The ouroboros or uroboros (/( ) /, also UK: //, US: /-/) is an ancient symbol depicting a serpent or dragon eating its own tail. Originating in ancient Egyptian iconography, the ouroboros entered western tradition via Greek magical tradition and was adopted as a symbol in Gnosticism and Hermeticism and most notably in alchemy. The term derives from Ancient Greek: οὐροβόρος, from οὐρά (oura), "tail" + βορά (bora), "food", from βιβρώσκω (bibrōskō), "I eat". The ouroboros is often interpreted as a symbol for eternal cyclic renewal or a cycle of life, death, and rebirth. The skin-sloughing process of snakes symbolizes the transmigration of souls, the snake biting its own tail is a fertility symbol. The tail of the snake is a phallic symbol, the mouth is a yonic or womb-like symbol. 
The first known appearance of the ouroboros motif is in the Enigmatic Book of the Netherworld, an ancient Egyptian funerary text in KV62, the tomb of Tutankhamun, in the 14th century BC. The text concerns the actions of the god Ra and his union with Osiris in the underworld. The ouroboros is depicted twice on the figure: holding their tails in their mouths, one encircling the head and upper chest, the other surrounding the feet of a large figure, which may represent the unified Ra-Osiris (Osiris born again as Ra). Both serpents are manifestations of the deity Mehen, who in other funerary texts protects Ra in his underworld journey. The whole divine figure represents the beginning and the end of time.
The ouroboros appears elsewhere in Egyptian sources, where, like many Egyptian serpent deities, it represents the formless disorder that surrounds the orderly world and is involved in that world's periodic renewal. The symbol persisted in Egypt into Roman times, when it frequently appeared on magical talismans, sometimes in combination with other magical emblems. The 4th-century AD Latin commentator Servius was aware of the Egyptian use of the symbol, noting that the image of a snake biting its tail represents the cyclical nature of the year.
The famous ouroboros drawing from the early alchemical text, The Chrysopoeia of Cleopatra (Κλεοπάτρης χρυσοποιία), probably originally dating to third century Alexandria but first known in a tenth century copy, encloses the words hen to pan (ἓν τὸ πᾶν), "the all is one". Its black and white halves may perhaps represent a Gnostic duality of existence, analogous to the Taoist yin and yang symbol. The chrysopoeia ouroboros of Cleopatra the Alchemist is one of the oldest images of the ouroboros to be linked with the legendary opus of the alchemists, the philosopher's stone.
An aim of alchemists and adepts, described as "individual self-perfection through physical transmutation and spiritual transcendence", was familiar to the alchemist and physician Sir Thomas Browne. It focused on the eternal unity of all things as well as the cycle of birth and death (from which the alchemist sought release and liberation). In his A Letter to a Friend, a medical treatise full of case-histories and witty speculations upon the human condition, he wrote:
... that the first day should make the last, that the Tail of the Snake should return into its Mouth precisely at that time, and they should wind up upon the day of their Nativity, is indeed a remarkable Coincidence ...
In Gnosticism, a serpent biting its tail symbolized eternity and the soul of the world. The Gnostic Pistis Sophia (c. 400 AD) describes the ouroboros as a twelve-part dragon surrounding the world with its tail in its mouth.
A 15th-century alchemical manuscript, The Aurora Consurgens, features the ouroboros, where it is used amongst symbols of the sun, moon, and mercury.
A highly stylized ouroboros from The Book of Kells, an illuminated Gospel Book (c. 800 AD)
An engraving of a woman holding an ouroboros in Michael Ranft's 1734 treatise on vampires.
Seal of the Theosophical Society, founded 1875
In Norse mythology, the ouroboros appears as the serpent Jörmungandr, one of the three children of Loki and Angrboda, which grew so large that it could encircle the world and grasp its tail in its teeth. In the legends of Ragnar Lodbrok, such as Ragnarssona þáttr, the Geatish king Herraud gives a small lindworm as a gift to his daughter Þóra Town-Hart after which it grows into a large serpent which encircles the girl's bower and bites itself in the tail. The serpent is slain by Ragnar Lodbrok who marries Þóra. Ragnar later has a son with another woman named Kráka and this son is born with the image of a white snake in one eye. This snake encircled the iris and bit itself in the tail, and the son was named Sigurd Snake-in-the-Eye.
The Ouroboro symbol is a common sight on 17th and 18th century gravestones. Varying from region to region some have legs and even torso like bodies while others are carved in a bracelet like circle. They appear alongside the Hourglass, skulls, bible and cherubs that are so common in depictions of mortality of this era. And like those other symbols they disappeared from gravestones in the 19th century Gothic revival.
Ouroboros symbolism has been used to describe the Kundalini. According to the medieval Yoga-kundalini Upanishad, "The divine power, Kundalini, shines like the stem of a young lotus; like a snake, coiled round upon herself she holds her tail in her mouth and lies resting half asleep as the base of the body" (1.82).
The alchemists, who in their own way knew more about the nature of the individuation process than we moderns do, expressed this paradox through the symbol of the Ouroboros, the snake that eats its own tail. The Ouroboros has been said to have a meaning of infinity or wholeness. In the age-old image of the Ouroboros lies the thought of devouring oneself and turning oneself into a circulatory process, for it was clear to the more astute alchemists that the prima materia of the art was man himself. The Ouroboros is a dramatic symbol for the integration and assimilation of the opposite, i.e. of the shadow. This 'feed-back' process is at the same time a symbol of immortality, since it is said of the Ouroboros that he slays himself and brings himself to life, fertilizes himself and gives birth to himself. He symbolizes the One, who proceeds from the clash of opposites, and he therefore constitutes the secret of the prima materia which ... unquestionably stems from man's unconscious.
I was sitting, writing at my text-book; but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by the repeated visions of the kind, could now distinguish larger structures of manifold conformation: long rows, sometimes more closely fitted together; all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis.
Martin Rees used the ouroboros to illustrate the various scales of the universe, ranging from 10−20 cm (subatomic) at the tail, up to 1025 cm (supragalactic) at the head. Rees stressed "the intimate links between the microworld and the cosmos, symbolised by the ouraborus", as tail and head meet to complete the circle.
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