The pineal gland is a small endocrine structure roughly the size of a grain of rice, sitting near the centre of the brain. Anatomically, it's unusual in two specific, documented ways: it contains photoreceptor cells — the same type of light-sensitive cells found in the retina — despite having no direct pathway to external light, and it's the only structure in the brain that isn't duplicated across the two hemispheres.
Its primary confirmed function is producing melatonin, the hormone that regulates circadian rhythm and sleep-wake cycles, in response to light signals relayed from the retina via a separate neural pathway. That much is uncontested clinical physiology.
The "third eye" across history
Long before Western neuroscience had a name for it, contemplative traditions had already located something significant at roughly the pineal gland's anatomical position. The Hindu yogic tradition's Ajna chakra — often translated as the "third eye" — corresponds to this same location and has been described in texts predating modern neuroscience by more than two thousand years as a seat of higher perception.
Leonardo da Vinci, in his anatomical notebooks around 1490, described the pineal gland as "the eye through which the soul regards itself" — a striking anticipation of its photoreceptive properties, arrived at through dissection centuries before the cellular biology could be confirmed.
Where the evidence ends and speculation begins
What's documented: the anatomy, the photoreceptor cells, the melatonin function, the magnetite crystals some studies have identified within the gland (which has led to speculation about electromagnetic sensitivity, though this remains an active and unsettled research area rather than confirmed function).