The epithalamus, a part of the diencephalon, is located superior and posterior to the thalamus and contains the pineal gland (also called the pineal body or epiphysis). The epithalamus also serves as boundary of the third ventricle. Physiological studies have established that the epithalamus plays a regulatory role in the sleep-wake cycle and in controlling mood.

The pineal gland attaches via a stalk to the posterior boundary wall of the third ventricle of the cerebrum. The babenular and posterior commissures are located on the superior and inferior aspects of the base of the pineal stalk.

Controversy regarding the physiological function of the pineal gland is longstanding. The pineal gland was called the "third eye" in medieval writings and René Descarte once proposed a theory of mind and body that asserted that the pineal gland was the "seat of the soul. Some bizarre pseudoscientific beliefs still assert that...