162 



SPECIAL PHYSIOLOGY. 



1. 

 I 



3. 



base of support, the equilibrium of the body becomes unstable, and 

 the body has a tendency to fall down. 



The levers employed in the body, are the three or- 

 dinary levers known in mechanics. A lever consists 

 of a rigid rod, movable in one plane, around a point 

 I, in the rod, called the fulcrum.. In actual practice, 

 this fulcrum is seated opposite the point of support ; 

 2. a weight or resistance also exists, which has to be 



P moved or overcome ; and lastly, there is a force or 

 i power to move the weight, or overcome the resistance. 

 I | The three kinds of levers are distinguished from each 



F w other by the relative positions in them of the fulcrum, 



the weight, and the power. In the first kind, 1, the 

 fulcrum F is placed between the weight w and the 

 F power P: it is employed in the balance, in working a 



I I pump-handle, and in raising the coals in a grate with 



a poker. The second, 2, has the weight w placed be- 

 \V tween the fulcrum F and the power P: it is used in 



raising a weight by putting a crowbar underneath it, 

 and also in wheeling a wheelbarrow. In the third kind, 3, the power 

 P is placed between the fulcrum F and the weight w : it is used in the 

 foot-piece of a lathe, and in drawing a ladder from the wall, by pull- 

 ing on one of the steps near the bottom, whilst the foot is placed to 

 prevent its slipping out. Fire-tongs and sugar-tongs are double levers 

 of the third kind. On comparing the three kinds of levers, it will be 

 seen that the terms on each side are twice repeated; but that the mid- 

 dle terms consist, respectively, of the three important elements of the 

 lever, arranged in methodical order, viz., in the first kind, the fulcrum 

 F, or point of support, is in the middle space ; in the second kind, the 

 weight w; and in the third kind, the moving power p. 



Fig. 48. 



Fig. 48. Example, from the body, of a lever of the first. order, shown in the balance of the head on the 

 top of the vertebral column. 1, position of the fulcrum, at the articulation of the cranium with the first 

 cervical vertebra; 2, the weight, or excess of weight, in the fore part of the head and face; 3, the power, 

 in the muscles at the back of the neck. 



In the bocly, the fulcra are sometimes in the joints, and sometimes 

 at the extremity of a limb, in contact with the ground, or with some 



