84 A CHEMICAL SIGN OF LIFE 
impulse is determined by the chemical change going on 
in the nerve at the time of stimulation, and that it is 
the resting respiration, or metabolism, which seems to 
determine how fast the nerve impulse should travel along 
the fiber. It is exactly as if, during rest, the nerve sub- 
stance was sustained in a very unstable state by the 
expenditure of energy by processes which set free the 
carbon dioxide. It is as if the irritable or unstable con- 
dition was like a stone rolled partly up a hill and kept 
there at the cost of considerable panting by the toiling 
demon of life. When the nerve impulse comes along, 
the stone escapes from his grasp and rolls downhill. 
During the period of rest or recovery which follows, 
this tiny, toiling Sisyphus, with infinite labor and pant- 
ing, pushes the stone uphill. The higher he gets it the 
more he gasps, the more unstable it becomes the more 
easily it escapes his grasp, the more rapidly does it crash 
down, and the more irritable is the nerve the more 
rapidly does the impulse travel. 
Conclusion.—Basing our conclusions on the foregoing 
experimental facts, we may express the relation between 
excitation, conduction, and respiration in nerves as 
follows: 
The maintenance of chemical activity, or metabolism, 
is responsible for that unstable condition in the nerve, 
whatever its nature, which we call the state of irrita- 
bility or excitability. All irritable tissue must respire. 
The tissue cannot be made irritable and then kept so 
without effort. Chemical energy must constantly be 
expended to keep the tissue irritable. The amount of 
this expenditure of energy is not the same at all points 
along the fiber, but it diminishes in one direction or the 
