Page:Popular Science Monthly Volume 35.djvu/398

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the brain; and since no muscle normally contracts except under tHe stimulus of a nervous impulse transmitted through a nerve-fiber from the central nervous system, my first thesis will be at once admitted, viz., that exercise of muscles necessarily involves exercise of their associated regions in the central nervous system, and that voluntary movements at least require the activity of certain areas of the brain.

It is admitted that the evolution of mind in the animal series and that of the bodily organs have kept pace with each other. The hemispheres of the brain increase in size and in complexity in the ascending zoölogical scale, the animal becoming mentalized in a direct ratio to the development of this part of the brain, which in man forms by far its largest subdivision. The doctrine, first definitely formulated by Fritsch and Hitzig, that the cortex of the brain contains special centers which govern definite groups of muscles,[1] is most significant in connection with this subject. They and their followers divide the brain cortex into two principal regions—one of sensory areas, which lie in the hinder part of the brain, and another of motor areas, which lie anteriorly; i. e., into a region engaged in receiving from the surface organs (the skin, the eye, etc.) impressions which excite the various sensations, and a region concerned in exciting and co-ordinating the movements of the body. The motor centers thus far definitely located are those which control the muscles of the face, arm, leg, and trunk. They lie on each side of one of the fissures of the brain,[2] in the order named from below upward—an arrangement which led Dr. Lauder-Brunton to suggest that it had occurred in accordance with the progressive evolution of the faculties, premising that the uppermost in position were the latest to be acquired and the highest functionally. Thus animals low in the scale seize their food with the mouth; the center for the face muscles was therefore earliest in order of development, as it is lowest in situation. Animals of a somewhat higher grade grasp their food with the anterior limbs—the next higher centers being those devoted to the arm-movements. Animals still further advanced in development have the power of running after their prey, using the posterior to assist the anterior limbs in accordance with the higher level of the centers concerned. Later still, the trunk muscles come to the assistance of the arms and legs in the all-important work of securing food, the first necessity. Coincident with these observations is the fact that the higher the center the more it requires education in the human being. The new-born infant has control of the muscles of the mouth to

  1. Known as the doctrine of localization.
  2. The fissure of Rolando, anterior to the fissure of Sylvius which separates the motor from the sensory areas.