It was believed for a long time that the cerebral hemispheres were insensible and inexcitable to direct stimulation. The Germans Fritsch and Hitzig discovered, however, that parts of the cerebrum would respond to a very gentle current of electricity. This beginning has been carefully followed up by Ferrier, Munk, Goltz, and many others, until we now have, amid much disagreement and uncertainty, some results that are interesting, to say the least.
All experiments on the cerebrum are of two kinds (stimulation of the surface and destruction of the surface), and are necessarily made on the lower animals. Dupuy offered an objection to experiment by electrical stimulation, which, if well founded, would destroy the entire value of the undertaking. He claimed that the effects produced by electricity at the surface of the hemispheres were due wholly to conduction of the current through the mass to the corpora striata below and so to the muscles. Dupuy proved that conduction did take place through the cell-mass of the hemispheres. He placed the leg of a frog in contact with the rear of a brain, and by application of electricity to the front of this brain produced strong movements in the limb. Ferrier's answers to Dupuy are a sufficient refutation of the objection.
If the effects observed under electrical stimulation are due to conduction, we could not have (as is the case) strikingly different results from application of the electrodes to very closely adjacent areas. Further, when the striata themselves are stimulated, there is always a general contraction of muscles on the entire opposite side of the body. There is no limitation of the movements to special groups of muscles, as always happens when particular centers on the brain-surface are stimulated. Again, there are many portions of the brain which give no response to electrical stimulus. How can this be so if such movements as are produced result from conduction, especially since many of these silent regions of the brain are no more remote from the striata than the responsive ones?
Experiment and pathology, despite all the contradictions, seem to point to the existence of a motor zone on the surface of the hemispheres. This means that certain parts of the brain are directly concerned with the movements of particular muscles and groups of muscles; also, that these parts can not be shown to be connected with sensations. The natural, primary occasions of their activity may be the states of consciousness which we call volitions; they are not, so far as evidence goes, the states of consciousness we call sensations.
It is of interest to observe that these motor regions are situated in the anterior portions of the hemispheres, and occupy here a relatively small space. They lie above the Sylvian fissure, and are mostly on the fourth frontal and ascending parietal convolutions.
The experiments have been performed on a great variety of animals, and repeated a large number of times. The monkey is, of course,
