cavity of the human ear and its consequent power of resonance, high
notes produce a particularly strong impression. When male animals utter
sounds in order to please the females, they would naturally employ
those which are sweet to the ears of the species; and it appears that
the same sounds are often pleasing to widely different animals, owing
to the similarity of their nervous systems, as we ourselves perceive in
the singing of birds and even in the chirping of certain tree-frogs
giving us pleasure. On the other hand, sounds produced in order to
strike terror into an enemy, would naturally be harsh or displeasing.
Whether the principle of antithesis has come into play with sounds, as
might perhaps have been expected, is doubtful. The interrupted,
laughing or tittering sounds made by man and by various kinds of
monkeys when pleased, are as different as possible from the prolonged
screams of these animals when distressed. The deep grunt of
satisfaction uttered by a pig, when pleased with its food, is widely
different from its harsh scream of pain or terror. But with the dog, as
lately remarked, the bark of anger and that of joy are sounds which by
no means stand in opposition to each other; and so it is in some other
cases.
There is another obscure point, namely, whether the sounds which are
produced under various states of the mind determine the shape of the
mouth, or whether its shape is not determined by independent causes,
and the sound thus modified. When young infants cry they open their
mouths widely, and this, no doubt, is necessary for pouring forth a
full volume of sound; but the mouth then assumes, from a quite distinct
cause, an almost quadrangular shape, depending, as will hereafter be
explained, on the firm closing of the eyelids, and consequent drawing
up of the upper lip. How far this square shape of the mouth modifies
the wailing or crying sound, I am not prepared to say; but we know from
the researches of Helmholtz and others that the form of the cavity of
the mouth and lips determines the nature and pitch of the vowel sounds
which are produced.
It will also be shown in a future chapter that, under the feeling of
contempt or disgust, there is a tendency, from intelligible causes, to
blow out of the mouth or nostrils, and this produces sounds like pooh
or pish. When any one is startled or suddenly astonished, there is an
instantaneous tendency, likewise from an intelligible cause, namely, to
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