Page:Principles of Psychology (1890) v1.djvu/51

Rh were not locally separated, but carried on each by the aid of the whole organ. Hitzig in 1870 showed, however, that in a dog's brain highly specialized movements could be produced by electric irritation of determinate regions of the cortex; and Ferrier and Munk, half a dozen years later, seemed to prove, either by irritations or excisions or both, that there were equally determinate regions connected with the senses of sight, touch, hearing, and smell. Munk's special sensorial localizations, however, disagreed with Ferrier's; and Goltz, from his extirpation-experiments, came to a conclusion adverse to strict localization of any kind. The controversy is not yet over. I will not pretend to say anything more of it historically, but give a brief account of the condition in which matters at present stand.

The one thing which is perfectly well established is this, that the 'central' convolutions, on either side of the fissure of Rolando, and (at least in the monkey) the calloso-marginal convolution (which is continuous with them on the mesial surface where one hemisphere is applied against the other), form the region by which all the motor incitations which leave the cortex pass out, on their way to those executive centres in the region of the pons, medulla, and spinal cord from which the muscular contractions are discharged in the last resort. The existence of this so-called 'motor zone' is established by the lines of evidence successively given below:

(1) Cortical Irritations. Electrical currents of small intensity applied to the surface of the said convolutions in dogs, monkeys, and other animals, produce well-defined movements in face, fore-limb, hind-limb, tail, or trunk, according as one point or another of the surface is irritated. These movements affect almost invariably the side opposite to the brain irritations: If the left hemisphere be excited, the movement is of the right leg, side of face, etc. All the objections at first raised against the validity of these experiments have been overcome. The movements are certainly not due to irritations of the base of the brain by the downward spread of the current, for: a) mechanical irritations will produce them, though less easily than electrical; b) shifting the