[3] The pineal gland is located in the epithalamus, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of the thalamus join.

[8] Reconstruction of the biological evolution pattern suggests that the pineal gland was originally a kind of atrophied photoreceptor that developed into a neuroendocrine organ.

The 17th century philosopher René Descartes regarded the gland as having a mystical purpose, describing it as the "principal seat of the soul".

[9] The word pineal, from Latin pinea (pine-cone) in reference to the gland's similar shape, was first used in the late 17th century.

[12] Unlike most of the mammalian brain, the pineal gland is not isolated from the body by the blood–brain barrier system;[13] it has profuse blood flow, second only to the kidney,[14] supplied from the choroidal branches of the posterior cerebral artery.

[15][16] According to research on animals, neurons of the trigeminal ganglion that are involved in pituitary adenylate cyclase-activating peptide neuropeptide signaling project the gland.

[17][16] The canonical neural pathway regulating pineal melatonin production begins in the eye with the intrinsically photosensitive ganglion cells of the retina which project inhibitory GABAergic efferents to the paraventricular nucleus of hypothalamus via the retinohypothalamic tract.

[19] The human pineal gland grows in size until about 1–2 years of age, remaining stable thereafter,[20][21] although its weight increases gradually from puberty onwards.

[22][23] The abundant melatonin levels in children are believed to inhibit sexual development, and pineal tumors have been linked with precocious puberty.

Nerve fibers then relay the daylight information from the SCN to the paraventricular nuclei (PVN), then to the spinal cord and via the sympathetic system to superior cervical ganglia (SCG), and from there into the pineal gland.

Pinealectomy performed on rodents produced no change in pituitary weight, but caused an increase in the concentration of FSH and LH within the gland.

[36] Pineal gland calcification is detrimental to its ability to synthesize melatonin[37][38] and scientific literature presents inconclusive findings on whether it causes sleep problems.

[42] These neoplasms are divided into three categories: pineoblastomas, pineocytomas, and mixed tumors, based on their level of differentiation, which, in turn, correlates with their neoplastic aggressiveness.

[47] The lancelet Branchiostoma lanceolatum, an early chordate which is a close relative to vertebrates, also lacks a recognizable pineal gland.

[47] Protochordates in general do not have the distinct structure as an organ, but they have a mass of photoreceptor cells called lamellar body, which is regarded as a pineal homologue.

The loss of parietal eyes in many living tetrapods is supported by developmental formation of a paired structure that subsequently fuses into a single pineal gland in developing embryos of turtles, snakes, birds, and mammals.

[58] Crocodilians and some tropical lineages of mammals (some xenarthrans (sloths), pangolins, sirenians (manatees and dugongs), and some marsupials (sugar gliders)) have lost both their parietal eye and their pineal organ.

[57] In most vertebrates, exposure to light sets off a chain reaction of enzymatic events within the pineal gland that regulates circadian rhythms.

[64] In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei (SCN) and the pineal gland.

[71] He introduced the name κωνάριο (konario, often Latinised as conarium) that means cone, as in pinecone,[72] in De usu partium corporis humani.

For example, English physician Thomas Willis described a glandula pinealis in his book, Cerebri anatome cui accessit nervorum descriptio et usus (1664).

Epiphysan – an extract derived from the pineal glands of cattle – was historically used by veterinarians for rut suppression in mares and cows.

Dermatology professor Aaron B. Lerner and colleagues at Yale University, hoping that a hormone from the pineal gland might be useful in treating skin diseases, isolated it and named it melatonin in 1958.

[82] The substance did not prove to be helpful as intended, but its discovery helped solve several mysteries, such as why removing a rat's pineal gland accelerated its ovary growth, why keeping rats in constant light decreased the weight of their pineals, and why pinealectomy and constant light affect ovary growth to an equal extent; this knowledge gave a boost to the then-new field of chronobiology.

[84] The notion of a "pineal-eye" is central to the philosophy of the French writer Georges Bataille, which is analyzed at length by literary scholar Denis Hollier in his study Against Architecture.

In this work Hollier discusses how Bataille uses the concept of a "pineal-eye" as a reference to a blind-spot in Western rationality, and an organ of excess and delirium.

[86] In the late 19th century Madame Blavatsky, founder of theosophy, identified the pineal gland with the Hindu concept of the third eye, or the Ajna chakra.

[9] The pineal gland has also featured in other religious contexts, such as in the Principia Discordia, which claims it can be used to contact the goddess of discord Eris.

In Season 16, episode 6 of "American Dad" Steve tries to "astral project" using his pineal gland to help him understand the meaning of life.