Page:Popular Science Monthly Volume 18.djvu/620

602 ones have their homologues in all animals. They have also their special names. When the excitant is moved about over the different convolutions, the existence of the motor zone is made manifest by the successive movements of the animal; the sensitive zone is known by signs of sensation, while the indications of an intellectual zone are not thus positive. Observe what passes when we irritate the so-called motor region. We excite a certain point of the convolutions, and a movement is produced. Groping about with our electrodes, we find the effect limited to a small tract where at all the points the same movement is provoked, and that alone. Next to this zone, we can in the same way find the limits of others presiding over other movements. The influence appears greatest at the center of the zone: exciting the periphery sometimes produces a light supplementary movement belonging to a neighboring zone. The special centers or zones being very near together, our means of electrizing them are not perfect enough to prevent a slight diffusion of the current into neighboring centers and faintly exciting them. To get precise separate effects we must excite the center of the special zone.

As the same movement invariably follows the stimulation of a special center (the name of a special zone), we conclude that the center in question has charge of this movement, and, as a great number of these centers, placed side by side, preside over most of the movements of the body, we infer the existence of a motor region of the brain, the seat of voluntary control of the body. But, since, as the galvanometer shows, the electricity is diffused over neighboring centers, what right have we, it may be asked, to assert that the movement observed, when a certain center is excited, is due to that excitation, when neighboring centers are also excited? We reply that to these facts we can oppose other facts. On removing the electrical conductors scarcely a centimetre, we excite perfectly definite unlike movements, although the current is diffused as before. Why are not the movements last produced the same as those observed at the first position? Why such distinct localization in spite of diffusion '? Although the occurrence of diffusion is proved by means of sensitive apparatus, it is physiologically insufficient. Besides, it can be prevented by proper precautions.

Again, it is asked if there may not be such a thing as diffusion beneath the surface? This is a grave question, for, under the motor region, there is a large ganglion containing motor fibers going to the muscles. If the current diffuses downward as far as this ganglion, we can not assume that the convolution alone is excited. There are three answers to this objection: 1. The excitation of the convolutions nearest to this ganglion gives the least results; sometimes no movement at all. Unless we admit that the effects are directly as the resistance, which is absurd, downward conduction can not be affirmed. 2. Braun has demonstrated that a section of the white fibers below the excited points, interrupting the physiological continuity, does not