MOTION AND LOCOMOTION. 145 



the muscle (orMcularis oris) which surrounds the mouth- 

 opening, and by its contraction narrows it and purses out the 

 lips; or the orlicularis palpebrarum which similarly sur- 

 rounds 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 me- 

 chanics are found in the Human Body. 



Levers of the First Order. In this form (Fig. 63) the 

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



I F \ 



P jj^ W 



FIG. 63. A lever of the first order. .F, fulcrum ; P, power ; W, resistance or 



weight. 



or 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 dis- 

 tance FW is the " weight-arm." When power-arm and 

 weight-arm are equal, as is the case in the beam of an ordi- 

 nary 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. When 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 dif- 

 ference 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 Tf, 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 move- 

 ments of the head, the fulcrum being the articulations be- 

 tween the skull and the atlas. When the chin is elevated 

 the power is applied to the skull, behind the fulcrum, by 



