6i8 THE POPULAR SCIENCE MONTHLY. 



at first with imperfect success, later with proper regulation of the 

 tone and effort necessary ; which, when acquired, is remembered, so 

 that the words are now clearly pronounced. But this learning to talk 

 is simply the acquiring of memories of definite combinations of mus- 

 cular action; in a word, of motor-memories. Other examples of motor- 

 memories are the memory of the motions made in playing a musical 

 instrument, swinging Indian clubs, writing, or using various imple- 

 ments of trade. These are all distinct memories, and any one of them 

 alone can be blotted out by disease. But, if the memory of the mo- 

 tions necessary to pronounce the words of your reply is affected, it is 

 evident that you will be as powerless to answer the question as though 

 you did not understand it. If this is the case, the disease will be in 

 a different part of the brain from that affected in the first case. It 

 will lie above and in front of the temporal region, in what is known 

 as the third frontal convolution of the brain. This, too, is established 

 by hundreds of examinations of persons who died with loss of speech. 



It thus becomes evident from the study of brain-disease that our 

 visual memories, our auditory memories, our memories of motion, and 

 our memories of speech may each be lost while other memories are 

 unaffected ; and further, that a loss of any one of these memories is 

 always due to disease in its own appropriate part of the brain. 



One other set of facts remains which confirms in a remarkable 

 manner the theory of the localization of functions. It is well known 

 that organs which are constantly used grow in strength by use. The 

 blacksmith's arm is the favorite example. It is no less true that an 

 organ which is not used withers away. If one carries his arm in a 

 sling for several weeks, it grows thin. Now, a sensory organ, like the 

 eye, is simply a mechanism for the reception and transmission to its 

 corresponding part of the brain of appropriate impulses. Sujjpose the 

 organ to be destroyed. It is evident that the part of the brain with 

 which it is joined is no longer called into action ; it is no longer used, 

 and the result is that it withers. If from a new-born animal you re- 

 move an eye, the tract to the posterior part of the brain and that part 

 of the brain will never be called into use, and hence they never de- 

 velop to a normal size. If a child is born blind, or loses his sight in 

 infancy, the same is true ; so that, when in old age he dies, the posterior 

 part of his brain will be found small and shrunken. It is probable 

 that the examination of the brain of a deaf and dumb person would 

 show an atrophy of the speech-centers, although this has not yet been 

 investigated. It is known that if a limb be amputated and the individ- 

 ual lives for twenty years or more, the part of the brain which for- 

 merly governed the movements of that limb, and which received sen- 

 sations from it, will be found shrunken and withered. So that from 

 this class of facts important evidence is derived regarding the parts 

 of the brain which preside over various functions and which j^reserve 

 their appropriate memories. 



