f that part, stimulation of which gives rise to trunk movements
(_vide_ fig. 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. Arrows
indicate also the passage of innervation currents from the centres in the
brain which preside over voluntary breathing. It will be observed that the
innervation currents which proceed from the brain pass over to the opposite
side of the spinal cord and are not represented as coming into relation
with the respiratory centre _R_. This centre, as we have seen, acts
automatically, and exercises especially its influence upon the diaphragm,
which is less under the control of the will than the elevators of the ribs
and the abdominal muscles.

The diagram also indicates why these actions of voluntary breathing and
phonation can be initiated in either hemisphere; it is because they are
always bilaterally associated in their action; consequently both the higher
centres in the brain and the lower centres in the medulla oblongata and
spinal cord are united by bridges of association fibres, the result being
that even if there is a destruction of the brain at _a-b_, still the mind
and will can act through both centres, although not so efficiently.
Likewise, if there is a destruction of the fibr