View source for Pupil

{{short description|Part of an eye}}
{{Other uses}}
{{pp-sock|small=yes}}
{{Infobox anatomy
| Name        = Pupil
| Latin       = pupilla. ([[plural]]: pupillae)
| Image       = Eye_iris.jpg
| Caption     = The pupil is the central opening of the iris on the inside of the eye, which normally appears black. The grey/blue or brown area surrounding the pupil is the [[iris (anatomy)|iris]]. The white outer area of the eye is the [[sclera]]. The central outermost transparent colorless part of the eye (through which we can see the iris and pupil) is the [[cornea]].
| Width       = 
| Image2      = Schematic diagram of the human eye en.svg
| Caption2    = Cross-section of the [[human eye]], showing the position of the pupil.
| Precursor   = 
| System      = 
| Artery      = 
| Vein        = 
| Nerve       = 
| part_of = [[Eye]]
| system = [[Visual system]]
| Lymph       = 
}}
The '''pupil''' is a hole located in the center of the [[Iris (anatomy)|iris]] of the [[eye]] that allows light to strike the [[retina]].<ref name="Cassin">Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company.</ref> It appears black because light rays entering the pupil are either absorbed by the [[biological tissue|tissues]] inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.<ref>{{Citation |last=Manion |first=Garrett N. |title=The Effect of Pupil Size on Visual Resolution |date=2025 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK603732/ |access-date=2025-08-10 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=38753941 |last2=Stokkermans |first2=Thomas J.}}</ref> The size of the pupil is controlled by the iris, and [[Pupillary response|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]].<ref>{{cite journal |last1=Arráez-Aybar |first1=Luis-A |title=Toledo School of Translators and their influence on anatomical terminology |journal=Annals of Anatomy - Anatomischer Anzeiger  |year=2015 |volume=198 |pages=21–33 |doi=10.1016/j.aanat.2014.12.003|pmid=25667112 }}</ref>

In humans, the pupil is circular, but its shape varies between species; some [[Cat eyes|cats]], [[reptile]]s, and [[fox]]es have vertical slit pupils, [[Goats#Anatomy and health|goats]] and [[Sheep#Characteristics|sheep]] have horizontally oriented pupils, and some [[catfish]] have annular types.<ref name=Malmstrom>{{cite journal |vauthors=Malmström T, Kröger RH |title=Pupil shapes and lens optics in the eyes of terrestrial vertebrates |journal=J. Exp. Biol. |volume=209 |issue=Pt 1 |pages=18–25 | date=January 2006 |pmid=16354774 |doi=10.1242/jeb.01959|bibcode=2006JExpB.209...18M |doi-access= }}</ref> 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 (iris)|collarette]], marking the junction of the embryonic pupillary membrane covering the embryonic pupil.

==Function==
{{See also|Iris (anatomy)}}
The [[iris (anatomy)|iris]] is a contractile structure, consisting mainly of [[smooth muscle]], surrounding the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. This is known as the [[pupillary light reflex]].

The iris contains two groups of smooth muscles; a circular group called the [[Iris sphincter muscle|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.

===Effect of light===
[[File:Pupillary light reflex.jpg|thumb|300x300px|The pupil diameter can vary greatly due to various factors (primarily the [[pupillary light reflex]]), from constriction to as small as 2 mm, to dilation larger than 8 mm in some individuals, though the maximal dilation also varies substantially by individual and decreases with age]]
The pupil gets wider in the dark and narrower in light. When narrow, the diameter be 1.5 to 4 millimeters.<ref name=":1" /> In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8&nbsp;mm.<ref name=":1" /> However, in any human age group there is considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4&nbsp;mm to 9&nbsp;mm with different individuals. After 25 years of age, the average pupil size decreases, though not at a steady rate.<ref>{{cite web |url=http://amateurastronomy.org/Events/EH361.html |title=Aging Eyes and Pupil Size |publisher=Amateurastronomy.org |access-date=2013-08-28 |archive-url=https://web.archive.org/web/20131023134125/http://amateurastronomy.org/Events/EH361.html |archive-date=2013-10-23 }}</ref><ref>{{cite journal|url=http://www.iovs.org/content/35/3/1132.full.pdf |title=Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects |journal=Investigative Ophthalmology & Visual Science |date=March 1994 |volume=35 |issue=3 |pages=1132–1137 |access-date=2013-08-28|last1=Winn |first1=B. |last2=Whitaker |first2=D. |last3=Elliott |first3=D. B. |last4=Phillips |first4=N. J. |pmid=8125724 }}</ref> At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as [[hippus]]. The constriction of the pupil and near vision are closely tied. In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity; in the dark, this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.<ref>"Sensory Reception:  Human Vision:  Structure and Function of the Eye"  Encyclopædia Brtiannicam Chicago, 1987</ref>

