CH. XL] THE DIPHASIC VARIATION 123 



current of rest docs not exist; it is really a current produced by 

 injury, and is now generally called a demarcation current : the more 

 the ends of the muscle are injured the more positive they become ; 

 and when they are connected to the uninjured centre, a current 

 naturally is set up as described by Du Bois Ueyrnond. If a muscle 

 is at rest and absolutely uninjured it is iso-electric ; that is, it gives 

 no current at all when two parts of it are connected together by a 

 wire. 



Since Du Bois Keymond's researches, the electrical changes 

 which occur during a single twitch have been studied, also, and 

 before we can understand the " negative variation " of tetanus, it is 

 obviously necessary to consider the electrical variation which 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 

 positive than it was before ; it then rapidly returns to its previous 

 condition. The increase of positivity indicates a disturbance of 

 the stability of the tissue ; the disappearance of this increased 

 positivity 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. 100) may 

 be most readily studied in a curarised muscle, that is, in a muscle 

 which is physiologically 

 nerveless. The electrical 

 variation travels at the 

 same rate as the visible 

 contraction, but precedes it. 



Suppose two points (p 

 and d) of the muscle (fig. 

 141) are connected by non- 

 polarisable electrodes to a 

 galvanometer, and that the 

 muscle-wave is started by a 

 single stimulus applied at 



A ; as soon as the wave FlG 141 



reaches p this point be- 

 comes positive 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, for when the wave reaches the 

 point d, that point is positive to p, and the galvanometer needle 

 moves in the opposite direction. 



* The time will vary with the distance between p and d. 



