CHEMICAL SIGNS OF IRRITABILITY 55 
in proportion to carbon dioxide production, then it 
means that the nerve when stimulated would take up 
only 0.03 c.c. of oxygen during ten hours’ stimulation. 
It is extremely difficult, as everyone who has tried it 
knows, to free any gas from such small amounts of 
oxygen as those which are required to keep up irrita- 
bility. Our experience in freeing gases from traces of 
carbon dioxide makes us realize the difficulty of getting 
the nerve in the first place in a gas quite free from 
oxygen, and we believe that many experiments have 
been tried in which there is still some probability that 
enough oxygen remained to supply these small amounts 
needed. More delicate determinations will have to 
be made before we feel certain that nerves have been 
found to be irritable for some time in atmospheres which 
are free beyond question from all traces of oxygen. How 
shall we know when the gas we use is free from oxygen 
in these minute amounts? Yet until we know this it is 
impossible to study accurately the relation of irritability 
to oxygen. Meanwhile, however, we may recall the 
fact that carbon dioxide production in the spider crab’s 
nerve is not only reduced in the absence of oxygen, 
but also that we cannot increase its production in such 
an atmosphere by a stimulation which in the presence 
of oxygen increased the production of carbon dioxide 
over 200 per cent. These facts show conclusively, 
negative evidence to the contrary notwithstanding, that 
oxygen is in some way involved in the anabolism or 
katabolism of nerve fibers. 
Summary.—The facts presented in this chapter 
prove that all kinds of nerves, medullated and non- 
medullated, when stimulated increase their output of 
