Because the human pineal gland is buried deep in the fleshy bits of the brain and was the last part of the endocrine system to be understood, it's picked up a bunch of mystical baggage. Often it's associated with telepathy and/or enlightenment; sometimes it's referred to as a "Third Eye". To be fair, The Other Wiki does say that in nonmammal vertebrates the pineal gland has cells similar to those in the retina. In some reptiles, fish, and amphibians, it even connects to an external opening and has light-sensing properties (which serve to regulate physiological day/night cycles rather than to perceive).
The pituitary gland can also be used in a similar way, possibly because writers don't know the difference. Even cerebrospinal fluid is sometimes treated as a liquid equivalent, thanks to the vampiric implications and the chance to have victims of CSF-theft actually survive their loss.
Often this runs in tandem with 90% of Your Brain. The Trope Maker is none other than Rene Descartes, who believed that the pineal gland was the "seat of the soul", and the connection point between mind and body. He wasn't the only one, as both scientists and occultists were quite enthusiastic about the idea for awhile.
The human pineal gland's accepted function is to produce melatonin, a hormone that helps to regulate sleeping patterns, plus a tiny amount of antioxidant. Mess with the thing, and the most you'll get as a result is insomnia, not ESP.
- The Diclonii in Elfen Lied start their lives as humans with a huge pineal gland. If their psychic "Vector" arms come in contact with a normal human's pineal gland, that human will only create Diclonii when they reproduce.
- In Bubblegum Crisis 2040, Dr. Stingray's Boomer technology is ultimately based on a reproduction of his wife's pineal gland implanted into his daughter Sylia's brain to grow into the first Boomer cores. The Knight Sabers' hardsuits are derived from the same basic technology, which greatly restricts who can use them successfully. In addition, the implant resonates with prototype unit Galatea to give all three Knight Sabers visions of Sylia's past.
- In Top 10, an alien's diet is based on the chemicals secreted by the pineal gland, requiring a round of brain eating once a year. Police initially think it's just a calendar-themed serial killer. This assumption is helped by the fact all the past victims are prostitutes.
- A Silver Age Superman comic had Superman exposed to radiation that caused his pineal gland to develop into a literal third eye. Of course, this was exactly what he needed to defeat the villain.
- A Silver Age Supergirl story had her visiting the future in response to a call for help from the Legion of Super-Heroes in Action Comics #287. To get her up to date on the pending crisis, they used a future-tech device to download the info into her "'Third Eye' pineal gland."
- The Psycho Serum in V for Vendetta (batch 5) was stated to be pituarin-pinealin.
- In Fear and Loathing in Las Vegas, the protagonists convince a police officer that there's a plot to murder suburbanites for their pineal glands. This later turns out to be true (sort of).
- In the Lovecraftian movie From Beyond, the mad scientist stimulates his pineal gland, allowing him to see into other dimensions. Freshly put to use, it grows to the extent that it pops out of his head on a stalk. To the further amusement of anyone familiar with brain anatomy, as the stalk extends in front, yet the pineal gland is near the back of the human brain.
- In Plan 9 from Outer Space, stimulating the pineal and pituitary glands of corpses turns them into zombies.
- In The Leech Woman, the juice from the pineal gland combined with a certain type of pollen forms an effective (but temporary) Fountain of Youth.
- In the 1959 monster movie The Alligator People, secretions from alligator pituitary glands cause rapid regeneration that repairs injured humans' wounds, only to cause them to sprout scales within a year of treatment.
- "The Stolen Body" by H. G. Wells posits the pineal gland as a point of contact between embodied humans and the world of disembodied spirits.
- In one of the Lensman books, an alien doctor was able to make the (human) hero regenerate by stimulating his pineal gland.
- Averted in the fourth Artemis Fowl book. Opal Koboi has a human pituitary gland grafted to her brain, not for any supernatural purpose, but so that the growth hormone it produces will cause her to gradually grow larger, in order to pass off as a human (at the time of the story, she is pretending to be a child). It also has the unintentional side effect of draining her magic for some reason, though she is unaware of it until she finally runs out at the worst possible time.
