THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 10.3 



In none of the fishes and amphibians do we find so well-differenti- 

 ated cortex as in reptiles. Why is this? The answer seems to be that 

 in all Ichthyopsida the entire forebrain is dominated by the olfactory 

 system to such an extent as to retain a measure of physiological 

 homogeneity, which is not favorable for cortical differentiation. The 

 basic feature of cortical function is the association of diverse com- 

 ponents of the action system with separate localization of the func- 

 tional systems involved. In fishes and amphibians this localization of 

 function in the forebrain is incipient, but it is not sufficiently ad- 

 vanced to evoke cortex of definitive type. In reptiles, on the other 

 hand, the great increase in the system of somatic sensory exterocep- 

 tive thalamic radiations is correlated with enlargement of the corpus 

 striatum complex, including an extensive area quite free from olfac- 

 tory and hypothalamic connections and the extension of some of the 

 fibers of the thalamic radiation to the dorsal pallial field without 

 interruption in the striatum. Thus the pallial field is subdivided into 

 three well-circumscribed areas, each with a physiological specificity 

 different from those of the others and one of which is emancipated 

 from dominance of the olfactory system. Now for the first time in 

 phylogenetic history the pallium possesses an intracortical system of 

 association fibers adapted for the specific cortical type of function 

 ('26, pp. 78, 123; '27, pp. 315 ff.; '33e; '34a). 



In a survey of the history of cortical evolution the birds occupy an 

 anomalous position. They are much more highly differentiated than 

 reptiles, but in an aberrant direction, with no mammalian affinities. 

 In most of them the olfactory system is reduced almost to the vanish- 

 ing-point, and the optic system is greatly enlarged. There is extensive 

 local differentiation of thalamic nuclei, but not in the mammalian 

 pattern. The system of ascending thalamic pi-ojection fibers is larger 

 than in reptiles, and most of these fibers end in the enormously en- 

 larged and complicated corpus striatum. Correlated with the latter 

 point is the striking fact that, despite the great increase in thalamic 

 projection fibers, the cortex of many birds is scarcely more extensive 

 than in reptiles and in some species is less well differentiated (Craigie, 

 '40) . Birds are more highly specialized in both structure and behavior 

 than are the lower mammals, and yet their cerebral cortex is rudi- 

 mentary in comparison with even the most primitive mammals. The 

 explanation for this is that the bird's more diversified behavior is 

 largely stereotyped in instinctive patterns, adequately served by 

 subcortical apparatus, while the patterns of mammalian behavior, 



