THE PLACE OF BIOLOGY. 85 



this gas in the arterial blood ; and it has been found 

 that it is to this that the activity of the centre responds 

 in regulating the breathing so exactly. Now, carbon 

 dioxide when dissolved in a liquid such as the blood 

 acts as a very weak acid, although its influence as an 

 acid is far less in the blood than in pure water. Careful 

 investigation has shown that the arterial blood is faintly 

 alkaline, and that what the respiratory centre is doing 

 in regulating the partial pressure of carbon dioxide 

 in the blood is in reality to regulate this faint alkalinity. 

 No existing physical or chemical method of demon- 

 strating changes in alkalinity approaches in delicacy 

 the discrimination of the respiratory centre ; but by 

 indirect means the proof has been given that the 

 variations in the activity of the centre are caused by 

 minute variations in the alkalinity of the blood. 



In the respiratory centre we thus have, to use orthodox 

 mechanistic language, a " mechanism " tuned to react 

 with astounding delicacy and constancy to the minutest 

 changes in the alkalinity of the blood. This supposed 

 mechanism consists of what is called " protoplasm " ; 

 and protoplasm is something which from the physical 

 and chemical standpoint is excessively unstable, and 

 sensitive to every slight change in its environment. 

 Yet this unstable mechanism reacts in the human body 

 hour after hour, day after day, year after year, true as 

 tempered steel, to one absolutely definite and absolutely 

 puny stimulus ! How can we conceive that such con- 

 stancy is maintained ? Knowing what we do of the 

 nature of living protoplasm, the answer must be that 

 the physiological environment of the respiratory centre 

 remains constant. But how, considering only the vary- 



