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- 



