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 atmospherie 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 
