CHAPTER II 
CHEMICAL SIGNS OF IRRITABILITY IN THE 
NERVE FIBER 
There are various chemical processes which occur in 
all forms of living matter and which we might examine 
in order to see whether they are associated with the 
property of irritability, but we naturally seek to make 
use of that one which is the easiest to detect. Among 
these chemical processes there are, in the first place, 
the processes concerned in growth. All living matter 
has the power of building up complex proteins, fats, 
and carbohydrates as long as it is vigorously alive. 
But it is clear that this process would be very hard to 
measure quantitatively without killing the living matter 
and determining how much substance it has produced. 
And there are also other objections to using growth as a 
measure of vitality. Another chemical process found 
in all, or nearly all, forms of matter is respiration. By 
' respiration we mean the gaseous exchange of living 
matter with its environment: the taking on of oxygen 
and the production of carbon dioxide. This is a very 
much more promising line of experiment to follow in 
measuring life and metabolism, for, in the first place, it 
is universal, as I shall presently show, and, in the second 
place, the oxygen may be measured, or the carbon 
dioxide given off may be determined, without injuring 
the living matter. It was for this reason that the 
carbon dioxide was selected for study as probably 
Io 
