A parietal eye ( third eye, pineal eye) is a part of the epithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland, which regulates and hormone production for thermoregulation.
The parietal eye was discovered by Franz Leydig, in 1872, from work with Lizard.
Discovery
Franz Leydig, a professor of zoology at the University of Tübingen, dissected four species of European lizards—the
slow worm (
Anguis fragilis) and three species of
Lacerta.
in 1872;
He found cup-like protrusions under the middles of their brains. He believed the protrusions to be glandular and called them frontal organs (German
Stirnorgan).
In 1886, Walter Baldwin Spencer, an anatomist at the University 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 the Parietal eye and the pineal stalk.
Presence in various animals
The parietal eye is found in the
tuatara, most lizards, frogs,
, certain
osteichthyes,
, and
.
It is absent in
but was present in their closest
Extinction relatives, the
, suggesting that it was lost during the course of the mammalian evolution due to it being useless in
endotherm animals.
It is also absent in the ancestrally endothermic ("warm-blooded")
such as
birds. The parietal eye is also lost in
Ectotherm ("cold-blooded") archosaurs like
Crocodilia, and in
Turtle, which may be grouped with archosaurs in
Archelosauria.
Despite being
Lepidosauria, as lizards and tuatara are,
lack a parietal eye.
Anatomy
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.
[
] The parietal eye is a part of the
epithalamus, which can be divided into two major parts—the
pineal organ (the pineal organ; or the pineal gland, if it is mostly endocrine) and the parapineal organ (often called the
parietal eye or, if it is photoreceptive, the
third eye). The structures arise as a single anterior
evagination of the
Pineal gland organ or as a separate outgrowth of the roof of the
diencephalon; 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 the
skull.
[ Light-sensitive organs that evaginate from the diencephalon - NCBI][
] The parietal eye's way of detecting light differs from the use of
and
in a normal vertebrate eye.
[
]
Many of the oldest fossil vertebrates, including , , , and early , have in their skulls sockets that appear to have held functional third eyes. The socket remains as a foramen between the in many living amphibians and reptiles, although it has vanished in birds and mammals.
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.[
]
Comparative anatomy
The parietal eye of amphibians and reptiles appears relatively far forward in the skull; thus it may be surprising that the human
pineal gland appears far away from this position, tucked away between the
corpus callosum and
cerebellum. Also the
, 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 in
and basal amphibians, but have moved further back in higher vertebrates.
Likewise in the brain of the frog, the
diencephalon, from which the
pineal stalk arises, appears relatively further forward, as the cerebral hemispheres are smaller but the optic lobes are far more prominent than the human
mesencephalon, which is part of the
brain stem.
In humans the
optic tract, commissure, and
optic nerve bridge the substantial distance between eyes and diencephalon. Likewise the pineal stalk of
Petromyzon elongates very considerably during metamorphosis.
Analogs in other species
at the nauplius 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 the
brine shrimp, retain the primary eye throughout all stages of their life. Most
have one or more simple eyes, called
ocelli, between their main,
.
See also
