642 



SCIENCE. 



[N. S. Vol. XIV. No. 356. 



muscles under the sway of the former 

 would weigh as much as one of the larger 

 muscles (say the triceps) of the arm. This 

 is sufficient to show that it is not muscle 

 power which determines the extent of the 

 motor areas in the cerebral cortex. It is 

 the degree of refinement in the movements 

 required, as well as the degree of variety 

 in muscle combinations, which apparently 

 determines the amount of ground covered 

 by a motor center. 



Still, the increase in the amount of cere- 

 bral cortex in man due to the greater re- 

 finement of movement acquired by different 

 groups of muscles is relatively small in 

 comparison with the increase which has 

 occurred in other regions from which no 

 motor fibers are sent out, and which there- 

 fore have no direct connection with muscles. 



The remarkable conclusions arrived at 

 by Flechsig, although not confirmed and 

 accepted in all their details, have tended 

 greatly to clear up much that was obscure 

 in the relations of the different districts of 

 cerebral cortex. More particularly has he 

 been able to apportion out more accurately 

 the different values to be attached to the 

 several areas of the cerebral surface. He 

 has shown that fully two thirds of the cor- 

 tex in the human brain constitutes what he 

 terms ' association centers.' Within these 

 the higher intellectual manifestations of 

 the brain have their origin, and judgment 

 and memory have their seat. They are 

 therefore to be regarded as the psychic 

 centers of the cerebral cortex. 



Now, it requires a very slight acquaint- 

 ance with the cerebral surface to perceive 

 that the great and leading peculiarity of 

 the human brain is the wide extent of these 

 higher association centers of Flechsig. 

 Except in connection with new faculties, 

 such as speech, there has been relatively no 

 striking increase in the extent of the motor 

 areas in man as compared with the cortex 

 of the ape or the idiot, but the expansion of 



the association areas is enormous, and the 

 increase in the frontal region and the back 

 part of the parietal region is particularly 

 well marked. It is this parietal extension 

 of surface which is chiefly responsible for 

 the pushing down of the lower frontier of 

 the parietal lobe and the consequent 

 enlargement of its territory. 



I have ah'eady referred to the views 

 which have been recently urged by several 

 independent observers, that in the men who 

 have been distinguished during life by the 

 possession of exceptional intellectual power 

 this region has shown a very special devel- 

 opment. 



It is a curious circumstance, and one 

 which is worthy of consideration, that in 

 the left cerebral hemisphere the Sylvian 

 fissure or the lower boundary of the parie- 

 tal lobe is more depressed than in the right 

 hemisphere, and, as a result of this, the 

 surface area occupied by the parietal lobe is 

 greater on the left side of the brain than 

 on the right side. To the physiologist it is 

 a matter of every-day knowledge that the 

 left cerebral hemisphere shows in certain 

 directions a marked functional preeminence. 

 Through it the movements of the right arm 

 and rights side of the body are controlled 

 and regulated. Within it is situated also 

 the active speech center. This does not 

 imply that there is no speech center on the 

 right side, but simply that the left cerebral 

 hemisphere has usurped the chief, if not the 

 entire, control of this all important function, 

 and that from it are sent out the chief part, 

 if not the whole, of the motor incitations 

 which give rise to speech. The significance 

 attached to the dominant power of the left 

 hemisphere receives force from the now 

 well-established fact that in left-handed in- 

 dividuals the speech function is also trans- 

 ferred over to the right side of the brain. 

 To account for this functional preeminence 

 of the left cerebral hemisphere numerous 

 theories have been elaborated. The inter- 



