Pineal body

The pineal gland (also called the pineal body or epiphysis) is a small endocrine gland in the brain. It is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join.

Location
The pineal gland is a reddish-gray body about the size of a pea (8 mm in humans) located above the superior colliculus and behind and beneath the stria medullaris, between the laterally positioned thalamic bodies. It 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.

Structure and composition
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.

The pineal body has nervous tissue, and consists of follicles lined by epithelium and enveloped by connective tissue. These follicles contain a variable quantity of gritty material, called brain sand, acervuli, or corpora arenacea, which is composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate.

Pinealocytes in lower vertebrate animals have a strong resemblance to the photoreceptor cells of the eye. Indeed, the human pineal gland can be evolutionarily retraced to the pineal (or third) eye in reptiles and birds. In some reptiles this pineal eye is an almost fully-developed eye with lens, vitreous humor, and retina. In birds, the pineal glad is located on the surface of the brain, directly under the skull. In some animals there is even a light-permeable membrane in the scalp and skull so that light may reach the photoreceptor-like pinealocytes. The function of this "third eye" is to allow these animals to use light as prime zeitgeber (time-giver) in their biological clock. In humans and other mammals, this function is taken over by the "regular" eyes.

Function
The pineal gland is responsible for the production of melatonin, which is regulated in a 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. The retina detects the light, and directly signals and entrains the suprachiasmatic nucleus (SCN). Fibers project from the SCN to the paraventricular nuclei (PVN), which relay the circadian signals to the spinal cord and out via the sympathetic system to superior cervical ganglia (SCG), and from there into the pineal gland.

The pineal gland is large in children, but shrinks at puberty. It appears to play a major role in sexual development, hibernation in animals, metabolism, and seasonal breeding. The abundant melatonin levels in children is believed to inhibit sexual development. When puberty arrives, melatonin production is reduced.

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 phototransducing pigment melanopsin in the pineal gland. Avian pineal glands are believed to act like the suprachiasmatic nucleus in mammals.

Reports in rodents suggest that the pineal gland may influence the actions of drugs of abuse such as cocaine and antidepressants such as fluoxetine (Prozac); and contribute to regulation of neuronal vulnerability. In addition, it has been proposed by Rick Strassman that the pineal gland is responsible for manufacture of endogenous dimethyltryptamine, or DMT, although this is only speculation. There are some who believe DMT has a role in dreaming and possibly near-death experiences and other mystical states, and, although not proven, it was hypothesized in 1988 by Jace Callaway that DMT is connected with visual dreaming.

Mythology
The pineal gland was the last endocrine gland to have its function discovered. Its location deep in the brain seemed to indicate its importance. This combination led to its being a "mystery" gland with myth, superstition and even metaphysical theories surrounding its perceived function.

Rene Descartes called the pineal gland the "seat of the soul", believing it is unique in the anatomy of the human brain in being a structure not duplicated on the right and left sides. This observation is not true, however; under a microscope one finds the pineal gland is divided into two fine hemispheres.

The pineal gland is occasionally associated with the sixth chakra (also called Ajna or the third eye chakra in yoga). It is believed by some to be a dormant organ that can be awakened to enable "telepathic" communication.

See Discordianism for the relevance of the pineal gland in this religion.