388 THE HUMAN BODT. 



and digested. The activity of the muscles and the digestive- 

 glands is dependent on processes which give rise to a large 

 production of carbon dioxide and, during the night, when 

 both are at rest, more oxygen is taken up than is contained 

 in the carbon dioxide eliminated. If a man works and 

 takes his meals at night, and sleeps in the day, the usual 

 ratios of his gaseous exchanges with the exterior are entirely 

 reversed. 



4. The amount of work that a man's organs do, is not 

 dependent on the amount of oxygen supplied to them, but. 

 the amount of oxygen used by him depends on how much 

 he uses his organs. The quantity of oxygen supplied must, 

 of course always be, at least, that required to prevent suffoca- 

 tion; but an excess above this limit will not make the tissues 

 work. Just as a man must have a certain amount of food 

 to keep him alive, so he must have a certain amount of 

 oxygen; but as extra food will not make his tissues or Mm 

 (who is physiologically the sum of all his tissues) work, 

 apart from some stimulus to exertion, so it is with oxygen. 

 Highly arterialized blood, or an abnormal amount of blood, 

 flowing through an organ will not arouse it to activity;. 

 the working organ, muscle (p. 257) or gland (p. 269), for ex- 

 ample, usually gets more blood to supply its extra needs 

 just as a healthy man who works will have a better appe- 

 tite than an idle one; but as taking more food by an idle 

 man will not of itself make him more energetic, so neither 

 will sending more arterial blood through an organ excite 

 it to activity. 



5. The preceding statement is confirmed by experiments 

 which show that an animal uses no more oxygen in an hour 

 when made to breathe that gas in a pure state, than when 

 allowed to breathe ordinary air. In other words, the 

 amount of oxygen an animal uses (provided it gets the 

 minimum necessary for health) is dependent only on how 

 much it uses its tissues. These (the rest in most cases sub- 

 ject to a certain amount of control from the nervous) de- 

 termine their own activity, and this, in turn, how much 

 oxygen shall be used in the systemic circulation and re- 



