The Mammalian Cerebral Cortex. 



151 



Shrew are reduced to small threads. In the Hedgehog these nerves are 

 considerably larger. In all three the fifth nerves are relatively very large. 



The Lamination of the Neopallium. 



General Eemarks. — The classification of the cortical layers adopted by the 

 writer is that introduced by J. Shaw Bolton. The latter considers that 

 the human cerebral cortex is constructed upon a five-layered type — viz., 

 I, Molecular; II, Pyramidal; III, Granular; IV, Inner line of Baillarger: 

 V, Polymorphic. Of these only three are primarily cell layers — viz., the 

 pyramidal, granular, and polymorphic, Layers I and IV being primarily fibre 

 layers, although containing nerve cells — the cells of Cajal in Layer I, and the 

 Betz cells (psycho-motor region) or solitary cells of Meynert (other regions) 

 in Layer IV. The outstanding features of this classification are: (1) The 

 recognition of the granular layer as separating the true pyramidal layer above 

 from the more or less pyramidal shaped cells which may be found below this 

 layer, for the cells of Layer IV are not " pyramidal " cells at all, the Betz cells 

 in the psycho-motor area constituting " the origin of the important tract for 

 skilled voluntary movement," whilst the solitary cells of Meynert in other 

 regions " probably possess a somewhat analogous function." (2) The con- 

 sideration of the pyramidal layer as forming one layer developmentally and 

 functionally. 



Bolton, as the result of his studies of the development of the human cerebral 

 eortical layers, and of their depth in the normal individual as well as in 

 various degrees of amentia and dementia, has come to the following conclusions 

 as to the functions of the three primary cell layers. The pyramidal layer 

 "subserves the psychic or associational functions of the cerebrum." The 

 granule layer " probably subserves the reception or immediate transformation 

 of afferent impressions, whether from the sense organs or from other parts of 

 the cerebrum," whilst the fifth, or polymorphic layer, " probably subserves the 

 lower voluntary functions of the animal economy." 



When dealing with the mammalian cortex generally one or two further 

 explanations are necessary. 



The term " granular " is used in a wide generic sense and as indicative of 

 a certain cortical layer rather than of the cell constituents of this layer, 

 which latter, in an adult animal, may take the form of angular, quadrilateral, 

 stellate or even small pyramidal-shaped cells, or a mixture of these elements. 

 In some regions of the cortex in certain animals these elements of the 

 granular layer may be scattered and comparatively few in number ; yet 

 their recognition is of importance, for such provides the means by which 

 the lower limits of the true pyramidal layer may be determined. Owing 



