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, 
Fio. 130.—Muscles of the Human Leg: 
sartorius, or “tailor’s muscle,” the 
longest muscle in the body, flexes the 
leg upon the thigh; rectus femoris 
and vastus 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. 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 tlje muscles are within the skele¬ 
ton. 
