54 A CHEMICAL SIGN OF LIFE 



oxygen consumption of the nerve appear to be on a 

 somewhat different footing. I say " appear to be/' 

 because the methods of determining the oxygen con- 

 sumption are still rather crude, and the studies have 

 been few. A nerve always gives off more carbon dioxide 

 when it is stimulated or active, whereas we know very 

 little about whether its intake of oxygen is increased 

 in anything like the same degree. The sciatic nerve 

 of a frog a medullated nerve can remain excitable 

 for a long time in the absence of atmospheric oxygen, 

 although its irritability diminishes under these cir- 

 cumstances, and, as already explained, its fatigability 

 increases. 



There is a considerable amount of evidence to show 

 that oxygen is very closely associated with* the state of 

 excitability. To harmonize these two facts, namely, 

 the independence of atmospheric oxygen and the fact 

 just stated, the oxygen-storage hypothesis has been 

 suggested, by which the exhaustion is attributed to 

 complete consumption of stored oxygen. Excitability 

 is restored when atmospheric oxygen is readmitted. 

 Without committing ourselves to this hypothesis, we may 

 add that according to Haberlandt's figure the resting 

 nerve of 10 mg. weight will consume only 0.0042 c.c. 

 oxygen in ten hours. If we take our figure of carbon 

 dioxide output and assume that one volume of oxygen 

 was necessary to produce one volume of carbon dioxide 

 (this assumption is made without any significance 

 except to give a liberal estimate), the carbon dioxide 

 production would require a consumption of about 

 0.015 c.c. of oxygen for ten hours. And if we assume 

 again that activity will increase oxygen consumption 



