50 ANIMAL PHYSIOLOGY. 



particularly erect. When the upper part of the "body is Lent 

 forward, the lower part is carried backwards; and if the 

 knees be bent, the projection forwards of the legs is balanced 

 by the bending backwards of the thigh. When the knees 

 are completely bent, the heel is raised from the ground, 

 because the joints at the balls of the toes are required to 

 supplement the ankle in bending the leg sufficiently down 

 to bring the centre of gravity forwards over the base of sup- 

 port; and that is the reason why, in such a position, the 

 heel is more raised when the trunk is upright than when it 

 is stooped forwards. 



27. In walking and running the weight of the body is 

 thrown from one limb to the other alternately, and, except in 

 exceedingly slow marching, balance is not yet completely 

 established on one limb when the weight is shifted to the 

 other. Before the limb which is advanced reaches the ground, 

 the body is propelled forwards by the straightening of the 

 ankle of the foot which is behind; it is then pulled over on 

 the hip of the advanced limb by the muscles on the outside 

 of that joint (glutens medius and minimus), and, especially 

 in long steps, it is drawn forwards to the advanced limb by 

 a strong mass of muscles extending between the pelvis and 

 inside of the thigh (the adductors). 



The difference between walking and running is two- 

 fold. First, in walking the heel is brought to the ground, 

 while in running it is not; yet the foot, at the moment 

 when it is used to propel the body in running, is as ad- 

 vantageously placed as in walking ; for the leg is so much 

 sloped forwards that the angle between it and the foot is 

 quite as sharp as it ever is in walking. Secondly, whereas 

 in walking the whole propulsive action is from the foot, in 

 running, the knee and hip-joints being both greatly bent, a 

 vast additional impulse is given by their simultaneous exten- 

 sion. So also in leaping, all the joints of thelower limb 

 are flexed in preparation for the leap, and the impetus given 

 by their sudden extension propels the body through the air. 

 But leaping differs from running, in that the limbs are 

 extended together instead of acting alternately. 



28. In the movements of the skeleton, all the three orders 

 of levers are employed. In extending the fore-arm, as in box- 



