CHAPTER III 



CHEMICAL SIGNS OF IRRITABILITY IN THE NERVE 



FIBER Continued 



Increased metabolism on stimulation. We have 

 already stated that all living matter, whether it is an 

 organism or an isolated tissue, normally undergoes 

 chemical changes and produces carbon dioxide as one 

 of the final products of its metabolic activity, and that 

 the nerve fiber is no exception to this rule. In other 

 words, respiration is one of the unfailing signs of life 

 and is a necessary condition for living processes. But 

 carbon dioxide production from a tissue is not by itself 

 a sufficient sign of life. For there are many chemical 

 compounds which spontaneously give off carbon dioxide, 

 among others sea-water, bicarbonate solutions, as well 

 as organic materials which are unstable. It would 

 obviously be a mistake to call these compounds living 

 because of the fact that they give off this gas. This 

 criterion alone, therefore, cannot be used for detecting 

 the vitality of the tissues. 



Not only is it common for many non-living matters to 

 give off carbon dioxide spontaneously, but there are 

 also some whose mode of gaseous exchange is remarkably 

 similar to that of the living process. Among these 

 substances there is none in which the parallelism to 

 vital respiration is more detailed and interesting than 

 ordinary linseed oil. The many curious resemblances 

 of the chemical processes involved in painting to proto- 



34 



