Diagram 64.—Position of the Brain in the Head.
A represents the nerve by which impulses are brought in. It runs straight up to the surface of the cortex, and there its branches end, interlaced with those of a many-branched distributing cell (B). The two cells (C and D) shaped like pyramids, which send up branched processes from their apexes, receive an impulse from the distributing cell, and transmit it along the fibre which runs downwards from the middle of their base. Where the fibre from the smaller one goes to we are not sure—probably to another part of the brain to insure harmonious working—but the large pyramidal cell sends its fibre right away through the lower parts of the brain, passing the cell-stations they contain, on into the spinal cord, till it reaches the centre there, which immediately works some particular limb.
Diagram 65.—Map of the Cerebral Hemisphere, showing the Areas in which Various Functions are localized.
Supposing we anæsthetized somebody, throwing him into deep unconsciousness, and then opened his skull, laying bare the brain as is done in [Diagram 64], only not quite in such a wholesale manner. If we then stimulated the part of the brain we are now considering at different places with electric needles, using a weak induction current, we should see him moving different members according to the different regions touched—now an arm, now a leg, now the whole head. If we were to place the electrodes in the centre for the hand, and then gradually increase the strength of the current, the activity of the hand centre would throw other centres into activity. The arm would move next, raising the hand towards the face. Then the eyes would turn, and the whole head to meet the hand. Lastly the mouth would open. The movements are those of putting something into the mouth—the ruling passion strong in unconsciousness.
Diagram 66.—Map of the Cerebral Hemisphere, showing the Areas in which Various Functions are localized.
Such experiments were, of course, first made upon animals, but they have been fully verified on the human subject. The story of how this was done is not, however, a romance with a martyr or a criminal for the central figure. The corpus vile was not provided by a volunteer, or kidnapped and bound in a dark cellar, but treated as a patient in the airy wards of a hospital. With increasing knowledge of the brain, it was found that epilepsy was cortical in origin. A little piece of the cortex becomes diseased and hyperexcitable. The sufferer suddenly becomes acutely conscious of one of his members—a hand or a foot, say—not because there is anything the matter with it, but because the corresponding area in the brain is morbidly active, and he refers the sensation to the part from which it receives its nerves. The next moment the limb begins to twitch, and the excitement spreading, as in the experiment we described above, to other centres which are not diseased, they, too, become morbidly active, and the whole body is thrown into convulsions. This is a disease which must be checked as soon as possible. The surgeon accordingly lays bare the part of the brain affected, knowing now where to look; finds the exact spot which is diseased by reproducing the first twitchings of a fit by electric stimulation, and removes the source of the trouble.
Diagram 67.—Showing the System of Nerve Cells in the Cortex.