APA Dictionary of Psychology

the view, now outmoded, that the brain consists of three layers reflecting its evolutionary development. The first and oldest is the archipallium, or R-complex (meaning reptilian complex and including the brainstem and cerebellum); the second is the paleomammalian system, or limbic system; and the most recently evolved is the neopallium, or neocortex, also known as the neomammalian brain. [proposed by U.S. physician and neuroscientist Paul D. MacLean (1913–2007)]