NERVOUS CONDUCTION AND EXCITATION 141 



light ; and the train of thought which prompted Helmholtz 

 to experiment is instructive. *' As long as physiologists thought 

 it necessary," he argued, " to refer nerve-actions to the move- 

 ments of an imponderable or psychical principle, it must 

 have appeared incredible that the velocity of the movement 

 could be measured within the short distances of the body. 

 At present we know from the researches of du Bois Raymond 

 upon the electromotive properties of nerve that those activities 

 by means of which the conduction of an excitation is 

 accomplished are in reality actually conditioned by or at least 

 closely connected with an altered arrangement of their material 

 particles. Therefore conduction in nerves must belong to 

 the series of self-propagating reactions of ponderable bodies. 

 . . ." Only six years before Helmholtz' experiment Johannes 

 Miiller had denied the possibility of ever attaining the means 

 of measuring the velocity of nervous transmission ; the history 

 of mechanistic thought is paved with such denials. 



The rate of propagation of the excited disturbance varies 

 greatly in different animals. In the following tables some 

 examples are given : — 



