374 THE MECHANICAL FUNCTIONS. 



lancing its body in various attitudes, and, in a very short 

 time, is unconscious that these actions require exertion. 



In walkins:, the first effort that is made consists in trans- 

 ferring the whole weight of the body upon one foot, with a 

 view to fix it on the ground; and, then, the other foot, being 

 at liberty, is brought forwards. By this action, the centre 

 of gravity is made to advance, till it passes beyond the base 

 of the foot: in this situation, the body, being unsupported, 

 falls through a certain space, and would continue its descent, 

 were it not that it is received on the other foot, wjiich, by 

 this time, has been set upon the ground. This falling of the 

 body would, if not immediately checked, become very sen- 

 sible; as happens when, on walking inattentively, the foot 

 we had advanced comes down to a lower level than we were 

 prepared for; in which case, the body, having acquired a 

 certain velocity by its greater descent, receives a sudden 

 shock when that velocity is checked, and thus a disagreea- 

 ble jar is given to the whole frame. 



While the weight of the body is thus transferred, alter- 

 nately, from one foot to the other, the centre of gravity not 

 only rises and falls, so as to describe, at every step, a small 

 arch, but also vibrates from side to side, so that the series of 

 curves it describes, are somewhat complicated in their form. 

 This undulation of the body, from one foot to the other, 

 would scarcely ever be performed w^ith perfect equality on 

 both sides, if we trusted wholly to the sensations communi- 

 cated by the muscles, and if we were not guided by the sense 

 of sight, or some other substitute. Thus, a person blind- 

 folded cannot walk far in a straight line; for, even on a le- 

 vel plane, he will incline unconsciously either to the right 

 or to the left. 



In all quadrupeds, and even also in the quadrumana, the 

 fore extremities more or less contribute to the support and 

 progression of the body: it is only in man that they are 

 wholly exempted from these ofhces, and are at liberty to be 

 applied to other purposes, and employed as instruments of 

 prehension and of touch. In the power of executing an in- 



