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 
