96 



THE MECHANISM OF LIFE 



animals higher than fishes to become the cerebral hemispheres. 

 The latter progressively increase in mass in the amphibia, 

 reptiles, and birds, until they become the enormous lobes that 

 we see in man, overarching and almost concealing all the other 

 parts of the brain. 



Thus we have the three developmental parts of the vertebrate 

 brain : 



The lower brain : corpora striata, optic thalami, mid-brain, 



incipient cerebellum, medulla; 

 The cerebellum ; 

 The cerebral hemispheres with their cortex. 



Cortex 

 cerebri 



Cerebmi 

 hemisphere 



Corfius obtic 

 striatum thalamus 



Cor/:) or a. 



<^uadrij^emina. 



Cerebellum 



Medulla. 



..^^f:)inal cord 



Fig. 25.— Further Stage in the Development of the Brain. 



The primitive lower brain in a fish, such as the cod, is very 

 simple (Fig. 26). The mid-brain, with its great optic lobes, is the 

 most important part. The fore-brain is the part lying between 

 the two corpora striata. The cerebellum is not greatly developed. 

 In its general characters such a brain as that which we have 

 just described represents what we shall refer to hereafter as the 

 " lower brain " of the higher mammal. Imagine the thin 

 membranous " pallium " which covers the corpora striata to 

 become enormously thickened and to grow backwards over all 

 the rest of the brain, and •suppose the cerebellum greatly to 

 increase in mass relatively to the other parts. Then we should 

 have the brain of the mammalian animal. 



