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The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil. The size of the pupil is controlled by the iris, and varies depending on many factors, the most significant being the amount of light in the environment. The term "pupil" was coined by Gerard of Cremona.
In humans, the pupil is circular, but its shape varies between species; some Cat eyes, , and have vertical slit pupils, goats and sheep have horizontally oriented pupils, and some catfish have annular types. In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop. The image of the pupil as seen from outside the eye is the entrance pupil, which does not exactly correspond to the location and size of the physical pupil because it is magnified by the cornea. On the inner edge lies a prominent structure, the collarette, marking the junction of the embryonic pupillary membrane covering the embryonic pupil.
The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
The sensory pathway (rod or cone, bipolar, ganglion) is linked with its counterpart in the other eye by a partial crossover of each eye's fibers. This causes the effect in one eye to carry over to the other.
When bright light is shone on the eye, light-sensitive cells in the retina, including rod and cone photoreceptors and melanopsin ganglion cells, will send signals to the oculomotor nerve, specifically the parasympathetic part coming from the Edinger-Westphal nucleus, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil. This is the pupillary light reflex, which is an important test of brainstem function. Furthermore, the pupil will dilate if a person sees an object of interest.
Certain drugs cause constriction of the pupils, such as opioids. Other drugs, such as atropine, LSD, MDMA, mescaline, psilocybin mushrooms, cocaine and amphetamines may cause pupil dilation.
The sphincter muscle has a parasympathetic innervation, and the dilator has a sympathetic innervation. In pupillary constriction induced by pilocarpine, not only is the sphincter nerve supply activated but that of the dilator is inhibited. The reverse is true, so control of pupil size is controlled by differences in contraction intensity of each muscle.
Another term for the constriction of the pupil is miosis. Substances that cause miosis are described as miotic. Dilation of the pupil is mydriasis. Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide.
Near-sightedness individuals have Mydriasis resting and dark dilated pupils than Far-sightedness and Emmetropia individuals, likely due to requiring less accommodation (which results in pupil constriction).
Some humans are able to exert direct control over their iris muscles, giving them the ability to manipulate the size of their pupils (i.e. dilating and constricting them) on command, without any changes in lighting condition or eye accommodation state. However, this ability is likely very rare and its purpose or advantages over those without it are unclear.
Osteichthyes generally have circular pupils with a nearly spherical lens behind it. But in some, especially nocturnal ones, the pupil is elongated in the rostral end to create a Aphakia gap.
Amphibian pupils can be circular (urodeles and aquatic anurans), horizontal oval (most anurans), horizontal diamond ( Hyperolius horstockii), vertical diamond ( Corythomantis greeningi), heart-shaped ( Bombinator pachypus), vertically pear-shaped ( Pelobates fuscus) or diagonally pear-shaped ( Caloptochephalus quoyi). The king penguin's pupil when contracting in bright light becomes square-shaped. The pupil of some sharks is a diagonal slit, as in nurse shark, tassled wobbegong, and angel shark, though it is more commonly a vertical oval slit. It is a square diamond in the tiger shark. A certain Scyliorhinidae sp. is reported to have diagonal slit pupil with multiple openings, similar to that of a gecko. Most seals have vertical slit pupils, but Bearded seal has diagonal slit pupils. Most have vertical slit pupils, except the with their circular pupils with an enormous dynamic range.
Only one bird, black skimmer, is known to have slit pupils.
