THE NERVOUS SYSTEM 239 



nerve forms the external cutaneous, supplying the lateral surface 

 of the thigh region. 



The anterior crural nerve is composed mainly of branches of 

 the fifth and sixth lumbar. It receives a small branch from the 

 fourth. It supplies the psoas muscles, which it pierces, and 

 then divides into several branches, one of which is the saphenous, 

 passing subcutaneously along the medial aspect of the leg. The 

 other nerves supply the muscles of the thigh on the cranial and 

 medial aspect. The obturator nerve is composed of branches 

 from the sixth and seventh lumbar. It is smaller than the pre- 

 ceding, and passes through the obturator foramen to supply the 

 obturator, adductor, gracilis and pectineus muscles. 



The sacral plexus is formed of the three sacral nerves with 

 branches from the sixth and seventh lumbar nerves. 



The great sciatic nerve is composed mainly of the ventral 

 branches of the seventh lumbar and first sacral nerves. It usu- 

 ally receives accessions from the other sacral nerves and the sixth 

 lumbar. The great sciatic is the largest peripheral nerve in the 

 body. It passes caudad from its origin around the greater 

 sciatic notch and thence along the caudal aspect of the thigh to 

 the popliteal space, where it divides into the internal and 

 external popliteal nerves. The former continues down the caudal 

 side of the tibia as the posterior tibial nerve to the internal mal- 

 leolus, where it divides into internal and external plantar nerves, 

 which supply the digits. The external popliteal or peroneal 

 nerve extends to the outer cranial aspect of the leg, where it 

 divides into the musculocutaneous and anterior tibial nerves. 

 The former extends between the extensor longus digitorum and 

 peronei muscles to its ramification on the dorsum of the foot. 

 It supplies the skin along its course and the peronei muscles. 

 The anterior tibial nerve passes down on the cranial aspect of 

 the tibia beneath the extensor longus digitorum muscle to the 

 tarsal region, where it anastomoses with a branch of the external 

 cutaneous, and supplies the skin, tibialis anticus, and the 

 extensor muscles. 



