834 HUMAN ANATOMY. 



above its external condyle, embedded in the substance of the vastus externus, dividing finally 

 into branches which take part in the formation of the circumpatellar anastomoses. The termi- 

 nation of the internal branch is similar, and its course is beneath the semimembranosus and 

 through the tendon of the adductor magnus into the substance of the vastus internus. 



{c) The internal inferior articular branch (a. genu inferior medialis) arises about opposite 

 or a little above the line of the tibio-femoral articulation and courses downward and inward 

 over the surface of the popliteal muscle, beneath the inner head of the gastrocnemius. It passes 

 beneath the internal lateral ligament of the knee-joint and winds around the tuberosity of the 

 tibia to join the circumpatellar anastomosis. 



{d ) The external inferior articular branch (a. genu inferior lateralis) arises a little lower down 

 than its fellow and passes almost transversely outward, at first beneath the external head of the 

 gastrocnemius and the plantaris, and winds around the outer tuberosity of the tibia, beneath 

 the long internal lateral ligament of the knee-joint, to join the circumpatellar anastomosis. 



{e) The azygos articular branch (a. genu media) is the smallest of all the articular branches. 

 It arises either from the anterior surface of the popliteal or from the external superior articular 

 branch, and pierces the posterior ligament of the knee-joint to be distributed to the crucial, 

 mucous, and alar ligaments. 



The circumpatellar anastomosis (rete patellae) (Fig. 732) is a rich net-work 

 of vessels which occurs in the superficial fascia surrounding the patella, and from which 

 branches are sent to the patella, the capsule of the knee-joint, and the neighboring 

 muscles. The following vessels take part in its formation. From above, the anasto- 

 motica magna from the femoral and the descending branch of the external circumflex ; 

 from the sidcs, the internal and external superior and the internal and external inferior 

 articular branches of the popliteal and the muscular branches of the same artery ; and 

 from below, the anterior tibial recurrent. 



3. The Cutaneous Branches. — These are variable in origin and number and 

 are distributed to the integument covering the popliteal space and the upper part of 

 the calf of the leg. One of them occasionally attains a considerable size and is 

 termed the posterior saphenous artery. It accompanies the short saphenous vein 

 down the back of the cms, sending off branches to the adjacent integument. 



The Collateral Circulation of the Popliteal Artery. — The passage of 

 blood to the leg after ligation of the popliteal artery is effected by means of the rich 

 anastomosis which exists around the knee-joint, and in which the branches of the 

 popliteal take part. In addition to these, however, it also receives from above the 

 anastomotica magna, the descending branch of the external circumflex, and the 

 terminal portion of the profunda artery, while there pass to it from below the superior 

 fibular and the anterior and posterior tibial recurrent arteries. 



THE POSTERIOR TIBIAL ARTERY. 



The posterior tibial artery (a. tibialis posterior) (Fig. 736) is the direct con- 

 tinuation of the popliteal down the posterior surface of the leg. It begins at the 

 bifurcation of the popliteal at the lower border of the popliteus muscle and passes 

 almost vertically downward, under cover of the more superficial muscles of the calf, 

 to the groove between the inner malleolus and the os calcis, where, opposite the tip 

 of the malleolus, it terminates by dividing into the internal and external plantar 

 arteries. Its course may be indicated by a line drawn from the centre of the popli- 

 teal space to a point midway between the inner malleolus and the os calcis. 



Relations. — A)iterior/v, the artery rests in succession, from above downward, 

 upon the tibialis posticus, the flexor longus digitorum, the posterior surface of the 

 lower part of the tibia, and the internal lateral ligament of the ankle-joint. It is 

 closely bound down to the muscles upon which it rests by the layer of the deep fascia 

 which covers them, the thickness and density of this fascia increasing towards the 

 lower part of the leg. Posteriorly, it is covered by the soleus and gastrocnemius 

 throughout the greater part of its course, but in the lower third of the leg it is super- 

 ficial, being covered only by the skin and fascise, except just before its termination, 

 where it lies beneath the internal annular ligament and the origin of the abductor 

 hallucis. A short distance below its commencement it is crossed obliquely, from 

 within outward, by the posterior tibial nerve. Internally, it is in relation with the 

 posterior tibial nerve for a short distance above, and in the malleolar groove it has 



