2Q2 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XVIII. 

 THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



The cerebral hemispheres of man are the largest and most 

 complex part of the nervous system. They are also proportion- 

 ately larger and more complex than in animals. The degree of 

 development of the cerebral hemispheres is correlated with the 

 mode of life of the animal. The greater size and the complexity 

 of internal structure of the hemispheres in man are a measure of 

 the degree of organization of his activities, the perfection of his 

 adjustment to the manifold aspects of his environment, and the 

 correlation of experience which makes such adjustment possible. 

 The cerebral hemispheres constitute a mechanism whose structure 

 is determined by all the experience of the race and of the individual, 

 and in the working of whose minute parts is found the means of 

 directing every one of the more complex activities of the man. 

 In order to understand this highly organized mechanism every 

 means of study must be employed, and nowhere has the compara- 

 tive method proved more useful than here. The differences 

 between the human cerebrum and that of lower animals would 

 seem at first sight to be so great as to make intelligent comparison 

 impossible. In fact, as will appear below, the most fundamental 

 parts of the cerebral hemispheres of man are present and have 

 the same structure in all vertebrates, while for the study of the 

 most highly specialized parts comparison with the various orders 

 of mammals is very fruitful. 



The hemispheres of man are divisible into three widely different 

 parts. The part by which the hemispheres are directly connected 

 with the rest of the brain is a thick bi-lobed mass consisting of 

 collections of nerve cells pierced by numerous large bundles of 

 fibers and is known as the basal ganglion or corpus striatum. It 

 is by way of the corpus striatum that the chief fiber tracts connecting 

 the hemispheres with the rest of the brain pass to or from the 



