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Page:Popular Science Monthly Volume 18.djvu/621

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arrest electrical conduction sufficiently to prevent movements. 3. Ferrier has shown that the direct excitation of this ganglion produces a general muscular contraction of the other side of the body, and not special isolated movements. So that we again conclude that, though electrical diffusion toward the corpus striatum (the name of the ganglion in question) may exist, it is physiologically insufficient.

But, it may still be asked if electrical diffusion does not excite the white fibers interposed between these convolutions and the ganglion beneath. This objection borrows a character of probability from the slight thickness of the layer of gray matter of the convolutions, and also from the alleged unexcitability of the gray substance, which has been proved in the case of the spinal cord, but conclusive proof in regard to the brain is yet wanting. Many physiologists claim that these cells are only excitable by the will. For the theory of motor centers they substitute that of psycho-motors. Reserving this discussion for another time and place, we may say that, whether we excite the cells or the fibers that arise from them, the result is the same.

All stimulation, whatever its nature or origin, acts upon a nerve according to its functions. Excite a motor nerve at any point of its course, and you produce movement; excite a sensory nerve, and the subject will feel a sensation which will vary with the nature of the nerve. Compress the eyeball, you excite the optic nerve and get the sensation of light; auditive nerves give sensations of sound, and so for all the nerves of special or general sensibility. If, then, in the brain we excite the motor region, the origin of the motor nerves is irritated, and we get movements; excite the sensitive region, and in place of movement we have sensation owing to the connection of these nerves with sensitive cells. So that the electrization of the gray matter of the convolutions acts in the same way as the electrization of a nerve on any point of its track; the only difference is that in one case we excite them at a point near their origin. From the anatomical relations that exist between the white fibers and gray cells, we infer that the cells play the role of center to the nerves.

We come now to the details of Ferrier's experiments. Ferrier operated chiefly on monkeys, because in them the will is in the ascendant, while in lower animals automatism preponderates. As the result of his experiments, he affirms the existence of three zones—the intellectual, motor, and sensitive—into which the surface of the brain can be divided. The one best known, and to which least objection has been made, is the motor zone. In passing the electrodes over its surface, we soon find the little, well-defined centers that preside over particular groups of muscles. Under the microscope there is seen at these points a limited mass of large cells, called nests by Betz. The functions of these centers are best shown when they are electrized at the central point, the current being then less likely to spread, and so produce more complicated movements that mask the true function of