RESPIRATION. 



Case of the rusting of the metals there is as much heat pro- 

 duced as when they are burned, but the heat is so slowly 

 generated that it is given off about as fast as it is pro- 

 duced, and we do not notice it. The oxidation produces 

 the waste matters, just as the burning of the various 

 substances produces waste. 



Oxidation of Live Tissues and Dead Matter. In our 



experiments with oxygen we see that substances which 

 burn in air will burn still more actively in oxygen. But 

 we must not infer from this that in our bodies the oxida- 

 tion of the tissues would be faster in pure oxygen. This 

 is not the case. The tissues take as much oxygen as they 

 need (if they can get it), and they will not take any more 

 than they need, no matter how much is offered them. It 

 does not injure the body, nor any part of it, to breathe 

 pure oxygen. It does not make one feverish, it does not 

 produce any more heat, nor make one "live faster." This 

 point should be specially noticed,- as it was formerly sup- 

 posed that the oxidation of the tissues of the body was 

 just like any combustion of dead material. But the tissues 

 are alive. They know their own needs. Each cell takes 

 what it requires and no more, just as it does of food 

 brought to it by the blood. The amount of oxygen pres- 

 ent does not determine the degree of muscular activity, 

 but the degree of muscular activity determines the amount 

 of oxygen consumed. 



Increased Blood Flow is the Result of Exercise. - 

 When we exercise, the muscles need more oxygen. They 

 also need to have removed the waste matters that they are 

 so rapidly producing at this time. How is the oxygen 

 brought and the waste removed ? By the blood, you 

 answer. True ; but what makes the blood come and 



