CHEMICAL SIGNS OF IRRITABILITY n 



being the easiest of quantitative determination and 

 undoubtedly correlated with the most fundamental 

 vital processes. 



The idea that respiration is one of the most funda- 

 mental of vital phenomena is by no means a novel view. 

 Even in the earliest times breathing was supposed to 

 be the process most intimately connected with life. 

 When a man stopped breathing he died. As early as 

 the second century Galen had the notion that there must 

 be a pneumatic spirit in the air which kept up life, 

 and he predicted that some day it would be discovered. 

 It was after the lapse of fifteen hundred years that this 

 prediction was verified or fulfilled when Mayow, an 

 English physician, discovered that there was a gas, or 

 spirit, in the air which was essential to life and com- 

 bustion. Later, oxygen was discovered by Priestly, and 

 it was Lavoisier who first showed that this oxygen after 

 entering the lungs came out again as carbon dioxide; 

 and he proved that animal heat was due to the com- 

 bustion of the materials of the body by the oxygen to 

 form water and carbon dioxide, and that the sole source 

 of energy of living things was this combustive change. 

 In selecting respiration as the chemical test of life we 

 are, therefore, selecting that most fundamental reaction 

 by virtue of which living things get their energy. It 

 is clear that it is this reaction, rather than any other 

 chemical reaction, which touches most closely the 

 phenomena of irritability; for, to move or to think, 

 we must have energy. It is much better to take this 

 reaction, rather than those chemical changes which 

 are related to growth or the repair of waste, as a 

 criterion of living, for the very essence of a living thing 



