38 
certain amount of kation-proteid to be on the point of break- 
ing down at points which are about to be made kathodic by 
the polarising current, then, if the strength of the polarising 
current be insufficient to decompose the whole, an additional 
excitation will be aided by the effect already present. If, 
however, the polarising current has decomposed all the ion- 
proteid most immediately available, irritability at kathodic 
points will decrease. 
We now touch upon the curious fact, that during the 
closure of a constant current, after the make twitch, no per- 
ceptible effect is usually produced in striated muscle until the 
current is broken. 
This depends upon two factors : the supb-rior stimulation 
efficiency of rapid variation of current density (to be con- 
sidered later) : and, secondly, the comparative exhaustion of 
ion-proteid material at the kathode after make. It is obvious 
that such exhaustion must take place sooner or later, and we 
need not be surprised at its taking place immediately after 
the initial twitch, for, as we have seen, the duration of the 
current has an effect upon the height of the make twitch, 
inasmuch as it augments it ;* that is to say, the constant cur- 
rent decomposes a large amount of ion-proteid material 
initially, to produce the make twitch. 
We can account for the fact that persistent closure con- 
traction takes place more usually, and to a greater degree, in 
smooth than in striated muscle, by the higher value in the 
former of the ''threshold number"— for an excess of free 
kations might be liberated by the current, sufficient to cause 
a considerable increase in tone of the muscle, and yet insuffi- 
cient to cause displacement, and so initiate a wave of nega- 
tivity. Net only is variation of current density ordinarily 
of importance, but the comparative exhaustion of ion-proteid 
material after the make greatly increases the necessity for 
such variation in a way that will be explained shortly. Hence 
we cannot wonder that in such highly sensitive • contractile 
material as striated muscle persistent closure contractions are 
not usually seen in a marked degree. 
Biedermannf states that a wave of contraction, initiated 
in an extra-polar tract, cannot pass the kathode of a polaris- 
ing current of certain intensity, while it can the anode. This 
is not due to the persistent closure contraction, because ''inhi- 
bition is most pronounced when a persistent descending cur- 
rent in the upper half of the muscle has reduced the original 
persistent closure contraction to a minimum." I can account 
* Biedermann : Eleotro-physiology : Trans, by F. A. Welby, 
vol. i., page 176. 
t Ibid., vol i., page 296. 
