[9]
We need not, however, go so far back in the zooelogical series to explain
the origin and significance of tickling in the human species. Sir J.Y.
Simpson suggested, in an elaborate study of the position of the child in
the womb, that the extreme excitomotory sensibility of the skin in various
regions, such as the sole of the foot, the knee, the sides, which already
exists before birth, has for its object the excitation and preservation of
the muscular movements necessary to keep the foetus in the most favorable
position in the womb.[10] It is, in fact, certainly the case that the
stimulation of all the ticklish regions in the body tends to produce
exactly that curled up position of extreme muscular flexion and general
ovoid shape which is the normal position of the foetus in the womb. We may
well believe that in this early developed reflex activity we have the
basis of that somewhat more complex ticklishness which appears somewhat
later.
The mental element in tickling is indicated by the fact that even a child,
in whom ticklishness is highly developed, cannot tickle himself; so that
tickling is not a simple reflex. This fact was long ago pointed out by
Erasmus Darwin, and he accounted for it by supposing that voluntary
exertion diminishes the energy of sensation.
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