associated with a pleasurable state of mind; for throughout a large
part of the animal kingdom vocal or instrumental sounds are employed
either as a call or as a charm by one sex for the other. They are also
employed as the means for a joyful meeting between the parents and
their offspring, and between the attached members of the same social
community. But why the sounds which man utters when he is pleased have
the peculiar reiterated character of laughter we do not know.
Nevertheless we can see that they would naturally be as different as
possible from the screams or cries of distress; and as in the
production of the latter, the expirations are prolonged and continuous,
with the inspirations short and interrupted, so it might perhaps have
been expected with the sounds uttered from joy, that the expirations
would have been short and broken with the inspirations prolonged; and
this is the case.
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.
A graduated series can be followed from violent to moderate laughter,
to a broad smile, to a gentle smile, and to the expression of mere
cheerfulness. During excessive laughter the whole body is often thrown
backward and shakes, or is almost convulsed; the respiration is much
disturbed; the head and face become gorged with blood, with the veins
distended; and the orbicular muscles are spasmodically contracted in
order to protect the eyes. Tears are freely shed. Hence, as formerly
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