The Story of the Mind

by James Mark Baldwin

Fig. 4.—Inner (mesial) surface of the right hemisphere of the brain (modified from Schäfer and Horsley). In both figures the shaded area is the motor zone.

Of all the special questions which have arisen about the localization of functions in the nervous system, that of the function of certain areas known as "motor centres" has been eagerly discussed. The region on both sides of the fissure of Rolando in Fig. 3 contains a number of areas which give, when stimulated with electricity, very definite and regular movements of certain muscles on the opposite side of the body. By careful exploration of these areas the principal muscular combinations—those for facial movements, neck movements, movements of the arm, trunk, legs, tail, etc.—have been very precisely ascertained. It was concluded from these facts that these areas were respectively the centres for the discharge of the nervous impulses running in each case to the muscles which were moved. The evidence recently forthcoming, however, is leading investigators to think that there is no cortical centre for the "motor" or outgoing processes properly so called, and that these Rolandic areas, although called "motor," are really centres for the incoming reports of the movements of the respective muscles after the movements take place, and also for the preservation of the memories of movement which the mind must have before a particular movement can be brought about (the mental images of movement which we called on an earlier page Kinæsthetic Equivalents). These centres being aroused in the thought of the movement desired, which is the necessary mental preparation for the movement, they in turn stimulate the real motor centres which lie below the cortex at the second level. This is in the present writer's judgment the preferable interpretation of the evidence which we now have.

Fig. 5.—The speech zone (after Collins).

The Speech Zone.—Many interesting facts of the relation of body and mind have come to light in connection with the speech functions. Speech is complex, both on the psychological and also on the physiological side, and easily deranged in ways that take on such remarkable variety that they are a source of very fruitful indications to the inquirer. It is now proved that speech is not a faculty, a single definite capacity which a man either has or has not. It is rather a complex thing resulting from the combined action of many brain centres, and, on the mental side, of many so-called faculties, or functions. In order to speak a man normally requires what is called a "zone" in his brain, occupying a large portion of the outside lateral region (see Fig. 5). It extends, as in the figure, from the Rolandic region (K), where the kinæsthetic lip-and-tongue memories of words are aroused, backward into the temporal region (A), where the auditory memories of words spring up; then upward to the angular gyrus in the rear or occipital region (V), where in turn the visual pictures of the written or printed words rise to perform their part in the performance; and with all this combination there is associated the centre for the movements of the hand and arm employed in writing, an area higher up in the Rolandic region (above K). In the same general zone we also find the music function located, the musical sounds being received in the auditory centre very near the area for words heard (A) while the centre for musical expression is also in the Rolandic region. Furthermore, as may be surmised, the reading of musical notation requires the visual centre, just as does the reading of words. In addition to this, we find the curious fact that the location of the whole speech zone is in one hemisphere only. Its location on the left or the right, in particular cases, is also an indication as to whether the person is right-or left-handed; this means that the process which makes the individual either right or left-handed is probably located in the speech zone, or near it. A large majority of persons have the speech zone in the left hemisphere, and are right-handed; it will be seen that the figure (5) shows the left hemisphere of the brain, and with it the right hand holding the pen.

Defects of Speech—Aphasia.—The sorts of injury which may befall a large zone of the brain are so many that well-nigh endless forms of speech defect occur. All impairment of speech is called Aphasia, and it is called Motor Aphasia when the apparatus is damaged on the side of movement.