PROBLEM OF THE CEREBRAL CORTEX 25 



cerebral hemisphere.^ The paUium may or may not 

 contain differentiated cerebral cortex, though the 

 terms "pallium" and "cortex" are often used as 

 synonyms. But the anatomical delimitation of the 

 pallium presents even greater difficulty than that of 

 the cortex, so the concept of the pallium does not 

 help to clarify our ideas of the limits of the cortex. 



Some have supposed that the superficial gray 

 matter of the cortex assumed this position and in its 

 more elaborated forms became convoluted in response 

 to the nutritive requirements, either to avoid too 

 great thickness of brain substance or to be reached 

 conveniently by the blood vessels, keeping close to 

 the source of the blood supply in the pia mater. It 

 has even been suggested that the cerebral fissures 

 were caused in the first place by the mechanical 

 pressure of the great superficial blood vessels. 



The nutritive requirements have apparently 

 played some part in shaping the form of the cerebral 

 hemisphere in primitive vertebrates, as I have else- 

 where endeavored to show (1921), and it is not 

 improbable that vascularization from the pia mater 

 is still a contributing factor in the maintenance of 

 the superficial position of the cerebral cortex. But it is 

 evident that gray matter may, on occasion, be devel- 

 oped in similar thin sheets quite independently of 



^The view expressed by Rabl-Riickhard In 1883 that the membra- 

 nous roof of the forebrain of some fishes is homologous with the pallium of 

 higher vertebrates is now discredited. 



