54 THE ANIMALS AND MAN 



levers of the third class. By them quickness of motion 

 is magnified. 



Thus by noting first what motions an animal makes, and 

 then, by dissection, examining the muscles, the bones, and 

 their points and means of attachment, we may come to 

 understand clearly the uses of the muscles and skeleton in 

 any animal. 



Necessity of oxygen and food. In the organs of motion 

 just studied, the muscles and bones are only the machinery 

 for motion. They make use of energy but cannot them- 

 selves provide it. Just as an engine and all the wheels and 

 levers connected with it make use of heat, which is one of 

 the forms of energy, to produce the needed motions, so the 

 muscles and bones make use of some form of energy to 

 produce the motions of the animal body. In the steam- 

 engine the special form used is heat, generated by the burn- 

 ing of coal, oil, or wood; by means of this heat, which 

 expands the steam, i.e., the vapor of water, energy is applied 

 to the piston in the form of a push. The motion of the 

 piston is passed over to the wheels and levers of the shop, 

 and by them are given all the different directions and veloci- 

 ties required by the different machines of that particular 

 shop. 



In the animal body the muscle is the engine, for in it the 

 energy is generated. In a way we do not yet exactly under- 

 stand this energy makes the muscular substance contract 

 and give a pull on the tendon, with the same effect as the 

 push of the steam on the piston, that is, to set the rest of 

 the machinery, the bones, in motion. The bones apply the 

 motion in the way required for the movement of the animal. 

 A striking difference, however, between the animal body and 

 a shop is this, that while in even a very large shop there may 

 be but one engine generating energy to run all the different 

 machines, in the body every muscle is a separate engine, and 

 one bone may be connected with a number of them. Never- 