- In Stephen King's Firestarter, one of these glands is a seat to Charlie McGee's power.
- In H. P. Lovecraft's story "From Beyond", a Mad Scientist creates a machine which stimulates the pineal gland, allowing humans to see really clearly. Apparently "fresh air" and "blue sky" are made out of amoeba-like monstrosities, and some unspeakable, unseen things stalk other lifeforms, and consume them into nothingness in seconds. This is the main risk for humans who open their eyes into this true reality, because what's actually going on is that they're seeing into normally-invisible dimensions that intersect with ours. The danger is that people who gain the ability to see into those dimensions also become visible to the things that live there.
- In the SF short story The Rose by Charles L. Harness, the two protagonists (a ballerina/opera singer and a composer) "suffer" from an odd form of cancer: their pineal glands grow rather impressively, forming a pair of "horns" which curve around and over the brain before bursting out of the skull (but not skin) at the temples. They also get a kind of hunchback from a tumour that appears to contain brain tissue it does, and if the skin over it gets split, it can engorge and tumesce from blood flow, although this may kill the person in question. They get a weird kind of projective telepathy apparently based on musical and colour theory, and spot patterns that no-one else can see.
- Rene Barjavel's Le Voyageur Imprudent treats the matter slightly more realistically. In the far future, a subspecies of humanity that specializes in improved eyesight has three eyes on stalks. One of them is the pineal gland, which has (re)gained full functionality as a visual organ.
- In Ethan of Athos by Lois McMaster Bujold, the Cetagandan Empire figures out how to bioengineer telepathy; their starting point is a homeless woman with a mutated pineal gland.
- In Principia Discordia (and thus also in Illuminatus!) the pineal gland is the organ that allows communication with Eris.
- In The Company Novels, the artificial hormone that makes cyborgs The Ageless is called "Pineal Tribrantine 3."
- In the novel Return to Doomstar, one of the planets visited by the protagonists has Functional Magic. In order to use it, the existing settlers compulsively cannibalize one another for their pineal glands, and attempt to do so to their guests. The absence of children is conspicuously noted even before the reveal.
- This is a running gag in The Middleman:
- A minor character gets his pineal gland surgically removed and gains the ability to see and hear ghosts.
- Another minor character has his pineal gland severed in a freak limbo accident and gains a telepathic link with his Mirror Universe Evil Twin (who also had his pineal gland severed in a freak accident).
- When the main characters encounter a flying fish that turns people into zombies, the antidote to the zombification is made from the fish's pineal fluid.
- In the unproduced 13th episode, the villain's plan requires that he steal a number of pineal glands from people or creatures with mystical power.
- In the Mystery Science Theater 3000 episode "The Leech Woman", the pineal gland can be pierced with a small implement (killing the victim, of course) and its secretions mixed with a flower pollen to induce artificial de-aging.
- The pineal gland is implied to be the source of superpowers in Heroes; it's also implied to be what Sylar goes after when he opens up his victims' skulls.
- In The Girl from Tomorrow, the pituitary gland has a telepathic function in humans.
- Fringe had an episode where a serial killer was killing women and stealing their pituitary glands. Walter Bishop deduced that it was related to a project he worked on for the government involving hyper-aging test-tube soldiers. The killer was one of the hyper-aging test subjects, and he was using the pituitary glands to maintain his age. Otherwise he'd grow old and die really fast.
- In American Gothic (1995) Lucas mentions the supposed importance of the pineal gland/third eye. Yancy then stabs him in the area in an attempt to kill him.
- In Earth 2 the changes to Uly's system after his Terrian healing were suspected to have started in his pineal gland. Julia, his doctor, lied about it to protect him and the matter was quietly dropped.
- Stargate SG-1 flirts with this trope in "Morpheus", in which the danger-of-the-week is a parasite that puts people to sleep and then eventually kills them by causing aneurysms near their pineal gland. They eventually figure out that the parasite is feeding on melatonin — which is what the pineal gland actually produces — so in the end it's more an example of Pineal Normality.
