HOW ANIMALS MOVE. 163 
hop. The Ostrich can for a time outrun the Arabian 
Horse; and the speed of the Cassowary exceeds that of 
the swiftest Greyhound. 
Fic. 129.—Muscles of the Human Leg: 
sartorius, or ‘‘tailor’s muscle,’ the 
longest muscle in the body, flexes 
the leg upon the thigh; rectus femo- 
ris and vastus externus and internus 
extend the leg, maintaining an erect 
posture; gastrocnemius, or ‘‘calf,” 
used chiefly in walking, for raising 
the heel. Another layer underlies 
these superficial muscles. 
Fig. 130.—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; jf, 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. 
