1905.] 



The Mammalian Cerebral Cortex. 



157 



by Dr. J. S. Bolton, may be taken as fairly typical also of the relative 

 differences in depth of the supra- and infra-granular layers especially, in the 

 corresponding regions in the other two animals. 



Micrometric Measurements of the Cortex of the Mole.* 



,, II. — Supra-granular 



„ IT. — Infra-granular 



T 



Area 1. — Motor 

 (average of 23). 



Area 2. — 

 General sensory 

 (average of 17). 



Lateral area 

 (average of 7). 



mm. mm. 

 0-162 



mm. mm. 

 0-144 



°' 093 l 0-367 

 0-274/° 6bl 



0-176 1 0-377 

 0-201/° 6 " 



mm. mm. 

 0-095 



Xot ' e .-to-471 

 parated J 



Total 



-786 mm. 



-888 mm. 



-886 mm. 



It will be observed that in Area I the supra-granular layer (II) is less 

 than one-third the depth of the infra-granular layer (IV and V), and that 

 in Area II it is only about one-quarter of the depth of the latter. If 

 Layers II and III (supra-granular and granular) are taken together they only 

 approximately equal in depth the infra-granular. 



A very abbreviated comparative summary of these measurements in the 

 case of the Mole, and those of Boltonf from the prefrontal cortex in the sixth 

 month's human foetus, the full-time new-born child and the normal human 

 adult, reveals the following facts, shown approximately in fig. 4 : — 



(1) The granular layer is of approximately the same depth in the Mole 

 •(Areas I and II) as in the normal human adult prefontal cortex. 



(2) The supra-granular (pyramidal) layer : (a) in the sixth month's human 

 foetus is nearly two and a-half times the depth of that of the Mole (Areas I 

 and II) ; (b) in the full-time child it is five to six times the depth of that 

 of the Mole ; (c) in the normal human adult nine times the depth. 



(3) The infra-granular layer. — In the sixth month's foetus and full-time 



* With regard to the lateral area of undifferentiated cortex, the cellular elements in 

 which are comparatively uuspecialised, it is obvious that its increase in depth as compared 

 with Area I is due chiefly to the infra-granular layers (IV and V"), the portion of the 

 •cortex from which the measurements were taken being in the neighbourhood of the rhinal 

 Assure, and so forming what would correspond to the apex of a convolution. 



t Bolton's observations as to the ontogenetic development of the cerebral cortical layers 

 have been confirmed by the writer in the cases of a fourth, sixth, and eighth month human 

 foetus, and the same relative order of development of the layers has been found by him 

 to hold good with regard to several foetal and new-born lower mammals belonging to 

 different orders. 



