150 THE ELECTRICAL PHENOMENA OF MUSCLE. [CH. xn. 



and then begins slowly to decline. This long tail denotes only the 

 gradual disappearance of polarisation of the mercury meniscus. 



The meaning of such photographic records becomes clear by testing the 

 electrometer with known differences of potential, and from such data it is 

 possible to construct what maybe called an interpretation diagram (fig. 174). 

 The horizontal line is that of equipotentiality 

 of the two surfaces of contact Q;) and (<#). 

 The curve P' expresses the relative negativity 

 of the surface Qy) ; the curve D', the cor- 

 responding relative negativity of the sur- 

 face (rZ). S' is a curve of which the ordinates 

 are the algebraic sums of the corresponding 

 ordinates of P' and D'. S is the photographic 

 curve which expresses S' ; P is the photo- 



p' graphic curve which expresses P' (monophasic 



variation). The numbers under the horizontal 

 line indicate hundredths of a second ; the dis- 

 tance tt' expresses the time taken by the wave 

 in its progress from Q?) to (cT). 



From these considerations we can now 

 pass to study what occurs when the 

 muscle enters into tetanus. The simplest 

 case is that which was first observed by 

 Du Bois Reymond. He placed his nou- 

 polarisable electrodes in the positions 

 indicated in fig. 173, one (p) on the 

 comparatively uninjured surface, the 

 other (d) on the devitalised cut end. 

 He sent in the tetanising series of shocks 

 at A. The electrical response is under 

 these circumstances a summation of the 

 individual electrical responses evoked by 

 instantaneous stimuli ; and the mono- 

 phasic character of the single response 

 explains easily that which occurs during 



Fig. 174. Interpretation dia- fpfprma tVio ^em-H-P nf tV>f> rmir>lp VIP 

 gram. (Burdon Sanderson.) 



comes more negative or less positive 



than it was before, and so the electrical difference of potential 

 between the centre and the injured end is lessened. But with 

 regard to uninjured muscle the problem is not so easy. It is at 

 first sight difficult to sec why the summed effects of a series of 

 diphasic variations should take the direction of the first phase, 

 as was found to be the case by Du Bois Reymond in experiments 

 with the frog's gastrocnemius. One would have anticipated that 

 " negative " variation in the arithmetical sense would be absent 

 altogether, and this is the case in absolutely normal muscles ; 



