"Big Brain: The Origins and Future of Human Intelligence"

figure 5.1

Big brains have different proportions than small ones. A slice through the brain (below left) reveals the clumped or "nuclear" structure of subcortical systems, in contrast to the overlying layered "carpets" of the cortex. The cortex of the big mammal (top right) has grown so large that it takes on a crumpled appearance. A section through the brain shows how dwarfed other brain divisions are next to the cortex. In small brains, the cortex flops over the rest of the brain; in big brains (below right), the cortex becomes repeatedly folded into the skull, like a carpet in a too-small room.