IN HYGIENIC PHYSIOLOGY. 149 



15. Why can ive exert greater force with the back teeth 

 than with the front ones ? 



(See Physiology, p. 35.) 



The lower jaw is a lever of the second class. In the former 

 case the resistance to be overcome, i.e., the weight, is situated 

 much nearer the power. 



16. Wliy do ive lean forward ivhen ive wish to rise from 



a chair ? 



(See Popular Physics, p. 57.) 



In order to bring the center of gravity over the feet. 



17' IVliy does the projection of the heel-bone make 

 ivalkiiig easier ? 



(See Frontispiece, and also Fig. 18 in Physiology.) 

 It brings the power further from the fulcrum or weight. 



18. Does a horse travel ivith less fatigue over aflat than 

 a hilly country ? 



No. The variety of travel in a hilly country, other things 

 being equal, tends to rest the horse, and enable him to better 

 endure the fatigue of the journey. 



19. Can you move your upper jaw ? 



All the bones of the face, except the lower jaw, are firmly 

 and immovably articulated with one another and with the cra- 

 nium. LEIDY. 



20. Are people naturally right or left-handed ? 



Many persons are naturally either right or left-handed ; but 

 most can and should learn to use either hand with equal 

 facility. 



21. Why can so few persons move their ears by the 

 muscles ? 



Perhaps because of lack of practice ; more probably, how- 

 ever, the muscles (see Physiology, p. 53 and Fig. 14) are de- 

 veloped in few persons. 



22. Is the blacksmith's right arm healthier than the 

 left? 



