WORK AND EESPIEATION. 133 



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

 (if they can get it) 5 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 no- 

 ticed, as it was formerly supposed 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 present does not determine the degree of muscular 

 activity, but the degree of muscular activity determines the 

 amount of oxygen consumed. 



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 go faster at this time ? 

 By reflex action, you reply. The muscles send a message to 

 a nerve center ; and this nerve center sends back a message 

 to the blood tubes, making them widen, and the heart also 

 may be made to beat faster. But would it do any good to 

 have the blood flow through the muscles faster, if it could 

 not bring more oxygen, and take away and get rid of more 

 wastes ? You will say no. To give the extra oxygen, and 

 take out the carbon dioxid, the lungs cannot, of themselves, 

 take in and send out air. The work of pumping air depends 

 on the muscles of respiration, the diaphragm, and the muscles 

 that elevate the ribs. These will not work faster unless they 

 are ordered to do so. A message must be sent to these tell- 

 ing of the need in the muscles that we are considering, say 

 one of the large muscles of the lower limbs. Thus, by a 



