HOW ANIMALS MOVE. 



165 



The order in which the legs of Quadrupeds succeed 

 each other determines the various modes of progression, 

 called the walk, trot, gallop, and leap. Many, as the 

 Horse, have all these movements ; while some only leap, 

 as the Frog and Kangaroo. In leaping animals, the hind 

 limbs are extraordinarily developed. In many Mammals, 

 like the Squirrel, Cat, and Dog, the fore legs are used for 

 prehension as well as locomotion. Monkeys use all four, 



Fig. 130.— Muscles of the Human Leg: 

 sartorius, or "tailor's muscle," the 

 longest muscle in the body, flexes the 

 leg upon the thigh; rectus fwnoris 

 and vastus externus and inter nus 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. 131 — Muscles of an Insect's Leg 

 (Melolontha vulgaris) : a, flexor, and 

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

 d, accessory muscle; e, extensor of 

 claw; /, 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- 

 ton. 



