READJUSTMENTS OF REGULATION 55 
improved the method further, and found that both 
in animals and in ourselves we got results wholly con- 
sistent with the diffusion theory, provided that the 
percentage of CO was kept very low. If sufficient 
CO was given to produce symptoms of oxygen want 
we got active secretion. We also got active secretion 
if oxygen want was produced in a group of muscles 
by fatiguing work. Nevertheless the human experi- 
ments gave on the whole a much less striking result 
than the former ones, and we could not at the time 
see any reason for this. 
The apparent acclimatisation to oxygen want in 
mountaineers or persons living at high altitudes then 
attracted our attention, and in conjunction with Yan- 
dell Henderson the Pike’s Peak Expedition (in which 
he, Douglas, Schneider and I participated, while Miss 
Fitz Gerald made observations at neighboring mining 
camps and towns) was planned. When we reached 
the summit of Pike’s Peak (14,100 feet) we were all 
more or less blue in the lips, as were other newcomers. 
We then suffered in various degrees for two or three 
days from mountain sickness, after which the blueness 
entirely disappeared, although our alveolar oxygen 
pressures remained nearly the same as while the blue- 
ness was present, and our haemoglobin percentages 
had not as yet risen appreciably. After this we made a 
number of determinations of the arterial oxygen pres- 
sure, and each one without exception showed a consid- 
erably higher pressure of oxygen in the arterial blood 
than in the alveolar air. On the other hand, when we _ 
breathed during the experiment air rich in oxygen, so 
