THE LOWER LIMB 523 



placed, being situated between the superficial and deep muscles, 

 and over the interval between the tibia and fibula. It then inclines 

 inwards, and in the lower third of the leg it lies over the back of 

 the tibia and to the inner side of the tendo Achillis, where it is com- 

 paratively superficial. The course of the vessel may be indicated 

 by drawing a line from the centre of the lower part of the popliteal 

 space to a point midway between the inner part of the point of the 

 heel and the tip of the internal malleolus. 



Relations — Superficial or Posterior. — In the upper two-thirds the 

 vessel lies beneath the gastrocnemius, soleus, and posterior or 

 transverse intermuscular septum ; and, in the lower third, it is 

 covered by the skin, superficial fascia, deep fascia, and the internal 

 annular ligament. Anterior or Deep. — ^The tibialis posticus (to 

 which it is bound by the posterior or transverse intermuscular 

 septum), flexor longus digitormn, posterior surface of the tibia, and 

 internal lateral ligament of the ankle-joint (in this order from above 

 downwards). External. — The tendo Achillis in the lower part of 

 the leg. The artery in this part of its course lies about midway 

 between the inner border of the tendo Achillis and the internal 

 border of the tibia. As the vessel passes beneath the internal 

 annular ligament it occupies a special fibro-osseous canal along 

 with its venae comites and the posterior tibial nerve, the latter 

 being on the outer side. In this situation the tendons of the tibialis 

 posticus and flexor longus digitorum lie on the inner side of these 

 structures, and the tendon of the flexor longus hallucis on their 

 outer side. 



The artery is accompanied by two venae comites, which closely 

 embrace its sides, and communicate with each other at frequent 

 intervals by transverse branches crossing superficial to the vessel. 

 These venae comites ultimately join to form one trunk, which unites 

 with that formed by the junction of the venae comites of the 

 anterior tibial artery to form the popliteal vein, the union taking 

 place at the lower border of the popliteus muscle. The posterior 

 tibial nerve is at first internal to the artery for about ij inches. 

 It then crosses over the vessel just below the origin of the peroneal 

 branch, and from this point onwards it hes on its outer side. 

 Beneath the internal annular ligament the artery has frequently 

 a nerve upon either side of it, an arrangement which is brought 

 about by an early division of the posterior tibial nerve into its 

 internal and external plantar branches. 



Branches. — ^These are as follows : peroneal, muscular, medullary 

 or nutrient, cutaneous, communicating, internal malleolar, internal 

 calcaneal, and terminal. 



The description of the peroneal artery, from its large size and 

 numerous branches, will be deferred to the last. 



The muscular branches are numerous, and are distributed to 

 the deep group of muscles and to the soleus. 



The medullary or nutrient artery is the largest of all the arteries 

 bearing that name. Arising from the upper part of the posterior 



