546 THE CHANGING WORLD 



grams, and they could all be packed into a cubic inch of space. It is 

 this restricted sheet of cortical cells that constitutes the marvelous 

 headquarters of control for human behavior. In it are centers or 

 patches of specialized nerve cells for the reception of impressions 

 received through each of the various sense organs, such as eyes, ears, 

 and touch endings, with neighboring areas devoted to the control of 

 bodily movements. It will be seen from the map of brain locali- 

 zation, shown in the figure, that the receptive center for hearing is 

 located in the temporal lobes of the cerebrum, that of touch in the 

 parietal lobes, and that of sight in the occipital lobe, while the out- 

 going control of muscular movements is spread along the edge of the 

 deep groove, the fissure of Rolando, marking the boundary between 

 the frontal and the parietal regions. 



The cortical centers of reception and disbursement are so hooked 

 up and interrelated that together they form an intricate but unify- 

 ing and efficient switchboard, reminding one of the central telephone 

 exchange in a large city. This arrangement makes possible associa- 

 tions of various sorts, and furnishes a mechanism for the formation 

 of ideas, as well as providing for the storage of garnered experiences, 

 that have been embalmed in the preservative of memory and kept 

 available for future reference. 



In the course of vertebrate evolution the sense of smell was the 

 first to acquire significant representation in the cortex, since it was 

 the most useful of the senses in the case of lowly animals sniffing 

 around with their noses close to the ground. As time went on, 

 however, particularly in connection with arboreal life of the primates, 

 cortical centers of sight, hearing, and muscular control gained a 

 relative ascendancy, enabling natural selection to take a fresh lease 

 on the task of sharpening the wits and elaborating the brain. The 

 process is by no means completed yet, but even now it has gone so 

 far that, in the case of man, a brain has been developed which en- 

 ables him to perform such wonderful feats as weighing a star, or 

 splitting off an electron from an atom, intellectual feats that are 

 quite unthinkable in the case of any other animal. 



Flint and Metal History 



Man, with his handy hands, is the master mechanic, and the only 

 animal that can use all sorts of tools. Monkeys and raccoons have 

 grasping hands, but they are not very successful tool-users, for 

 the reason that their brains have not caught up with their hands. 



