THE LEVER AND MUSCULAR MECHANISM. 11 



gravitation to come outside of the base and provoke a fall. Therefore, a horse 

 having a heavy body mounted upon long, slender members drawn close to the 

 median plane will be endowed with an equilibrium relatively unstable. As the 

 base of support has the form of an elongated rectangle, with the horse at rest, 

 the displacement of the centre of gravity from in front backward will be much 

 less dangerous for the integrity of its equilibrium than those which take place 

 from side to side, because the line of gravitation will be moved much easier from 

 the base of support in the latter case than in the former. This explains to us 

 why falls during rapid paces scarcely ever take place upon a straight line, but 

 are produced, on the contrary, in turning ; why the ambling horse, which moves 

 his members by lateral bipeds, goes faster and falls more frequently than the 

 trotting horse, which moves his members by diagonal bipeds ; why the leaping 

 horse, which displaces the centre of gravity greatly, falls down so often ; why 

 sliding sideways is more dangerous than moving forward or backward, etc. 



That instability of equilibrium gives the measure of speed has been justly 

 said. It is easy to account for this. If, from the fact of its displacements, or 

 from its situation being more or less raised, it is carried outside of the base of 

 support, it will cause the members to be displaced with a greater rapidity to sup- 

 port the body, as the fall is more imminent. 



The race-horse lengthens his body, extends his head and neck, and seems 

 almost to lie down upon the ground in such a manner as to bring, as much as 

 possible, the centre of gravity in advance of the movement. The dressed horse 

 raises himself in a strong proportion in order to execute more easily the varied 

 movements which at every instant his rider exacts from him. His paces are 

 shortened, raised; his equilibrium is more stable, the displacements of his 

 centre of gravity less extended but more numerous. On account of his acquired 

 speed, and especially because of the instability of his equilibrium, the hippo- 

 drome-horse can scarcely make progress upon a straight line ; the slightest lateral 

 displacement of the centre of gravity, a little sudden, causes him to fall. 



CHAPTER III. 



THE LEVER AND MUSCULAR MECHANISM. 



The lever is defined as a "rigid and inextensible rod supported upon a 

 fixed point." The form and nature of the substance of which this stalk is com- 

 posed have no importance from the point of view which concerns us. It is seen, 

 then, that the bones of the skeleton are properly considered as levers, since they 

 enter into the definition which we have just given. 



Every lever may be submitted to the action of several forces, but, whatever 

 may be their number, we know that it is always easy to reduce them to two. 

 When it is a question of ascertaining the conditions of the equilibrium of this 

 machine, only two forces are usually recognized. The former will be in equi- 

 librium when the resultant of the forces which move it is annulled by the reac- 

 tion of the point of support. If equilibrium does not result, the effect of the 

 forces is to determine a rotation of the stalk around the fixed point. 



