184 Cortical Structure of the Gyms Foniicatiis [CHAP. 



radiations of Meynert do not form composite bundles but leave the white substance 

 in a more or less continuous extended line. 



The association system of fibres and interradiary plexus are not well represented ; in 

 short the general fibre-wealth of the part is much below par, the general plan of arrangement 

 is upset, the cortex altogether is shallow, and no fibres of large size are to be found. 



As to nerve cells general mal-representation is again evident. 



A plexiform layer is recognisable, but the remaining cortex is made up of a continuous 

 lamina of rather closely-packed, irregularly arranged, medium-sized pyramidal cells, not unlike 

 those to be noted as occupants of the third and fourth layers of the gyrus fornicatus above 

 the corpus callosum, only smaller, not so deeply stained, and especially indistinct as regards 

 their processes. In the depths of the cortex there are just a few intensely-stained elongated 

 pyramidal cells similar to cells to which I shall presently make special reference. 



As we pass towards the base of the brain the surface structure of this gyrus gradually 

 loses the appearance of cortex. 



The small olfactory area of Broca lying immediately internal and anterior to the mesial 

 olfactory root is covered by an undeveloped type of cortex like that described above, and 

 its fibre-arrangement is broken up by offshoots from the root. 



GYRUS FORNICATUS OR GYRUS CINGULI. 



I have found that the structure of the gyrus fornicatus can be most conveniently studied 

 in a series of sections extending in radiate fashion from the corpus callosum to the margin 

 of the hemisphere. Such sections show the structure of the gyrus in question and the 

 more peripheral convolutions lying side by side ; and when those sections, stained for nerve 

 fibres, which pass through the paracentral lobule and allow of a contrast between the dense 

 and deeply stained motor and the pallid fornicate cortex, are inspected, it' will be granted 

 immediately that the latter is deserving of separate representation on a cerebral map. 



Arrangement of Nerve Fibres. (Plate XIX, fig. 1.) 



Perhaps the chief distinguishing feature of this type of cortex is that it contains 

 absolutely no large evenly-medullated fibres like those seen in the central convolutions and 

 calcarine region, and indeed practically none of medium calibre ; they are all fine and wavy, and 

 in most cases varicose, hence the pallor of the cortex when compared with that of adjoining 

 fields. It is also to be noted that this region does not present the bizarre features seen 

 in the hippocampus. 



The zonal layer, as might be expected, is sparsely supplied and indistinct ; the supraradiary 

 field is fairly rich in fibrils running in all directions ; a line of Baillarger can always be 

 made out, but it has no striking constituents ; the radiations of Meynert lack solidity on 

 account of the absence of large fibres, but they contain an abundance of wavy fibrils ; 

 (Text-figure 18) in the spaces between the radiating fasciculi there is a rich enough plexus, 

 but the component fibres are only discernible with the stronger lenses (+ ^ fi ). The system 

 of association fibres is not well represented, and the white substance immediately underlying 

 the cortex has a pallid look, which it also owes to the absence of large fibres. 



