This is a condition arising from a disease of the brain which occasions loss or impairment of the idea of language or its expression. It differs from aphonia in that in the latter disease, speech is impaired or lost from disease of the vocal apparatus, although the memory of words and the power of expressing them by writing remains unimpaired, while in aphasia the vocal apparatus remains intact, but the memory of words and the power to use them is destroyed or impaired.

The most common cause of aphasia is injury to the brain from apoplexy. The portion of the brain supposed to be injured in these cases is a part called the "island of Reil" of the left side of the brain. It is thought that the memory of words and control of the organs of speech reside chiefly in the left side of the brain, from the fact that in nearly all cases in which aphasia results from injury of the brain, examinations after death have shown the injury to be on the left side. This is not always found to be the case, however, and it is probably true that both sides of the brain possess this faculty, but that from force of habit, the left side is chiefly used, just as one eye, one hand, or one ear is generally employed in preference to the other. It has been claimed that in right-handed people aphasia is due to injury of the left side of the brain, while in left-handed people it is to be attributed to injury on the opposite side, owing to the well-known anatomical fact that the nerves of the right side of the body originate in the left side of the brain, and vice versa.

A person suffering with aphasia may be' unable to utter a syllable, or may simply be deprived of the power of using words correctly. Sometimes the power of speech will be lost, while the memory of words remains, so that an individual can write as well as ever. This is not generally the case, however. Patients are sometimes aware of their inability to use words correctly, and at other times seem to be wholly oblivious to the mistakes they are constantly making. We recently had under treatment a patient suffering with this difficulty as a result of apoplexy. If she wished to say door, she was much more likely to say chair, table, carpet, or window. Apparently aware of the mistakes made in speaking, she often repeated a long list of names, hoping to find the right word. If the desired word was suggested to her, she would at once recognize it, and would repeat it without difficulty. Aphasia sometimes results from epilepsy, hysteria, and various other functional disturbances of the brain.