5 io SCIENCE PROGRESS 



This incomplete historical survey affords us one more 

 instance of what is so interesting in the progress of science — 

 the tendency towards concreteness in conception. We begin 

 in Antiquity with "spirits" in the nerves; the science of the 

 Renaissance converts these into succus nerveus, an incom- 

 pressible fluid such as was being investigated by the physicists 

 of that time ; the eighteenth century gives us the vis nervosa, 

 which later is identified with the electric current then being 

 studied both in Italy and in England. In the nineteenth 

 century we have nerve-impulses not only measured as to the 

 velocity of their travelling, but actually rendered visible through 

 their concomitant electrical effects. Nerve-impulses are not 

 electricity, but they produce it and can be manifested by it. 

 Thus each generation must think and express itself in the 

 language of its own time. 



