106 A CHEMICAL SIGN OF LIFE 
bicycle running rapidly will not fall down. The process 
of standing up may be compared to the vital process 
in the resting state. The degree of irritability will be 
measured by the force necessary to change the angle it 
makes with the ground. Perhaps the amount of life may 
be compared to the angle the bicycle makes with the 
ground. The metabolic activity is the force which 
moves the wheel. The locomotion of the wheel is the 
functional activity. There is, however, this difference— 
the faster the bicycle moves the more stable it is, whereas 
the faster the respiration drives the less stable is the 
irritability of the tissues. Our simile breaks down here. 
And indeed it is but a poor picture, of not much value. 
But whatever view we may take of the matter, we may 
at least be sure of this much: that chemical change is in- 
volved in irritability. The transmission of a nerve 
impulse involves material decomposition in the fiber. 
The impulse may be nothing else than the increased 
‘ metabolism itself. The nerve impulse is a very real 
thing, and it has a material basis which we may hope to 
discover. So far we have found two facts about it: 
first, it liberates carbon dioxide as it passes over the 
fiber; and, second, it depends on the nerve fiber having 
been previously oxidized or exposed to oxygen. Evi- 
dently combustion is involved in the process somewhere, 
but it appears at present more probable that it is involved 
in the creation of the irritable substance rather than in 
the very act of excitation itself. In other words, the 
oxidation is part of the process of repair or the recovery 
of the tissue—the process by which the state of irrita- 
bility is maintained—and not the process of transmission 
of the impulse itself. 
