CHAP, ii.] THE CONTRACTILE TISSUES. 113 



i.e. A will be negative relatively to B, and this will be shewn by 

 a deflection of the needle. A little later B will be entering into 

 contraction, and will be becoming negative towards the rest of the 

 fibre including the part under A, whose negativity by this time 

 is passing off, that is to say B will now be negative towards A, 

 and this will be shewn by a deflection of the needle in a direction 

 opposite to that of the deflection which has just previously taken 

 place. Hence between two electrodes placed along a fibre a single 

 wave of contraction will give rise to two currents of different 

 phases, to a diphasic change ; and this indeed is found to be 

 the case. 



This being so it is obvious that the electrical result of tetanizing 

 a muscle when wave after wave follows along each fibre is a com- 

 plex matter ; but it is maintained that the apparent negative 

 variation of tetanus can be explained as the net result of a series of 

 currents of action due to the individual contractions, the second 

 phase of the current in each contraction being less marked than 

 the first phase. We cannot however enter more fully here into a 

 discussion of this difficult subject. 



Whichever view be taken of the nature of these muscle-currents, 

 and of the electric change during contraction, whether we regard 

 that change as a 'negative variation' or as a 'current of action/ 

 it is important to remember that it takes place entirely during the 

 latent period. It is not in any way the result of the change of 

 form, it is the forerunner of that change of form. Just as a 

 nervous impulse passes down the nerve to the muscle without any 

 visible changes, so a molecular change of some kind, attended by 

 no visible events known to us at present, but only by an electrical 

 change, runs along the muscular fibre from the end-plate to the 

 ends of the fibre, preparing the way for the visible change of form 

 which is to follow. This molecular invisible change is the work 

 of the latent period, and careful observations have shewn that it, 

 like the visible contraction which follows at its heels, travels along 

 the fibre from a spot stimulated towards the ends of the fibres, in 

 the form of a wave having about the same velocity as the contrac- 

 tion, viz. about 3 metres a second 1 . 



The Changes in a Nerve during the passage of a Nervous 



Impulse. 



68. The change in the form of a muscle during its contrac- 

 tion is a thing which can be seen and felt ; but the changes in a 

 nerve during its activity are invisible and impalpable. We stimu- 

 late one end of a nerve going to a muscle, and we see this followed 



1 In the muscles of the frog; but as we have seen having probably a higher 

 velocity in the intact mammalian muscles, within the living body, and varying 

 according to circumstances. 



F. 8 



