CHAPTER VII 



HOW ANIMALS TURN FOOD AND AIR INTO 

 FLESH AND ENERGY 



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 themselves 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, gen- 

 erated by the burning 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 velocities 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 ex- 

 actly understand this energy makes the muscular sub- 

 stance 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, how- 

 ever, between the animal body and a shop is this, that 



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