THE CHEMISTRY OF RESPIRATION. 413 



(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 nof arouse it to activity; the 

 working organ, muscle, or gland, for example, usually gets 

 enough more blood to supply its extra needs just as a healthy 

 man who works will have a better appetite 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 subject to a certain 

 amount of control from the nervous) determine their own 

 activity, and this, in turn, how much oxygen shall be used in 

 the systemic circulation and restored in the pulmonary. In 

 other words, the physiological work of an animal, which of 

 course is largely dependent upon how external forces act upon 

 it, determines how much oxygen it uses daily; and not the 

 supply of oxygen how much its tissue activity shall be, unless 

 the supply sinks below* the starvation limit. 





