292 GEOFFREY JEFFERSON 
Further, the maximum and minimum thickness of the gray 
matter of man are about 4.0 mm. and 1.25 mm. respectively. 
So that the maximum depth of the furrow that could run be- 
tween contiguous areas, if specialisation were the only cause of 
furrows, would be 2.75 mm. This in itself is enough to show 
that other factors besides cortical specialisation are involved 
in the production of the sulci. 
With regard next to growth antagonism: It is evident that 
growth antagonism must play an important part, otherwise 
all the areas would be spread out evenly upon the surface of 
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Fig. 1 Cortical map of Marmoset, Hapale jacchus [K. Brodmann (5)]. Shows 
that specialisation alone is not sufficient to produce furrows. When these areas 
have increased in extent and new ones have been added, furrows will have to 
appear. Their sites can already be forecasted on this map, as the edges of the 
areas correspond remarkably with the position of the furrows in higher brains. 
the brain, and fissures would be absent. This state of affairs 
is almost possible in the lower monkeys whose cortical surfaces 
have but few areas to accommodate, and accordingly in their 
case the brain is to a large extent smooth. In the human brain, 
where place has to be found for upwards of forty distinctly 
different histological areas, the displaying of all these areas in 
toto is impossible, as an enormous cranium would he necessary. 
Only some 36 per cent of the cortical areas are exhibited on the 
surface of the human brain: 64 per cent are hidden in the fur- 
rows (4). This means that the cranium would have to be three 
