APA Dictionary of Psychology

a small, cone-shaped gland attached by a stalk to the posterior wall of the third ventricle of the brain; it is part of the epithalamus. In amphibians and reptiles, the gland appears to function as a part of the visual system. In mammals, it secretes the hormone melatonin and is an important component of the circadian system regulating biological rhythms. Because it is an unpaired organ located in the middle of the brain, René Descartes believed it was the seat of the rational soul and the connection between mind and body. Also called pineal body.