RESPIRATION 241 



All the facts just mentioned confirm the direct evidence in 

 favor of oxygen secretion in the lungs. Part of the exhaustion of 

 hard physical work is due to imperfect saturation of the arterial 

 blood with oxygen and the consequent effects on the respiratory 

 center and central nervous system as already described in Chap- 

 ters VI and VII. In persons who are in good physical training 

 these effects are in abeyance because as one part of physical train- 

 ing the lung epithelium has become much more capable of re- 

 sponding to the stimulus calling forth increased secretion of 

 oxygen, just as in the case of a man who has become acclimatized 

 to a high altitude, or to breathing air containing a small per- 

 centage of CO. It is the training of the lung epithelium, and not 

 anything else, that makes the specific difference. This is shown 

 at once by the fact that acclimatization to high altitudes or CO 

 poisoning takes place whether a man takes exercise or not. 



In this connection I may mention the result of an experiment 

 which I made for a specific object during the war. It seemed 

 desirable to find out how soon the fall in oxygen percentage in the 

 air of a submarine would begin to have serious effects. I there- 

 fore shut myself in an air-tight respiration chamber which was 

 provided with the same sort of purifier for absorbing the CO 2 

 produced by respiration as was used in British submarines. The 

 oxygen percentage was also allowed to fall at the same slow rate 

 as that at which it had been found to fall in the most crowded 

 submarines then in use. After a few hours a light would no longer 

 burn in the air, and in a few more hours even a lighted pipe 

 handed in through a small air lock would no longer keep alight. 

 After 56 hours the oxygen percentage had fallen below 10. I then 

 terminated the experiment as the purifier was failing, and the 

 immediate object of the experiment, which was to find out whether 

 the air in a submarine would last easily for 48 hours without any 

 addition of oxygen, had been attained. I had no trace of mountain 

 sickness or any other symptom of anoxaemia, and my lips were 

 just as red as usual, though from other experiments described 

 in Chapter VII, I knew that without acclimatization I should have 

 broken down hopelessly in the existing atmosphere. A laboratory 

 attendant who afterwards went into the chamber along with me 

 became blue and uncomfortable, and finally collapsed and had to 

 be pulled out hurriedly. 



In this experiment the fall in oxygen percentage had been so 

 slow that acclimatization had kept pace in me with the fall in 

 oxygen percentage, just as when a man ascends only very gradu- 



