LOCOMOTION OF MAMMALIA. 67 



In walking, the Horse, if the right side be in advance, moves 

 first the left hind-leg, second the right fore-leg, third the right 

 hind-leg, fourth the left fore-leg ; propelling the centre of gravity 

 forward over a space equal to the length of the first step. When 

 the left hind-leg is in the act of advancing, the trunk is supported 

 on the other three legs and is balanced on a triangular instead of 

 a parallelogrammical basis. A succession of movements of the four 

 legs, in the above order, constitutes the progression by walking 

 in most quadrupeds ; its rapidity depends on the time occupied 

 in the series of movements by which the limbs effect the step. In 

 a large well-made Horse one foot may move the length of a step 

 in a second of time, when each leg may swing during one quarter 

 and rest on the ground three quarters of a second. Rapid walkers 

 do it in less time, and the interval between putting down one leg 

 and lifting another becomes inappreciable. In quadrupeds with 

 limbs unusually long in proportion to the trunk there is a modifi- 

 cation of the act of walking : the Camel and Giraffe seem to 

 swing along by moving the two right limbs together and alter- 

 nately with the two left limbs. But, though in a quick walk the 

 two legs of the same side seem to be moved forward simul- 

 taneously, and are both off the ground at the same time through 

 the greater part of the step, yet on close inspection the hind-leg 

 is seen to be first lifted from the ground, and after a very brief 

 interval the fore-leg of the same side. 1 In this way of walk the 

 trunk is balanced on a linear basis of support, alternately trans- 

 ferred from one side to the other. In the Giraffe the long neck 

 is then stretched out in a line with the back, giving the animal a 

 stiff and awkward appearance; but this is lost when they commence 

 their graceful undulating amble : 35 



the motions of the legs are now 

 peculiar; the hind-pair are lifted 

 alternately with the fore, and are 

 carried outside of and beyond them 

 by a kind of swinging movement. 2 



In the pace of the Horse called 

 the 'trot,' the legs move in pairs 

 diagonally, those marked B, e, fig. 



35, e.g. being raised as soon as A, D, e %^ j^ LjPL_A 

 strike the ground : the bases of sup- i> ^""^^jT 



port are alternately in the lines A, D, B, E ; and the undulations from 

 the projection of the trunk are in the vertical, not as when walking 



1 xcvir. p. 244. 2 lb. p. 244. 



f2 