and have horizontal rectangular pupils with a pair of serrated edges, such that when the pupil shrinks, the edges mesh together like two rows of fangs. This structure is the umbraculum. In horses, gazelles, and goats, the umbraculum is much smaller, and in these species, the umbraculum is usually called the corpora nigra or sometimes the granula iridica. Newborn horses have round pupils, but at around 5 years old, the pupil reaches its final form. The horse has 3 or 4 large ones on the top edge, and 5 or 6 small ones on the bottom edge. The sheep has up to 20, the highest recorded. Many have a similar pupil. The rock hyrax has a circular pupil with a spade-shaped umbraculum, attached to the upper edge of the pupil, and it contains muscles that can extend or contract independently of the pupil. Under a strong light, the pupil would contract, and the umbraculum would extend, shielding the pupil. This allows them to look into the sun, leading to a Zulu legend that they are blind. Similar structures appear in some Loricariidae, , the bottlenose dolphin, the beluga whale. In these species, it is usually called the operculum or the operculum pupillare. In the bottlenose dolphin, it has the function of allowing equivalent visual acuity both in air and in water. The operculum of skates (such as the clearnose skate or the thornback skate) is particularly complex, as it is a spade with frills on its edges. Thus, at maximal extension, the pupil becomes a series of pinholes arranged in a U-shape. In a few amphibians, the tadpole has a similarly shaped structure named the elygium, although it is disputed on whether it differs from the umbraculum.
Although human pupils are normally circular, abnormalities like can result in unusual pupil shapes, such as teardrop, keyhole or oval pupil shapes.
There may be differences in pupil shape even between closely related animals. In felids, there are differences between small- and large eyed species. The domestic cat (Felis sylvestris domesticus) has vertical slit pupils, its large relative the Siberian tiger (Panthera tigris altaica) has circular pupils and the Eurasian lynx (Lynx lynx) is intermediate between those of the domestic cat and the Siberian tiger. A similar difference between small and large species may be present in canines. The small red fox (Vulpes vulpes) has vertical slit pupils whereas their large relatives, the gray wolf (Canis lupus lupus) and domestic (Canis lupus familiaris) have round pupils.
The W-shape of the cuttlefish reduces light from the dorsal visual field significantly more than it reduces light from the horizontal band. This is hypothesized to benefit cuttlefish in its natural habitat, where the scene is brighter near the sea surface. Also, it preserves more of the frontal and caudal visual acuity compared to a circular pupil.
Many pupils, at maximal constriction, would become two or more pinholes. The pinholes are vertically aligned in some geckos and the cat. In the geckos, there are usually 4 diamond pinholes. The pinholes are horizontally aligned in many species with prominently horizontal rectangular pupils, such as the goat, the horse, and many . The pinholes are U-shaped in some skates. Similarly, in species with a roughly horizontal pupil and an operculum, when the operculum is maximally extended, it would bisect the pupil into two vertical slits. This is usually assumed to have certain optical advantages. For example, it is shown that by dividing the pupil in the middle by an operculum, a nocturnal animal may have a slit pupil that protects the retina, and yet still pass high spatial frequencies. One hypothesis for the gecko's multi-pinhole eye is that it allows the eye to use defocus blur to estimate distance, similar to Scheiner's disc used in clinical eye examination. Generally, animals that form multiple pinholes under bright light have rod-rich retinas, have no fovea, and are neither strictly nocturnal nor strictly diurnal.
A 2015 study confirmed the hypothesis that elongated pupils have increased dynamic range, and furthered the correlations with diel activity. However it noted that other hypotheses could not explain the orientation of the pupils. They showed that vertical pupils enable ambush predators to optimise their depth perception, and horizontal pupils to optimise the field of view and image quality of horizontal contours. They further explained why elongated pupils are correlated with the animal's height.
In a surprising number of unrelated languages, the etymology of the term for pupil is "little person". This is true, for example, of the word pupil itself: this comes into English from Latin pūpilla, which means "doll, girl", and is a diminutive form of pupa, "girl". (The double meaning in Latin is preserved in English, where pupil means both "schoolchild" and "dark central portion of the eye within the iris".)" pupil, n.2.", Oxford English Dictionary Online, 3rd. edn (Oxford University Press, 2007). This may be because the reflection of one's image in the pupil is a minuscule version of one's self. In the Old Babylonian period (c. 1800-1600 BC) in ancient Mesopotamia, the expression "protective spirit of the eye" is attested, perhaps arising from the same phenomenon.
The English phrase apple of my eye arises from an Old English usage, in which the word apple meant not only the fruit but also the pupil or eyeball. apple, n.", Oxford English Dictionary Online, 3rd ed. (Oxford University Press, 2008), § 6 B.
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