234 Types of Cell Lamination [CHAP. 



reduction in depth, most marked when " precentral " and " prefrontal " sections are compared ; 

 and, secondly, by a lowering of the intensity of coloration, a feature recognisable with the 

 naked eye. Lastly, I would say that for purposes of demonstration I am in the habit 

 of showing sections from the upper end of the " precentral " area alongside sections of the 

 " prefrontal " cortex and remarking that they represent extremes of cortical fibre wealth. 



TYPES OF CELL LAMINATION. (Plate XXIV, figs. 1 and 2.) 



The reason for separating this division of the frontal lobe from the " intermediate 

 precentral " area and for further subdividing it into two main territories, arising out of the 

 examination of the characters of its cortical fibre arrangement, finds support and confirmation 

 when sections stained for the display of nerve cells are inspected ; indeed, the manner in 

 which each change in fibre arrangement is immediately followed by a corresponding alteration 

 in cell lamination is one of the most instructive revelations of cortical histology. 



But it is not to be denied that the structural variations are of a subtle description 

 and only discoverable by a cautious examination of a large expanse of cortex in serial 

 sections ; for not only is the entire field enveloped by cortex showing the usual seven 

 laminae to which I have made constant reference throughout this research, but the changes 

 affecting these laminae are not of the gross character seen in other regions ; moreover, the 

 several subdividing lines of demarcation are by no means so sharp as the diagram indicates. 

 Therefore in putting an account of the lamination on paper it becomes necessary to use 

 as a descriptive basis the appearances seen in portions of cortex lying towards the centre 

 of each area, that is portions which may be regarded as typical, and to give point to the 

 survey by adding comparative remarks concerning each type. 



The individual layers will now be analysed in the usual way. 



Plexiform Layer. 



For purposes of territorial demarcation the plexiform layer is again of little use ; all that 

 I will say is that it appears to lose in depth as we proceed forwards. 



Layer of Small Pyramidal Cells. 



This layer also shows nothing peculiar ; its constituents are numerous and closely aggregated 

 but only slightly more so than in other parts. 



Layer of Medium-sized Pyramidal Cells. 



The arrangement of these cells also follows the common type ; they form a layer of good 

 depth, the upper and lower borders of which are indefinite, and although my observations 

 on this point do not pretend to be exact I believe that they -suffer a reduction in size 

 as we go in the frontal direction. 



External Layer of Large Pyramidal Cells. 



The changes affecting this layer are of undoubted importance and constitute our chief 

 guide to regional differentiation. We have seen that in the " intermediate precentral " area 

 there is a well-constituted layer containing numerous cells of large size, pear-shaped, and 



