174 SPECIAL PHYSIOLOGY. 



metrical movements on the two sides of the body, performed at the 

 ankle, knee, and hip-joint, the trunk being kept, as nearly as possible, 

 in a state of equilibrium, though, as we shall immediately show, its 

 centre of gravity is not merely carried forwards, but undergoes both 

 vertical and lateral oscillations. One leg is first lifted from its base 

 of support, with a slight flexion of the knee and foot, so as to prevent 

 the latter from touching the ground, and is advanced a certain distance, 

 chiefly by swinging, as will be presently mentioned, but also by flexion 

 of the thigh upon the pelvis, and by extension of the leg and foot ; it 

 is soon permitted to touch the ground in advance of the body, the 

 centre of gravity at the same time descending a little, as well as ad- 

 vancing forwards, and also inclining over in the direction of the ad- 

 vanced limb. As the forward foot advances, the hinder one inclines 

 in the same direction, and the centre of gravity, now moved beyond 

 the original base of support, is slightly lowered. When the for- 

 ward foot has touched the ground, the hinder one is raised by ex- 

 tension of the foot, which, continuing to press on the ground, assists 

 in urging the centre of gravity forwards, a little upwards, and still 

 more over to the opposite side. The centre of gravity having now, 

 reached a secure point of support, over the advanced and stationary 

 limb, the hinder limb completely leaves the ground ; the thigh is slightly 

 bent on the pelvis, the leg is a little bent on the thigh, and the foot 

 somewhat on the leg ; in this position of the segments, it is shortened 

 by about one-ninth part of its length, so that the toes should keep 

 clear of the ground. The limb, in its turn, is now swung forwards, 

 to be planted on the ground in advance of the body, the centre of 

 gravity being again carried forwards, downwards, and over to the same 

 side, and the foot being finally planted on the ground, as before. A 

 repetition of the same movements, with the same results, is performed 

 by the two limbs alternately. Regarding the body as a whole, the 

 centre of gravity is not moved directly forward, at a uniform height 

 from the ground, in any mode of progression. Such a condition does 

 mot take place in any living animal, though it happens in the case of 

 inanimate objects, such as carts, locomotives, or masses of matter 

 dragged over horizontal surfaces. In the living body, to advance im- 

 plies an elevation of the centre of gravity, followed by a slight descent : 

 in other words, the body is lifted and falls at every step forwards, and 

 so describes a vertical oscillation, which has been estimated at about 

 one inch and a quarter in extent. As above shown, lateral oscilla- 

 tions also occur, due to the alternate shiftings of the centre of gravity 

 from a point over one leg to a point over the other. In walking, the 

 advancing foot invariably touches the ground, before the hinder foot 

 is lifted ; so that in this mode of progression, there is a short period, 

 during which both limbs touch the ground, alternating with a longer 

 period, in which only one limb rests upon the base of support. The 

 general rate at which man can walk, depends on the length of his lower 

 limbs, the thigh being unusually long. The pace, in particular cases, 

 is regulated by the length of the leg, and by the muscular efforts em- 

 ployed to secure rapidity of step. In rapid walking, almost every 

 muscle of the body is exercised; the duration of the step is shortened, 

 and so also is the length of time during which both feet touch the 



