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 





Section through cerebral cortex of the frog. 



< After E K -> 



a 



in the floor of the out- FIG. 207. 



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 prosencephalon. At its first appear- 

 ance 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 olfactory 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 meshworli of fibres 

 which run tangentially to the surface and come in contact with innumer- 

 able 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 eredity. In the 

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



415 



