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 Arbacia. 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. 91.) 



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 



