836 HUMAN ANATOMY. 



internally and in front of it the tendon of the flexor longus digitorum, and internal to 

 that the tendon of the tibialis posticus. Externally, the posterior tibial nerve accom- 

 panies it throughout the greater portion of its course, and at the ankle-joint the nerve 

 lies external and posterior to the artery, between it and the tendon of the flexor longus 

 hallucis. The artery is accompanied throughout its course by two venae comites 

 which lie respectively to its outer and inner side. 



Branches. — In addition to numerous muscidar braiiches which are distributed 

 to the neighboring muscles, and cutaneous branches to the inner and posterior sur- 

 faces of the leg, the posterior tibial gives origin to ( i ) a nutrient branch to the tibia, 

 (2) the peroneal artery, (3) a coninuinicating branch, (4) an internal malleolar 

 branch, (5) an internal calcaneal and the two terminal branches, (6) the internal, 

 and (7) the external plantar arteries. 



Variations. — Although apparently the principal artery of the flexor surface of the leg and 

 the direct continuation of the popliteal, developmentally the posterior tibial is a secondary 

 vessel, the original main vessel being the peroneal. The history of the posterior tibial seems to 

 have been somewhat as follows. The saphenous artery, whose origin has been mentioned in 

 connection with the variations of the femoral artery (page 823), in the lower part of the leg 

 winds around to the posterior surface and passes behind the internal malleolus, where it termi- 

 nates by dividing into the plantar arteries. From the upper part of the peroneal artery a branch 

 arises which passes down the tibial side of the leg, beneath the superficial flexor muscles, and 

 at the internal malleolus anastomoses with the saphenous. This vessel is the posterior tibial, 

 and, its calibre enlarging, exceeds that of the peroneal, which thus sinks to the rank of a branch 

 of the artery to which it gave birth. A reason for this increase of calibre in the posterior tibial 

 is to be found in the degeneration of the saphenous artery (page 849), whereby the tibial be- 

 comes the channel of supply for the plantar arteries, which seem to be its continuation. 



The majority of the principal variations of the posterior tibial are readily explained in the 

 light of such a history. Thus there may be no posterior tibial, or it may be represented by a 

 small vessel whose distribution is confined to the upper part of the leg. In such a case, as 

 the saphenous artery degenerates, anastomoses between it and the terminal portion of the pero- 

 neal may enlarge so that the plantar arteries come to take their origin from that vessel. Or, 

 again, the development of the posterior tibial may proceed normally, but the lower portion of 

 the saphenous may not degenerate completely, but persists, as has been observed, as a branch 

 of the tibial, passing upward upon the leg in company with the long saphenous nerve. 



Other variations of the posterior tibial which have been observed, however, cannot appar- 

 ently be explained as resulting from modifications of the normal course of development, but are 

 rather to be regarded as progressive variations due to the enlargement of what are usually more 

 or less insignificant anastomoses. Of this nature is the origin from the posterior tibial, at about 

 the middle of the leg, of a branch which pierces the interosseous membrane and divides into 

 an ascending and a descending branch, which together represent the anterior tibial artery. Or, 

 again, the posterior tibial has been observed to perforate the lower part of the interosseous 

 membrane and to be continued down the dorsum of the foot as the dorsalis pedis artery, 

 the plantar arteries arising from the peroneal. Occasionally, also, the posterior tibial may 

 terminate by inosculating with the peroneal, probably by the enlargement of the communicating 

 branch, the peroneal in this case also giving rise to the plantar arteries. 



The high and low origins of the posterior tibial have already been mentioned in connection 

 with the variations of the popliteal (page 831). 



Practical Considerations. — The posterior tibial artery on account of its deep 

 position beneath the large superficial calf muscles is rarely wounded and, by reason 

 of the support which it receives in its upper two-thirds from those muscles and the 

 deeper muscular layer on which it lies, and in its lower third from the dense fascia 

 covering it, it is seldom the subject of aneurism. Except for a short portion of its 

 course immediately above the ankle, it is separated from the tibia by the deep calf 

 muscles, and is therefore not often involved in fractures of that bone. 



The bifurcation of the popliteal is not infrequently the region at which an em~ 

 bolus carried down from the popliteal is arrested, and such a clot may block both 

 the tibial arteries. Their free anastomosis prevents gangrene if only one of them is 

 occluded ; but if both are involved, and especially if the succeeding additions to the 

 clot invade the anterior tibial recurrent — interfering with anastomosis from above — 

 gangrene almost certainly follows. 



Co7npression of the posterior tibial is scarcely possible abo\'e its lower third. 

 Above the ankle and behind the inner malleolus it may be flattened against the tibia 

 by pressure directed outward and a little forward. 



Ligation of the posterior tibial mav be done at any part of its course, but in its 

 upper third is an operation of some difficulty. 



