it also becomes mute, unless special educational measures are employed; and in adults destruction of the auditory center interferes sadly with talking, while destruction of the motor speech center does not seem to interfere at all with the understanding of speech.
When reading is first undertaken, the auditory and motor speech centers with their association fibers are already well developed. The visual center, V, now begins to work with them. When impulses from the eyes reach this center, the sensation of sight occurs. The appearance of each letter calls up the memory of its sound through the association fibers, V A, and a number of these elementary sounds uttered in quick succession are recognized by the learner as a word. Its meaning is awakened by the auditory center, and at first it is necessary to read aloud in order to make the association impulses from this center sufficiently exact and vigorous. Later, the memories of the sound and of its utterance suffice without its actual reproduction, but in most persons these memories remain an essential part of the reading process throughout life. As this is read, the reader is doubtless conscious of the sound and of the incipient utterance of each word.
In learning to write, the motions of the hand become automatically associated with the memory of the corresponding utterance. It might be supposed that in writing the appearance of each word is recalled and copied; but this is not the case, although the visual memory may be an aid to correct spelling. Words may be written with no recollection of their appearance.
The correctness of the foregoing statements is proved by the effects of disease of the language centers, as shown by observation of speech defects during life, followed by post-mortem examination of the brain.
Destruction of the visual centers of both sides causes blindness; but when these centers themselves are unharmed, disease may be so situated as to cut off their communication with other centers. In this case the patient sees, but does not recognize what he sees, and is said to be mind-blind. If the affection is so slight that he can still recognize ordinary objects, but not written or printed words, which are more difficult, he is only word-blind. To a person afflicted with word-blindness the print of his own language is like that of a foreign one. George Eliot, with her usual sure touch in medical matters, gives an interesting illustration of this affection in the case of Tito's foster-father, Baldasarre; yet Romola was finished in 1863, when very few physicians were aware of the existence of such cases. Reading in such a case is, of course, impossible; but writing is not prevented, although the patient can not read what he has just written. Speaking and the understanding of speech are not interfered with at all.