The impressive nerve-path, o a K, is centripetal; the expressive, M h z, centrifugal; v, intercentral.
When the normal child learns to speak, o receives the sound-impressions; by a the acoustic-nerve excitations are passed along to K, and are here stored up, every distinctly heard sound (a tone, a syllable, a word) leaving an impression behind in K. It is very remarkable here that, among the many sounds and noises that impress themselves upon the portions of the brain directly connected with the auditory nerve, a selection is made in the sound-field of speech, K, since all those impressions that can be reproduced, among them all the acoustic images necessary for speech, are preserved, but many others are not, e. g., thunder, crackling. Memory is indistinct with regard to these. From K, when the sound-images or sound-impressions have become sufficiently strong and numerous, the nerve-excitement goes farther through the connecting paths v to M, where it liberates motor impulses, and through h sets in activity the peripheral apparatus of speech, z.
Now, speech is disturbed when at any point the path o z is interrupted, or the excitation conducted along the nerve-fibers and ganglionic cells upon the hearing of something spoken or upon the speaking of something represented in idea (heard inwardly) is arrested, a thing which may be effected without a total interruption of the conduction, e. g., by means of poison and through anatomical lesions.
On the basis of these physiological relations, about which there is no doubt, I divide, then, all pure disturbances of speech, or lalopathies, into three classes:
(1) Periphero-Impressive or Perceptive Disturbances.
The organ of hearing is injured at its peripheral extremity, or else the acusticus in its course; then occurs difficulty of hearing or deafness. What is spoken is not correctly heard or not heard at all: the utterance is correct only in case the lesion happened late. If it is inborn, then this lack of speech, alalia, is called deaf-mutism, although the so-called deaf and dumb are not in reality dumb, but only deaf. If words spoken are incorrectly heard on account of acquired defects of the peripheral ear, the patient mis-hears, and the abnormal condition is called paracusis.
(2) Central Disturbances.
a. The higher impressive central paths are disturbed: centro-sensory dysphasia and aphasia, or word-deafness. Words are heard but not understood. The hearing is acute. "Patients may have perfectly correct ideas, but they lack the correct expression for them; not the thoughts but the words are confused. They would understand the ideas of others also if they only understood the words. They are in the position of persons suddenly transported into the midst of a people using the same sounds but different words, which strike upon their ear like an unintelligible noise." (Kussmaul.) Their articulation is without defect, but what they say is unintelligible because the words are mutilated and used wrongly. C. Wernicke discovered this form, and has separated it sharply from other disturbances of speech. He designated it sensory aphasia. Kussmaul later named this abnormal condition word-deafness (surditas verbalis).
b. The connections between the impressive sound-centers and the motor speech-center are injured. Then we have intercentral conductive dysphasia and aphasia. What is spoken is heard and understood correctly even when v is completely interrupted. The articulation is not disturbed, and yet the patient utters no word of himself. He can, however, read aloud what is written. (Kussmaul.) The word that has just been read aloud by the patient can not be repeated by him, neither can the word that has been pronounced to him; and, notwithstanding this, he reads aloud with perfect correctness. In this case, then, it is impossible for the patient of his own motion, even if the memory of the words heard were not lost, to set in activity the expressive mechanism of speech, although it might remain uninjured.
c. The motor speech-center is injured. Then we have centro-motor dysphasia and aphasia. If the center is completely and exclusively disturbed, then it is a case of pure ataxic aphasia. Spontaneous speaking, saying over of words said by another, and reading aloud of writing, are impossible. (Kussmaul.) On the other hand, words heard are understood, although the concepts belonging with them can not be expressed aloud. The verbal memory remains; and the patient can still express his thoughts in writing and can copy in writing what he reads or what is dictated to him.