266 POSTERIOR TIBIAL REGION. 



space down the middle of the posterior part of the leg to the heel, 

 bounding it inferiorly by a transverse incision passing between the 

 two malleoli. Turn aside the flaps of integument and remove the 

 fasciae from the whole of this region ; the gastrocnemius muscle will 

 then be exposed. 



The GASTROCNEMIUS (ya^oxv^/ay, the bellied part of the leg) 

 arises by two heads from the two condyles of the femur, the inner 

 head being the longest. They unite to form the beautiful muscle so 

 characteristic of this region of the limb. It is inserted, by means of 

 the tendo Achillis, into the lower part of the posterior tuberosity of 

 the os calcis, a synovial bursa being placed between that tendon 

 and the upper part of the tuberosity. The gastrocnemius must be 

 removed from its origin, and turned down, in order to expose the next 

 muscle. 



Relations. By its superficial surface with the deep fascia of the leg 

 which separates it from the external saphenous vein, and with the ex- 

 ternal saphenous nerve. By its deep surface with the lateral portions 

 of the posterior ligament of the knee-joint, the popliteus, plantaris, and 

 soleus. The internal head of the muscle rests against the posterior 

 surface of the internal condyle of the femur ; the external head against 

 the outer side of the external condyle. In the latter a sesamoid bone 

 is sometimes found. 



The PLANTARIS (planta, the sole of the foot), an extremely diminu- 

 tive muscle situated between the gastrocnemius and soleus, arises from 

 the outer condyle of the femur ; and is inserted, by its long and deli- 

 cately slender tendon, into the inner side of the posterior tuberosity of 

 the os calcis, by the side of the tendo Achillis ; having crossed ob- 

 liquely between the two muscles. 



The SOLEUS (solea, a sole), is the broad muscle upon which the 

 plantaris rests. It arises from the head and upper third of the fibula, 

 from the oblique line and middle third of the tibia. Its fibres con- 

 verge to the tendo Achillis, by which it is inserted into the posterior 

 tuberosity of the os calcis. Between the fibular and tibial origins of 

 this muscle is a tendinous arch, beneath which the popliteal vessels and 

 nerve pass into the leg. 



Relations. By its superficial surface with the gastrocnemius and 

 plantaris. By its deep surface with the intermuscular fascia, which 

 separates it from the flexor longus digitorum, tibialis posticus, flexor 

 longus pollicis, from the posterior tibial vessels and nerve, and from the 

 peroneal vessels. 



Actions. The three muscles of the calf draw powerfully on the os 

 calcis, and lift the heel ; continuing their action, they raise the entire 

 body. This action is attained by means of a lever of the second 

 power, the fulcrum (the toes) being at one end, the weight (the body 

 supported on the tibia) in the middle, and the power (these muscles) 

 at the other extremity. 



They are, therefore, the walking muscles, and perform all move- 

 ments that require the support of the whole body from the ground, as 



