Chap. XXIIL] OF THE HUMAN BRAIN. 449 



type of Cortex is most distinct in those parts of tlie frontal and 

 parietal convolutions which constitute the excitable or so-called 



• motor area ' of Ferrier (see p. 575), thoncrh in by far the largest 

 l^art of the Hemispheres the convolutions have rather a six- 

 laminated type : (3) that in the five-lamitiated regions the so- 

 called 'giant-cells' of the fourth layer have generally a grouped ov 

 clustered arrangement, owing to these bodies existing in irregular 

 aggregations (the 'nests * of Betz) ; the principal exception to this 

 lying in the fact that at the bottom of the ' sulci ' (where the 

 grey layer has also less depth than at the summit and sides of 

 the Convolutions), even in these regions, such large cells are dis- 

 posed in a regular but solitary manner, so that in vertical sections 

 they appear to be ranged in linear series : (4) that in the much 

 more extensive six-laminated areas of Cortex, in addition to the 

 existence of the extra layer of small pyramidal and angular nerve- 

 elements above referred to, another distinctive character is to be 

 found in the fact that the large cells have in all parts of the con- 

 volutions that laminar or solitary arrangement which in the so-called 

 ' motor area ' exists only at the bottom of the ' sulci ' * : (5) that 

 transition regions, or convolutions, exist where the six-laminated 

 arrangement seems to be giving place to the five-laminated 

 arrangement, and that almost precisely similar transitions are to 

 be seen even in the five-laminated regions on passing from the 

 bottom of the ' sulci ' to the sides of the Convolutions. 



Although they differ so mucli in size the proper nerve 

 elements of the second, third, and fourth layers are 

 essentially similar in shape, and there is really no good 

 ground for separating these strata from one another. It 

 may be warrantable as a mere artifice for facilitating 

 description, but is not warrantable if the fact of such 



* The fact that these two layers {i.e., the ' fourth ' and the 



* fifth ' of the six-laminated areas) are, as Bevan Lewis points out, 

 always developed in inverse proportion ; and the fact that whero 

 the former is nominally absent {i.e., in the five-laminated areas) 

 "small angular cells" still exist, intermixed with the so called 

 ' giant cells ', make it possible that we have here the above two 

 layers merged into one, owing to the extreme development of some 

 of the nerve elements otherwise existing as small pyramidal cells. 



