WALKING, SWIMMING, AND FLYING. 



531 



cellent illustration. Here we have not only the sharp angles, but lever- 

 like adjustment of the several bones. 



From these arises the power possessed by many animals to bound 

 or leap enormous distances. The kangaroo has been known to leap 

 twenty feet. The jerboa, when pursued, will pass over nine feet at a 

 bound, and repeats so rapidly that a swift horse can scarcely overtake 

 it. The greyhound and the hare will pass over sixteen feet at a stride. 

 Animals of great weight and moderate speed have nearly straight 

 limbs. Those of the deer are more angular than those of a horse, and 

 those in the wing of a bird more angular than those of the fleetest 

 quadruped. 



Fig. 3. 



Skeleton of the Deeb (after Pander and d' Alton). The bones in the extremities of this, the 

 fleetest of quadrupeds, are inclined very obliquely toward each other, and toward the scapular 

 and iliac bones. This arrangement increases the leverage of the muscular system, and confers 

 tfreat rapidity on the moving parts. It augments elasticity, diminishes shock, and indirectly 

 begets continuity of movement. 



The forms of joints which predominate in the animal kingdom are 

 the hinge and the ball and socket. The latter gives to the extremities 

 their extraordinary range of motion, and a power of rotation so indis- 

 pensable, as we will see, to the effectiveness of all the organs of loco- 

 motion. 



It has been shown that a spiral configuration occurs in the bones 

 and joints of the wing of the bat and the bird, and in the extremities 

 of most quadrupeds. " The bones of animals are, as a rule, twisted 

 levers, and act after the manner of screws." Thus it is that their 

 traveling surfaces in progression may be turned at almost any angle, 

 getting from the resisting media in which they move as much pro- 

 pelling power as possible, with a minimum of slip or waste. 



It is because the traveling surfaces of animals " are screws struct- 



