iv ELECTROMOTIVE ACTION IN MUSCLE 381 



the moment at which the beginning of the excitatory wave has 

 reached, the point named, i.e. so long after each individual 

 stimulus as the wave requires in order, to travel over the 

 distance (P, a), a, .perceptible deflection of the magnet may be 

 expected in the sense that (a) will be negative to the second 

 led -off jfoint (b), if it is correct that each point under the 

 excitatory wave is negative to each point beyond it. If the 

 closure of the galvanometer circuit is advanced still further in 

 the same direction, so that other superficial points of the curve 

 of variation are excluded, the effects must at first increase in 

 the same direction, reach a maximum when the excitatory 

 wave is at its acme, and finally decline to zero when the 

 entire wave of excitation has passed the point (a). Starting 

 from Bernstein's computation of 10 mm. for the length of the 

 wave, with the two leading-off electrodes at more than 10 mm. 

 from each other, the end of the excitatory wave will have 

 passed the point (a) before the first part reaches (b), and the 

 same still obtains a little later, provided the electrodes are 

 sufficiently far apart. At a certain adjustment of the rheotome 

 slider, corresponding with this interval, there will therefore be 

 hardly any difference in potential between (a) and (&). It is not 

 till the closure of the galvanometer circuit is so delayed after each 

 single stimulus that the first part of the excitatory wave has 

 already reached the point (b) that there will again be any 

 marked deflection, and that in a direction diametrically opposite 

 to the earlier variation, since (b) is now negative to (a). The 

 difference in potential increases as before with further advance- 

 ment of the galvanometer closure, attains a maximum, and 

 finally, when the end of the excitatory wave has passed under 

 (&), declines to zero. Thus, on leading-off from two symmetrical 

 longitudinal points of a muscle, rhythmically excited (tetanised) 

 by induction shocks, there is, after each single stimulus, a double 

 variation, or, more properly, a diphasic current of action. Bern- 

 stein, to whom we owe the discovery of this fact, named the 

 current which appears while the wave of excitation is passing 

 under the point a, and has the direction of the lower arrow 

 in the muscle (Fig. 120), the negative variation, that which 

 follows it in the direction of the upper arrow, the positive 

 variation. It is evident that the absolute magnitude of 

 the deflection produced by the first action current should be 



