THE ELECTRICAL RESPONSE. 



421 



middle of the muscle, the distal near its tibial end, which in the one 

 case was devitalised, but not in the other. It is seen that the ascending 

 limb is identical in the two curves, but that, from the point at which 

 the diphasic curve approaches its culmination, the " injury curve " 

 diverges from it, continuing to ascend. The inclination, however, 

 rapidly diminishes until, from T f v to y^ of a second after excitation, 

 the line becomes first horizontal and then begins slowly to decline. 

 Shortly after the beginning of the decline, the two leading-off contacts 

 become equipotential. The meniscus, however, does not reach its zero 

 position until later. During the period which corresponds to the second 

 phase in the uninjured muscle, the difference of potential in the injured 

 muscle, which at the moment corresponding to the divergence of the 

 two curves is still strongly negative, diminishes at first rapidly, then 

 very gradually, until it finally disappears. 



The estimate usually given of the duration of the 

 monophasic variation is mainly founded on a beautiful 

 experiment of Helmholtz's, in which he measured the 

 latency-period in the same muscle when excited under 

 different conditions. In one case the nerve was excited 

 directly by an induction shock, and the muscle through 

 it; in the other, the nerve of the same muscle was 

 excited by the monophasic variation of another muscle, 

 of which the nerve was excited directly. In the first 

 case the period of latency was shorter than in the 

 other by about ^ho second. He therefore concluded 

 that the rheoscopic limb receives the excitation to 

 which it responds, T (foo of a second later than it 

 would have received it had its nerve been directly 

 excited — or, as he then supposed, at the middle of 

 the period of latent stimulation of the exciting muscle. 

 Our photographic records tell us that at that moment 

 the column of the electrometer is shot up by the 

 sudden rise of tension at the proximal electrode ; and 

 it can scarcely be doubted that it is by this change 

 that, in Helmholtz's experiment, the nerve of the 

 recording muscle is excited. ' ' ' ' ' — _J 



At the moments of culmination of the diphasic and 20 Sec. 



monophasic variations respectively, the difference of Fig. 234. 



potential is expressed in much larger proportion 



by the inclination of the curve at the point corresponding to that moment, 

 than by the distance of that point from the zero line. Thus, in the examples 

 published in 1895, 1 the maximum "negativity" was found to be from T ^ 

 to ygy of a volt, of which amount only y^u or nnnr were due to the height 

 of the meniscus above its zero position. Fig. 234, which must be understood 

 to be nothing more than a diagram, serves to illustrate this. The black 

 area is that of the diphasic variation, the black and shaded together make 

 up that of the monophasic. The line oabd is that of the " tension curve " 

 of the diphasic, the line oar J that of the monophasic. The difference 

 of potential between the contacts at the moment of culmination of the first 

 phase is two or three times as great as that of the "negative variation," 

 as estimated by du Bois-Reymond. This important fact was proved 

 some years ago by a method of investigation employed by myself in association 



1 Burdon Sanderson, "The Electrical Response to Stimulation of Muscle," Joum. 

 Physiol., Cambridge and London, 1895, vol. xviii. p. 117. 



