Page:Popular Science Monthly Volume 7.djvu/66

56 was obvious from the fact that he had no access to water, and was already comfortably housed. He was simply working out what was impressed upon his nervous organization by countless generations of dam-builders before him, independent of circumstances or uses. In this class, however, are commenced two grand improvements in the cerebral development: first, the uniting of the two parts constituting the hemispheres by a broad band of fibres, known as the "great transverse commissure," indications of which appear in the rodents; and, second, "indications of a middle lobe marked off from the anterior by the fissure of Sylvius." To these soon are added the "convolutions" or corrugation of the outer or cortical layer of the hemispheres, so as to secure more surface of active nerve-material without additional bulk; and lastly, in the apes, appears the commencement of the third or "posterior lobe."

In the human brain no addition of parts is presented, but only improvements in those parts found in animals next below. One of these improvements is the more perfect communication established between the diffrent parts composing the brain; the great "transverse commissure" connecting the two hemispheres is much better developed than in any of the preceding races, also the fibres connecting the cerebrum with the sensorium upon which it lies; the "convolutions," only indicated by slight depressions even in monkeys and apes, become conspicuous in the human brain, giving it the appearance of being gathered up in deep folds; and the "posterior lobe," which, as we have seen, first makes its appearance in the highest apes, is much increased in man, even in the uncultivated tribes, as is also the comparative bulk of the whole cerebrum.



These various improvements, gradually succeeding each other, are accompanied by psychical developments equally marked; gradually the strong instincts and limited perception of surroundings, which