CHEMICAL SIGNS OF IRRITABILITY 33 
duction per gram is reduced thereby. But one thing 
seems to be certain, ie., that to attribute the carbon 
dioxide production in the nerve fiber to the connective 
tissue cells surrounding the nerve trunk, as Bayliss does, 
is rather ridiculous. Nerve cells evidently breathe 
at about the same rate as nerve fibers, and not faster, as 
one might suppose. Table IV summarizes the carbon 
dioxide production by various nervous tissues, some of 
which contain cells and others only fibers. 
Summary.—wWe have thus far shown, then, that the 
living nerve fiber is no exception to the rule that all 
living matter undergoes chemical changes. It respires. 
‘There is a chemical sign of irritability in the nerve. 
By the use of a proper apparatus of sufficient delicacy 
we can demonstrate experimentally the formation of 
carbon dioxide in all nerves; and by estimating the 
amount produced under various conditions—conditions 
which we know affect the state of irritability of the 
normal nerve in the body—we find a very close parallel- 
ism between the amount of carbon dioxide produced and 
the state of irritability. We are justified, therefore, in 
concluding that the gas thus measured is the expression 
of the metabolic activity of the nerve. We may now 
pass on to discover whether this carbon dioxide is 
increased in case of stimulation. 
