258 THE ORIGIN OF THE NERVOUS SYSTEM 



being effectively transmitted to greater distances down 

 than up a gradient. If the electric current produced 

 at a region of excitation is the transmitting factor this 

 difference is readily understood (p. 97). As structural 

 differentiation of transmission paths progresses, the 

 morphological features of this relation to the gradients 

 become more and more evident, until the conditions 

 found in the higher animals and man are attained. 

 Moreover, as regards the functional relations of neurons, 

 even if we admit that conduction may occur without 

 decrement in the single axon, the probability exists of 

 quantitative differences in excitation in different neu- 

 rons and of the relation of these differences to levels of 

 the gradients and to the changes in the gradients which 

 occur during development and evolution. 



These suggestions do not involve the assumption 

 that the growth of the axons is determined by the 

 passage of nervous impulses, but if the suggestions 

 advanced in chapters iv and xi have any basis in fact, 

 the growth of the axon and the transmission of excitation 

 are both determined by the electrical factor. The 

 growth of the axon is the physiological effect of increase 

 in internal positivity produced by the electric currents 

 in a physiological gradient, and transmission is the 

 physiological effect of the electrical currents associated 

 with a local excitation, this effect being primarily, 

 according to Lillie, an increase in internal positivity. 

 From this viewpoint the outgrowth of the axon and 

 the transmission of excitation are simply reactions of 

 protoplasm to essentially the same electrical conditions, 

 and it is not strange that they should exhibit certain 

 similarities. 



