48 
tion is -kept sufficiently dilute to ensure complete dissociation 
of the salts, the frequency of the beat will be greater the 
greater the concentration. And, obviously, the frequency 
will be less the p-reater the threshold number. 
We further notice that if the ions diffusing into the 
muscle gathered unchecked on the muscle side of the sur- 
face, diffusion would shortly cease because of the approxi- 
mation of the concentrations on both sides — the process could 
not be kept up. But we know that this is not the case ; a 
periodic discharge of anions takes place which, by releasing 
kations, starts waves of negativity, giving rise to the contrac- 
tions, or, when the heart is bathed in a solution with a posi- 
tive stimulation efficiency, tihe periodic discharge is one of 
kations starting, as before, a wave of negativity. The con- 
centration after each contraction is, therefore, on the muscle 
side, kept automatically constant, as far as anions are con- 
cerned : on the fluid side it is kept absolutely constant by cir- 
culation, but during the intervals between contractions the 
difference between the concentrations on the two sides is not 
constant, but continually falls off. Another fact to be con- 
sidered is that the difference between the velocities of en- 
trance of the anions and kations will diminish progressively 
during the intervals between contractions owing to the electro- 
static repulsion, due to the excess of one kind of ion which has 
entered, tending to accelerate the other kind of ion and 
retard the ion bearing a similar charge. Finally, we have 
to take into account the reciprocal influence of kations and 
anions in altering the threshold number — kations will aug- 
ment the threshold number for anions, and anions will aug- 
ment the threshold number for kations. Hence the 
threshold number will be greater the less the difference be- 
tween the velocities of the anions and kations on entering the 
muscle. Also, it is possible that kations of one kind may 
raise the threshold number for kations of another kind (when 
the solution contains two or more salts). Hence the threshold 
number, and consequently the extent of contraction, will 
vary considerably in different solutions. 
It is obvious that a number of conditions must be satis- 
fied in order that a solution may be able to keep a heart beat- 
ing. Thus, the threshold number must be reached on the 
muscle side by the faster-moving ions before their velocity 
has been reduced to that of the slower-moving ions, by the 
electrostatic force which they develop on the muscle side. 
This involves the difference between the velocities of the 
anions and kations, the influence of one sort of ion in rais- 
ing the threshold number for another sort, and the difference 
between the concentrations of the ions on the two sides of the 
muscle. Then, again, if the frequency of the beat is too 
