THE SPEECH CENTRES.
The speech centre in its entirety includes certain cortical areas of the lateral surface of the hemisphere and, in right-handed individuals, is located on the left side.
a. The motor speech centre, Broca's centre presiding over the ability to speak, lies within Broca's convolution embracing the base of the gyrus frontalis inferior. It extends, perhaps, also to the adjacent part of the lowest region of the precentral convo lution and to the anterior part of the insula. Upon the integrity of this centre depends the ability to carry out the co-ordinated movements necessary in speaking. Damage of the centre leads, therefore, to abolition of the execution of motor speech. Voluntary speech, repeating words or reading aloud are no longer possible. This centre, therefore, is also termed the centre of motor aphasia.
b. The sensory speech centre, the tone-picture or auditory centre, lies within the posterior third of the gyrus temporalis superior and the adjoining part of the gyrus supramarginalis. It is also known as Wernicke' s centre and represents the cortical region where the memory-pictures of the heard and spoken words are retained. If the centre be injured, the patient, while still hearing the spoken word, can no longer compre hend what he hears. He has lost speech-understanding.
The centre is also designated as the centre for word-deaf ness or sensory aphasia.
c. The visual cen tre, where the memory-pic tures of written characters are stored, lies within the gyrus angularis. Injury of the centre is followed by inability to recognize the printed or written letters, or to form words from them. The centre is also termed the centre for word-blindness or alexia.
d. A special writing centre is still often assumed to lie within the base of the gyrus frontalis medius. Its existence, however, is scarcely longer to be accepted, since the centre for writing is blended with the motor centre for the hand within the middle region of the precentral convolution.
These speech centres are, therefore, centres of memory, namely, for the movement conceptions of articulation, the acoustic pictures of speech and the visual pictures of written speech. Individuals, who, in consequence of lesions of these centres, have lost the memory of the motor, auditory and visual conceptions of speech, are neither paralyzed, deaf, nor blind, but only wanting in speech-understanding. We are, therefore, warranted
in assuming the existence of memory centres beside the projection centres ; further, it must be noted, that the projection centres serve not only sensation and innervation, but also memory, and, on the other hand, that the regions adjoining the projection centres are not to be regarded as exclusive commemorative centres, since the existence of projection tracts to them has been proven.
Concerning the association function of the cerebrum, we must assume that the binding together of conceptions or recollections of the same kind occurs in the cortex within the individual cortical fields, but that all the higher association processes are con nected with the collective activity of many, perhaps of all, the cortical regions.
Finally, it must be especially emphasized, that the two cerebral hemispheres are functionally by no means identical. In connection with localization of the speech centre, it has been pointed out, that in right-handed individuals the left hemisphere is concerned. Not only for speech does the left hemisphere outweigh the right, but also for manipu lation. For proof of this we are indebted especially to the investigations of Liepmann, who has made us acquainted with the clinical picture of apraxia. By apraxia is under stood the inability to execute the appropriate movements during continued motion ; that is, the apraxic patient is still able to carry out certain simple movements, as flexing, lower ing, raising or extending the arm, but has lost the ability to perform combinations of consecutive movements, such as made in greeting, beckoning, or threatening. Such expressive movements are executed in an entirely abnormal manner, likewise the imitation • of definite movements, and objects are no longer properly used or handled. In many lesions of the left hemisphere, followed by paralysis and apraxia of the right hand, a similar affection of the left hand may be recognized. Moreover, in numerous cases of extensive lesion of the corpus callosum, dyspraxia of the left hand is present. According to Liepmann, one is, therefore, warranted in assuming that the recollection of certain acquired dexterities and also the supervision of the execution of the same are in predom inating degree concerns of the left hemisphere, which are conveyed to the right hemisphere by means of the corpus callosum.