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



takes place during a twitch, for tetanus is made up of a fused 

 series of twitches. 



The electrical change during a twitch is called a diphasic 

 variation. The contracting part of a muscle becomes first more 

 negative than it was before ; it then rapidly returns to its 

 previously positive condition. The increase of negativity indicates 

 a disturbance of the stability of the tissue ; the return of the 

 galvanometer needle is the result of a return of the muscular 

 tissue to a state of rest. If the muscle is stimulated at one end, 

 a wave of contraction travels along it to the other end. This 

 muscle wave (see p. 125) may be most readily studied in a 

 curarised muscle, that is, in a muscle which is physiologically 



nerveless. The electri- 

 cal variation travels at 

 the same rate as the 

 visible contraction, but 

 precedes it. 



Suppose two points 

 (p) and (d) of the 

 muscle are connected 

 by non-polarisable elec- 



A LJ trodes to a galvano- 



meter, and that the 

 muscle wave is started 

 by a single stimulus 

 applied at A : as soon 

 as the wave reaches (p) this point becomes negative to (d), 

 and therefore a current flows from (d) to (p) through the 

 galvanometer (G). A moment later the two points are equi- 

 potential and no current flows ; a minute fraction of a second * 

 later this balance is upset, and now when the wave reaches the 

 point (d), that point is negative to (/>), and the galvanometer 

 needle moves in the opposite direction. 



The electrical variations may also be investigated by the 

 capillary electrometer ; the mercury moves first in one direction, 

 and then rapidly returns to its former position. The deep black 

 curve in the next figure (fig. 172) shows the record obtaining by 

 photographing the movement of the column of mercury on a 

 rapidly travelling photographic plate. 



The capillary electrometer has the advantage of giving us the means of 

 measuring the time of onset and duration of the electrical disturbance, and 

 experiments made with this instrument confirm the earlier experiments made 

 with the rheotome. They show that the change only lasts a few thousandths 



* The time will vary with the distance between (//) and (//). 



Fig. 171. 



