4 2o SCIENCE PROGRESS 



reflex-arc preparation in a fit functional condition, the effect is 

 bound to follow the cause : and the whole process works with 

 the same inevitable certainty as the law of gravitation. This 

 fact is no longer questioned; nor is it questioned that so far as 

 the developed nervous system has been brought within the 

 range of observation, the impulses propagated through it are 

 likewise a physico-chemical manifestation, the precise nature of 

 which is still a subject of discussion. Now under the vitalist 

 hypothesis, there occur at certain points in the nervous system 

 sudden interferences with the normal processes ; as a result of 

 which a cause no longer produces its effect, and physical laws 

 or universally founded uniformities are suspended. Let us 

 attempt to visualise the process. 



In order to obtain a conception of a nerve impulse, let us 

 take a large number of billiard balls and arrange them in a 

 straight line at a few inches' distance from each other. Let us 

 now propel one of the balls at the end of the line against the 

 centre of the next ball. What happens ? The end ball gives 

 up its entire motion to the second ball J : the end ball comes to 

 a dead stop, while the second ball carries on the motion to the 

 third. In this way the original impulse travels right down 

 the line : each ball in turn takes up the motion from the one 

 behind it, and passes it on to the one in front, immediately 

 coming to rest itself. At the end of the experiment all the balls 

 will remain in the same straight line and at the same distances 

 from one another as at first, except, of course, for the last ball 

 of all, which will travel away with precisely the same velocity 

 that was originally impressed upon the first ball. 



Now this, of course, is a very rough representation of the 

 nervous impulse. It symbolises the fact, however, that, in the 

 nervous impulse, something is passed on from molecule to 

 molecule. That something is not motion, indeed; it appears 

 to be some kind of electromotive change : but whatever it is, 

 the molecule or other unit of the nerve-substance passes it 

 on, and then immediately reverts to its former quiescence. 



Now the vitalistic conception requires and affirms that 

 at certain points in the propagation of impulses a vital or 

 spiritual force intervenes and causes a diversion of the current 

 from the channel into which the material forces would by them- 

 selves have guided it. With the help of our analogy of the 



1 I assume that there is no friction, and therefore that the balls do not roll. 



