n] General /{t'liHirkK <ni Corficn/ Fibre Arniiii/rnicHt 11 



notice that Ramon y C'ajal dors not employ the name' zonal layer, luit speaks of "the 

 tangential and other fibres in the plexiform layer," meaning by the latter the first or 

 molecular cell layer of other authors. 



The zonal Jayer varies greatly in depth, in fibre wealth, and in essential constitution, 

 in different regions; further, its representation varies with the age of the individual. Its 

 outer margin is usually sharp and definite, but the inner limit is often less distinct; and, 

 as Kaes .-iiid Ramon y (.'ajal have pointed out, it frequently rests upon or is succeeded by 

 a thin zone pallid and poorly supplied with fibres. Fibres placed obliquely or vertically, 

 making connection with subjacent layers, are numerous, some of these' may be descending 

 collaterals of autochthonous fibres, others ascending axis cylinders of Martinotti cells 1 . 



It is composed of parallel rows of fibres among which three types may be distinguished, 

 viz. (1) large, non-varicose, distinctly medullated tubes, (2) large, and (3) small varicose 

 fibres, but the large varicose fibres generally predominate, and it is to their presence that 

 the band formation owes most of its distinctness. Ramon y Cajal believes that the largest 

 fibres of all are axis cylinders of cells special to the first layer, and both he and Exner 

 indicate that they are more or less confined to the outer parts of the layer. When we 

 come to discuss regional variations, I shall show that these gross fibres are only present 

 in certain situations. 



The fibres of the zonal layer are said to constitute part of the so-called association 

 system, by which we have to infer that their function is to connect, or bring into associa- 

 tion, cells in convolutions or parts more or less widely separated from one another; but, as 

 I shall indicate presently, in the case of the precentral gyrus, there is an isolated rich 

 development of the zonal layer quite confined to that gyrus and showing no tendency to 

 spread to convolutions either before or behind, and surely this hardly agrees with its supposed 

 function as an association tract. Kaes likewise questions the associating function of the 

 zonal layer, and points out in addition that the layer is better developed in sulcal walls than 

 on the free surface of gyri. Now in most cases there is little doubt that the fact just 

 stated is a manifestation of condensation, the result of pressure; this explanation, however, 

 will not apply to the precentral gyrus, for here the rich tangential formation is almost as 

 well-marked on the free surface as on the covered part of the convolution; here also, and 

 this is a point which needs emphasising, there is an extremely strong development of all 

 the systems of fibres in the deeper parts of the cortex, and it seems unquestionable that 

 these superficial and deep systems are essentially correlated, and that the zonal band is 

 nothing but an expression or a reflection on the surface of the dense formation of fibres 



in subjacent parts. 



Line of Baillarger. 



We will discuss this line next because it constitutes an important stratification landmark 

 in the cerebral cortex. The writer whose name it bears recognised its presence with the 

 naked eye and had little knowledge of its composition. 



Roughly speaking, the formation is to be sought about mid-way between the surface 

 and the white substance. In no part of the brain is it better developed than along the 

 calcarine fissure ; indeed in this situation no lens or microscope is needed to detect its 

 presence, a fact which did not escape the notice of Gennari and Vicq d'Azyr; but to 



i Large pyramidal cells of which the axis cylinder, instead of proceeding downwards, turns and runs up to the surface. 



22 



