76 ORGANISM AND ENVIRONMENT 
the heart, or the nervous system, which is the regu- 
lator, but the metabolic activity of the body as a whole. 
The blood circulates at such a rate as is sufficient to 
keep its composition approximately constant at any 
part of the body, and the rate of flow seems to be 
greater or less at any one part in proportion as the 
causes tending to disturb the composition of the blood 
are greater or less at the same part. Among the chief 
of these causes is consumption of oxygen and libera- 
tion of CO,. Hence the circulation rate is to a large 
extent determined by the activity of the latter pro- 
cesses, and varies, just as the breathing varies, in such 
a way as to keep the gas pressures in each part of the 
body approximately constant. 
This is not an isolated fact in physiology. Claude 
Bernard pointed out in 1878 in his Lecons sur les 
phénoménes de la vie that the blood is a fluid of re- 
markably constant composition, and practically pro- 
vides a constant internal environment for the living 
cells of which the body of a compound organism is 
made up. He seems to have been led to this conclu- 
sion by his well-known studies on the sugar of the 
blood. While still under the influence of the old ideas 
of the blood as a very variable liquid he began his 
investigations under the expectation that the amount 
of sugar in the blood would vary in proportion to the 
sugar absorbed by the intestine, and would disappear 
when no sugar or other food was taken. To his 
astonishment, however, he found sugar still abun- 
dantly present in the blood during starvation, and that 
any increase which he could produce in the blood 
