16). Whatever its situation, it must be connected by
association fibres with the centres of phonation and articulation.
[Illustration: FIG. 18]
[Description: FIG. 18.--The accompanying diagram is an attempt to explain
the course of innervation currents in phonation.
1. Represents the whole brain sending voluntary impulses _V_ to the regions
of the brain presiding over the mechanisms of voluntary breathing and
phonation. These two regions are associated in their action by fibres of
association _A_; moreover, the corresponding centres in the two halves of
the brain are unified in their action by association fibres _A'_ in the
great bridge connecting the two hemispheres (Corpus Callosum). On each side
of the centre for phonation are represented association fibres _H_ which
come from the centre of hearing; these fibres convey the guiding mental
images of sounds and determine exactly the liberation of innervation
currents from the centre of phonation to the lower centres by which the
required alterations in tension of the laryngeal muscles for the production
of the corresponding sounds are effected. Arrows are represented passing
from the centre of phonation to the lower centres in the medulla which
preside over the muscles of the jaw, tongue, lips, and larynx.
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