Page:Popular Science Monthly Volume 25.djvu/629

Rh to be connected by means of insulated white nerve-fibers with the eye. The area which, one says, governs touch, the other says is connected with the skin. The area which one proves to be concerned with voluntary movements, the other finds to be joined to the muscles. Thus the two independent lines of evidence unite in indicating that each region of the brain has its own work to do, its own memories to preserve.

While the anatomical evidence in favor of the localization of memories is as strong in the case of man as it is in that of the dog or ape, the physiological evidence is wanting. Physiologists lament that they can not experiment upon man, and psychologists are slow to admit that these experiments throw any light upon man's mind and its action. Just here, however, the study of disease comes in to help out our knowledge. Disease may be regarded as an experiment of Nature to satisfy both physiologists and psychologists, and its results are the more satisfactory, since man is an animal who can describe his sensations during the experiment, as no other animal can. The nature and value of the evidence for the localization of memories to be derived from the study of disease will be clear after the blood-supply of the brain in man is understood. Every artery divides and subdivides as it passes outward from the great central artery of the body—the aorta—so that the vascular system may be likened to a tree, with trunk, boughs, branches, and twigs. Each terminal division of an artery supplies with blood a little cone-shaped mass of brain, the base of the cone being the gray surface of the brain, and its apex being the point of entrance of the little artery. In the brain the terminal branches of the arteries do not run into each other, as in some organs, so that each little cone, like the leaf on the tree, is independent of adjacent cones and hangs upon its own arterial twig. Now, it is evident that anything which plugs up the artery is going to cut off the blood, and therefore the nutriment from the little cone of brain, and then the little cone will wither and die. The larger the artery plugged, the greater the surface of brain destroyed. This is the process of disease known as embolism or thrombosis. But such a destruction of brain-tissue in man corresponds to the artificial destruction of brain-tissue in the dogs experimented upon, with this advantage in the case of man, that the shock of the operation is avoided. The experiments of Nature and of the physiologist are therefore parallel. The only difference is in the order of the observation. The physiologist cuts out a definite part and observes the result. The pathologist observes the result of Nature's experiment by watching the symptoms of his patient, and, after the patient's death, he can ascertain the position of the part diseased. Now, if the old theory be true, according to which the brain acts as a whole, and its various parts do not possess distinct mental functions, a limited area of disease in one part may impair the mental powers but will not produce a loss of one function. If, on the contrary, the new theory