HOW ANIMALS MOVE 



375 



that is, farther forward. Birds and men are the only 

 true bipeds, the former standing on their toes, the latter 



FIG. 327. Muscles of the Human Leg: 

 sartorzus, or " tailor's muscle," the 

 longest muscle in the body, flexes the 

 leg upon the thigh: rectus femoris 

 and i>astus externus and internus ex- 

 tend the leg, maintaining an erect 

 posture; gastrocnemius, or "calf," 

 used chiefly in walking, for raising the 

 heel. Another layer underlies these 

 superficial muscles. 



FIG. 328. Muscles of an Insect's Leg 

 (Melolontha vulgaris): a, flexor, and 

 b, extensor, of tibia; c, flexor of foot; 

 </, accessory muscle ; e , extensor of 

 claw; f, extensor of tarsus. The 

 joints are restricted to movements 

 in one plane; and therefore the mus- 

 cles are simply flexors and extensors. 

 All the muscles are within the skele- 



on the soles of the feet. Terrestial birds walk and run ; 

 while birds of flight usually hop. The ostrich can for a 

 time outrun the Arabian horse ; and the speed of the 

 cassowary exceeds that of the swiftest greyhound. 



