SACRAL PLEXUS. 327 



Sacral Nerves. 



The sacral nerves are five in number. The upper 

 four emerge from the anterior and posterior sacral 

 foramina. The fifth sacral, which is very small, makes 

 its exit between the sacrum and coccyx. The sacral 

 nerves are derived from the cauda equina and have 

 the longest roots. Their upper three are large ; the 

 fourth is small. Their anterior divisions unite with the 

 lumbo-sacral cord to form the sacral plexus. 



Sacral Plexus. 







This plexus is simple in construction, being formed 

 by the union of the lumbo-sacral cord, first sacral, second 

 sacral, and third sacral, witli some filaments from the 

 fourth sacral nerve, in a large, broad, flat band of nerve- 

 fibres one inch wide, which sends oft' the following 

 branches: 



The great sciatic is the largest nerve in the body. It 

 is practically the continuation of the sacral plexus and 

 emerges from the pelvis by the greater sacro-sciatic 

 foramen below the pyriform muscle. It passes down the 

 posterior median part of the thigh and leg into the 

 foot, and' is distributed to the muscles of the posterior 

 part of the thigh and of the leg and foot, and also sup- 

 plies the skin of nearly the entire lower extremity with 

 sensation. Its branches are articular to the hip-joint, 

 muscular, and it divides above the popliteal space into 

 internal popliteal the larger and external popliteal the 

 smaller branch. 



The internal popliteal descends through the middle 

 of the popliteal space to the leg, where it lies upon the 

 deep muscles. It sends off cutaneous and muscular 

 branches and the external saphenous, which is distributed 

 to the integument at the back of the calf of the leg. It 



