THE USE OF MUSCLES IN THE BODY 119 



as for example the muscle (orbicularis oris) which surrounds the 

 mouth-opening, and by its contraction narrows it and purses out 

 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. Examples of the first form of lever 

 (fulcrum between power and weight, Fig. 49), are not numerous in 

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



{ F \ 



P Jj^ W 



FIG. 49. A lever of the first order. F, fulcrum; P, power; W, resistance or 

 weight. 



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 

 vertebral column to the occiput; the resistance is the excess in 

 the weight of the part of the head in front of the fulcrum over 

 that behind it, and is not great. To depress the chin as in nodding 

 does not necessarily call for any muscular effort, as the head will 

 fall forward of itself if the muscles keeping it erect cease to work, 

 as those of us who have fallen asleep during a dull discourse on a 

 hot day have learnt. If the chin however be depressed forcibh r , 

 as in the athletic feat of suspending one's self by the chin, the 

 muscles passing from the chest to the skull in front of the occipital 

 condyles are called into play. Another example of the employ- 

 ment of the first form of lever in the Body is afforded by the 

 curtsey with which formerly one lady saluted another. In curtseying 

 the trunk is bent forward at the hip-joints, which form the fulcrum; 

 the weight is that of the trunk acting as if all concentrated at 

 its center of gravity, which lies a little above the sacrum and be- 

 hind the hip-joints; and the power is afforded by muscles passing 

 from the thighs to the front of the pelvis. 



