68 Type of Arrangement of Nerve Fibres [CHAP. 



A. ARRANGEMENT OF NERVE FIBRES. (Plate V, fig. 1.) 



Zonal layer. 



If sections comprising a view of the cortex on both walls of the Rolandic fissure be 

 examined, and the zonal layer of the two compared, it will be seen that it lacks repre- 

 sentation on the postcentral side, and this weak development of the zonal layer, found over 

 the whole extent of the postcentral area, is the first point of distinction between postcentral 

 and precentral cortex (cf. Plate III, fig. 1). The layer is composed, mainly, of varicose fibrils, 

 with which larger varicose fibres are intermingled. Coarse medullated fibres are present, but 

 they have to be carefully sought for and are not found in every section. 



Supraradiary layer. 



In depth, this layer is approximately equal to the same layer in the precentral cortex, 

 and the general arrangement of elements is similar, but a pronounced inferiority in fibre 

 wealth constitutes an outstanding point of difference. 



Line of Baillurger. 



In thick sections this line is plainly visible with the naked eye, and assumes the form 

 of a dark broad band, but in thin preparations, the microscope is required to disclose it, 

 and when so examined it is found to be composed mainly of small fibres, interlacing and 

 intertwining in all directions, mingled with which is an appreciable number of horizontally 

 directed fibres of medium size ; to the latter the line in a large measure owes its depth 

 of colour, in thick sections. 



Radiary Zone. (Text figs. 1 and 5.) 



It is the appearance presented by the radiating fasciculi of projection fibres, the inter- 

 radiary plexus, and the association fibres, in short the combined radiary zone, which above 

 all gives chai-acter to the postcentral cortex and distinguishes it from that of any other field 

 in the brain. 



In describing the precentral cortex it was stated that the radiate fasciculi were numerous, 

 richly stocked with fibres and voluminous, that the interradiary collaterals were extremely 

 abundant, that the association system of fibres was particularly well-represented, and that 

 these sets of fibres altogether combined to form a zone of great depth, and of such density 

 and bewildering complexity that it could be readily recognised with the naked eye by the 

 attendant intensification of colour. Contrasting this with the arrangement in the postcentral 

 cortex, we find that the radiate fasciculi are attenuated instead of voluminous ; the plexus 

 formed by the association fibres and interradiary collaterals combined, open instead of dense ; 

 and no dark-stained zone, but only a relatively thin and pallid band is seen capping the 

 medullary projection. In the walls of the Rolandic fissure, these differences are quite as marked 

 as they are along the crown of the convolutions, and taken altogether a single glance serves 

 to distinguish one gyrus from the other. 



