66 MECHANISM OF EQUINE LOCOMOTION. 



We have seen that the greater the abihty to straighten the leg, the 

 higher will be the speed of propulsion ; and vice versa. Hence, if a horse 

 is, for instance, unable from peculiarity of conformation to straighten 

 his hocks (a condition called " sickle-hocks," p. 314), he will not be so 

 fast as he would have been, had he greater freedom in these joints. We 

 may test the truth of this principle in our own selves ; for if, when 

 swimming in a bath (p. 61), we wanted to touch and turn, but were 

 unable to straighten our knee on account of an injury or from some 

 other cause, we would be capable of giving only a comparatively poor 

 shove-off. The great beauty of a " straight-dropped hind leg " in the 

 horse, as a conformation favourable to speed, will again be alluded to 

 on p. 314. 



The action of the fore limb in raising the fore-hand, which is essential 

 for the regulation of the position of the centre of gravity in locomotion, 

 is also dependent on its difference of length when bent and when 

 straightened out. It is manifest that this difference is due to the action 

 of the shoulder joint, elbow joint, fetlock, and pastern joints. Hence, 

 obHque shoulders and sloping pasterns are " points " of speed and of 

 leaping power. If the shoulder-blade and pastern be upright, the limb 

 will be capable of but slight extension. Here, the knee does not 

 come into play, because it is kept straight, when the fore foot is put 

 on the ground. 



In heavy draught, full straightening out of the limbs is not desirable ; 

 for the last part of this action is accomplished at a great mechanical 

 disadvantage, to which I have alluded on page 59. At present it is 

 sufficient to compare the action of the limbs to that of the oars of a boat, 

 in which the practically useful work is finished after the oars have passed 

 the line at right angles to the length of the boat ; the remainder of the 

 propulsive effect being obtained by a wasteful expenditure of force. 

 Consequently, an increase of speed is procured by an amount of work 

 which is greatly out of proportion to the result. Thus, to double the 

 speed during a certain period of time, it may be necessary for the horse 

 to do five or six times the amount of muscular exertion. For instance, 

 it may be more fatiguing for a horse to go twenty miles in one hour, 

 than fifty miles in five hours. The cart-horse, when in heavy draught, 

 moves his load with his hind limbs in a more or less bent condition (Fig. 

 126), which gives his hind-quarters the crouching appearance which 

 must be famiUar to us all. Sloping pasterns from a load-pulling point 

 of view are objectionable in the cart-horse ; for the more oblique they are, 

 the greater is the mechanical disadvantages at which they work (p. y^,). 



Action of the Muscles which Extend the Vertebrae. 



— When a horse prepares to kick, he, as a general rule, lowers his head 

 and arches his loins (" puts his back up "), by doing which he relieves 



