A more recent contribution of natural science to the advancement of psychology has come from investigations in the physiology and pathology of the central nervous system since the discovery about 1870 of the so-called speech center by Broca, and of the motor areas of the brain cortex by Fritsch and Hitzig. Some have placed a rather low value on this contribution and, noticing the errors and immature conceptions of this or that investigator, have arrived at the conclusion that psychology can learn nothing worth mentioning from the work of these men. This, it seems to me, is a great mistake.
Quite aside from innumerable details, psychology owes to the investigations made in recent years concerning the physiology of the brain two fundamental conceptions. In the first place it has come to be generally recognized that the search of centuries for the exact seat of the soul in the brain--for the point where mind and body come into interaction--is without an object. There is no seat of the soul in this sense; the brain is the embodiment of almost absolute decentralization. Our mind receives the impressions of the external world by means of widely separated parts of the brain, as different sensations, according to the peripheral organs stimulated. And our mind controls our actions by means of widely separated parts of the brain according to the local differences of the muscle groups which are called into action. All the parts of the brain are connected, but they function in relative independence, without being controlled from a single point. Now, it is clear that insight into this fact is of no little significance for our conception of the nature of mind.
In the second place it is only through the work of these neurologists that psychologists have come to realize how enormously complicated are even those mental functions which have always been regarded as comparatively simple. That the speech function, for example, involves consciousness of sound, of movement, and sometimes of sight, may be recognized immediately, and has been recognized. That our images of things are directly nothing but revived sense impressions of various kinds, visual, auditory, olfactory, and so on, and that our skill in handling things depends upon our experience obtained through running our fingers over them, is also recognized. But that all these images are more than abstractions, that they have a concrete significance even though the subject may not be aware of them, has been recognized only after the study of pathological cases, where, in consequence of peculiar lesions of the brain a dissociation has occurred among those factors which usually work together harmoniously, and where some of them are perhaps entirely lost. It was not until these pathological facts were known that psychology was able to give a definite formulation to certain of its problems. It then became clear that many former problems which took their origin from those popular simplifications, will, judgment, memory, or from the seeming simplicity of ideas and movements, were perfect nonsense, considering the actual complexity of the facts. Now, after having learned how to formulate its problems, psychology can at last hope to understand the phenomena of mental life.