THE NERVOUS SYSTEM 



363 



developed in apes. The brains of such fossil types as the Java and Peking 

 man are transitional between those of modern man and apes. 



Nor is the gap between the brain size of mammals and that of reptiles 

 formidable. In dorsal view, all five divisions of the primitive brain are 

 visible alike in monotremes and alligators. The overgrowth of the hemi- 

 spheres, begun in reptiles, reaches its climax in man, whose domination 

 in the animal world may be ascribed to the enlargement of his conscious 

 control centers in the cerebral hemispheres. 



The cortical enlargement in mammals, however, involves more than 

 an increase of gray matter. Correlated with the multiplication of cells 



C. TARSIER. D. MARMOSET 



Fig. 322. — Diagrams of the brains of insectivores and of lower primates viewed from 

 the left side. The figures show the increasing dominance of the centers of vision over 

 those of smell. A, brain of jumping shrew. B, brain of tree shrew. C, brain of the 

 primate Tarsius. D, brain of the marmoset. (Redrawn after G. Elliot Smith.) 



is an increase in the number of nervous interconnexions. Association 

 fiber tracts connect all parts of the enlarging brain so that all regions, 

 however remote from one another, are interconnected. The brain pro- 

 duced by these evolutionary changes is an organized and integrated whole, 

 no part of which appears to function independently of the rest. 



In primates a marked retrogression of the olfactory lobes accompanies 

 the enlargement of the hemispheres. The olfactory centers in the hip- 

 pocampus persist, but other regions of the cortex enlarge disproportion- 

 ately. Vision in primates is more important than smell, and changes in the 

 brain express relative functional values. 



Cortex. Two parts of the primitive vertebrate brain participate 

 especially in the great enlargement of the cerebral hemispheres, the striate 



