Page:Popular Science Monthly Volume 25.djvu/634

618 at first with imperfect success, later witb 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 muscular 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 implements 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 motions 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. Suppose 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 remove 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 develop 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 ago 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 individual lives for twenty years or more, the part of the brain which formerly governed the movements of that limb, and which received sensations 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 preserve their appropriate memories.