xiv THE MUSCULAR SYSTEM 253 



of the tibio-fibula ; the other head joins the tendon of the 

 triceps. The muscle tapers distally where it is inserted by 

 a very strong tendon, which passes over the ankle joint and 

 spreads over the plantar surface of the foot. It acts as a 

 flexor of the leg and an extensor of the foot. 



The tibialis posticus lies on the posterior side of the leg 

 beneath the gastrocnemius. It arises from the greater por- 

 tion of the length of the tibio-fibula and ends in a tendon, 

 which, running in a groove on the inner side of the distal 

 end of the tibia, passes between the ends of the tibio-fibula 

 and the bones of the ankle to be inserted into the anterior 

 side of the proximal end of the tibiale. When the foot is 

 flexed, it acts as an extensor and pronator, and when the 

 foot is fully extended, it flexes it to within about 45 of a 

 continuation of the long axis of the cms. 



The tibialis anticus longus is a narrow muscle on the 

 front of the cms. It arises by a long, narrow tendon from 

 the distal end of the femur. Distally it divides into two 

 parts which are inserted into the proximal ends of the tibi- 

 ale and fibulare. It acts as an extensor of the leg and a 

 flexor of the foot. 



The peroncus lies on the postaxial side of the preceding 

 muscle, partly covered by gastrocnemius. It arises by a 

 narrow tendon from the distal end of the femur. One part 

 of the muscle is inserted into the distal end of the fibula, 

 the other passes over the ankle joint and joins the outer 

 angle of the head of the fibulare. The muscle acts to 

 extend the leg ; when the foot is partially extended, it acts 

 to extend it still farther, and pronates it, i.e. turns its plan- 

 tar surface downward ; when the foot is strongly flexed, it 

 acts as a flexor, bringing it up close to the cms ; it also pulls 

 the ankle postaxially. 



The extensor cruris is a slender muscle lying on the 



