724 THE POPULAR SCIEXCE MONTHLY. 



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 char- 

 acter, 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 auto- 

 matic 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 

 j)rint, 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 sus- 

 pended by disease or the power of utterance of language is ham- 

 pered 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. 



* Brain, 1891, p. 284. 



