[Illustration: FIG. 17]

[Description: FIG. 17.--Diagram to illustrate the Speech Zone of the left
hemisphere (Bastian). This scheme is used to explain the mechanism of
speech, but probably the centres are not precisely limited, as shown in the
diagram; it serves, however, to explain disorders of speech. Destruction of
the brain substance in front of the central fissure gives rise to what is
termed Motor Aphasia and Motor Agraphia, because the patient no longer
recalls the images of the movements necessary for expressing himself in
articulate speech or by writing. Destructive lesions behind the central
fissure may damage the portion of the brain connected with the mental
perception of the sounds of articulate language, or the portion of the
brain connected with the mental perception of language in the form of
printed or written words--Sensory Aphasia; the former entails inability to
speak, the latter inability to read.

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_