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 CO2 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 COg, 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 CO2 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 



