24 Precentral or Motor Area [CHAP. 



PRECENTRAL AREA. 



DETAILED ACCOUNT OF STRUCTURE. 



The type of structure present in the "precentral " area is of such a special and distinctive 

 character as to afford an index of the possession by the area of a special function, and I shall 

 now proceed to describe it in detail. 



TYPE OF ARRANGEMENT OF NERVE FIBRES. (Plate III. Fig. 1.) 



Fibreless Layer. 



The fibres it contains are few in number, delicate in calibre, placed in all directions 

 and non-varicose. 



Zonal Layer. 



The zonal layer is remarkably well -developed over the whole "precentral" area, being 

 recognisable even with the naked eye. It measures about 68 ft, in depth, is sharply defined as 

 regards its lower border, and is composed of several layers of closely packed fibres of which 

 delicate varicose fibrils form the predominating element ; coarse varicose fibres are, however, 

 present in considerable abundance and evenly medullated fibres of large size are likewise seen. 

 The latter are usually situated along the inner border of the band, and an idea of their frequency 

 may be obtained from the tact that one can generally be found in each high power field of 

 the microscope. In a transverse section of the precentral gyms it is to be observed that 

 the zonal layer is more strongly developed on the Rolandic side than along the crest of the 

 convolution, and more on the crest than on the anterior side. 



Supraradiary Layer. 



Compared with other convolutions, the fibre wealth of this layer is great, although to the 

 naked eye it has quite a pallid, almost white appearance. Roughly speaking, the supply of 

 fibres is equally distributed over the layer. The majority of the fibres have a transverse trend, 

 but vertical and oblique ones are present. 



The transverse fibres are almost all of fine calibre and varicose, only in the lower parts 

 are ones of larger size noticeable, and in all likelihood these pertain to the association system. 

 Those fibres which are arranged vertically or obliquely are usually of greater size than the 

 transverse ones, but it is uncommon to find a typical large medullated fibre in this layer. 

 Neither a definite line of Bechterew nor a line of Kaes can be identified. 



In this layer the superiority of fibre wealth on the Rolandic side of the section is not 

 so pronounced as it is in the case of the zonal layer. 



Line of Baillarger. 



A line of Baillarger is distinctly difficult to define, but to say that the fibres which 

 constitute the linear formation are not represented would be wide of the truth ; a statement, 

 however, to the effect that it is obscured and its elements hidden in the wealth of neighbouring 



