160 



The Mammalian Cerebral Cortex. 



mammals possess a relatively poor supra-granular layer. Mauy of these 

 lower mammals have adopted a safe mode of life, others have resorted to 

 fecundity. With these, which may for present purposes be termed 

 extraneous aids to survival, their essentially instinctive activities have been 

 relatively sufficient to ensure their continued existence. There has therefore 

 in these mammals been little necessity for the development of a supra- 

 granular layer, the infra-granular providing most of the necessary cortical 

 physical basis required for practical behaviour. 



The infra-granular layer, with the reservation to which reference has 

 been made, thus constitutes the earlier developed and more fundamental 

 associational system of the cerebral cortex ; the supra-granular, a higher 

 and accessory system, super-added, and of any considerable functional 

 importance only in certain regions in lower mammals such as tbe Insectivora. 



In view of the above conclusions, attention may be briefly directed to the 

 following points. Areas I and II — motor and general sensory — in the Mole 

 and Shrew (and the combined field in the Hedgehog) appear in every sense 

 to be the most completely developed regions of neopallium which these 

 animals possess, and are the only areas in the two former animals having 

 a supra-granular layer of any considerable depth and complexity as regards 

 its individual cell elements. In the Hedgehog, in the area which is believed 

 to have visual functions, there is as regards individual cells a moderately 

 well-developed, though thin, supra-granular layer, whilst this is practically 

 absent in the comparatively blind Mole and Shrew in the analogous region. 



Note to Fig. 4. — The infra-granular layer in the 6-months' foetus and new-born child 

 were practically of the same depth in the specimens measured, viz., - 442 to '424. In the 

 figure the darkly-shaded part (infra-granular layer) in the third column is therefore 

 represented somewhat too deep. 



