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Experiments on animals and observations on human beings show that the
centres presiding over the muscles of the larynx are situated one in each
hemisphere, at the lower end of the ascending frontal convolution in close
association with that of the tongue, lips, and jaw. This is as we should
expect, for they form a part of the whole cerebral mechanism which presides
over the voice in speech and song. But because the muscles of the tongue,
the lower face muscles, and even the muscles of the jaw do not necessarily
and always work synchronously and similarly on the two sides, there is more
independence in their representation in the cerebral cortex. Consequently a
destruction of this region of the brain or the fibres which proceed from it
to the lower executive bulbar and spinal centres is followed by paralysis
of the muscles of the opposite side. Likewise stimulation with an
interrupted electric current applied to this region of the brain in monkeys
by suitable electrodes produces movements of the muscles of the lips,
tongue, and jaw of the opposite side only. Not so, however, stimulation of
the region which presides over the movements of the muscles of the larynx,
for then _both_ vocal cords are drawn together and made tense as in
phonation.
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