360 



CLXXXIX. Of leaping. Leaping, in man, is performed, principally 

 by the sudden extension of the lower limbs, whose articulations were in r. 

 previous state of flexion, The alternate angles of the foot, of the knee 

 and hip, disappear, and the extensors,, contract in almost a convulsive 

 manner. This straightening is not limited to the lower limbs, in violent 

 leaping: it, likewise, affects the vrtebral column, which acts as a bow in 

 unbending. Professor Barthez, who has the merit of having suggested 

 this explanation, which Borelli and Mayow had very imperfectly under- 

 stood, perhaps goes too far, in considering as imaginary, a power of re- 

 pulsion in the ground. This re-action, admitted by Hamberger and by 

 Haller, clearly operates, when we leap on an elastic floor; it enables tum- 

 blers to rise, without much effort, on the rope which bears them. But 

 though ail physiologists do not admit that, in leaping there is are-action 

 from the ground ; it is universally admitted, that there must be a certain 

 resistance, from the ground on which we tread* In fact, a moving sand, 

 yielding to the pressure of the body, would, by giving way to a consider- 

 able degree, render it impossible to leap. The instantaneous contraction 

 of the extensor muscles is so powerful, in extending the lower extremi- 

 ties, and in communicating to the body a power of projection, so as to 

 raise it, that frequently, during this effort, the tendons of these muscles, 

 or even the bones into which they are inserted, break across. It is on this 

 account that dancers are very apt to fracture their patella. This accident 

 happens, at the moment when their body, in rising from the ground, is 

 powerfully elevated to a certain height. 



If leaping consists merely in the sudden straightening of the lower ex- 

 tremities, whose articulations are bent in alternate directions, it must be 

 more considerable, according as these are longer, more bent on one an- 

 other, and as the muscles which straighten them contract more powerful- 

 ly. .Hence animals that move by leaps, as the hare, the squirrel, and the 

 jerboa, have posterior extremities of considerable length, in proportion 

 to their fore legs. Their different parts are, besides, capable of consi- 

 derable flexion. All these animals, strictly speaking, are incapable of 

 walking or running-, and they move by leaps or bounds succeeding each 

 other with different degrees of rapidity. Some, however, as the rabbit 

 and the hare) are capable of running, when climbing up a steep place, as 

 the slope, in this case, lessens the effect of the impulse communicated by 

 the extension of the posterior limbs; an impulse which, from the strength 

 and length of these extremities, throws the whole weight of the body on 

 the fore legs, which are weaker and shorter, with such a degree of force 

 that the animal is obliged to stiffen these and to keep them straightened, 

 and in a state of extension, to avoid striking the ground with his head, 

 while leaping on an horizontal plane. Frogs, but especially grasshoppers 

 and fleas, between whose hind extremities and the rest of the body, there 

 is the greatest disproportion, astonish us by the very considerable space 

 which they can clear at a leap; but the wonder ceases, when we consi- 

 der that powers communicate to the masses equal degrees of velocity, 

 when proportionate to one another; now, the space gone over, depend- 

 ing entirely on the velocity, since the body that leaps, loses, by a grada- 

 dion which nothing can lessen, that which it had acquired : these mo- 

 tions must be nearly alike in small and in large animals- 



Swammerdam says, that the height to which grasshoppers rise, in 



