viii MUSCLE MOVEMENTS 75 



III. \Yhen the foot is bent, as in raising the toes, the fulcrum 

 is at the ankle joint, the weight at the toes, and the 

 power applied by the flexor muscles between the two. 

 Lever of the 3rd class. 



Other examples of lever mechanisms in the body are : 



Levers of the ist Class. The nodding movement of the 

 head backwards and forwards on the atlas as the 

 fulcrum. Raising the body from the stooping position, 

 with the fulcrum at the hip joints. 



Lever of the 2nd Class. When one thigh is kept bent up 

 towards the body as in hopping ; the fulcrum in this 

 case is at the hip joint, the power (mainly due to the 

 extensor muscles on the front of the thigh) is applied 

 at the knee, and the weight of the limb falls between 

 the two. 



Levers of the 3rd Class. Movement of the lower jaw 

 upwards in mastication. Raising the body from the 

 recumbent to the sitting posture ; the fulcrum in this 

 case is at the hip joints, the weight is that of the head 

 and body, and the power is applied by the muscles 

 passing from the front part of thigh to the hip bones. 



Walking. In walking one foot is placed on the ground 

 in front before the toes of the other foot have left it. The 

 leg is stepped forward chiefly by the action of the muscles 

 passing from the pelvis to the front of the thigh ; and the knee 

 is slightly bent by the action of the muscles of the back of 

 the thigh, to enable the toes to clear the ground. The body 

 is then raised on the toes of the leg that is behind and lifted 

 forward. This is chiefly done by the action of the muscles 

 of the calf raising the weight of the body, which meets the 

 foot at the ankle joint, the fulcrum being at the toes ; in fact, 

 this part of the act is that of a lever of the second class 

 described above. In easy walking very little effort is required, 

 for a leg can swing like a pendulum, that is, at a rate in 

 accordance with its length, the longer the pendulum the 

 slower the rate of swing. This explains why it is that, when 

 walking at a natural step, a short-legged man naturally takes 

 quicker steps than a long-legged man. 



Running. In running both feet are off the ground at the 



