Page:Popular Science Monthly Volume 1.djvu/359

Rh cases, notwithstanding the stimulus of the water on the integuments, the frog will remain immovable. If, now, you slowly withdraw the board below the frog without disturbing his position, he will remain motionless; but, if you tip it one side, the frog at once wakes from his quietude. The loss of equilibrium acts more energetically than the stimulus of the water on the skin.

Tip a carp to the right or left, at once it recovers its normal attitude. If you place a duck on one of its sides, either on the ground or in the water, it at once corrects itself and comes upright. The cerebellum alone controls equilibrium, as is easily proved by experiment. When it is wounded or destroyed, the animals He indifferently on one side or the other, and make no movements to recover the lost equilibrium. In certain cases, even, they cannot maintain an equilibrium, but tend to fall on one side.



We may conclude that the movements of animals, whether superior or inferior, are produced by certain special mechanisms, or by locomotive centres, situated at the base of the brain. These centres are essentially passive; they have no spontaneous action, and come into activity only when excited by peripheral stimulus or by the brain.

We ought, then, no longer to admit, in the habitual movements which appear perfectly voluntary, a direct action of the brain on each muscle. We must remember that there exist at the base of the brain motor centres which serve to intermediate between the will and external acts. The will calls into action such or such centres, and these immediately determine the action of certain muscular groups. We know besides, that,