THE POSTERIOR TIBIAL NERVE 177 



deeply seated, and lies beneath the inner head of the gastrocnemius 

 muscle, where it crosses the flexor perforatus. It then emerges from 

 beneath the head of the gastrocnemius and appears on the inner aspect 

 of the limb in front of the tendo-Achilles. It lies beneath the deep 

 fascia of the leg. For the greater part of its course it is some distance 

 behind the posterior tibial artery, for the latter makes its appearance at 

 the inferior extremity of the muscular portion of the flexor accessorius, 

 and then runs down the limb closely applied to the posterior aspect of 

 the tendon of that muscle. Just above the hock, however, the artery 

 forms an S-shaped curve, the first convexity of which is directed 

 downwards and the second upwards, so that the vessel here approximates 

 much more closely to the nerve. Moreover, from the second convexity 

 of the S curve a retrograde branch is given off, which ascends in immediate 

 relationship to the anterior aspect of the nerve. Notwithstanding the 

 great reduction in size which the internal popliteal nerve has undergone 

 in consequence of the number of muscular branches which it detaches, 

 the posterior tibial nerve — its continuation — is still a nerve of considerable 

 volume. Unlike the great sciatic nerve, which in the upper portion of 

 the limb is a flat band, the posterior tibial nerve is rounded and cord-like. 

 On a level with the summit of the tuber calcis the posterior tibial nerve 

 terminates by dividing into the two plantar nerves, which pass down- 

 wards through the tarsal sheath with the perforans tendon. At a point 

 which averages five inches above the summit of the tuber calcis the 

 posterior tibial nerve gives off an important cutaneous branch, which 

 runs downwards and forwards across the seat of spavin, and to which 

 attention will be drawn later in dealing with posterior tibial neurectomy. 

 Other small cutaneous branches are detached from the posterior tibial 

 nerve, but these are of little importance. 



