REGULATION OF ENVIRONMENT 87 
diminution in the alkalinity of the blood. But the 
duration of the responses is modified by stimuli de- 
pendent on inflation or deflation of the lungs, while 
the extent of inflation or deflation which is effective 
in this direction depends on the strength of the 
primary chemical stimulus. The effect of this primary 
stimulus is also dependent on the supply of oxygen to 
the centre, and is increased if the oxygen supply is 
defective. If we prefer to put the matter in another 
way, deficiency of oxygen is itself a stimulus to the 
centre, but is dependent for its effect on the reaction 
of the blood, and is quite ineffective if the alkalinity 
increases slightly. Other substances, such as morphia, 
chloral, or chloroform, diminish the responses of the 
centre to a given diminution in blood alkalinity ; and 
from the analogy of other tissues we may be quite 
sure that slight changes in the concentration of the 
salts and other substances in the blood, or changes 
in its temperature, must similarly affect the response 
of the centre in one direction or another. We can even 
imagine the respiratory centre responding, not, as 
normally, to changes in alkalinity, but to changes in the 
concentration of, say, calcium salts. 
When we seek for the “cause” of a physiological 
reaction we are thus landed in a maze of contributory 
causes. We can wander in this maze for as long as we 
like, but there is no end to it. So far as it is possible 
to judge, those who seek in physiological phenomena 
for the same kind of causal explanations as can 
usually be assigned in connection with inorganic phe- 
