This speech zone acts as a whole, and many disorders of speech may arise
from destructive lesions within its limits. It has a special arterial
supply, viz. the middle cerebral, which divides into two main branches--an
anterior, which supplies the motor portion, and a posterior, which supplies
the posterior sensory portion. The anterior divides into two branches and
the posterior into three branches, consequently various limited portions of
the speech zone may be deprived of blood supply by blocking of one of these
branches. The speech zone of the left hemisphere directly controls the
centres in the medulla oblongata that preside over articulation and
phonation; innervation currents are represented by the arrows coming from
the higher to the lower centres.]
These several cortical regions are connected by systems of subcortical
fibres to two regions in front of the ascending frontal convolution (_vide_
fig. 17), called respectively the "glosso-kinaesthetic" (sense of movement
of tongue) and the "cheiro-kinaesthetic" (sense of movement of hand) centres.
Now a person may become hemiplegic and lose his speech owing either to the
blood clotting in a diseased vessel, or to detachment of a small clot from
the heart, which, swept into the circulation, may plug one of the arteries
of the brain.
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