READJUSTMENTS OF REGULATION 47 



not return. The animal dies of want of oxygen, or 

 failure of the circulation, without making any effort 

 to breathe. Hence if we reduce the CO 2 pressure 

 of the blood low enough no amount of oxygen want 

 will excite the respiratory centre. Oxygen want is 

 thus not by itself an adequate stimulus to the respira- 

 tory centre; but it helps the action of CO 2 , or if we 

 like to put it otherwise, causes the respiratory centre 

 to react in presence of a degree of blood alkalinity 

 which would be too high to excite it under normal 

 conditions. 



Although a slight, or even a considerable, deficiency 

 in the oxygen pressure of the air breathed produces 

 no immediate effect on the breathing, yet a long-con- 

 tinued deficiency has a very distinct effect; and the 

 study of the effects of a long-continued deficiency has 

 furnished, I think, one of the most interesting chap- 

 ters in recent physiology. To observe the effects of 

 long-continued deficiency it is only necessary to go 

 to places at high altitudes, where the barometric pres- 

 sure is low, but where men nevertheless live under 

 perfectly healthy conditions. The Anglo-American 

 expedition to Pike's Peak in 1911 had for its object 

 the careful study of these effects. 



On going to a very high altitude the breathing is 

 increased at once, and the alveolar CO 2 pressure falls 

 correspondingly ; but if the altitude is only very mod- 

 erate there is at first no effect on the breathing, just 

 as happens when air containing a moderately reduced 

 percentage of oxygen is breathed in the laboratory 

 for a short time. After some days, however, it will 



