Page:Popular Science Monthly Volume 28.djvu/115

Rh would enable the other hemisphere to do the work; but all clinical facts say that, once destroyed, the loss is never recovered.

If we examine this motor region of the cortex with the microscope, we of course find these large corpuscles, which we have learned are those which alone give energy to the muscles. But you must not imagine that the motor region consists solely of these corpuscles. On the contrary, as you see in this diagram, we have several layers of corpuscles. I shall return to this arrangement of the corpuscles directly.

Looking back at the surface of the brain, you notice that I have only accounted for but a small portion of the cortex. Dr. Ferrier was the first to show that the portion of cortex which perceived (and I use the word in its strictest sense) the sensation of light was this part, and it is therefore called the "visual center or area." From recent researches it would appear that we must give it the limits drawn on this diagram; below it we find the center for hearing. Thus we know where two sense perceptive centers are situated.

Microscopical investigation shows that this sensorial portion of the cortex is very deficient in large corpuscles, and is correspondingly rich in small cells. Here in this diagram you see these two kinds of structure in the cortex cerebri. Note the greater number and complication of the small corpuscles in the sensory part of the cortex, and the comparatively fewer though much larger corpuscles in the motor region.

It seems to me that several beliefs are justified by these facts: In the first place, the movements produced by the action of these motor centers are always the same for the same center; consequently, it has only one thing to do, one idea, as it were. Thus, for instance, bending of the arm: this action can only vary in degree, for the elbow will not permit of other movements. Hence we may look upon it as one idea. Now, observe that where one idea is involved we have but few corpuscles. Next, consider the multitude of ideas that crowd into our mind when we receive a sensation. One idea, then, rapidly calls up another, and so we find anatomically that there are a corresponding much greater number and complication of nerve-corpuscles. To sum up, I believe we are justified in asserting that where in the nervous system a considerable intensity of nerve-energy is required—e. g., for the contraction of muscles—you find a few large corpuscles and fibers provided; and that where numerous ideas have to be functionalized, there numerous small corpuscles are arranged for the purpose.

But, now, the special interest attaching to the sensory perceptive areas is that they, unlike the motor areas, tend to be related to both sides of the body. With our habit of constantly focusing the two eyes on one object, it will strike you at once that habitually we can only be attentively conscious of one object at a time, since both eyes are engaged in looking at it, and, as you know, we can not as a matter of fact look at two things at once.

Hence, I take it, both sensory perceptive centers are always fully