READJUSTMENTS OF REGULATION 49 
age results of measurements of alveolar CO, pressure 
made by Miss Fitz Gerald, in connection with the 
Pike’s Peak Expedition, on persons residing perma- 
nently at different altitudes. It will be seen that the 
alveolar CO, pressure diminishes regularly with alti- 
tude, starting from sea level. That this diminution 
_ is a response to the diminished alveolar oxygen pres- 
sure there can be no doubt. If the barometric pres- 
sure is kept steady, and the oxygen pressure is dimin- 
ished by lowering the percentage of oxygen, the re- 
sults are precisely the same, so far as can be judged by 
the available observations; and, as was first clearly 
pointed out by Paul Bert, practically all the physiologi- 
cal disturbances produced by low barometric pressures, 
or high altitudes, are due to lowering of the oxygen 
pressure. 
From Figure 3 it is pretty evident that if the oxygen 
pressure is raised above the normal value at sea level, 
the alveolar CO, pressure will rise still higher. That 
this is actually the case has recently been shown by 
Hasselbalch and Lindhard, who have confirmed in a 
steel chamber many of the Pike’s Peak results, and 
have added further observations of their own. It 
appears from their results that the alveolar CO, — 
pressure does not rise much higher after the normal 
oxygen alveolar oxygen percentage has been exceeded ; 
but the fact that there is a rise is of great interest, as 
showing that even the “normal” alveolar CO, pressure 
depends on the existing alveolar oxygen pressure. 
What is the significance of the fall in alveolar CO, 
pressure at low barometric pressures? It might be 
