ORGANIC DETERMINATION 87 



unusual want of oxygen or excess of carbon 

 dioxide in the blood. 



Now it seemed extremely improbable that 

 such a theory could be correct ; since living 

 organisms do not regulate their affairs in this 

 mechanical manner. Thus guided, we rein- > 

 vestigated the whole question in the Oxford 

 Physiological Laboratory, and found that as a 

 matter of fact the breathing is, under normal 

 conditions, so regulated in man as to re 

 spond with almost incredible exactness to the 

 slightest variation in the output of carbon 

 dioxide or partial pressure of carbon dioxide 

 in the alveolar air. The vagus nerve-fibres play 

 no part in the main regulation, to which their 

 influence is only subsidiary, though not unim 

 portant. 



Under ordinary conditions the constancy in 

 the pressure of carbon dioxide in the lung 

 alveoli provides for the supply of oxygen to 

 the lungs and blood ; but quite evidently the 

 breathing is, under abnormal conditions, regu 

 lated also in direct relation to the oxygen 

 supply ; and the action of the lung epithelium, 

 the concentration of haemoglobin in the blood, 

 and the mode in which oxyhsemoglobin is 



