1887.] NATURAL SCIENCES OF PHILADELPHIA. 63 



is moving, or in the case of the terrestrial animal, the surface of the 

 ground. 



The resistance of the air and the water are so much less than 

 that of the earth that the acts of flying and of swimming become 

 radically different from those of walking, of running, or of any 

 allied movement. In flying and swimming the resistance made by 

 the limb against the medium in effecting an impetus does not arrest 

 the movement of the pinion or the foot ; whereas in terrestrial 

 movements the instant that the foot strikes the earth the resistance 

 is great and the arrest is complete. 



In the swimming turtle the first stage of the recover drives the 

 foot in spite of the resistance of the water to the point at which the 

 second stage begins. With some slight modifications the same is 

 true of fossorial animals. Thus in flying, in swimming, and in 

 burrowing the limb describes a continuous movement which unites 

 the path of the stroke to that of the recover. In the animal moving 

 on the surface of the ground, the foot being brought to rest, an 

 absolute break occurs between the beginning of the act of recover 

 and its completion, the time which would be required to describe 

 the interval and thus to complete the union corresponds to the period 

 that the foot is on the ground. This weriod constitutes the stroke. 



The limb rests on the ground until the trunk moves beyond the 

 point at which it can maintain itself It is lifted at intervals which 

 are dependent upon the momentum of the moving mass. One, two^ 

 or three limbs may be on the ground at the same time. The rates 

 at which the succession of the foot-falls occur, in their turn, depend 

 not only upon the rate of sjieed at which the animal is moving, 

 but on the gait as well. 



KINDS OF WORK DONE BY THE LIMBS. 



The kinds of work done by the limbs are two in number, viz., that 

 done by the fore limbs and that done by the hind limbs. The hind 

 limbs are more powerful than the fore limbs, and in some animals, 

 as the kangaroo and the jumping mouse, are the main eflectives. 

 No terrestrial animal depends for support upon the fore limbs. 

 AVhen all the limbs are equal or nearly equal in length, the prepon- 

 derance is still in favor of the hind limbs owing to the ftict that the 

 great backward movement of these limbs on the trunk is made possible 

 by the fixation of the bones to the pelvis and through this structure 

 to the vertebral column. Not only is this the case but the hind limbs 

 alone possess the power of propelling the body so as to throw upon the 



