[11] This explanation is,
however, inadmissible, for, although we cannot easily tickle ourselves by
the contact of the skin with our own fingers, we can do so with the aid of
a foreign body, like a feather. We may perhaps suppose that, as
ticklishness has probably developed under the influence of natural
selection as a method of protection against attack and a warning of the
approach of foreign bodies, its end would be defeated if it involved a
simple reaction to the contact of the organism with itself. This need of
protection it is which involves the necessity of a minimal excitation
producing a maximal effect, though the mechanism whereby this takes place
has caused considerable discussion. We may, it is probable, best account
for it by invoking the summation-irradiation theory of pain-pleasure, the
summation of the stimuli in their course through the nerves, aided by
capillary congestion, leading to irradiation due to anastomoses between
the tactile corpuscles, not to speak of the much wider irradiation which
is possible by means of central nervous connections.
Prof. C.L. Herrick adopts this explanation of the phenomena of
tickling, and rests it, in part, on Dogiel's study of the tactile
corpuscles ("Psychological Corollaries of Modern Neurological
Discoveries," _Journal of Comparative Neurology_, March, 1898).
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