Page:Popular Science Monthly Volume 46.djvu/744

724 movement of her body. Yet she was blind. And when these false perceptions occur in the domain of hearing, either taking the form of definite commands, such as "You shall not eat" "You can not move," "You are a lost soul" or such a terrible order as "You must cut your throat," the impression which is produced may be so intense as to dominate the mind and hamper all mental action; and if these commands are not recognized as products of internal disease, they may lead to actions, such as suicide or homicide, which a sound judgment would condemn.

Our knowledge of the localization of brain functions, so far as sensory impressions are concerned, has enabled us to explain not only defects of perception and of memory, but also these false perceptions which we term hallucinations. It has made it evident that such hallucinations are the result of irritative disease in the definite region of the surface of the brain in which the memory pictures were stored. Disease excites the cortical cell to activity. The mental result is a perception which consciousness has no means of distinguishing from an actual perception, and which in a diseased state it regards as real.

The localization of motor functions is no less precise than that of the sensory functions. Every movement of a voluntary character, from the coarse act of grasping an object to the fine touch of the pianist, or the delicate stroke of the artist, or the graceful balancing and light movements of the dancer, originates in a well-defined portion of the brain surface. Destroy this portion of the surface and no amount of volition will produce the desired act; or let this portion be irritated by disease and, without the will, the act will be performed over and over again in an automatic manner and apparently without purpose.

The mechanism of speech has also been determined by these investigations into the localization of brain functions. We know that the understanding of words when spoken or when seen in print, and the articulation of words in speech or their production in writing, are all dependent upon the integrity of definite regions upon the brain surface. So positive are we of this that in certain cases, when either the comprehension of language is suddenly suspended by disease or the power of utterance of language is hampered by disease, we can put our finger upon the spot in the brain which is affected; and if that spot is pressed upon by a clot of blood or by a new growth, we can remove it and thus restore the power of understanding speech or the power of utterance. Dr. McBurney has reported the case of a physician whose speech was thus restored after three months of silence by the removal of a clot from the motor speech center.