CHAPTER X. 



FRONTAL AND PKEFRONTAL AREAS. 



OF the frontal lobe there still remains for consideration that part uncovered by " inter- 

 mediate precentral " and " limbic " cortex ; this comprises the anterior half of the marginal 

 gyrus, on the mesial surface of the hemisphere, much of the superior, middle, and inferior 

 frontal convolutions, on the lateral surface, and their downward extensions on the orbital face. 

 Although this expanse is covered all over by cortex showing a type of fibre arrangement 

 and cell lamination approximately uniform in character, it is nevertheless possible to split 

 it up into two fields, the hinder of which forms a skirt to the " intermediate precentral " 

 an/a and will for convenience be called " frontal," while the anterior, centred on the tip of 

 the frontal lobe, will be designated " prefrontal." 



TYPE OF FIBRE ARRANGEMENT. (Plate XXIII, figs. 1 and 2.) 



Since it is impossible to localise any minor areas in the frontal lobe which are marked 

 by a specialised cortical structure, and since the variations of which I have to write affect 

 degree more than kind, I will not describe the formation in each of these subdivisions 

 separately, but will take a comprehensive view of the frontal cortex from the " intermediate 

 precentral " area forwards, making incidental comparisons of the structural grades by which 

 each is characterised. 



Zoned Layer. 



The " precentral " area has been pointed to as a part in which the zonal layer reaches 

 a maximum of representation, gaining a superiority in fibre wealth over all other regions; 

 in the " intermediate precentral " field we noticed a decided fall in the density and definition 

 of the layer, but it still contained large varicose fibres ; in the " frontal " area the deterioration 

 is more marked and the large varicose fibres disappear ; and, finally, in the " prefrontal area " 

 the development is so poor that a few scattered short wavy fibrils alone remain to denote 

 the existence of the layer. 



Supraradiary Layer. 



A like description holds for the Supraradiary layer; in the "intermediate precentral" 

 cortex the general fibre supply was relatively speaking rich, moreover, long, ascending fibres of 

 Martinotti, and long, horizontally-placed, association fibres of medium size occasionally traversed 

 the layer ; in the " frontal " cortex the general supply is distinctly less and the long medium- 

 sized elements are virtually absent, and when we come to the "prefrontal" area only a few 



