52 ORGANISM AND ENVIRONMENT 
results of Miss Fitz Gerald’s observations on the 
haemoglobin percentages in persons permanently living 
at different altitudes. These observations were all 
made by the colorimetric method of determination 
which I introduced a few years ago, and with a care- 
fully standardised instrument. It will be seen that 
just as the alveolar CO, rises with fall in the baro- 
metric pressure, so the haemoglobin percentage rises. 
It appears also that in an atmosphere with a higher 
oxygen pressure than air at sea level a decrease in the 
haemoglobin percentage below what is termed “nor- 
mal” would occur. Here also, then, the setting of the 
regulation of haemoglobin percentage is altered by 
change in environment. 
Using the carbon monoxide method of Lorrain 
Smith and myself, we found that on going to a high 
altitude not only the percentage amount, but also the 
total amount of haemoglobin in the blood is increased. 
The total volume of the blood seems to diminish 
at first, thus raising the concentration of haemo- 
globin; but after a few days the volume of the blood 
increases above normal. The regulation of total 
haemoglobin, concentration of haemoglobin, and blood 
volume are thus all dependent on the oxygen pressure 
of the air breathed. 
I now come to what was the most striking result of 
the expedition. In the lungs the blood is separated 
from the alveolar air by an extremely thin membrane 
consisting of the “protoplasm” of flattened epithelial 
cells. Do these cells play any active part in the gaseous 
exchange between the air and the blood? Or does 
