6 2 ATLAS OF NERVE CELLS 



THE CEREBRAL CORTEX 



GENERAL TOPOGRAPHY 



PLATES XXXIII. to LIII. show the structure of the cerebral cortex. This is the most 

 intricate portion of the nervous system. It has formed the subject of numerous careful 

 studies, but there remain many points which are still obscure, and there is as yet much 

 uncertainty regarding the arrangement of its cells and the direction of its fibres. The 

 methods of Golgi have, however, been particularly successful in demonstrating the constituent 

 parts of the cortex, in revealing the existence of numerous varieties of cells, in tracing the 

 division and destination of their branches, and in unravelling the tangled mass of fibres 

 which permeate it everywhere. The description here given rests largely upon the study of 

 the facts presented by Golgi, Cajal, and Retzius, and illustrated in the plates. 



Plate XXXIII. shows a section through the cortex of the Rolandic region of a human 

 embryo of eight months. It gives a general idea of the topography of the cortex at a 

 period in the development when the structure is simple, and the various layers can be easily 

 distinguished. It is evident that there are several layers in the gray matter. In this plate the 

 superficial layer is so deeply stained that little of its structure can be seen, although some fibres 

 parallel to the surface, so-called tangential fibres, can be made out in the lighter parts. The 

 characteristics of this layer are shown in Plates XXXIV. to XXXVII. Beneath the super- 

 ficial layer a comparatively clear region is seen having a striated appearance, because 

 traversed by long fibres at right angles to the plane of the surface. These fibres are really 

 the long, slender, upright apical extensions of the pyramidal-shaped cells which form the 

 second and third layers. A careful inspection, however, will show here and there in this 

 clear zone some small cells with little apices extending upward toward the surface. And 

 when the layer of large pyramidal cells is studied, it will be noticed that these cells lie at 

 different levels, a number of smaller ones being nearer to the surface than the very large 

 ones which first catch the eye. A number of deeper lying cells are visible at the right of the 

 plate. There is really no sharp dividing line between the layers of small and large pyramid 

 cells; and although modern authors divide them into two distinct layers, the plates demon- 

 strate that they intermingle. Beneath the layer of cells is another fairly clear zone occupied 



