532 THE MUSCLES AND FASCIA 



gradually becoming thicker and narrower, joins with the tendon of the Gastroc- 

 nemius, 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 posticus, 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 (tendo calcaneus), 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 commences about the middle of the leg, but receives fleshy 

 fibres on its anterior surface nearly to its lower end. Gradually becoming con- 

 tracted below, it is inserted into the lower part of the posterior surface of the 

 os calcis, a synovial bursa (bursa tendinis calcanei [Achillis]) (Fig. 395) 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 integument, a bursa (bursa subculanea calcaned) (Fig. 395) being often inter- 

 posed between the tendon and the fascia. The tendon is separated from the deep 

 muscles and vessels by a considerable interval filled up with areolar and adipose 

 tissues. Along its outer side, but superficial to it, is the external saphenous vein. 



The Plantaris (m. 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, terminating 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 wanting. Occasion- 

 ally 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, and the 

 Plantaris by the fourth and fifth lumbar and first sacral nerves through the internal popliteal. 

 The Soleus is supplied by the fifth lumbar and first and second sacral nerves through the internal 

 popliteal and posterior tibial. 



Actions. The muscles of the calf are the chief Extensors of the foot at the ankle-joint. They 

 possess considerable power, and are constantly called into use in standing, walking, dancing, 

 and leaping; hence, the large size they usually present. In walking, these muscles draw power- 

 fully upon the os calcis, raising the heel, and with it the entire body, from the ground; the body 

 being thus supported on the raised foot, the opposite limb can be carried forward. In standing, 

 the Soleus, taking its fixed point from below, steadies the leg upon the foot, and prevents the 

 body from falling forward, to which there is a constant tendency from the superincumbent 

 weight. The Gastrocnemius, acting from below, serves to flex the femur upon the tibia, assisted 

 by the Popliteus. The Plantaris is the rudiment of a large muscle which exists in some of the 

 lower animals and is continued over the os calcis to be inserted into the plantar fascia. In man 

 it is an accessory to the Gastrocnemius, extending the ankle if the foot is free, or bending the 

 knee if the foot is fixed. Possibly, acting from below, by its attachment to the posterior liga- 

 ment of the knee-joint, it may pull that ligament backward during flexion, and so protect it 

 from being compressed between the two articular surfaces. 



The Deep Layer (Fig. 397). 



Popliteus. Flexor longus digitorum. 



Flexor longus hallucis. Tibialis posticus. 



Dissection. Detach the Soleus from its attachment to the fibula and tibia, and turn it down- 

 ward, when the deep layer of muscles is exposed, covered by the deep transverse fascia of the leg. 



1 These two muscles with a common tendon are by some anatomists classed together as one muscle, the Triceps 

 surae, the two heads of origin of the Gastrocnemius and the Soleus constituting the three heads of the Triceps, 

 and the tendo Achillis the single tendon of insertion. 



