41 
9. — The Influence of Varying Current Density. 
It seems probable that the reason for the importance of 
the steepness of increase in current density for evoking jnus- 
cular contractions lies in the diffusion of the kations away 
from the points which form the physiological kathode. When 
the kations are only very slowly liberated they diffuse away 
from the points where they are liberated, so that they never 
become concentrated at any point, and their mass at any 
point is never appreciable in comparison with the mass of 
ion-proteid with which they come in contact. Hence the 
kations diffuse through the whole muscle without the poten- 
tial having risen at any point high enough to evoke a per- 
ceptible contraction. This view is supported by Bieder- 
mann's statement that "the transmission of excitation from 
the seat of direct stimulation would seem, in the last resort, 
to be produced and conditioned by a rapid variation in the 
current."'^ 
Persistent closure contractions, however, appear to be 
due to a number of kations liberated by the action of the 
current at the different points in the muscle forming the 
physiological kathode. These kations are insufficient to 
cause a wave of negativity from any of these points, but by 
raising the potential at such points they evoke a persistent 
contraction. If such were the case we should expect to find 
that persistent closure contractions were more apt to occur in 
muscles in which the threshold number is large ; and this is the 
case, for ''the visible manifestations of persistent excitation 
fall into the background, w4iile the excitatory effects of cur- 
rent variation come prominently forward in proportion as 
the excitable protoplasm is more highly mobile, "■■!■ and we 
have seen that the less mobile a tissue is the greater is the 
threshold number (section 8). Thus we see why the dis- 
charge of the initial ''wave of negativity" tends to inhibit 
persistent closure contraction in striated muscle. i Only the 
more stable ion-proteid compounds are left at the kathode., 
and these, besides being fewer in number for the current to 
act on, present a greater resistance to the dissociative effect 
of the current, so that very few ions will be liberated at any 
given moment, and these will diffuse into the spongioplasm 
before any accumulated effect is possible. 
* Biedermann : Electro-physiology: Trans, by F. A. Welby, 
vol. i,. page 193. 
t Ibid., vol. i.. page 192. 
t Vide remarks on polar excitatiioii in muscle, section 9. this 
paper. 
