226 THE ORIGIN OF THE NERVOUS SYSTEM 



already been pointed out that if the nerve fiber is a 

 physiological gradient when the impulse passes, a 

 decrement may occur, even though the reaction be "all 

 or none." During its growth period the axon possesses 

 a very active growing tip, and for the time being a 

 gradient probably extends a greater or less distance 

 from this tip. When the axon reaches its end organ 

 the tip as a region of special activity disappears, and 

 the gradient associated with the tip probably disappears 

 soon after. Exactly such changes as these occur in 

 many plant axes of limited growth. A gradient present 

 during the growth period in relation to a growing tip 

 may disappear completely after the tip ceases its activity 

 at the end of the growth period, and in some cases new 

 gradients may arise. In general the axon grows down 

 a general or local gradient, and after its growth ceases 

 it lies in the electrical field of the gradient which deter- 

 mined its growth, provided this gradient persists. It 

 seems probable that under such conditions the different 

 levels, particularly of the longer axons, must often be 

 in different physiological condition. Moreover, the 

 relation of the axon to the cell body, whether it be a 

 matter of transportative or of transmissive correla- 

 tion, may determine differences of condition at different 

 levels of the axon. 



The evidence, as far as it goes, indicates that these 

 gradients do exist in some axons, at least. Tashiro 

 (1914, 1915a, b) has found a gradient in C0 2 production 

 in the resting nerve, decreasing in the direction of func- 

 tional conduction, and in certain of the nerves investi- 

 gated by Tashiro I have found a susceptibility gradient 

 in the same direction, as indicated by the structural 



