Consider for a moment the passage of the nerve impulses through the brain that would have to occur. At the outset we find that the sensory perceptive centers would have to he engaged with two different ideas at once; but Lewes showed long ago that introspection tells us this is impossible, that "consciousness is a seriated change of feelings": he might equally well have said ideas. And, again, we know that when two streams of energy of like character meet, they mutually arrest each other's progress by reason of interfering with the vibration waves.
I will show directly that this is actually the case in the action of the cortex when the above-mentioned dilemma is presented to it. The experiment I have devised for this purpose is extremely simple. A person who is more or less ambidextrous, and who has been accustomed for a long time to draw with both hands, attempts to describe on a flat surface a triangle and circle at the same moment. I chose these figures, after numerous trials, as being the most opposite, seeing that in a triangle there are only three changes of movement, while in a circle the movement is changing direction every moment. To insure the attempt to draw these figures simultaneously succeeding, it is absolutely necessary that the experimenter should be started by a signal.
When the effort is made, there is a very definite sensation in the mind of the conflict that is going on in the cortex of the brain. The idea of the circle alternates with that of the triangle, and the result of this confusion in the intellectual and sensorial portions of the brain is that both motor areas, though remembering, as it were, the determination of the experimenter to draw distinct figures, produce a like confused effect, namely, a circular triangle and a triangular circle. If the drawing is commenced immediately at the sound of the signal, it will be found that the triangle predominates; thus, if I determine to draw a triangle with my left hand and a circle with my right, the triangle (though with all its angles rounded off) will be fairly drawn, while the circle will be relatively more altered, of course made triangular. On the other hand, if the two figures are not commenced simultaneously, it will be found that usually the one begun last will appear most distinct in the fused result, in fact, will very markedly predominate.
Now, the course of events in such an experiment appears to be clear. The idea of a triangle and circle having been presented to the intellect by the sensory centers, the voluntary effort to reproduce these is determined upon. Now, if we had a dual mind, and if each hemisphere was capable of acting per se, then we should have each intellectual area sending a message to its own motor area, with the result that the two figures would be distinct and correct, not fused.
The other evidence that I referred to above, which is adduced in favor of the synchronously independent action of the two hemispheres is from the account of such cases as the following: Professor Ball, of
