Definition
Brain anatomy describes how the brain is physically organized — which structures sit where, and what each broadly does. Behave offers a deliberately simplified map to make the rest of its argument legible: a three-layer model of the brain that, while not literal evolutionary history, is a useful organizing fiction.
Layer one is the automatic, regulatory brain — the brainstem and related structures that keep the body running. Layer two is the limbic system, the emotional brain. Layer three is the cortex, the cognitive brain, with the frontal cortex as its most distinctly human part.
Why it matters
How it works
The three layers are a heuristic, not a hierarchy of conquest. The cognitive cortex does not simply rule the emotional limbic system; the two are wired together so tightly that emotion shapes thought and thought shapes emotion. The automatic layer below them links both to the body.
Within the cortex, the frontal region handles executive control. Beneath it, the limbic system — anchored by the amygdala — generates emotion. The brainstem and hypothalamus handle survival functions and connect the brain to hormonal systems. Behave uses this map repeatedly: every behavior it explains is ultimately a pattern of activity distributed across these regions.