150 Temporal Lobe and Auditory Areas [CHAP. 



safely ascribe to the fact that lying within the fissure of Sylvius it has been denied any 

 chance of expansion. 



Let us next examine the various layers seriatim. (Plate XII. fig. 1.) 



Zonal Layer. 



Although evenly-medullated coarse fibres like those found in the precentral cortex are 

 rarely seen, still the layer must be described as well-developed, and it is also more distinct 

 here than in other parts of the temporal lobe. An abundance of fibres of the fine varicose 

 type and members of the coarse variety constitute the layer, and a direct continuity be- 

 tween some of the latter and vertically placed fibres in the immediately subjacent layers can 

 be proved. 



Supraradiary Layer. 



Commensurate with the prominence of the zonal layer, the supraradiary field is richly- 

 stocked with fibres, and it is interesting to notice that this is one of the few regions in 

 which one can make out a special collection of fibres lying midway between the surface 

 and the line of Baillarger, sufficiently obvious to constitute what is called a line of Kaes. 

 Delicate and medium-sized fibres running parallel with the surface chiefly make up the line, 

 but occasionally an isolated coarse medullated fibre is included. 



Line of Baillarger. 



The line of Baillarger is of great breadth and distinctly visible with the naked eye, 

 but on account of the great fibre wealth of the parts immediately above and below, its 

 limits are not clearly definable under the microscope. It is chiefly composed of fibres of 

 medium size, the majority of which run in the horizontal direction, but its prominence is 

 accentuated by the presence of numbers of long coarse fibres running in the same plane. 



Radiations of Meynert. 



The radiations form stout bundles and contain many fibres of gross size in addition 

 to the usual smaller elements. They do not exhibit a tendency to pierce the line of 

 Baillarger as they do in other parts of the temporal lobe. 



Interradiary Plexus and Association Fibres. 



We come now to a layer the characters of which absolutely stamp this field of cortex 

 and in great measure explain the intensity of coloration noticed with the naked eye. It 

 is not so much the density of the interradiary plexus proper which calls for special remark, 

 as the presence in great abundance of long fibres, of great size, which intersect the radiations 

 at all angles and at all levels (Plate XIII, fig. 1 ). The horizontal or oblique position of 

 these fibres as seen in transverse sections of .the temporal gyri, coupled with the point that 

 they decrease in number as the free surface of the convolution is approached, seem to indicate 

 that, emanating from the white substance, they become concentrated along the innermost 

 part of the gyri of Heschl, prior to their further distribution on the surface of the brain; 



