x] Frontal <in<l Pn-frontal, Areas ~2:Y1 



that ass the simian brain is so much .smaller, and the arrangement of elements less com- 

 plicated, the differentiation of the types is considerably easier. 



Since tin- human architectural plan is to a great extent repeated, it will be superfluous 

 to describe the structure in detail. I will merely state that although the differentiation can 

 be effected by an examination of cell elements and has been checked by such examination, 

 still sections stained for nerve fibres are more useful for working purposes. When in the 

 latter sections, the " frontal " is compared with the " intermediate precentral " cortex a general 

 reduction in fibre wealth is first observed, but more important than this the "frontal" cortex 

 is to be distinguished by a complete absence of coarse medullated fibres, both in the radiary 

 projections and association systems. At the same time it must be expressly stated that fibres 

 of medium calibre persist and appear in appreciable numbers. 



Comparing the " prefrontal " with the " frontal " cortex a still further reduction in the 

 general fibre wealth is found. The fibres of medium size are now wanting and none but 

 delicate wavy fibrils are seen in any of the systems, the radiations have grown more slender, 

 the association system is distinctly deficient, and the subjacent white substance indeed the 

 whole cortex has a pallid, ill-developed look. If we pass into the sulci in the same area we 

 may see a very shallow radiary zone, composed of stunted radiary fasciculi and a delicate inter- 

 radiary plexus, fringing the white substance; and while in most other parts of the brain 

 on examining the white substance carefully with a high power objective it is possible to 

 discover large medullated fibres, scarcely any can be found here. 



DISTRIBUTION. 

 A. " Frontal " Area. 



On the mesial aspect a human feature is repeated, for this type of cortex again covers 

 a small portion of the marginal gyrus lying above the level of the genu of the corpus 

 callosum. And I might mention that in one of my chimpanzees' and in the orang's hemi- 

 sphere, the field is limited anteriorly by a frontal offshoot of the prelimbic division of the 

 calloso-marginal fissure, which runs obliquely forwards and upwards to incise the margin of 

 the hemisphere. Of the constancy of this offshoot I am unable to speak. 



On the lateral surface the area appears to be relatively less extensive than it is in 

 the human brain. Viewing the hemisphere from the front I should describe the anterior 

 border as crossing from the inner to the outer surface at a point about 4 cm. above the 

 junction of the internal and orbital margins, that is 4 cm. above the lowest part of the 

 keel-shaped orbital projection, so characteristic of the simian brain. From here our anterior 

 border runs vertically downwards, parallel to the "intermediate precentral" area, until it 

 reaches the middle of the sulcus rectus, or as I prefer to call it the sulcus frontalis medius. 

 It then takes a short turn forwards and drops on to the orbital surface, where it runs 

 parallel to and a few millimetres in advance of the " intermediate precentral " area, and on 

 this surface the human distribution is copied. 



Returning to compare the extent of the field on the lateral surface of the hemisphere 

 with what we have seen in the human subject, the first point which strikes us is that while 

 the area retains its sagittal breadth it shows a pronounced reduction in vertical depth; and 

 looking for an explanation of this very important difference we soon recognise that it is 

 attributable to, first, the inferior volume of the superior and middle frontal convolutions, 