- In The X-Files the Black Cancer (or Oil), an alien life-form capable of taking control of its host, nests around the pineal body. It's mentioned briefly in Terma but never elaborated upon. Interestingly, the most prominent place the Oil is shown to reside is in the eyes. Cue Wild Mass Guessing.
- An alien version occurs on Star Trek: Voyager in which an alien psychopath was born with a detached equivalent of the pineal gland, leaving him prone to violent behavior. This changes when the Doctor treats him for head injuries sustained in a No-Holds-Barred Beatdown and accidentally reattaches the gland in question, triggering an immediate HeelFace Turn.
- In Promethean: The Created, the pineal gland is the source of the ectoplasmic humor that sustains the Ulgans. The pineal gland isn't actually special in humans, though — in humans, the humors are metaphorical, not literal, and there's no such thing as an ectoplasmic humor, because the theory of humors was about physical and mental traits, not spiritual.
- Mage: The Awakening has a legacy in which on the first attainment the pineal gland fully develops into a black eye that pops out of the back of the Mage's skull. It enables them to have 360° vision, see the past and prophecy the future.
- In Clanbook: Malkavian for Vampire: The Masquerade there are a few references made to the pineal gland.
- In GURPS: Thaumatology one of the ways to draw on the hermetic decan of Alleborith, associated with "pure" magic, is to use a pineal gland.
- In Aberrant, Novas draw their power from the Node, a part of their brain that grows in size when their power is awakened and grows further as they become more powerful. While no mention of it being the pineal gland has been made, it is implied to follow this rationale.
- In Kingdom of Loathing, there used to be a quest where you had to collect the pineal glands of zombies to make a vaccine. Zombie Pineal Glands still have a purpose: they're a small but essential component of a literal Ninja Pirate Zombie Robot. The event ended a long time ago, and for a good while pineal glands were extremely expensive in the Mall. Eventually, the Item of the Month known as "the Deck of Every Card" gave players a way to fight zombies from the event, giving a new way for pineal glands to enter the Kingdom.
- Averted in the Ever17 where there's a brief discussion of the pineal gland and how it relates to the third eye, but it's dismissed as nonsense. They even mention Descartes' theories about it and the evolutionary history of it as a light sensitive organ.
- Resident Evil Gun Survivor shows how Tyrants are made - their key ingredient is a hormone secreted in the pineal gland of adolescent boys feeling extreme terror or stress. Umbrella's answer for harvesting such: remove the gland without anesthetic. Needless to say, it works.
- In Postal: Apocalypse Weekend, when you go to the insane asylum, you find scrawlings within that detail plans to eat the inmates pineal glands with a fork. These show up in several areas, so its not quite clear whether the inmates, the staff, or both were planning on harvesting the inmates.
- Team Fortress 2:
While boxing gloves were made to cushion your well-placed uppercuts, steel fists were made to beat the snot out of your enemies' pineal glands. "What are pineal glands?", you might ask! We at Mann Co have no idea ourselves, but what we do know is that these babies are made of freaking steel!
- In the Fancy vs. Nasty update, the Fists of Steel description says:
- In the comic "Old Wounds", Medic injects a whale's pineal gland into Sniper's brain stem, and dismisses Sniper's vision of going to Heaven and meeting his adoptive parents again as a mere hallucination.
- In the Dr. Orpheus' premier episode in The Venture Bros., Dr. Venture's Lotus-Eater Machine "The Joy Can" operates by creating a direct link to the subject's pineal gland.
- In Futurama Dr. Zoidburg cured the Professor's "yetism" by feeding him the pineal gland of a yeti, the explanation being that it produced a counter-toxin to the one in the yeti's claws.
- Descartes claimed the pineal gland was the connection between the mind and the body; this is probably the Trope Maker (as well as Completely Missing the Point of the problem he was trying to solve). There has been some debate about whether he actually thought this, or was simply giving it as an entirely arbitrary answer to the continual questions of a particularly obnoxious princess he was tutoring.
- The pineal gland was the last endocrine gland to be "explained", so Truth in Television.
- Some fish, amphibians, and reptiles have an actual third eye associated with the pineal gland. They probably can't see much, though.