The reptilian brain is the informal name for the basal ganglia, which are structures derived from the floor of the forebrain during development i.e. it is the most primitive and evolutionarily oldest part of the brain.

The term derives from the fact that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. The reptilian brain is thought to be responsible for species typical instinctual behaviors involved in aggression, dominance, territoriality, and ritual displays.

Brain layers

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