794 THE NERVOUS SYSTEM. 



lower third of the thigh. When the division occurs at the plexus, the two nerves 

 descend together side by side ; or they may be separated, at their commencement, 

 by the interposition of part or the whole of the Pyriformis muscle. As the nerve 

 descends along the back of the thigh, it rests upon the posterior surface of the 

 ischium, the nerve to the Quadratus femoris, and the External rotator muscles, in 

 company with the small sciatic nerve and artery, being covered by the Gluteus 

 maximus ; lower down, it lies upon the Adductor magnus, and is covered by the 

 long head of the Biceps. 



The branches of the nerve, before its division, are articular and muscular. 



The articular branches arise from the upper part of the nerve ; they supply the 

 hip-joint, perforating the posterior part of its fibrous capsule posteriorly. These 

 branches are sometimes derived from the sacral plexus. 



The muscular branches are distributed to the flexors of the leg : viz., the 

 Biceps, Semitendinosus, and Semimembranosus, and a branch to the Adductor 

 magnus. These branches are given off beneath the Biceps muscle. 



The Internal Popliteal Nerve, the larger of the two terminal branches of the 

 great sciatic, descends along the back part of the thigh, through the middle of the 

 popliteal space, to the lower part of the Popliteus muscle, where it passes with the 

 artery beneath the arch of the Soleus and becomes the posterior tibial. It is over- 

 lapped by the hamstring muscles above, and then becomes more superficial, and 

 lies to the outer side of, and some distance from, the popliteal vessels ; opposite 

 the knee-joint it is in close relation with the vessels, and crosses to the inner side of 

 the artery. Below, it is overlapped by the Gastrocnemius. 



The branches of this nerve are articular, muscular, and a cutaneous branch, 

 the communicans tibialis nerve. 



The articular branches, usually three in number, supply the knee-joint : two of 

 these branches accompany the superior and inferior internal articular arteries, and 

 a third, the azygos articular artery. 



The muscular branches, four or five in number, arise from the nerve as it lies 

 between the two heads of the Gastrocnemius muscle ; they supply that muscle, 

 the Plantaris, Soleus, and Popliteus. The filaments which supply the Popliteus 

 turn round its lower border and are distributed to its deep surface. 



The communicans tibialis descends between the two heads of the Gastrocnemius 

 muscle, and about the middle of the back of the leg pierces the deep fascia, 

 and joins a communicating branch (communicans peronei) from the external 

 popliteal nerve to form the external or short saphenous (Fig. 422). The exter- 

 nal saphenous nerve, formed by the communicating branches of the internal and 

 external popliteal nerves, passes downward and outward near the outer margin of 

 the tendo Achillis, lying close to the external saphenous vein, to the interval 

 between the external malleolus and the os calcis. It winds round the outer mal- 

 leolus, and is distributed to the integument along the outer side of the foot and 

 little toe, communicating on the dorsum of the foot with the musculo-cutaneous 

 nerve. In the leg its branches communicate with those of the small sciatic. 



The Posterior Tibial Nerve (Fig. 423) commences at the lower border of the 

 Popliteus muscle, and passes along the back part of the leg with the posterior 

 tibial vessels to the interval between the inner malleolus and the heel, where it 

 divides into the external and internal plantar nerves. It lies upon the deep 

 muscles of the leg, and is covered in the upper part by the muscles of the calf, 

 lower down by the skin and fascia. In the upper part of its course it lies to the 

 inner side of the posterior tibial artery, but it soon crosses that vessel, and lies to 

 its outer side as far as the ankle. In the lower third of the leg it is placed 

 parallel with the inner margin of the tendo Achillis. 



The branches of the posterior tibial nerve are muscular, calcaneo-plantar, and 

 articular. 



The muscular branches arise either separately or by a common trunk from the 

 upper part of the nerve. They supply the Soleus, Tibialis posticus, Flexor longus 

 digitorum, and Flexor longus hallucis muscles ; the branch to the latter muscle 



