under the armpits. I touched with a bit of paper the sole of the foot
of one of my infants, when only seven days old, and it was suddenly
jerked away and the toes curled about, as in an older child. Such
movements, as well as laughter from being tickled, are manifestly
reflex actions; and this is likewise shown by the minute unstriped
muscles, which serve to erect the separate hairs on the body,
contracting near a tickled surface.[806] Yet laughter from a ludicrous
idea, though involuntary, cannot be called a strictly reflex action. In
this case, and in that of laughter from being tickled, the mind must be
in a pleasurable condition; a young child, if tickled by a strange man,
would scream from fear. The touch must be light, and an idea or event,
to be ludicrous, must not be of grave import. The parts of the body
which are most easily tickled are those which are not commonly touched,
such as the armpits or between the toes, or parts such as the soles of
the feet, which are habitually touched by a broad surface; but the
surface on which we sit offers a marked exception to this rule.
According to Gratiolet,[807] certain nerves are much more sensitive to
tickling than others. From the fact that a child can hardly tickle
itself, or in a much less degree than when tickled by another person,
it seems that the precise point to be touched must not be known; so
with the mind, something unexpected a novel or incongruous idea which
breaks through an habitual train of thought appears to be a strong
element in the ludicrous.
The sound of laughter is produced by a deep inspiration followed by
short, interrupted, spasmodic contractions of the chest, and especially
of the diaphragm.[808] Hence we hear of laughter holding both his
sides. From the shaking of the body, the head nods to and fro. The
lower jaw often quivers up and down, as is likewise the case with some
species of baboons, when they are much pleased.
Moderate Laughter and Smiling. Plate III
During laughter the mouth is opened more or less widely, with the
corners drawn much backwards, as well as a little upwards; and the
upper lip is somewhat raised. The drawing back of the corners is best
seen in moderate laughter, and especially in a broad smile the latter
epithet showing how the mouth is widened. In the accompanying figs.
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