yn 



EXAMPLES OF LEVERS 317 



(c) The raising of the body upon the toes, m standing 

 on tiptoe, and in the first stage of making a step forwards. 



(Fig. 96, II.) , , . 



Here the fulcrum is the ground on which the toes rest ; 

 the power is applied by the muscles of the calf to the 

 heel (Fig 6 1.); the resistance is so much of the weignt 

 of the body as is borne by the ankle-joint of the foot, 

 which of course lies between the heel and the toes. 



Three examples of levers of the third order are- 



(a) The spine, head, and pelvis, considered as a rigid 



bar/which has to be kept erect upon the hip-jomts. 



^^Hei\he fulcrum lies in the hip-joints, the weight is 

 hi-h above the fulcrum, at the centre of gravity of the 

 head and trunk ; the power is supplied by the extensor 

 muscles (Fig. 6, 2) in the front of, or the flexor muscles 

 (Fig. 6, II.) at the back of, the thigh, and acts upon points 

 comparatively close to the fulcrum. 



(6) Flexion of the forearm upon the arm by the biceps 

 muscle, when a weight is held in the hand. ^ ^ . , 



In this case, the weight being in the hand and the ful- 

 crum at the elbow-joint, the power is applied at the point 

 of attachment of the tendon of the biceps, close to the 



latter. (Fig. 92.) ,.,,••. 



(c) Extension of the leg on the thigh at the knee-jomt 

 Here the fulcrum is the knee-joint ; the weight is at 

 the centre of gravity of the leg and foot, somewhere be- 

 tween the knee and the foot ; the power is applied by the 

 muscles in front of the thigh (Fig. 6, 2 and Fig. 97), 

 through the ligament of the knee-cap, or patella, to the 

 tibia, close to the knee-joint. 



In studying the mechanism of the body, it is very im- 

 portant to recollect that one and the same part of the 

 bodv may represent each of the three kinds of levers, 

 according to circumstarices. Thus it has been seen that 

 the foot may, under some circumstances, represent a lever 

 of the first, in others, of the second, order. But it may 



