376 ELECTRO-PHYSIOLOGY CHAP. 



excitatory wave naturally varies considerably. Bernstein observed 

 this, and Kiihne, to whose experiments we shall return later, 

 found that the velocity, and with it the length of the excitatory 

 wave, varied considerably. In the most unfavourable cases the 

 former was 25 cms. per sec., in other cases, on the contrary, 

 more than 2 m. This recalls the striking fact that the same 

 muscle may propagate slow and rapid waves of contraction, and 

 we are in fact in both cases concerned with the same phenomenon, 

 since there is nothing to hinder the identification of " excitatory 

 wave " and " wave of excitation." It only remains, therefore, 

 to establish the relations between this latter and the " wave of 

 contraction." The fact that muscular contraction implies a latent 

 period, which, according to Bernstein, is absent in the " excitatory 

 wave," is a priori evidence that the " excitatory 

 wave outruns the wave of contraction, partially 

 at least, in an excited muscle fibre." 



As early as 1854, indeed, Helmholtz stated 

 that the negative variation, at any rate in its 

 steepest part where the secondary twitch is 

 excited, was the precursor of contraction. He 

 located it at the middle, v. Bezold later on 

 at the beginning, of the latent period. Helm- 

 holtz (22) arranged his experiment as fol- 

 FIO. ii9.-peri<xi of nega- lows : The nerve A of a muscle (Fig. 119) 

 tive variation. Helm- connected with the writing - point of a 



holtz s experiment. x 



myograph, bridged over the longitudinal and 

 transverse sections of the muscle J5, the nerve of which was 

 excited by a break induction shock, so that the negative variation 

 of the muscle current of B produced secondary contraction in 

 the muscle A. The measurable interval between the moment 

 of exciting the primary preparation and the beginning of the 

 secondary twitch of A was the sum of the four following time- 

 values : (i) interval between arrival of nerve excitation in A 

 and beginning of contraction, i.e. latency period of A ; (ii) 

 interval corresponding with propagation of excitation in nerve 

 of muscle A, from point of excitation to muscle ; (iii) interval 

 between arrival of excitation in B, and moment at which the 

 negative variation excites nerve A ; (iv) period of conductivity 

 in nerve of B. By deducting the known intervals (found by 

 other experiments), 1, 2, and 4 from the sum the required 



