right-handed person produces no loss of speech; but if the person is
left-handed there is aphasia, because he, being left-handed, uses the third
_right_ inferior frontal convolution for speech. These facts have for long
been accepted by most neurologists, but recently doubts have been cast upon
this fundamental principle of cerebral localisation by a most distinguished
French neurologist, M. Marie; he has pointed out that a destructive lesion
of the cortex may be accompanied by subcortical damage, which interrupts
fibres coming from other parts of the brain connected with speech.

In the study of speech defects it is useful to employ a diagram; a certain
part of the brain corresponds to the _Speech Zone_ there indicated, and
lesions injuring any part of this area in the left hemisphere cause speech
defects (_vide_ fig. 17). All neurologists, M. Marie included, admit this,
and the whole question therefore is: Is a destruction of certain limited
regions of the superficial grey matter the cause of different forms of
speech defects, or are they not due more to the destruction of subcortical
systems of fibres, which lie beneath this cortical speech zone?

There is a certain portion of the speech zone which is assumed to be
connected with the revival of written or printed language, and is called
the _visual word-centre_. There is another region connected with the memory
of spoken words--the _auditory word-centre_; you will observe that it is
situated in the posterior third of the first temporal convolution, but this
does not comprise nearly the whole of it, for there is an extensive surface
of grey matter lying unseen within the fissure, called the transverse
convolutions, or gyri. Lesions of either of these regions give rise to
_Sensory Aphasia_, which means a loss of speech due to inability to revive
in memory the articulate sounds which serve as verbal symbols, or the
graphic signs which serve as visual symbols for language.