CHEMICAL SIGNS OF IRRITABILITY 15 
Tn order to study the whole of the respiratory metab- 
olism of a tissue, we should at least determine the 
oxygen consumption as well as the carbon dioxide 
production, and also generally the heat production. 
Inasmuch as the present problem, however, is concerned 
only with presenting direct evidence for the existence 
of metabolic activity in nerve fibers, we shall attempt 
to measure the carbon dioxide production alone; for 
while the lack of consumption of atmospheric oxygen 
may not necessarily indicate the absence of chemical 
changes, the production of carbon dioxide will surely 
prove the presence of metabolism, provided, of course, 
that we can prove that such carbon dioxide is formed by 
physiological processes. Furthermore, as carbon dioxide 
is the only universal expression of the respiratory 
activity in almost all anaérobic and aérobic plant and 
animal tissues in normal condition, metabolic activity 
is probably better represented by carbon dioxide pro- 
duction than by oxygen consumption, although we 
must restate here, of course, that the study of carbon 
dioxide alone will never reveal completely the nature of 
the metabolic activity. 
Method.—The method which was finally devised to 
detect and measure quantitatively the very minute 
amounts of carbon dioxide which it might be expected 
would be formed consisted essentially in determining 
the amount of carbon dioxide which was just sufficient 
to produce a deposit of barium carbonate in a film of 
half-saturated barium hydroxide solution. Barium car- 
bonate is- almost entirely insoluble in such a barium 
hydroxide solution, and a very small amount of precipi- 
tate can be detected with the aid of a small lens. The 
