114 Visuo-Sensory and Visuo-Ps>/chie Areas [CHAP. 



The Radiations of Meynert. 



The individual bundles are by no means voluminous, and they are composed mainly of 

 fine or medium-sized fibres, those of large calibre being distinctly uncommon. 



Interradiary Plexus and Association Fibres. 



Immediately beneath the line of Gennari there occurs a zone of considerable depth in 

 which both the interradiary plexus proper and the association system are poorly represented; 

 hence this part has a pallid appearance, and in consequence the line of Gennari is given 

 a clearness of definition which it would not otherwise possess. Under a low power of the 

 microscope this pallid zone forms quite a striking feature, and it evidently corresponds in 

 position with the lamina of small stellate cells to be noted hereafter. 



In the remaining portion of the radiary zone, that is in the part lying between this 

 pallid lamina and the white substance, the fibre wealth is great ; not only do the fibres of 

 delicate calibre show a numerical increase, but evenly-medullated fibres of gross calibre make 

 their appearance. These are placed either obliquely or horizontally, and are most numerous 

 in the immediate neighbourhood of the white substance ; and it is important to mention that 

 they do not tend to join the radiary fasciculi, but on the contrary cross them at all angles. 

 (Text figure 10.) 



Now to my mind the presence of these fibres constitutes almost as important a feature 

 of the calcarine type of cortex as does the line of Gennari ; their great size, their oblique 

 or horizontal direction, and the more or less open formation of the layer which they traverse, 

 are all striking characters, and a similar appearance is seen only in two other parts of the 

 cortex, viz. in the postcentral area (along the parietal wall of the fissure of Rolando), and 

 in the transverse temporal gyri. 



It is impossible to trace these large fibres to their destination, or speak concerning their 

 intimate cortical connections from an examination of specimens stained for medullated fibres 

 only, but Ramon y Cajal gives a very full and lucid description of them, and rightly credits 

 them with much importance. He distinguishes them by the name " optic fibres," and his 

 figures, drawn from chrome-silver preparations, clearly represent that the majority of their 

 terminal branches end in and have important connections with the elements of the stellate 

 layer and line of Gennari. " Bei weitem die meisten der aus den Opticusfasern hervorgehenden 

 Aeste vertheilen sich also in den Schichten der Sternzellen und man muss folglich diese Schichten 

 als den Hauptort der grauen Substanz bezeichnen, in welchem sich das optische Bild projicirt 

 und an dem die optische Empfindung vor sich geht." 



Concerning the central course of these fibres all my sections show their presence in 

 great numbers in the white substance immediately subjacent to the calcarine cortex, some 

 cut obliquely, some transversely, and some horizontally, and the specimens as plainly demonstrate 

 that they constitute the cortical projections of the radiations of Gratiolet ; hence I am 

 compelled to agree with every word which Ramon y Cajal says concerning their physiological 

 significance as conductors of visual impulses. 



