CHEMICAL SIGNS OF IRRITABILITY II 
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 
