The mechanism by which expression is given to thought is most complex, and the modes in which the defects thereof comprehended in the designation "aphasia" may arise are correspondingly varied. The faculty of articulate speech bespeaks an accumulation of sound-memories, and that of written or printed speech an accumulation of visual memories as well. In addition, the motor mechanism of speech must in each instance be intact. A lesion of the auditory center in the first temporal convolution of the left cerebral hemisphere may give rise to total inability to appreciate the significance of sound—mind deafness, while a more circumscribed lesion will be attended with inability to understand or to repeat spoken language and with inability to read and to write from dictation, while the power of voluntary speech and writing, and of writing by transcript, is preserved, although words may be improperly used—paraphasia. This condition is known as word-deafness.