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 fibres proceeding from the brain
centres to the lower medullary and spinal centres, the will is still able
to act upon the muscles of phonation and breathing of both sides of the
body because of the intimate connection of the lower medullary and spinal
centres by association fibres.
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