EXPERIMENTS IN STIMULATION, 61 
persons susceptible to this kind of stimulation, the 
muscular action passes completely beyond the power 
of the will. Lastly, I may further observe, what 
I do not think has ever been observed before, that 
even in the domain of psychology the action of this 
principle admits of being clearly traced. We find 
it, for instance, in the rhythmical waves of emotion 
characteristic of grief, and at the other extreme we 
find it in the case of the ludicrous. We can endure 
for a short time, without giving any visible re- 
sponse, the psychological stimulation which is 
supplied by a comical spectacle; but if the latter 
continues sufficiently long in a sufficiently ludicrous 
manner, our appropriate emotion rapidly runs up 
to a point at which it becomes uncontrollable, and 
we burst into an explosion of ill-timed laughter. 
But in this ease of psychological tickling, as in the 
previous case of physiological tickling, some persons 
are much more susceptible than others. Neverthe- 
less, there can be no doubt that from the excitable 
tissues of a plant, through those of a jelly-fish and 
a frog, up even to the most complex of our psycho- 
logical processes, we have in this recently discovered 
principle of the summation of stimuli a very 
remarkable uniformity of occurrence. 
Effects of Temperature on Excitability. 
I shall now conclude this chapter with a brief 
statement of the effects of temperature on the 
excitability of the Medusze; and before stating my 
results, I may observe that in all my experiments 
in this connection I changed the temperature of the 
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