Page:Principles of Psychology (1890) v1.djvu/83

63 FUNCTIONS OF THE BRAIN. 68 irascible and amative in an extraordinary degree, and their sides grow bare with perpetual reflex scratching ; but they show no local troubles of either motion or sensibility. In monkeys not even this lack of inhibitory ability is shown, and neither stimulation nor excision of the prefrontal lobes produces any symptoms whatever. One monkey of Horsley and Schaefer's was as tame, and did certain tricks as well, after as before the operation.* It is probable that we have about reached the limits of what can be learned about brain- functions from vivisecting inferior animals, and that we must hereafter look more exclusively to human pathology for light. The existence of separate speech and writing centres in the left hemisphere in man ; the fact that palsy from cortical injury is so much more complete and endur- ing in man and the monkey than in dogs ; and the farther fact that it seems more difficult to get complete sensorial blindness from cortical ablations in the lower animals than in man, all show that functions get more specially local- ized as evolution goes on. In birds localization seems hardly to exist, and in rodents it is much less conspicuous than in carnivora. Even for man, however, Munk's way of mapping out the cortex into absolute areas within which only one movement or sensation is represented is surely false. The truth seems to be rather that, although there is a correspondence of certain regions of the brain to certain regions of the body, yet the several parts within each bodily region are represented throughout the toJiole of the corre- sponding brain-region like pepper and salt sprinkled from the same caster. This, however, does not prevent each ' part ' from having its focus at one spot within the brain- region. The various brain-regions merge into each other in the same mixed way. As Mr. Horsley says : " There are border centres, and the area of representation of the face merges into that for the representation of the upper limb. If there was a focal lesion at that point, you would have the movements of these two parts starting together." f f Trans, of Congress of Am. Phys. and Surg. 1888, vol. i. p. 343. Beevor and Horsley's paper on electric stimulation of the monkey's brain is the most beautiful work yet done for precision. See Phil. Trans., vol. 179, p. 205, especially the plates.
 * Philos. Trans., vol. 179, p. 3.