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 



