144 THE HUMAN BODY. 



directly ; and similarly other muscles arising at the back 

 of the orbit are directly fixed to the eyeball in front and 

 serve to rotate it on the pad of fat on which it lies. Many 

 facial muscles again have 110 direct attachment whatever to 

 bones, as for example the muscle (orbicular is or is) which 

 surrounds the mouth-opening and by its contraction nar- 

 rows it and purses out the lips; or the orbicularis palpe- 

 brarum 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. 56) the 

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



P W 



FIG. 56. 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 

 distance 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 de- 

 pressed, 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 propor- 

 tionate to the difference in the lengths of the arms. For 

 example if PF is twice as long as FW, then half a kilo- 

 gram applied at P will balance a whole kilogram at W, and 



