Page:Popular Science Monthly Volume 25.djvu/821

Rh it will be better not to do so; you suppress the yawn without moving a muscle. What happens is this: An inhibitory nerve-impulse is sent from the cortex, and puts a stop to the indiscreet activity of a nerve-center elsewhere in the brain. Further, when the cortex is set in activity in a particular way by one impulse, another impulse reaching it may inhibit the first activity, or, in terms of the localization theory, one nerve-center in the cortex may send out inhibitory impulses to any other nerve-center of the cortex.

I need not further multiply instances of inhibition. I wish, however, to lay stress on this, that it is highly probable that impulses traveling from any peripheral nerve-ending to a nerve-center, or from any one nerve-center to any other, may, under certain circumstances, diminish or annul the functional activity of the nerve-center—that is, may inhibit it. And there is equal reason to believe that, under certain other circumstances, the effect produced will not be inhibition, but an increase of activity of the center. The exact conditions which determine whether one effect or the other takes place have not as yet been made out. For the present the facts must suffice us. We may now return to the mesmerized frog.

Whatever the will may be, its action is accompanied by a certain activity of the cortex of the brain; if this activity is prevented from taking place, the will can no longer act. From the physiological standpoint, then, the mesmerized frog lies motionless because an inhibition of a particular activity of the nerve-cells of the cortex has taken place. We may distinguish two chief causes of this inhibition.

The tactile stimuli sent to the central nervous system when the frog lies on its back are obviously different from those sent when the frog is in its normal position. The unusual nerve-impulses traveling from the skin in the unusual position of the frog are inhibitory nerve-impulses. There is reason to believe that they act first on some lower center of the brain, and that from this impulses are sent which diminish or annul the activity of the cortical nerve-cells which is necessary for the exercise of the will.

The second chief cause of inhibition is in the cortex itself. Handling the frog in the way which is done when it is mesmerized produces a certain emotional condition which we may call fright. But, when the animal is frightened, the nerve-cells of the cortex are set in activity in a special manner. This mode of activity inhibits other modes of activity, and the will is paralyzed. We can not at present, I think, put in any more definite form the effect of one state of the cortex of the brain upon its other possible states. We do not know enough of the relations of the cortex of the brain to the psychical functions to say more. In some cases fright seems to play a very small part, if