92 ORGANISM AND ENVIRONMENT 
in a circle when we attempt to explain the constancy 
of the internal environment by the specific characters 
of bodily structure. The fact is that both the internal 
environment and the “structure” of the body remain 
approximately constant; but of this fact no explana- 
tion has been reached. 
The explanation cannot lie in the external environ- 
ment, since this is far less constant than the internal 
environment, which it constantly tends to disturb. 
It is nevertheless the case that the external environ- 
ment, in so far as it is in relation with the organism, 
exhibits constancy. The composition and amount of 
the food and drink in the alimentary canal approxi- 
mate to a certain average; the partial pressures of 
oxygen and carbon dioxide in the air which is in 
contact with the body, in the lungs, remain also nearly 
constant under most conditions ; the impressions trans- 
mitted inwards from without are similarly more or 
less constant on an average; and excesses of heat or 
cold are generally avoided. Just as the internal en- 
vironment seems, at first sight, to be regulated by the 
organism, so also does the external environment, but 
to a far less intimate extent. In both external and 
internal environment, the regulation is the expression 
of a balancing of opposing processes of loss or gain 
of material or energy ; and the processes involving loss 
are no less persistent on the whole than those involving 
gain. 
It is mainly through the nervous system that the 
body is, in the higher organisms, in relation with the 
external environment. When we look broadly at the 
