slightly forward. Some momentum is thus obtained, the limb being simultaneously 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 completely this principle is applied in swimming and flying will be presently 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 suspended they oscillate freely, and gravity brings them to a position of rest. How much the muscles are saved from exhaustion in ordinary locomotion, 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 elevating 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 greatest 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 rudimental.
Fig. 8.
Swimming of the Fish (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
