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 diverticula from the front part of an 

 outgrowth of the first cerebral vesicle. 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 fun- 

 damental part on which 

 has been built up all the 

 rest of the cerebral hemi- 

 spheres. 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 out- 

 growth from the fore-brain. 



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 prosen- 

 cephalon. 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 that of the centres forming the brain stem or the ol- 

 factory lobe itself in that it provides for a very rich association of impulses 

 between all its parts. The fibres entering the cortex break up into a fine mesh- 

 work of fibres which run tangentially to the surface and come in contact 

 with innumerable dendrites of nerve-cells situated at some little distance 

 below the surface (Fig. 207). We have here the first germ of an apparatus 

 in which the nerve-paths can be determined by education, i.e. in consequence 

 of inhibitions by pain, rather than by the limits set by heredity. In 

 the amphibian brain, and still more in the brain of the reptile, the cerebral 



416 



FIG. 207. 



Section through cerebral cortex of the frog. 

 (After EDLNGER.) 



