Visuo-Scnsory ami VisuO'Psychic Areas [CHAP. 



TYPE OF ARRANGEMENT OF NERVE FIBRES. 



In regard to fibre arrangement this field of cortex is distinguished by the following 

 features. (Plate X, fig. 2, and text-figure 12.) 



Zonal Layer. 



The zonal layer is well-developed better than in the calcarine region and its lower 

 margin is distinctly defined. Large medullated fibres are found only occasionally, but fibres 

 of the coarse varicose type are present in abundance and account in large measure for the 

 distinctness of the stratum. The remainder of the layer is composed of fine varicose fibres 

 like those found in this layer in all other parts of the cortex. 



Supraradia ry Layer. 



A moderately dense network of fine fibres occupies the whole of the supraradiary field. 

 This network follows the usual rule in being of increased density in the part adjoining 

 the line of Baillarger, and it is composed mainly of short, irregularly placed fibres. Fibres 

 of medium size and of greater length are seen also : a few of these are placed vertically 

 and may be regarded as fibres of Martinotti ; others have a transverse direction, and in 

 certain parts aggregate at a point midway between the surface and the line of Baillarger, 

 so forming a line of Kaes. This formation, better seen in thick than in thin sections, is 

 to be sought round the lips of sulci, for as the wall of the sulcus is descended the fibres 

 composing the line seem to commingle with the more deeply situated association fibres, while 

 in ascending, the linear formation loses in distinctness as the crest of the convolution is 

 approached. 



The fibre wealth of this layer is greater than it is in the calcarine area or in the 

 adjoining parietal and temporal cortex. 



Line of Baillarger. 



The appearance presented by the line of Baillarger is not the least important feature 

 of this type of cortex. Its breadth is great, greater than that of the corresponding line 

 of Gennari in the calcarine area, its fibre wealth also is considerable ; but instead of being 

 sharply defined and compact like the calcarine line, it here tends to spread upwards and 

 downwards into adjoining layers, hence its margins are blurred : furthermore, the absence of 

 the underlying pallid zone, noted in the calcarine region, accentuates the indistinctness of 

 its lower edge. 



In constitution it resembles the calcarine line in being mainly composed of short fibres 

 of delicate calibre, but it differs in containing many more long, transverse fibres of medium 

 calibre. 



Radiations of Meynert. 



The projection bundles form prominent objects, and on analysing their constituents we 

 find that in addition to ordinary fine wavy fibres each fasciculus contains one or more coarse 

 evenly-medullated fibres, and it is to the presence of these large fibres that the fasciculi 

 principally owe their distinctness. The bundles tend to pierce the line of Baillarger so as 

 to reach the supraradiary layer. 



