MOTION AND LOCOMOTION 103 



the lips; or the orbicularis palpebrarum which similarly surrounds 

 the eyes and when it contracts closes them. 



Levers in the Body. When the muscles serve to move bones 

 the latter are in nearly all cases to be regarded as levers whose 

 fulcra lie at the joint where the movement takes place. Examples 

 of all the three forms of levers recognized in mechanics are found 

 in the Human Body. 



Levers of the First Order. In this form (Fig. 51) the ful- 

 crum or fixed point of support lies between the "weight" or 



F 



W 



FIG. 51. A lever of the first order. F, fulcrum; P, power; W, resistance or 

 weight. 



resistance to be overcome and the "power" or moving force, 

 as shown in the diagram. The distance PF, from the power to 

 the fulcrum, is called the "power-arm"; the distance FW is the 

 "weight-arm." When power-arm and weight-arm are equal, as 

 is the case in the beam of an ordinary pair of scales, no mechanical 

 advantage is gained, nor is there any loss or gain in the distance 

 through which the weight is moved. For every inch through 

 which P is depressed, W will be raised an equal distance. W^hen 

 the power-arm is longer than the other, then a smaller force at P 

 will raise a larger weight at W, the gain being proportionate to the 

 difference in the lengths of the arms. For example if PF is twice 

 as long as FW, then half a kilogram applied at P will balance a 

 whole kilogram at W, and just more than half a kilogram would 

 lift it; but for every centimeter through which P descended, W 

 would only be lifted half a centimeter. On the other hand, when 

 the weight-arm in a lever is longer than the power-arm, there is 

 loss in force but a gain in the distance through which the weight 

 is moved. , 



Examples of the first form of lever are not numerous in the 

 Human Body. One is afforded in the nodding movements of the 

 head, the fulcrum being the articulations between the skull and 

 the atlas. When the chin is elevated the power is applied to the 

 skull, behind the fulcrum, by small muscles passing from the 



