26 A CHEMICAL SIGN OF LIFE 
decidedly less carbon dioxide than the nerves of normal 
frogs or nerves taken from frogs whose circulation had 
been suspended for a period of time equal to that of 
etherization. A perfect parallelism was found to exist 
between the carbon dioxide production and the state 
of excitability of the nerve. Thus small quantities of 
anesthetics have often the effect of increasing at first 
the excitability of the nerve, and it was found that such 
quantities also produced at first an increase in the carbon 
dioxide. A further consideration of the effects of 
anesthetics on the metabolism of the claw nerve of the 
spider crab will be found in chapter iv. The important 
fact is that since these agents are known to affect the 
normal uncut nerve im situ and also to modify carbon 
dioxide production in an isolated nerve, and in a manner 
parallel with their known actions on irritability, it is 
certain that at least the larger part of the carbon dioxide 
we measure in an isolated resting nerve must have been 
produced by a physiological process. 
Carbon dioxide production in a hydrogen atmos phere.— 
Although many nerves remain alive for a long time in an 
atmosphere free from oxygen, they generally exhibit a 
lowered irritability when compared with nerves in normal 
air. It has been found, for example, that if nerves 
remain in the body after the circulation of a frog has 
ceased, so that they have not been supplied with oxygen 
for some time, they are by no means so easily stimulated 
by a salt solution as are normal nerves. Their vitality 
is reduced. A similar change occurs in nerves taken 
out of the body and put in hydrogen gas. In them, also, 
irritability is decidedly diminished. If, now, carbon 
dioxide is produced in these nerves by a vital process, 
