward 

 behind the internal iliac vessels, divides into anterior and posterior branches, which 

 fuse respectively with similar 



branches of the fifth lumbar, the Fig. hi 2. 



two trunks thus formed compris- 

 ing the lumbo-sacral cord 

 (truncus lumbosacralis). This 

 double structure emerges from 

 the mesial margin of the psoas 

 magnus, passes down over the 

 brim of the pelvis and constitutes 

 the lumbar contribution to the 

 sacral plexus. The first and 

 second sacral nerves leave their 

 foramina, pass laterally, anterioi 

 to the pyriformis, and split into 

 anterior and posterior branches. 

 The third sacral nerve or n. bi- 

 gemi?ius divides, not into ante- 

 rior and posterior branches, but 

 into upper and lower, the upper 

 becoming a constituent of the 

 sacral and the lower a portion 

 of the pudendal plexus. Con- 

 verging toward the lower por- 

 tion of the great sacro-sciatic 

 foramen, the posterior portion 

 of the lumbo-sacral cord and the 

 posterior branches of the first 

 and second sacral nerves fuse 

 and form the external popliteal 

 or perojieal and some vmnor pos- 

 terior nerves. The anterior por- 

 tion of the lumbo-sacral cord, 

 the anterior branches of the 



first and second sacral nerves and the upper part of the third sacral unite in 

 the internal popliteal or tibial nerve and some small a w/(?r/^r branches (Fig. 11 12). 

 The resulting composite structure, the sacral plexus, is a broad triangular felt-work 

 of nerve-strands, whose base points toward the sacrum and whose apex presents at 

 the great sacro-sciatic foramen. The plexus is an occupant of the pelvis, on whose 

 posterior wall it is situated, lying upon the pyriformis muscle and under cover 

 of the parietal portion of the pelvic fascia. In relation with it anteriorly are the 

 ureter, the pelvic colon and the internal iliac artery and vein. The ilio-lumbar vessels 

 pass above the lumbo-sacral cord and between the cord and the first sacral nerve are 

 found the superior gluteal vessels. The interval between the second and third sacral 

 nerves is occupied by the sciatic artery and vein. 



In size the roots of the sacral plexus vary considerably, the largest, the fifth 

 lumbar nerve, measuring about 7 mm. in diameter and the smallest, the third sacral, 

 3.5 mm. As regards length, the contribution from the fourth lumbar has the long- 

 est course and that from the third sacral the shortest. 



Branches. — The branches of the sacral plexus and their classification centre 

 around the great sciatic nerve and its distribution. This nerve comprises two 



GRtAT SCIATIC 



Diagram illustrating plan of sacral plexus. 



