605 



LOCOMOTION OF ANIMALS. 



LOCOMOTION OF ANIMALS. 



508 



Fig. 16. 



Ill quickest walking, the advanced foot reaches the ground in the 

 vertical line which passes through the head of the thigh-bone, as in 

 fig. 17. Here the centre of gravity being entirely supported by the 

 forward leg, the hinder leg is in a condition to rise from the ground 

 the instant the other reaches it, and the time wherein both legs are 

 simultaneously on the ground becomes evanescent. If the joints of 

 the legs did not possess, as we have seen, a considerable freedom of 

 motion, we should not be enabled to vary our speed as we now do, 

 because as the length of the step increases the height of the centre 

 of gravity decreases ; and to accomplish the latter, the forward leg 

 must be much more bent when it reaches the ground than in slow 

 walking, as seen in figt. 1 6 and 17, the velocity of the man in fig. 16 being 

 ittle more than one-half of that in fig. 17. It is also in consequence of 



Fig. 17. 



the power we possess of bending the legs that we are enabled to move 

 the centre of gravity nearly horizontally, and thereby to move with 

 a much greater velocity than we could do if our limbs were inflexible ; 

 for a man with inflexible wooden legs is restricted from walking 

 beyond a velocity within very small limits, however great may be 

 his muscular power. For example, when a man is walking with 

 wooden legs, aa in fig. 18, the centre of gravity describes small arcs of 

 a circle, of which each leg is alternately the radius. Now, according 

 to Dr. Young, if the velocity could be sufficiently great to create a 

 centrifugal force exceeding that of gravity, each leg would be raised 

 from the ground immediately after touching it, which would constitute 

 running ; for in walking the body is always supported either by one 

 or two legs; and supposing the inflexible leg to be three feet iu 



length, the centrifugal force would become equal to that of gravity 

 when the velocity in walking became equal to that which a heavy body 

 acquires in falling through half the length of the leg, or one foot and 

 a half, which is very nearly ten feet in a second, or seven miles in an 

 hour. This then is the extreme limit of velocity which a man could 

 reach with wooden legs, or with legs whose joints have been rendered 

 useless by disease ; but in reality he cannot move with anything like 

 this speed, because he must place his swinging leg on the ground as 

 much before the vertical through his centre of gravity as the other 

 leg is behind it, and therefore his steps must be very short, and taken 

 at a greater mechanical disadvantage than in the slowest walking of 

 ordinary persons. In consequence of the flexibility of the legs, the 

 path taken by the centre of gravity undulates without forming any 

 abrupt angles during its elevation and depression, as seen in fiy. 20, 

 where the actual path resulting from the flexibility of the limb is 

 delineated; whilst infy. 19 we see the abrupt manner in which the 

 centre of gravity moves, and the curves begin and terminate ; and we 

 can readily imagine the jars to which the trunk would be subject in 

 locomotion if the legs were destitute of joints at the knee and ankle. 



Fig. 18. 



Fig. 19. 



The greatest velocity with which a person can walk (unless by an 

 enormous expenditure of muscular action, which could not be main- 

 tained) is when the time of a step is equal to half the duration of the 

 motion of the swinging leg ; that is, the time which elapses from the 

 raising of that leg until it is again placed on the ground, having 

 described half its arc of oscillation, the hind leg during the same time 

 pushing the trunk sufficiently forward, so that the centre of gravity 

 may be vertically over the base of support, as iu fig. 17. Hence, if 

 we suppose the leg capable of describing its arc freely in "730 of a 

 second, the least time of the step will be '730 divided by 2, or '375 of 

 a second. When the swinging leg is first raised from the ground the 

 trunk propels the head of the thigh-bone horizontally forwards, and 

 communicates a retrograde motion to the lower extremity of the leg 

 in the direction of the tangent of the curve in which the leg oscillates 

 This retrograde force tends to retard the movement of the leg forward, 

 and vould materially lengthen the time of a step, but the leg being at 

 the same time bent, and consequently shortened, to allow it to swing 

 freely above the ground, its movement is thereby as much accelerated 

 as the retrograde action tends to retard it, and the result is that the 

 leg swings in- the same time as if these accelerating and retarding 

 influences did not exist. The velocity in walking then, in the same 

 person, depends on the time taken in making each step, and on the 

 length of the steps ; and both of these are again dependent on the 

 height at which the centre of gravity, or the heads of the thigh-bones, 

 are carried above the ground ; for as the height of the latter dimi- 

 nishes the length of the step is increased and the time of the step is 

 decreased, and vice versa. The velocity of walking in different indi- 

 viduals depends greatly on the relative proportions of their framework, 

 and on the vigour of their muscular system ; but it must be borne in 

 mind that it is always the hind leg which has the work to accomplish, 

 and by throwing it into the required position, and regulating its 

 extension, the speed may be adjusted to the figure of the individual. 

 It is indeed owing to the dimensions of the several organs concerned 

 in locomotion, and to the habit of the individual in applying them, 

 that each person has a step peculiar to himself, so that the very sound 

 produced by the contact of the foot with the ground is sufficient to 

 enable us to recognise the approach of individuals with whom we are 

 familiar long before we see them. Compared with numerous species 

 of the lower animals, the velocity of man in walking is very inferior. 

 The best constituted persons are incapable of acquiring a speed of 

 little more than five miles in au hour ; and even at this rate of motion 

 they are quickly exhausted. Our expenditure of muscular power for 



