50 MECHANISM OF EQUINE LOCOMOTION 



" the parallelogram of forces." Thus, if the line a b in Fig. 36 represents the 

 horizontal force (derived from the speed) and ac the vertical one (obtained 

 from the right leg), and if we draw ^^ parallel to ab, and ^^ parallel to ac, 

 we shall have the resultant force represented by the line a d, and the angle 

 of elevation equal to the angle dab. 



The upward motion given by the limb to the body is necessary to keep up 

 the centre of gravity, which, if we wish the labour to be accomplished with a 

 minimum amount of muscular effort, should be maintained as nearly as 

 possible at one uniform height from the ground; for the distance through 

 which the centre of gravity is moved will be a measure of the work done. 

 Let us suppose at each step of a yard long by a horse, that the centre of 

 gravity falls 4 inches, and that the animal has to go a distance of 1000 yards 

 on a horizontal plane. It is evident that, in this case, the muscles of the 

 horse's limbs would not alone have to carry the weight of the body 1000 

 yards, but would also have to raise it 333^ feet (1000 X -3), which would be 

 approximately equivalent to going over a hill that was 333I feet high and 



a & 



Fig. 36.— Angle of Projection of Centre of Gravity. 



had a base 1000 yards broad. I need hardly say that the less the fall and 

 rise at each step, the lower this suppositious hill would be, and, conse- 

 -quently, the easier it would be to walk over. 



Although the duty of forward propulsion is chiefly performed by the hind 

 limbs ; the greater part of the work of adjusting the position of the centre of 

 gravity during ordinary locomotion, falls on the fore limbs. When a horse, 

 for instance, performs the high-school feat of cantering to the rear, the 

 respective rdles would obviously be reversed. The question of the adjustment 

 of the centre of gravity of the body, so that the muscles of locomotion may 

 act to the best possible advantage, will be considered further on. 



Comparative Speed in the Action of the Limbs.— The speed 

 with which the body is projected forward, is directly proportionate to th^ 

 speed with which the limb or limbs are straightened out, and has nothing to 

 do with the strength of the muscles that move the parts. Hence, any excess 

 of muscular development beyond that required for the due working of the 

 limbs, will tend to diminish the speed by unnecessarily adding to the weight 

 to be carried. For this reason, we never see great race-horses of the weight- 

 carrying hunter build. Some of the best (St. Simon and Tim Whififler, for 



