WALKING, SWIMMING, AND FLYING. 535 



slightly forward. Some momentum is thus obtained, the limb be- 

 ing simultaneous] y advanced. " The throwing forward of the body 

 may be said to inaugurate the movement of walking." The same 

 occurs with a horse, but, if attached to a load, great impetus is attained 

 by the body before either of the limbs is lifted. Momentum thus 

 relieves muscular strain in the limbs and economizes force. How com- 

 pletely this principle is applied in swimming and flying will be pres- 

 ently noticed. 



In ordinary walking of man or quadruped, the limbs swing forward 

 without muscular effort. According to Prof. Weber, they swing by 

 the force of gravity as a pendulum, and obey the same laws. If sus- 

 pended they oscillate freely, and gravity brings them to a position of 

 rest. How much the muscles are saved from exhaustion in ordinary lo- 

 comotion, by gravity, becomes obvious when we attempt to overcome it 

 by climbing or leaping. The foot being upon the ground, the limb 

 rotates upon it as an axis, carrying the body forward and slightly ele- 

 vating it, but the elevation is in the arc of a circle, and when the 

 other foot reaches the ground the body is slightly lowered. Thus in 

 progression the trunk continually rises and falls ; it really undulates 

 along a given line. 



But other motions than those referred to are developed in the act 

 of walking. «The movements of the arms and feet are complementary ; 

 the right foot and left arm advance together, and vice versa. This 

 begets a diagonal movement which produces oscillation or twisting of 

 the trunk, which is excessive in awkward walkers. To repress this 

 oscillatory swing is indispensable, if great velocity is to be attained. 

 Trained runners flex their arms and hold them steadily at their great- 

 est speed, and every school-boy does the same instinctively, without 

 considering why the act is important. The swiftest-running birds 

 have small wings; those of the ostrich are scarcely more than rudi- 

 mental. 



Fig. 8. 



I 



Swimming of the Fisu— (After Borelli.) 



The diagonal twist or movement referred to is expressed in a 

 spiral wave of motion which traverses the trunk in the direction of its 

 length. This motion is obvious in fishes in the act of swimming. It 

 is a resultant of motions, which are in all vertebrates essentially the 

 same. In the walking of a cat or panther, this wave of curvature is 

 continuous along the spine. It is really a lateral undulation. We will 



