Aparietal eye (third eye,pineal eye) is a part of theepithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with thepineal gland, which regulatescircadian rhythmicity and hormone production forthermoregulation.[1] The hole that contains the eye is known as thepineal foramen orparietal foramen, because it is often enclosed by theparietal bones.
The parietal eye was discovered byFranz Leydig, in 1872, from work withlizards.[2]
Franz Leydig, a professor of zoology at theUniversity of Tübingen, dissected four species of European lizards—theslow worm (Anguis fragilis) and three species ofLacerta.[2] in 1872;[3] He found cup-like protrusions under the middles of their brains. He believed the protrusions to be glandular and called them frontal organs (GermanStirnorgan).[2]
In 1886,Walter Baldwin Spencer, an anatomist at theUniversity of Oxford, reported the results of his dissection of 29 species of lizards; he noted the presence of the same structure that Leydig had described. Spencer called it the pineal eye or parietal eye and noticed that it was associated with theparietal foramen and the pineal stalk.[4] In 1918,Nils Holmgren, aSwedish zoologist, found the pineal eye infrogs anddogfish.[5] He noted that the structure contained sensory cells that looked like thecone cells of the retina,[6] and hypothesised that the pineal eye could be a primitive light-sensing organ (photoreceptor). The organ has become popularly known as the "third eye".[5]
The parietal eye is found in thetuatara, most lizards, frogs,salamanders, certainbony fish,sharks, andlampreys.[7][8][9] It is absent inmammals but was present in their closestextinct relatives, thetherapsids, suggesting that it was lost during the course of the mammalian evolution due to it being useless inendothermic animals.[10] It is also absent in the ancestrally endothermic ("warm-blooded")archosaurs such asbirds. The parietal eye is also lost inectothermic ("cold-blooded") archosaurs likecrocodilians, and inturtles, which may be grouped with archosaurs inArchelosauria.[11] Despite beinglepidosaurs, as lizards and tuatara are,snakes lack a parietal eye.[12][13]
The third eye is much smaller than the main paired eyes; in living species, it is always covered by skin, and is usually not readily visible externally.[14] The parietal eye is a part of theepithalamus, which can be divided into two major parts—theepiphysis (the pineal organ; or the pineal gland, if it is mostly endocrine) and the parapineal organ (often called theparietal eye or, if it is photoreceptive, thethird eye). The structures arise as a single anteriorevagination of thepineal organ or as a separate outgrowth of the roof of thediencephalon; during development, it divides into two bilaterally somewhat symmetric organs, which rotate their location to become a caudal pineal organ and a parapineal organ. In some species, the parietal eye protrudes through theskull.[15][16] The parietal eye's way[further explanation needed] of detecting light differs from the use ofrod cells andcone cells in a normal vertebrate eye.[17]
Many of the oldest fossil vertebrates, includingostracoderms,placoderms,crossopterygians, and earlytetrapods, have in their skulls sockets that appear to have held functional third eyes. The socket remains as aforamen between theparietal bones in many living amphibians and reptiles, although it has vanished in birds and mammals.
Lampreys have two parietal eyes, one that developed from the parapineal organ and the other from the pineal organ. These are one behind the other in the centre of the upper surface of the braincase. Because lampreys are among the most primitive of all living vertebrates, it is possible that was the original condition among vertebrates, and may have allowed bottom-dwelling species to sense threats from above.[14]Saniwa, an extinctvaranid lizard, probably had two parietal eyes, one that developed from the pineal organ and the other from the parapineal organ.Saniwa is the only knownjawed vertebrate to have both a pineal and a parapineal eye. In most vertebrates, the pineal organ forms the parietal eye, however, in lepidosaurs it is formed from the parapineal organ, which suggests thatSaniwa re-evolved the pineal eye.[18]
The parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the humanpineal gland appears far away from this position, tucked away between thecorpus callosum andcerebellum. Also theparietal bones, in humans, make up a portion of the rear of the skull, far from the eyes. To understand further, note that the parietal bones formed a part of the skull lying between the eyes insarcopterygians and basal amphibians, but have moved further back inhigher vertebrates.[19] Likewise in the brain of the frog, thediencephalon, from which thepineal stalk arises, appears relatively further forward, as thecerebral hemispheres are smaller but theoptic lobes are far more prominent than the humanmesencephalon, which is part of thebrain stem.[20] In humans theoptic tract, commissure, andoptic nerve bridge the substantial distance between eyes and diencephalon. Likewise the pineal stalk ofPetromyzon elongates very considerably during metamorphosis.[21]
Crustaceans at thenauplius stage (first-stage larva) have a single eye atop the head. The eye has a lens and senses the direction of light but can not resolve details. More sophisticated segmented eyes develop later on the sides of their heads, but the initial eye also stays for some time. Thus it is possible to say that, at some stage of development, crustaceans also have a "third eye". Some species, like thebrine shrimp, retain the primary eye throughout all stages of their life. Mostarthropods have one or more simple eyes, calledocelli, between their main,compound eyes.[22]
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