It is an equally obscure point why the corners of the mouth are
retracted and the upper lip raised during ordinary laughter.
The mouth must not be opened to its utmost extent, for when this occurs
during a paroxysm of excessive laughter hardly any sound is emitted;
or it changes its tone and seems to come from deep down in the throat.
The respiratory muscles, and even those of the limbs,
are at the same time thrown into rapid vibratory movements.
The lower jaw often partakes of this movement, and this
would tend to prevent the mouth from being widely opened.
But as a full volume of sound has to be poured forth, the orifice
of the mouth must be large; and it is perhaps to gain this
end that the corners are retracted and the upper lip raised.
Although we can hardly account for the shape of the mouth
during laughter, which leads to wrinkles being formed beneath
the eyes, nor for the peculiar reiterated sound of laughter,
nor for the quivering of the jaws, nevertheless we may infer
that all these effects are due to some common cause.
For they are all characteristic and expressive of a pleased
state of mind in various kinds of monkeys.
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