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 



