1. p. 266.
The sound of laughter is produced by a deep inspiration followed
by short, interrupted, spasmodic contractions of the chest, and especially
of the diaphragm.[8] 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.
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. 1-3, Plate III., different degrees
of moderate laughter and smiling have been photographed.
The figure of the little girl, with the hat is by Dr. Wallich,
and the expression was a genuine one; the other two are
by Mr. Rejlander. Dr. Duchenne repeatedly insists[9] that,
under the emotion of joy, the mouth is acted on exclusively
by the great zygomatic muscles, which serve to draw the corners
backwards and upwards; but judging from the manner in which the upper
teeth are always exposed during laughter and broad smiling,
as well as from my own sensations, I cannot doubt that some
of the muscles running to the upper lip are likewise brought
into moderate action.
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