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- 

 medulla ted, when stimulated increase their output of 



