The pineal body is located above the superior colliculus and behind and beneath the stria medullaris , between the laterally positioned thalamic bodies. The pineal body is part of the epithalamus .
The pineal gland is a midline structure and is often seen in plain skull X-rays as it is often calcified.
The pineal gland consists mainly of pinealocytes, but four other cell types have been identified: interstitial cells, perivascular phagocyte, pineal neurons and peptidergic neuron-like cells.
It is responsible for the production of melatonin, which has a role in regulating the circadian rhythm. Melatonin is a derivative of the amino acid tryptophan. The production of melatonin by the pineal gland is stimulated by darkness and inhibited by light. Light can be detected by the suprachiasmatic nucleus (SCN) which has directs connections to the retina. Fibers extends from SCN to the spinal cord into superior cervical ganglia and from there into the pineal gland.
In fact ancient amphibians such as Ichthyostega, which existed in the Late Devonian Period, had an orifice on the top of the skull through which the pineal gland was exposed and received light input. Over the course of time and for unknown reasons, the pineal gland migrated into the skull of later tetrapods and the skull orifice sealed. Modern birds and reptiles have been found to express the phototranducing pigment melanopsin in the pineal gland.
It has been found that the pineal gland manufactures trace amounts of the psychedelic chemical dimethyltryptamine, or DMT. This endogeous chemical in the human brain is believed to play a role in dreaming and possibly near-death experiences and other mystical states. It has been suggested by the researcher Jace Callaway that DMT is connected with visual dreaming.
Rene Descartes called the pineal gland the "seat of the soul", believing it was unique in the anatomy of the human brain in being a structure not duplicated on the right and left sides.
The pineal gland is occasionally referred to as the "third eye" in occult religions, and is believed by some to be a dormant organ that can be awakened to enable telepathic communication.
"Consult your pineal gland" is a common saying in Discordianism, a religion which emphasizes chaos. Discordians seem to regard the pineal gland as the source of answers to life's most difficult questions.