PROPULSION EFFECTED BY THE LIMBS. 



6i 



place when the horse starts from the halt into the walk ; for by the 

 straightening of one or both hind legs, he brings the centre of gravity 

 of his body beyond the toe of the most advanced fore foot, with the 

 result that the other fore leg has to be carried forward in order to restore 

 the equihbrium. In doing this, the first foot to quit the ground will 

 usually be a fore one. A succession of these displacements and re- 

 coveries by a man will constitute the walk, run, or other pace. 

 Similar movements occur when leaping, except when the spring is 

 made vertically upwards from a state of rest, in which case the 

 centre of gravity is not displaced beyond the base of support. 



Manner in which Propulsion is Effected by the Limbs. 



— Displacement of the centre of gravitj-, in the propulsion of the 

 body, is accomplished by the straightening out of the limb or limbs, 



Fig. 25. Fig. 26. Fig. 27. 



Displacement of Centre of Gravity of >[an when Walking. 



as the case may be. We can readily see how this gives the required 

 impetus, if we consider the manner in which the shove-off is done by 

 an expert swimmer in a bath, when he comes to the end of it, and wishes 

 to touch, turn, and strike off without loss of time. He, as we all know, 

 will, as much as possible, draw up the leg with which he is going to 

 give the shove off, and having applied it to the side of the bath (Fig. 28), 

 will suddenly straighten it out, with the result that his body will be shot 

 forward (Fig. 29). Here the drawn-up leg, which acts as a spring, is 

 placed between two objects : one (the body) movable, the other (the side 

 of the bath) immovable. When the leg is extended, the movable object 

 is naturally the one to be displaced. Identically the same action 

 occurs in the various progressive movements of the horse. We may 

 see it, also, in the run of the pedestrian, which is depicted in Figs. 36, 



