THE CEREBRAL HEMISPHERES 



SECTION XVI 



GENERAL STRUCTURAL ARRANGEMENTS OF 

 THE CEREBRUM 



THE cerebral hemispheres form the most important part of the 

 brain. It is to the development of this part that is due the rise in 

 type in vertebrates. In development they are formed as two diverti- 

 cula from the front part of an outgrowth of the first cerebral vesicle. 



FIG. 207. Section through cerebral cortex of the frog. 

 (After EDINGER.) 



In the lowest vertebrates these outgrowths are connected entirely with 

 the olfactory sense-organs, and we may regard the olfactory part of the 

 brain as a fundamental part on which has been built up all the rest of 

 the cerebral hemispheres. In a cartilaginous fish the whole of the 

 upper brain is connected with the organ of smell, and consists of a 

 thickening in the floor of the outgrowth from the fore-brain to which 

 is attached in front the olfactory lobe. The roof of the outgrowth 

 is formed of simple epithelium. With the development of the visual 

 sensations in the bony fishes there is still very little corresponding 

 growth of the fore-brain, most of the fibres from the optic nerves 

 going to the roof of the mid-brain (the optic lobes). The beginning 

 of the cerebral hemispheres is associated with the development of 

 nervous tissue in the roof of the prosencephalon. At its first appearance 

 this higher brain material still receives chiefly olfactory impressions. 

 But the structure of the cerebral cortex thus laid down differs from 



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