REGULATION OF ENVIRONMENT 83 
environment we find that they are no less persistent 
than the activities which maintain its constancy. The 
muscles still continue to consume oxygen and form 
heat, even though they are for the time at rest, and 
though all loss of heat from them is prevented. The 
kidneys still absorb oxygen when they are not secret- 
ing. Ina sense, too, they are still secreting, even when 
there is no external sign of secretion, for the absence 
of external secretion is only the expression of an equal 
balance between constant intake and constant output 
of material, When the muscles and sense-organs are 
not at work on the getting of food, or in other con- 
servative processes, they seem to employ themselves 
otherwise—for instance in what we know as play. 
No physiological facts are more significant than 
those relating to the persistence of the fundamental 
metabolic phenomena. In Liebig’s time it was observed 
that the excretion of urea rises and falls with the 
amount of nitrogenous food consumed, although dur- 
ing starvation there is still a certain minimum excre- 
tion of urea. This was interpreted as signifying that 
all superfluous nitrogenous food simply falls a prey to 
oxygen, and is wasted. When, however, the facts 
were further investigated it was found that within 
wide limits the oxidation in the body does not increase 
or diminish with increase or diminution of the nitrog- 
enous food consumed. Even after long starvation 
the oxygen consumption per unit of body weight is 
practically undiminished during rest. When more 
nitrogenous food is consumed in the body and oxidised 
to urea, less fat or carbohydrate is consumed. 
