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, 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,, 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, 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 
