264 THE MUSCLES OF THE LOWER LIMB. 



Relations. The soleus rests upon the flexor longus hallucis, flexor longus digitorum, and 

 tibialis posticus muscles, together with the posterior tibial vessels and nerve, from which, 

 however, it is separated by the deep aponeurosis. 



Varieties. To the soleus an accessory portion is occasionally added at its lower and inner 

 part ; this usually ends on the inner side of the tendo Achillis, but it is sometimes attached 

 separately to the os calcis, or to the internal annular ligament. 



The tibial head of the soleus is almost peculiar to man : among the lower animals it 

 occurs, of small size, only in the gorilla and sometimes in the chimpanzee. 



The tendo Achillis, the thickest and strongest tendon in the body, is formed 

 by the union of the flat tendons of the gastrocnemius and soleus. Broad at its 

 commencement near the middle of the leg, it contracts as it proceeds downwards 

 and becomes thicker to within about an inch and a half of the heel ; it then expands 

 slightly to be inserted into the middle part of the posterior surface of the tuberosity 

 of the os calcis. A synovial bursa is interposed between the upper part of the tube- 

 rosity and the tendon. 



The two heads of the gastrocnemius and the soleus, with their common tendon 

 of insertion, constitute the triceps extensor surce. 



The plantaris muscle arises from the femur immediately above the external 

 condyle, and from the posterior ligament of the knee-joint ; its muscular part is from 

 three to four inches in length, and terminates in a long slender tendon, which 

 inclines inwards between the gastrocnemius and soleus, and, running along the 

 inner border of the tendo Achillis, is inserted by the side of that into the posterior 

 part of the calcaneum. 



Varieties. The plantaris varies in its mode of termination ; it frequently joins the tendo 

 Achillis, or ends in the deep fascia of the leg, or in the internal annular ligament. Its tendon 

 is sometimes enclosed in the lower part of the tendo Achillis. It is often absent (7*5 per cent., 

 Gruber). 



Like the palmaris longus, this muscle, which is little developed in man, is the remains of a 

 superficial flexor of the digits. In many animals it is of large size, and is continued over the 

 calcaneum into the plantar fascia. It is, however, generally wanting in the anthropoids. 



The DEEP GROUP of posterior muscles of the leg is in close contact with the 

 bones ; it consists of the popliteus, flexor longus digitorum, flexor longus hallucis, 

 and tibialis posticus. 



The popliteus, a short muscle placed below the knee, arises by a rounded 

 tendon, about an inch in length, from the lower part of the groove on the outer 

 surface of the external condyle of the femur, within the external lateral ligament and 

 capsule of the knee-joint : it is in contact with the external semilunar fibre-cartilage, 

 and receives additional fibres from the posterior ligament of the joint. The 

 muscular fibres diverge as they pass downwards and inwards, and are inserted into 

 the triangular surface of the tibia above the oblique line, and into the aponeurosis 

 covering the muscle. Its tendon occupies the groove on the femur only when the 

 knee-joint is fully flexed. 



Relations. The popliteus is bound down by an aponeurosis, principally derived from the 

 tendon of the semimembranosus muscle. The plantaris and gastrocnemius muscles, the 

 popliteal vessels and internal popliteal nerve lie upon its posterior surface. The synovial 

 membrane of the knee-joint sends a prolongation downwards between its tendon and the back 

 of the outer tuberosity of the tibia. 



Varieties. This muscle has been seen with an additional head of origin from the 

 sesamoid bone in the outer head of the gastrocnemius. The pojrtitcHs wlnor is a muscular 

 slip of rare occurrence, arising from the femur on the inner side of the plantaris. and inserted 

 into the posterior ligament of the knee-joint. Th& perotyo^ibiali* is a small muscle found by 

 W. Gruber in the proportion of 1 in 7 in a large number of subjects. It arises from the inner 

 side of the head of the fibula, and is inserted into the upper end of the oblique line of the 

 tibia, being situated beneath the popliteus. It is constant in apes. (Arch. f. Anat., 1877, p. 

 401, and 1878, p. 484.) 



