CHEMICAL SIGNS OF IRRITABILITY 43 



sodium salt solutions of varying concentrations and has 

 already discovered confirmatory evidence for the increase 

 of metabolism during chemical stimulation. It may 

 be added here in passing that the different solubility of 

 carbon dioxide in these salt solutions cannot alone explain 

 our results, for there is not enough difference in solu- 

 bility of this gas in such dilute equimolecular solutions 

 of potassium and sodium chloride whose effects on carbon 

 dioxide production are so divergent, the former salt 

 diminishing, the latter increasing, it. 



The fact that during chemical stimulation the nerve 

 gives off more carbon dioxide is made evident, also, by 

 the use of low concentrations of anesthetics. If the 

 concentration is so low as to give a primary stimulation 

 to the nerve, the production of this gas is greatly acceler- 

 ated at the beginning of immersion of the nerve in the 

 narcotics. This is an additional evidence that there 

 is a relation between excitation and metabolic activity. 



Stimulation in hydrogen- -The last experiment which 

 we shall describe in this connection is on the quantitative 

 estimation of the carbon dioxide production in a nerve 

 when the latter is in an atmosphere of hydrogen and when 

 it is being stimulated by an electrical current. We 

 expected to find here one of two things. First, there 

 is evidence, to which reference has already been made, 

 that nerves left in hydrogen gas show diminished irri- 

 tability and that they give off smaller amounts of carbon 

 dioxide than do the same nerves in air. This fact led 

 us to anticipate that these nerves, being thus less irritable 

 than normal nerves, would produce less than the usual 

 increment of carbon dioxide on excitation. This would 

 be the case if the increment were a proper measure of the 



