56 ORGANISM AND ENVIRONMENT 
as to bring the alveolar oxygen to about the normal at 
sea level, the difference between arterial and alveolar 
oxygen pressure almost disappeared. We then deter- 
mined the arterial oxygen pressure in a newcomer who 
was still blue, but did not become mountain-sick till 
some hours later. It was very little above the alveolar 
oxygen pressure; but three days later when he was 
acclimatised and well, his arterial oxygen pressure was 
as high as our own. The mean result was that on 
Pike’s Peak, after acclimatisation, the arterial oxygen 
pressure was during rest only about 13 mm. lower than 
at sea level, but was 35 mm. higher than the alveolar 
oxygen pressure. The complete absence of any blue- 
ness after acclimatisation was thus easily intelligible. 
The lungs were actively secreting oxygen into the 
blood, even during rest. Nevertheless the blueness 
reappeared temporarily during prolonged muscular 
exertions, as in a long climb. The lung epithelium 
could thus apparently be fatigued by the extra work 
thrown upon it. 
As already seen, the lung epithelium is at all times 
capable of actively secreting oxygen inwards if the 
requisite stimulus arising from oxygen want in the 
tissues is present. But at high altitudes this capacity 
is greatly increased, and secretion goes on continuously 
after acclimatisation. The stimulus, moreover, is 
essentially the same stimulus as produces the changes 
in the regulation of blood alkalinity and in the haemo- 
globin of the blood. The stimulus is want of oxygen 
in some form; but how does the want of oxygen act? 
The haemoglobin of the arterial blood must, after 
