8o A CHEMICAL SIGN OF LIFE 



fishes, mammals, crabs, and arachnids. We feel certain, 

 therefore, that the existence of a metabolic gradient 

 in nerve fibers correlated with the functional activity, and 

 related to the direction the nerve impulse takes, must be 

 a very general phenomenon. From these facts we come 

 to the very simple conclusion that is summarized in the 

 following statement: The normal nerve impulse passes 

 from a point of higher toward a point of lower carbon 

 dioxide production from the more irritable to the less 

 irritable parts. There is also a decrement in the nerve 

 impulse. It cannot proceed indefinitely along a nerve; 

 it will ultimately die out. 



Velocity of the nerve impulse and its relation to respira- 

 tion. If the nerve impulse cannot pass through the 

 fiber without consuming substance, it is reasonable to 

 expect that there may be some relation between the 

 rate of production of carbon dioxide in the resting nerve 

 and the velocity of the nerve impulse. The reason for 

 such a supposition is clear. It has already been indi- 

 cated that the more irritable a nerve is the more carbon 

 dioxide does it produce in the resting state. Hence, 

 the more it respires, the more irritable it is, the faster 

 should it conduct the impulse. This is actually found, 

 within limits, to be the case. If one compares corre- 

 sponding nerves of different animals, or different nerves 

 of the same animal, it is found, other things being equal, 

 that there is a relation between the speed of contraction 

 of the muscles supplied by the nerve and the velocity 

 with which the nerve supplying the muscle conducts 

 the impulse. Obviously it would be foolish to have 

 a very rapidly contracting muscle, or limb, supplied with 

 a nerve which conducted the impulse to the muscle at a 



