THE POSTERIOR TIBIO-FIBVLAR REGION. 



437 



tibia and from the middle third of its internal border ; some fibres also arise from 

 a tendinous arch placed between the tibial and fibular origins of the muscle, be- 

 neath which the popliteal vessels and internal 

 popliteal nerve pass. The fibres pass back- 

 ward to an aponeurosis which covers the pos- 

 terior surface of the muscle, and this, gradually 

 becoming thicker and narrower, joins with the 

 tendon of the Gastrocnemius, and forms with 

 it the tendo Achillis. 



Relations. By its superficial surface, 

 with the Gastrocnemius and Plantaris ; by 

 its deep surface, with the Flexor longus 

 digitorum, Flexor longus hallucis, Tibialis 

 p'osticus, and posterior tibial vessels and 

 nerve, from which it is separated by the 

 transverse intermuscular septum or deep 

 transverse fascia of the leg. 



The Tendo Achillis, the common tendon 

 of the Gastrocnemius and Soleus, 1 is the 

 thickest and strongest tendon in the body. 

 It is about six inches in length, and com- 

 mences about the middle of the leg, but re- 

 ceives fleshy fibres on its anterior surface 

 nearly to its lower end. Gradually becoming 

 contracted below, it is inserted into the lower 

 part of the posterior surface of the os calcis, 

 a synovial bursa being interposed between 

 the tendon and the upper part of this surface. 

 The tendon spreads out somewhat at its lower 

 end, so that its narrowest part is usually about 

 an inch and a half above its insertion. The 

 tendon is covered by the fascia and the in- 

 tegument, and is separated from the deep 

 muscles and vessels by a considerable interval 

 filled up with areolar and adipose tissue. 

 Along its outer side, but superficial to it, is 

 the external saphenous vein. 



The Plantaris is an extremely diminutive 

 muscle placed between the Gastrocnemius 

 and Soleus, and remarkable for its long and 

 delicate tendon. It arises from the lower 

 part of the outer prolongation of the linea 

 aspera and from the posterior ligament of the 

 knee-joint. It forms a small fusiform belly, 

 about three or four inches in length, termi- 

 nating in a long slender tendon which crosses 

 obliquely between the two muscles of the calf, 

 and, running along the inner border of the 

 tendo Achillis, is inserted with it into the 

 posterior part of the os calcis. This muscle 

 is occasionally double, and is sometimes want- 

 ing. Occasionally, its tendon is lost in the internal annular ligament or in the 

 fascia of the leg. 



Nerves. The Gastrocnemius is supplied by the first and second sacral nerves, 



1 These two muscles with a common tendon are by some anatomists classed togethei\as one muscle, 

 the Triceps surce, the two heads of origin of the Gnstrocnemius and the Soleus constituting the three 

 heads of the Triceps, and the tendo Achillis the single tendon of insertion. 



Tendons of 



PERONEUS LONGUS 



and BREVIS. 



FIG. 259. Muscles of the back of the leg. 

 Superficial layer. 



