208 STUDIES IN PHYSIOLOGY 



flight of bats is much less rapid, however, than that of 

 birds. 



Locomotion on Land. All vertebrates living on land, with 

 the exception of snakes and a few rare lizards and amphi- 

 bia, are provided with two pairs of appendages. The legless 

 snakes move forward by pushing with the posterior edge of 

 their ventral scales, something as the earthworm uses its 

 bristles. If compelled to move in a straight line their 

 progress is slow; but by curving the body from side to 

 side (as the fish moves its tail) they can glide along with 

 considerable rapidity. 



Most flying birds (robins, sparrows) use their posterior 

 appendages for perching on a support or for hopping and 

 walking along the ground. Running birds (ostriches), on 

 the other hand, and most domestic fowls (hens, ducks, 

 geese) run about on their legs much like the human being. 



The hind legs of the four-footed mammals are used 

 mainly for pushing the body forward, the front legs serving 

 rather as a means of support for the head end. In dray 

 horses, the thigh muscles have a great development, while 

 in the animals that spring for their prey (lions, cats, tigers) 

 the extensors in the calf of the leg are highly developed. 



