64 



MECHANISM OF EQUINE LOCOMOTION. 



Direction of Propulsion, and Distance through which 

 the Centre of Gravity of the Body is Moved. — The 



direction of tlie propulsion given bj' a limb is necessarily through its 

 column of bones. If we examine the illustrations of the progressive 

 movements of the horse, in Chapters XII. and XIII., we shall see that in 

 every case, just before a limb leaves the ground, it is directed backward 

 and downward, as, for instance, the off hind in Fig. 80, and the off 

 fore in Fig. 188. Hence the direction of propulsion in these cases must 

 be forward and upward. The speed at which the body is moving will 

 greatly influence the direction of the propulsion. Thus in Fig. 38, 

 which is one of the series (Fig. 37 to Fig. 41) that shows the running 

 high leap of a man, the impetus from the right leg is given vertically ; 

 yet the centre of gravity is projected forward at an angle of about 45° 

 to the ground. The reason for this is, that in this case there are two 

 forces of projection, namely, that derived from the extension of the right 

 leg, and that due to the speed at which the pedestrian ran up to the 



a 



Fig. 42. — Angle of Projection of 

 Centre of Gravity. 



jump before he " took off." We have here the operation of " the 

 parallelogram of forces." Thus, if the line a 6 in Fig. 42 represents 

 the horizontal force (derived from the speed) and a c the vertical one 

 (obtained from the right leg), and if we draw c d parallel to a b, and bd 

 parallel to a c, we shall have the resultant force represented by the line 

 a d, and the angle of projection 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 accom- 

 plished 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 1,000 yards on a horizontal plane. It is evident that, in 

 this case, the muscles of the horse's limbs would not only have to carry 

 the weight of the body 1,000 yards, but would also have to raise it 333 J 

 feet (1000 X \), which would be approximately equivalent to going 

 over a hill that was 333^ feet high and had a base 1,000 yards broad. 



