NERVES. 



77 



cussion or by a rapid succession of percussions (Heiden- 

 hain) ; (6) by chemical agents which either deprive the 

 nerve of water or disintegrate it. 



In all measurable modes of excitation, the muscular 

 effect increases with the stimulus up to a certain limit, 

 beyond which there is no further augment. Excitations 

 just sufficient to produce the maximum effect are called 

 "maximal"; others "over maximal" or "minimal" as 

 the case may be. The effect of a minimal excitation is 

 increased when the seat of excitation is in cathelectrotonos ; 

 the effect of a maximal is diminished when it is in anelec- 

 trotonos. 



Excitation of one or more of the constituent fibres of a 

 nerve is without effect on the others : it is incapable of 

 propagation from the excited fibre to any other struc- 

 ture excepting the end-organ in which it terminates. 



The phenomena known as the " paradoxical twitch " and the " secondary 

 twitch from the nerve," which are apparent exceptions to the above statement, 

 are due to electrotonic variation. 



If by the myograph or otherwise the time is measured 

 which elapses between an instantaneous excitation of a 

 motor nerve and the beginning of the contraction of the 

 muscle which it supplies, first with the seat of excitation 

 close to the muscle, and then with the seat of excitation 

 at 2 '6 centims. distant, it is found that there is a slight 

 difference amounting to about one-thousandth of a second 

 between the two measurements. 



Living nerve is electromotive. The phenomena closely 

 agree with those of muscle. In an undivided nerve no 

 electrical differences manifest themselves either in the 

 normal state or during excitation. In a severed nerve, 

 the cut surface is found to be negative to the sound sur- 

 face ; the difference is, however, much less than in muscle. 

 An instantaneous excitation of any part of a severed 

 nerve produces a momentary diminution of the relative 



