THE CRURAL MUSCLES. 649 



{aa) The Superficial Layer. 



I. Gastrocnemius. 2, Soleus. 



3. Plantaris. 



The main mass of the calf of the leg is formed by two muscles, the gastroc- 

 nemius and the soleus, which unite below in a common tendon, the ^e?ido Achillis 

 (tendo calcaneus), inserted into the posterior surface of the tuberosity of the calca- 

 neum, a bursa (bursa tendinis calcanei) intervening between the tendon and the upper 

 part of the tuberosity. Since the gastrocnemius arises by two heads, these two 

 muscles together are sometimes spoken of as the triceps sura. 



Gastrocnemius (Fig. 618). 



Attachments. — The gastrocnemius takes origin by two heads. The outer 

 head arises from the posterior surface of the femur, just above the lateral condyle, 

 by a short, strong tendon which sometimes contains a sesamoid cartilage ; the inner 

 head arises also by a short tendon just above the medial condyle of the femur. 

 Above, the two heads are separated from each other by a groove, but below they 

 unite to form a thick belly, the fibres of which pass over into a broad, flat tendon 

 inserted below with the tendo Achillis. 



Nerve-Supply. — By the internal popliteal (tibial) division of the greater sciatic 

 nerve from the first and second sacral nerves. 



Action. — To extend the foot and to assist in flexing the knee-joint. 



Relations. — The gastrocnemius is in relation by its posterior surface with the 

 short saphenous vein and nerve. On its deep surface it is in contact with the 

 plantaris and soleus muscles (Fig. 617), and in its upper part with the capsule of 

 the knee-joint, the popliteus, and the popliteal vessels and nerves. 



A bursa (bursa m. gastrocnemii medialis) intervenes between the inner head and 

 the capsule of the knee-joint, with the synovial cavity of which it is frequently 

 continuous; the bursa m. gastrocnemii lateralis frequently presents similar relations to 

 the outer head. 



Variations. — Absence of the entire muscle or of the outer head has been observed, but the 

 most frequent anomaly is the occurrence of a third head which arises from some portion of the 

 popliteal surface of the femur. 



2. Soleus (Fig. 619). 



Attachments. — The soleus is a broad, flat muscle which arises from the head 

 and upper posterior portion of the fibula, from the oblique line of the tibia, and from 

 a tendinous arch which passes across between the tibial and fibular origins. Its 

 fibres pass downward to a broad tendon which joins with the tendo Achillis below. 



Nerve-Supply. — By the internal popliteal (tibial) division of the greater sciatic 

 "nerve from the first and second sacral nerves. 



Action. — To extend the foot. 



3. Plantaris (Fig. 619). 



Attachments. — The plantaris is a small spindle-shaped muscle which passes 

 over into a long, slender tendon extending downward between the gastrocnemius 

 and soleus. The muscle arises from the femur, just above the outer condyle, internal 

 to the lateral head of the gastrocnemius, and from the adjacent part of the posterior 

 ligament of the knee-joint. The tendon traverses almost the entire length of the leg 

 and \s inserted either into the tuberosity of the calcaneum along with, but to the 

 inner side of, the tendo Achillis, sending also some fibres to the internal annular liga- 

 ment, or into the tendo Achillis itself. 



Nerve-Supply. — By the internal popliteal (tibial) division of the greater sciatic 

 nerve from the fifth lumbar and first sacral nerves. 



