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. [Illustration: DIAGRAM 67.—SHOWING THE SYSTEM OF NERVE CELLS IN THE CORTEX. A, Afferent fibre; B, distributing cell; C, small pyramidal cell; D, large pyramidal cell.] Returning to general considerations, an important point is the way in which the different centres are connected by fibres, which put them into relation. The brain may consist of many centres, just as the body consists of many organs, but both body and brain must live as a whole. If the heart and lungs get out of harmony there is trouble, and if the bridge which connects hearing with motion in the brain breaks down, as occasionally happens for a time in an overworked man, he is mentally at a discount. He can hear and understand, but he cannot write or talk sense: he is sane, but quite helpless, and generally very frightened. Still more important are the intermediate stations and sidings on these lines of communication, for it is here that the most exhaustive weighing and comparing of incoming stimuli is carried on—the final balancing before a voluntary action; in a word, thought.