422 MUSCLES AND FASCIA. 



is separated by a synovial bursa, which, in some cases, communicates with the 

 cavity of the knee-joint. The tendon of the outer head contains a sesamoid 

 tibro-cartilage (rarely osseous), where it plays over the corresponding outer 

 condyle : and one is occasionally found in the tendon of the inner head. 



The Gastrocnemius should be divided across, just below its origin, and turned downwards, in 

 order to expose the next muscles. 



The Soleus is a broad flat muscle, situated immediately beneath the preceding. 

 It has received its name from its resemblance in shape to a sole-fish. It arises 

 by tendinous fibres from the back part ot the head of the fibula, and from the 

 upper half of the posterior surface of its shaft ; from the oblique line of the 

 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, and beneath which the posterior tibial vessels and nerve pass. The 

 fibres pass backwards to an aponeurosis which covers the posterior 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 Digitorurn, Flexor Longus Pollicis, 

 Tibialis Posticus, and posterior tibial vessels and nerve, from which it is sepa- 

 rated by the transverse intermuscular septum or deep fascia of the leg. 



The Tendo Achillis, the common tendon of the Gastrocnemius and Soleus, is 

 the thickest and strongest tendon in the body. It is about six inches in length, 

 and formed by the junction of the aponeuroses of the two preceding muscles. 

 It commences about the middle of the leg, but receives fleshy fibres on its an- 

 terior surface, nearly to its lower end. Gradually becoming contracted below, 

 it is inserted into the lower part of the posterior tuberosity of the os calcis, 

 a synovial bursa being interposed between the tendon and the upper part of the 

 tuberosity. The tendon spreads out somewhat at its lower end, so that its nar- 

 rowest part is usually about an inch and a half above its insertion. The tendon 

 is covered by the fascia and the integument, 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 saphe- 

 nous vein. 



The Plantaris is an extremely diminutive muscle, placed between the Gastroc- 

 nemius and Soleus, and remarkable for its long and delicate tendon. It arises 

 from the lower part of the outer bifurcation of the linea aspera, and from the 

 posterior ligament of the knee-joint. It forms a small fusiform belly, about 

 two inches in length, which terminates 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. 

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

 of the leg. 



Nerves. These muscles are supplied by the internal popliteal nerve. 



Actions. The muscles of the calf possess considerable power, and are con- 

 stantly called into use in standing, walking, dancing, and leaping; hence the 

 large size they usually present. In walking, these muscles draw powerfully 

 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 forwards. In standing, the Soleus, taking its fixed point from 

 below, steadies the leg upon the foot, and prevents the body from falling for- 

 wards, 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 serves as a tensor of the plantar 

 fascia. 



