ORIGINS OF THE CEREBRAL CORTEX 83 



inferior to those of most mammalsj are far in advance 

 of anything found in fishes and amphibians. *'Wise 

 as serpents" does not mean wiser than all men, but 

 it does mean wiser than any fishes; it does mean im- 

 proved capacity on the part of the individual to take 

 care of himself in a very complex and unfriendly 

 world. 



THE MAMMALIAN TYPE 



The earliest steps in the elaboration of the mam- 

 malian cortex are entirely unknown to us and prob- 

 ably must remain so forever. We know from endo- 

 cranial casts of fossil skulls that the brains of very 

 primitive mammals were more reptilian in external 

 form than are those of any existing species, but their 

 internal structure can only be surmised. The brains 

 of the most primitive living types of mammals show 

 many reptilian features, but the cerebral cortex of 

 these species is nevertheless distinctly mammalian, 

 not reptilian. In these forms the two salient features 

 of the reptilian cortex already cited are accentuated 

 and the internal structure is far more complex, a 

 development that goes on progressively as the mam- 

 malian series is ascended. 



The organization of the reptilian cortex is similar, 

 though not identical, in all of the three cortical fields 

 (Fig. 11) — a thin sheet of densely crowded cells, each 

 of which is typically a double pyramid with a den- 

 dritic brush at both inner and outer pole. Figures 22, 



