664 LOCO MOTOR MECHANISMS. [BOOK m. 



bat also a series of horizontal curves, inasmuch as at each step the 

 line of gravity is made to fall alternately on each standing foot. 

 While the left leg is swinging, the line of gravity falls within the 

 area of the right foot, and the centre of gravity is on the right side 

 of the pelvis. As the left foot becomes the standing foot, the 

 centre of gravity is shifted to the left side of the pelvis. The 

 actual curve described by the centre of gravity is therefore a 

 somewhat complicated one, being composed of vertical and 

 horizontal factors. The natural step is the one which is de- 

 termined by the length of the swinging leg, since this acts as 

 a pendulum; and hence the step of a long-legged person is 

 naturally longer than that of a person with short legs. The 

 length of the step however may be diminished or increased by a 

 direct muscular effort, as when a line of soldiers keep step in spite 

 of their having legs of different lengths. Such a mode of marching 

 must obviously be fatiguing, inasmuch as it involves an unnecessary 

 expenditure of energy. 



In slow walking there is an appreciable time during which, 

 while one foot is already in position to serve as a fulcrum, the 

 other, swinging, foot has not yet left the ground. In fast walking 

 this period is so much reduced, that one foot leaves the ground the 

 moment the other touches it ; hence there is practically no period 

 during which both feet are on the ground together. 



When the body is swung forward on the one foot acting as a 

 fulcrum with such energy that this foot leaves the ground before 

 the other, swinging, foot has reached the ground, there being 

 an interval during which neither foot is on the ground, the person 

 is said to be running, not walking. 



In jumping this propulsion of the body takes place on both feet 

 at the same time ; in hopping it is effected on one foot only. 



The locomotion of four-footed animals is necessarily more com- 

 plicated than that of man. The simple walk, such as that of the 

 horse, is executed in four times, with a diagonal succession : thus, 

 right fore leg, left hind leg, left fore leg, right hind leg. In the 

 amble, such as that of the camel, the two feet of the same side are 

 put down at one and the same time, this movement being followed 

 by a similar movement of the other two legs ; it corresponds there- 

 fore very closely to human walking. In the trot, which corresponds 

 to human running, the two diagonally opposite feet are brought to 

 the ground at the same time, and the body is propelled forwards on 

 them. Concerning this however, as well concerning the still more 

 complicated gallop and canter observers are not agreed and much 

 discussion has arisen. 



The other problems connected with the action of the various 

 skeletal muscles of the body are too special to be considered here. 



