CHEMICAL SIGNS OF IRRITABILITY 41 
metabolic activity, as shown by the output of carbon 
dioxide. 
Mechanical stimulation.—Since the ordinary method 
for mechanical stimulation cannot be used directly on 
the nerve in the biometer in its present form, in view of 
the fact that the chamber has to be kept shut, we used 
a different method, namely, that of injuring the nerve 
by crushing. That when protoplasm is smashed vigor- 
ous chemical changes result is well established. Fletcher 
reports that injured muscle gives off more carbon dioxide 
than normal muscle; later he and Hopkins discovered 
that muscle under a similar condition is richer in lactic 
acid. Mathews observed a similar increase in carbon 
dioxide in the crushed eggs of Avrbacia. We have 
discovered that if a nerve is crushed with a rough edge 
of a glass rod it gives off more carbon dioxide than the 
normal one; that is, an injury increases the carbon 
dioxide output of the nerve. Since this increase of 
carbon dioxide cannot be produced by crushing an 
unexcitable nerve, we consider this injury to be a form 
of mechanical stimulation. (For further consideration 
of this subject see p. gt.) 
Chemical stimulation—The study of the nature of 
chemical stimulation has been so thoroughly made that 
it might seem ideal to study quantitatively the increased 
production of the gas following the stimulation of the 
nerve by various salt solutions. But there are com- 
plications which seriously interfere with the use of this 
method. We found, for instance, that the presence of 
minute quantities of a foreign liquid is a seriously 
disturbing factor for a quantitative estimate of carbon 
dioxide. Qualitatively, however, we found various 
