THE CEREBRUM. 



301 



I Stratum 

 i zonale 





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 

 10-30 ti in width, composed of a subpial condensation of neuroglia and (6) 

 a deeper zone characterized by numerous fibres or processes, which course 

 parallel to the surface, and a meagre 

 number of nerve-cells whose most dis- 

 tinctive representatives are small fusi- 

 form elements (Ca/a/'s cells) provided 

 with long tangentially directed proces- 

 ses. . The latter give off short collaterals, 

 which ascend towards the surface, and 

 intermingle with the numberless terminal 

 filaments derived from the peripherally 

 coursing processes of the pyramidal and 

 other cells lying at deeper levels and 

 from the corticipetal fibres which con- 

 tinue 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 thick- 

 ness, 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 (7-10^) and pyramidal 

 form, at least in the deepest part of the 

 layer. In the superficial part the cells 

 are rounded or irregularly triangular, 

 but they assume the distinctive pyram- 

 idal outline as they approach the sub- 

 jacent layer, whose elements they re- 

 semble in possessing apical and lateral 

 processes. 



The layer of large pyramidal 

 cells contains the most distinctive neu- 

 rones of the cerebral 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 increase in 

 size but diminish in numbers as they are 

 traced from the second layer inwards, 

 the largest (20-40 /JL in width) and most 

 characteristic lying in the deepest part 

 of the stratum. The typical pyramidal cell possesses a conical body, trian- 

 gular in section, the apex of which is continued into a long tapering dendrite, 

 the apical process, which extends towards 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 direct- 

 ed, 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 











. Large pyra- 

 midal cells 



Polymorphic 



[cells 



FIG. 348. Section of cerebral cortex. X 90. 



