Page:Popular Science Monthly Volume 46.djvu/170

158 of the study of cortical localization, and indeed for the removal of psychology from the realm of speculation to that of scientific demonstration.

The method of removing certain cortical areas of the brain and then noting the effect is termed the extirpation method; it was by this method, practiced upon monkeys, that Prof. Schäfer, of London, and myself established the position of the center for vision in that animal. The monkey's brain is so similar to the human brain that with some modification results may be transferred from one to the other.

Fig. 8 shows the part of the monkey's brain which, when removed, produces blindness in the corresponding half of each eye, and Fig. 9 shows the parts which, when removed, produce complete and permanent blindness in both eyes. Now, while the positions of the areas for the other special senses have not been so satisfactorily demonstrated, the existence of such centers can not be doubted.

With these data, and the aid of Fig. 10, a fundamental step in the process of mental development may be investigated.

By way of the route as indicated in the figure, an impression made upon a specially constructed end organ, the eye, is transmitted along the cell process constituting the optic nerve, and so onward till it reaches a cell, or probably cells, in the occipital lobe; thence by means of the communicating fibers constituting the white matter, and previously described, the arm center is excited and a motor impulse passes out to the muscles moving the arm, and the hand is put into the flame; immediately a second impression is conveyed inward to the pain center and thence to the arm center, from which a second impulse emanates, resulting in a withdrawal of the hand. Finally, after one or more