THE TELENCEPHALON. 



1177 



FIG. 1015. 



which, from without inward, are : (i) the stratum zonale, (2) the layer of small 

 pyramidal cells, (3) the layer of large pyramidal cells, and (4) the layer of poly- 

 morphic cells. Although each presents characteristics which are distinctive, with 

 the exception of the junction between the first and second layers where the change 

 is well defined, no sharp demarcation separates the strata, each passing insensibly 

 into the adjoining layer. Neither are the modifications which distinguish the 

 cortex of certain regions abruptly assumed, one type of cortical structure being 

 gradually replaced by another without sudden transition. 



The stratum zonale, also known as the molecular stratum, underlies the pia 

 and measures about .25 mm. in thickness. The layer contains few nerve-cells and 

 appears subdivided into (a) a narrow peripheral zone, from .010 .030 mm. in width > 

 composed of a subpial condensation of neuroglia 

 and (^) a deeper zone characterized by numer- 

 ous fibres or processes, which course parallel to 

 the surface, and a meagre number of nerve-cells 

 whose most distinctive representatives are small 

 fusiform elements (Cajat's cells") provided with 

 long tangentially directed processes. The latter 

 give off short collaterals, which ascend towards 

 the surface, and intermingle with the number- 

 less terminal filaments derived from the periph- 

 erally coursing processes of the pyramidal and 

 other cells lying at deeper levels and from the 

 corticipetal fibres which continue from the 

 white core of the gyrus into the outermost 

 layer of the cortex. 



The layer of small pyramidal cells is 

 marked off from the stratum zonale, which it 

 about equals in thickness, with some distinctness 

 since, in contrast to the last-mentioned zone, 

 it contains very many cells. These, as indicated 

 by the name of the stratum, are of small size 

 (.007 .010 mm.) and pyramidal form, at 

 least in the deepest part of the layer. In the 

 superficial part the cells are rounded or irregu- 

 larly triangular, but they assume the distinctive 

 pyramidal outline as they approach the sub- 

 jacent layer, whose elements they resemble in 

 possessing apical and lateral processes. 



The layer of large pyramidal cells con- 

 tains the most distinctive neurones of the cere- 

 bral cortex. It measures usually about 1.25 

 mm. in thickness, but in some localities much 

 more, and blends with the adjoining layers 

 without sharp boundaries. The cells in- 

 crease in size but diminish in numbers as 

 they are traced from the second layer inward, 

 the largest (from .020 .040 mm. in width) and 

 most characteristic lying in the deepest part 

 of the stratum. The typical pyramidal cell 

 possesses a conical body, triangular in section, 

 the apex of which is continued into a long 

 tapering dendrite, the apical process, -which 

 extends toward the periphery for a variable but usually considerable distance, 

 depending upon the position of the cell. Upon gaining the stratum zonale, towards 

 which the apical dendrite is always directed, the process breaks up into a 

 number of end-branches that run parallel with the surface and contribute to the 

 fibre-complex of the outer layer. During its journey to the surface, the apical 

 dendrite gives off an uncertain number of branches that continue horizontally and, 



Section of cerebral cortex. X 90. 



