222 LEAPING. 



eyes blinded, to reach a spot at quite a short distance 

 from the starting-point. 



Dr. B. In ascending an inclined plain, or a flight 

 of steps, greater muscular exertion is required than in 

 walking on a level, because when the foot first moved 

 is brought to the ground, it is bent, instead of being 

 straight ; consequently, when the other foot is brought 

 forward to the same level, the first limb must be straight- 

 ened with the whole weight of the body resting upon it, 

 in order that the body may be raised to the erect position. 



Emily. And it is this repeated exertion of raising 

 the whole weight of the body on one leg, which produces 

 the fatigue we soon feel in walking up a hill or a flight of 

 steps, is it not ? But the same fatigue is experienced in 

 descending a hill, though perhaps not quite so soon ; I 

 do not exactly understand how this should be. 



Dr. B. Here, in bringing down the foot first moved 

 to a level lower than the one on which the foot behind 

 stands, the latter is obliged to sustain the whole weight 

 of the body with the joints all bent, which, you know, re- 

 quires considerable exertion. 



Eimly. The peculiar difference then between ascend- 

 ing and descending an inclined plain, is, that in the former 

 the limb is to be straightened with the whole weight of 

 the body resting on it, and in the latter, it is to be bent, 

 while sustaining the same weight. Arn I not right r 



Dr. B. Yes, and quite a good distinction it is. In 

 leaping, all the joints are put into a state of flexion, and 

 then extended by a sudden and powerful motion. The 

 force used in extending the limbs is so much greater 

 than is necessary for overcoming the weight of the body, 

 that it is carried from the ground, and, in consequence 

 of the impulse received from the feet, in a forward di- 

 rection. This sudden straightening is not confined to 

 the limbs merely, but takes place in the back, and you 

 can easily conceive how much this motion is assisted by 

 the sudden extension of the spine, when you recollect 

 that all the vertebrae are separated from one another by 



