LAYERS OF CELLS IN THE CORTEX. 167 



accordingly less conspicuous on the lateral convex aspect of the hemispheres, and 

 more so on the convolutions situated in the longitudinal fissure which approach the 

 white surface of the corpus callosum, and on those of the under surface of the brain. 

 It is especially well marked on the hippocampal gyrus, and it has been there 

 described under the name of the reticulated ivhite substance. 2. Immediately beneath 

 the white layer just described, is found a layer of grey or reddish grey matter, the 

 colour of which, as indeed of the grey substance generally, is deeper or lighter 

 according as its very numerous vessels contain much or little blood. 3. A layer, 

 appearing in section as a thin whitish line (line of Vicq d'Azyr, outer line of 



Fig. 115. SECTIONS OF CEREBRAL CONVOLUTIONS 

 (after Baillarger). 



The parts are nearly of the natural size. 1, shows 

 the six layers ordinarily seen in the cerebral cortex when 

 carefully examined with the naked eye ; 2, the appear- 

 ance of a section of a convolution from the neighbour- 

 hood of the calcarine fissure. 



Baillarger). 4. A second grey stratum, y. A second thin whitish layer (inner line 

 of Baillarger). 6. A yellowish grey layer which lies next to the central white matter 

 of the convolution. In some convolutions, especially those bordering on the calcarine 

 fissure, the line of Vicq d'Azyr is very distinct, but the inner line of Baillarger is not 

 visible (fig. 115, 2). 



Medullated fibres radiate from the white centre of each convolution in all direc- 

 tions into the grey cortex, having a course for the most part perpendicular to tho 

 free surface. In passing through the grey substance they are arranged in bundles 

 about TsVoth of an inch in diameter, and thus separate the nerve-cells into elongated 

 groups, and give the section a columnar appearance (fig. 110). The direction of 

 the fibres varies according to the part of the convolution in which they occur, 

 whether near the summit or the base, and the radiating direction is somewhat lost 

 in the sulci between the convolutions, where the arched fibres which connect the 

 adjacent convolutions seem to obscure the radiating bundles. 



Layers of cells in the cortex. The form and arrangement of the cells vary 

 at different depths of a convolution, and in this way several layers are distinguished, 

 having more or less definite characters, but not sharply marked off from one another. 

 Their relation to the stratification distinguishable by the naked eye is not everywhere 

 clearly made out. The most common type is that which is seen in the convolutions 

 of the parietal lobe. In this most observers agree with Meynert in recognising five 

 layers as follows (fig. 116) : - 



1. Superficial or molecular layer. This, the most external layer, is narrow, and 

 forms about T a oth of the whole thickness of the grey cortex. It is composed chiefly of 

 neuroglia, but contains some nerve-cells. A few medullated nerve-fibres occur in it, 

 forming a thin superficial white stratum almost immediately underneath the pia mater. 



As already intimated these fibres are much more developed in the hippocampal 

 region than in other parts of the cortex. The layer also contains non-medullated 

 fibres, which ramify in it, and most of which are derived from the peripherally- 

 directed processes of the pyramidal cells of the deeper layer. 



The neuroglia-cells of the superficial layer are mostly elongated and set perpen- 

 dicularly to the surface, where a principal process of each cell usually terminates in 

 a foot or enlargement (fig. 117). 



The nerve-cells of the layer are small. They vary in shape, many being fusiform 

 and set parallel with the surface. Both their dendrites and their axis-cylinder 

 processes, which give off numerous collateral branches, are for the most part confined 

 to this layer. Many of them have two or three axis-cylinder processes, and these 



