P LE TU Y SMOGB: A P HIC EXPERIMENTS.
457
the tendency to contraction increases with the number of
induction shocks per second. For example, in one case we may
have an intensity of 150, a duration of 10" and a rate of one shock
per second. In a second case we may suppose the intensity to he 10,
the duration to he 10" and the number of shocks per second to be 15.
The total stimulation in both cases is the same, but, in the first instance
we might have dilatation, and in the second contraction.
The effect of the induction shocks is cumulative. The stimulus
imparted by the induction shock is practically instantaneous, but its
effect abides for an appreciable length of time, and gradually subsides.
If a second shock is received before the effect of the first is completely
worn away, and then another shock at an equal interval of time, and so
on, the effect of the stimulus will go on increasing up to a certain point.
Hence, a single induction shock, or a limited number of such shocks,
which alone are not of sufficient intensity to produce any effect on the
blood-vessels, may be able to do so, if they are repeated in a sufficient
length of time.
It would be idle to assume that the effect of a powerful shock wears
away as quickly as that of a weak one, but we are not justified in
assuming that the length of time during which the effect of such a
stimulus persists is directly proportional to the intensity of the shock.
It is very possible that a weak shock may produce a greater proportional
disturbance in the nerve than a strong one. No doubt a portion of the
electrical energy of the shock is converted into some form of molecular
disturbance in the nerve ; but we have no reason to assume that the
amount of energy thus appropriated is always in exact ratio to the
intensity of the shock. Hence, it appears probable that in two cases
where the total stimulation is the same, the irritation may be greater
with rapidly interrupted weak shocks, than with shocks of greater
intensity following one another at greater intervals.
As the susceptibility of different frogs to shocks of varying intensity
and frequency varies, so may that of the same frog under different, con¬
ditions.
It is evident that the apparatus and methods used in these investi¬
gations can be profitably employed in studying many problems in vaso¬
motor physiology that remain to be investigated.
The plates are exact photographic reproductions of tracings. The
size of the tracings was reduced one-ninth to accommodate them to the
size of the page. Much of the delicacy and sharpness of the original
tracings is lost in the reproductions, which are in other respects perfectly