CHAP, ii.] THE CONTRACTILE TISSUES. 63 



no currents of rest exist. We have stated (p. 61) that the surface 

 of the uninjured inactive ventricle of the frog's heart is isoelectric, 

 no currents being observed when the electrodes of a galvanometer 

 are placed on two points of the surface. Nevertheless a most 

 distinct current is developed whenever the ventricle contracts. 

 This may be shewn either by the galvanometer or by the rheos- 

 copic frog. If the nerve of an irritable muscle-nerve preparation 

 be laid over a pulsating ventricle, each beat is responded to by 

 a spasm of the muscle of the preparation. In the case of ordinary 

 muscles too instances occur in which it seems impossible to regard 

 the electrical change manifested during the contraction as the 

 mere diminution of a preexisting current. 



Accordingly Hermann and those who with him deny the 

 existence of 'natural' muscle-currents speak of a muscle as de- 

 veloping during a contraction a ' current of action,' occasioned as 

 they believe by the muscular substance as it is entering into the 

 state of contraction becoming negative towards the muscular 

 substance which is still at rest, or has returned to a state of 

 rest. In fact, they regard the negativity of muscular substance 

 as characteristic alike of beginning death and of a beginning 

 contraction. So that in a muscular contraction a wave of 

 negativity, starting from the end-plate when indirect, or from 

 the point stimulated when direct stimulation is used, passes 

 along the muscular substance to the ends or end of the fibre. 

 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, unattended 

 by any visible events, known to us, at present, only by an electrical 

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

 terminations 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 . 



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. 



