THE REGULATING MECHANISM 173 



96. A change passes along a fibre when stimu- 

 lated. This may be termed a nervous impulse. 

 We do not know what this change is ; no 

 movement of matter can be observed ; 

 obscure chemical phenomena have been noted, 

 as shown by the necessity for oxygen and the 

 production of carbonic acid. Electrical 

 charges can be detected which seem, like the 

 nervous impulse, to pass along a nerve, but 

 are not to be confounded with it ; and the 

 impulse travels along the fibre with a velocity 

 of only 200 feet per second, incomparably 

 slow, as compared with the velocities of 

 electricity or sound. Recent observations, 

 made with a new form of galvanometer 

 Einthoven's string galvanometer a very sen- 

 sitive instrument, seem to show that the 

 velocity is considerably greater than has 

 been supposed. It would seem also that when 

 the fibre is stimulated at any point the 

 impulse travels in both directions. Nerve- 

 fibres are conductors, but, unlike an electrical 

 conducting arrangement, they are not only 

 conductors, because, at the point stimulated, 

 a change is there generated which is then 



