When discussing cerebral localization, it was pointed out, that in right-handed, therefore, in the majority of individuals, the speech-zone, with its different centres, was located within the left hemisphere. The chief centres include (Fig. 182): the sensory (A), within the posterior third of the superior temporal convolution, where the memory-pictures of the heard words are deposited—the centre, therefore, for the memory of word-tones—and the motor (M), within the posterior third of the inferior frontal convolution, in whose cells the memory-pictures of the spoken words lie and on whose integrity depends the ability to carry out the coordinated movements of certain muscles necessary for speech.
These two speech-centres, the sensory and the motor, stand in close relation with each other, the latter dependent upon the former, since speech is acquired by repetition of the word-sounds heard. On observing the development of speech in the child, we find in the connection of these two centres the basis for the possibility of pronouncing by repeating but without understanding. The development of speech teaches, moreover, that speech proper, that is, the intelligent utterance of sounds, in contrast to their mere repeat ing, is preceded by an understanding of speech without speaking—a stage of " normal deaf-mutism." The child understands much, but speaks little or nothing of what it understands ; it is, for the time, deaf-unite. Therefore, an intimate connection is early established between the memory of the word-sound or the acoustic word (A) and the idea (B). In Fig. 182, this close connection, as well as that between the sensory and motor speech-centres, is represented by the double line, A — B. In this relation it should be emphasized, that the idea-centre (B) is represented as a definitely bounded cortical area only as a schematic expedient, and that, as a matter of fact, we must con ceive the formation of the idea as a complex process involving, more or less, the entire cerebral cortex.
From this speech-comprehension without speaking (a' — — a — A — B) and the first mere repeating of spoken words (a' — — a —A—M—m— m' — later comes the repeating of words with the understanding of speech; that is, speech proper. The latter first takes the path : B— A— M— m — ml — ; later, in consequence of the connection B— it follows : .B— M—m—ml — The centre, m, represents the motor centre proper, in the lower third of the precentral convolution (the motor centre for the face, tongue, and larynx). The path, is the motor cortico-bulbar tract, which passes through the knee of the internal capsule, the crusta or base of the cerebral peduncle to the nuclei of the appropriate motor cerebral nerves. Path is the periph
eral motor neurone from the motor nucleus to the muscle. Likewise, close to the sensory speech-centre (A), the auditory centre proper is represented (a). The path a' shows the course of the auditory tract as far as the medial geniculate body ; the path is the last neurone in the auditory tract, which extends, by way of the internal capsule, from the geniculate body to the cortical auditory centre.
The foregoing connections represent speech in the more limited sense; later, as the result of learning written speech—reading and writing—the expansion to speech in its widest sense follows. By written speech is understood the speech of the letters ; the latter are to be regarded not as signs for ideas, as hieroglyphics, but as signs for sounds. We learn to separate the individual words into syllables and letters, each simple sound, vocals and consonants, being associated with a visual letter-picture ; by copying the optical picture of the letter we learn to write. The sensory speech-centre or the acoustic word now comes, therefore, into closer relation with the visual apparatus. Not only the acoustic word, but also the motor word, or the centre for the motor memory-pictures of the words, becomes connected with the visual letter-centre, or visual centre, 0, within the gyms angularis ; here the memory-pictures of the written characters are deposited, since for reading the sensory and motor speech-centres are necessary. For writing, more over, connection is established between the visual centre, 0, and the motor centre for the upper extremity within the middle part of the precentral convolution, the centre, N, for the musculature of the hand, wherein the grapho-motor memories are developed through practice. In Fig. 182, this locality is represented by two superimposed ovals, since the existence of a distinct writing-centre is not accepted.
Reading is accomplished, therefore, by the path : A or B; spontaneous writing by : A or The path represents the first neurone of the visual path leading to the lateral geniculate body; the path is the second neurone from the geniculate body, by way of the internal capsule, to the visual centre proper, o. The latter is shown in the diagram in the occipital pole, but, as well known, the centre is localized chiefly within the cortex of the cuneus, particularly sur rounding the calcarine fissure. The path, Al, represents the course of the motor tract from the arm centre through the internal capsule and the brain-stem to the spinal cord; the path, is the peripheral motor neurone to the muscles of the hand.