Fig. 6.—Left Hemisphere of Monkey's Brain. Outer Surface.
Even when the entire motor zone of a dog is removed, there is no permanent paralysis of any part, but only this curious sort of relative inertia when the two sides of the body are compared; and this itself becomes hardly noticeable after a number of weeks have elapsed. Prof. Goltz has described a dog whose entire left hemisphere was destroyed, and who retained only a slight motor inertia on the right half of the body. In particular he could use his right paw for holding a bone whilst gnawing it, or for reaching after a piece of meat. Had he been taught to give his paw Before the operations, it would have been curious to see whether that faculty also came back. His tactile sensibility was permanently diminished on the right side.[12] In monkeys a genuine paralysis follows upon ablations of the cortex in the motor region. This paralysis affects parts of the body which vary with the brain-parts removed. The monkey's opposite arm or leg hangs flaccid, or at most takes a small part in associated movements. When the entire region is removed there is a genuine and permanent hemiplegia in which the arm is more affected than the leg; and this is followed months later by contracture of the muscles, as in man after inveterate hemiplegia.[13] According to Schaefer and Horsley, the trunk-muscles also become paralyzed after destruction of the marginal convolution on both sides (see Fig. 7). These differences between dogs and monkeys show the danger of drawing general conclusions from experiments done on any one sort of animal. I subjoin the figures given by the last-named authors of the motor regions in the monkey's brain.[14]
Fig. 7.—Left Hemisphere of Monkey's Brain. Mesial Surface.
In man we are necessarily reduced to the observation post-mortem of cortical ablations produced by accident or disease (tumor, hemorrhage, softening, etc.). What results during life from such conditions is either localized spasm, or palsy of certain muscles of the opposite side. The cortical regions which invariably produce these results are homologous with those which we have just been studying in the dog, cat, ape, etc. Figs. 8 and 9 show the result of 169 cases carefully studied by Exner. The parts shaded are regions where lesions produced no motor disturbance. Those left white were, on the contrary, never injured without motor disturbances of some sort. Where the injury to the cortical substance is profound in man, the paralysis is permanent and is succeeded by muscular rigidity in the paralyzed parts, just as it may be in the monkey.
Fig. 8.—Right Hemisphere of Human Brain. Lateral Surface.
