THE POPULAR SCIENCE MONTHLY 



Avas obvious from tlic lUct tliat lie liad no access to -water, and was 

 already comfortably housed. He Avas simply Avorkiug out what was 

 impressed upon his nervous organization by coiintless 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 trans- 

 verse commissure,'''' indications of Avhich appear in the rodents; and, 

 second, " indications of a '>^<7V/f?/e /oJe' marked oft* from the anterior 

 l)y the fissure of Sylvius." To these soon are added X\\e '^ convolu- 

 tions,'''' or corrugation of the outer or cortical layer of the hemis])heres, 

 so as to secure more surface of active nerve-material without addi- 

 tional bulk ; and lastly, in the apes, appears the commencement of tlie 

 third or ^^ posterior lobe.'''' 



In the human brain no addition of parts is presented, but only im- 

 provements in those parts found in animals next below. One of these 

 improvements is the more perfect communication established between 

 the diftcrent parts composing the brain ; the great " transverse com- 

 missure " connecting the two hemispheres is much better developed 

 than in any of the preceding races, also the fibres connecting the cere- 

 brum with the sensorium upon which it lies ; the " convolutions," only 

 indicated by slight depressions even in monkeys and ajjes, become con- 

 spicuous in the human brain, giving it the appearance of being gath- 

 ered 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. 





Fig. C Human Brain: 1. Cerebrum: 2. Cerebellum: 3. Spinal Cord. ScDSorium covered by 

 Cerebrum ; a, b, c, Anterior, Middle, and Posterior Lobes of the 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 



