314 ELEMENTARY PHYSIOLOGY less. 



pig is killed. And it is not that the colouring matter is 

 merely, as it were, washed out ; the dye is permanent, 

 but the bones nevertheless become parti-coloured. In the 

 shaft of a long bone, for instance, a certain time after 

 feeding with madder, a deep red layer of bone in 

 the middle of thickness of its wall will be found 

 to have colourless bone on its medullary and on its perio- 

 steal face. And the longer the time which has elapsed 

 since the feeding with madder, the more completely 

 will the deep red bone be replaced and covered up by 

 colourless bone. 



10. The Mechanics of Motion. The System of 

 Levers. — To understand the action of the bones, .as levers, 

 properly, it is necessai'y to possess a knowledge of the 

 different kinds of levers and be able to refer the various 

 combinations of the bones to their appropriate lever- 

 classes. 



A lever is a rigid bar, one part of which is absolutely 

 or relatively fixed, while the rest is free to move. Some 

 one point of the movable part of the lever is set in 

 motion by a force, in order to communicate more or less 

 of that motion to another point of the movable part, 

 which presents a resistance to motion in the shape of a 

 weight or other obstacle. 



Three kinds of levers are enumerated by mechanicians, 

 the definition of each kind depending upon the relative 

 positions of the point of support, or fulcrum ; of the 

 point which bears the resistance, weight, or other 

 obstacle to be overcome by the force ; and of the point 

 to which the force, or po'wer employed to overcome the 

 obstacle, is applied. 



If the fulcrum be placed between the power and the 

 weight, so that, when the power sets the lever in motion, 

 the weight and the power describe arcs, the concavities 

 of which are turned towards one another, the lever is said 

 to be of the first order. (Fig. 96, I.) 



If the fulcrum be at one end, and the weight be between 



