494 



COMPARATIVE ANATOMY 



Increase in the size of mammalian brains is accompanied by com- 

 plication in form and structure. The cortex becomes cellular, and 

 consequently gray in color. The amount of cortical material increases 

 manyfold, so that if the human cortex were spread out fiat it would cover 



/TELA CHORIOIDEA 



xCEREBOJJJM /TELA CHORIOIDEA 



/■MEDULLA OBLONGATAX 



DIENCEPHALON 



CHORIOID PLEXUS, 

 PARAPHYSISi 

 MONROE'S FORAMEN 



TELENCEPHALON, 

 OLFACTORY 



OLFACTORY/ 

 LDBE 



LAT VENTRICLE' 



HIPPOCAMPUS' 

 HEMISPHERE' 



'^ — ^^\- 



VENTRICLE© 

 VtELA CHORIOIDEA 



Fig. 410. — Brains of Petromyzon (A), Scymnus {B), and Rana (C) in dorsal aspect. 



(Redrawn after Plate.) 



a surface eighteen inches square. The number of neurons runs into the 

 billions, and five cell layers may be distinguished. The increase in the 

 mass of the cerebral hemispheres as we pass from lower to higher mammals 

 is the result, not of multiplication of layers of neurons in the cerebral 

 cortex, but of folding of the cortex. 