When bright light is shone on the eye, light-sensitive cells in the retina, including rod and cone photoreceptors and [[melanopsin]] [[retinal ganglion cell|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.{{citation needed|date=May 2020}}

==Clinical significance==
{{further|Pupillary response}}

===Effect of drugs===
If the drug [[pilocarpine]] is administered, the pupils will constrict and accommodation is increased due to the parasympathetic action on the circular muscle fibers, conversely, [[atropine]] will cause paralysis of accommodation (cycloplegia) and dilation of the pupil.

Certain drugs cause constriction of the pupils, such as [[opioids]].<ref>{{Cite journal|last=Larson|first=Merlin D.|date=2008-06-01|title=Mechanism of opioid-induced pupillary effects|journal=Clinical Neurophysiology|volume=119|issue=6|pages=1358–64|doi=10.1016/j.clinph.2008.01.106 |pmid=18397839|s2cid=9591926}}</ref> Other drugs, such as [[atropine]], [[LSD]], [[MDMA]], [[mescaline]], [[psilocybin mushrooms]], [[cocaine]] and [[amphetamines]] may cause pupil dilation.<ref>{{Cite journal|url=https://www.csam-asam.org/sites/default/files/pdf/misc/Howtospot.pdf|title=How to spot illicit drug abuse in your patients|last=Johnson|first=Michael D.|date=October 1, 1999|journal=Postgraduate Medicine|volume=106|issue=4|pages=199–200, 203–6, 211–4 passim|doi=10.3810/pgm.1999.10.1.721|pmid=10533519|access-date=March 22, 2018}}</ref><ref>{{Cite journal|last1=Alderman|first1=Elizabeth M.|last2=Schwartz|first2=Brian|date=1997-06-01|title=Substances of Abuse|url=https://pedsinreview.aappublications.org/content/18/6/204|journal=Pediatrics in Review |volume=18|issue=6|pages=204–215|doi=10.1542/pir.18-6-204 |s2cid=73382801 |url-access=subscription}}</ref>

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]].

=== Diseases ===
A condition called ''bene dilitatism'' occurs when the optic nerves are partially damaged. This condition is typified by chronically widened pupils due to the decreased ability of the optic nerves to respond to light. In normal lighting, people affected by this condition normally have dilated pupils, and bright lighting can cause pain. At the other end of the spectrum, people with this condition have trouble seeing in darkness. It is necessary for these people to be especially careful when driving at night due to their inability to see objects in their full perspective. This condition is not otherwise dangerous.

=== Size {{anchor|Diameter}} ===
{{Main|Mydriasis|Miosis|Pupillary response}}
The size of the pupil (often measured as [[diameter]]) can vary from 1.5 mm. to 8mm.<ref name=":1">{{Cite journal |last1=McDougal |first1=David H. |last2=Gamlin |first2=Paul D. |year=2015 |title=Autonomic Control of the Eye |journal=Comprehensive Physiology |volume=5 |issue=1 |pages=439–473 |doi=10.1002/cphy.c140014|pmid=25589275 |pmc=4919817 |isbn=978-0-470-65071-4 }}</ref> Pupil size can be a [[symptom]] of an underlying disease. Dilation of the pupil is known as [[mydriasis]] and contraction as [[miosis]].
[[File:Voluntary pupil dilation.gif|thumb|A human adult exhibiting voluntary control over his iris muscles, which grants him the ability to dilate and constrict his pupils on command]][[File:Human eye in dim light.jpg|thumb|260x260px|Pupil dilated naturally to 9 mm due to dim light. The subject is an extreme case, as most individuals are not able to naturally dilate their pupils to that extent]]
[[File:Human eye in bright light.jpg|thumb|260x260px|Pupil constriction can be in response to negative emotional states]]
Not all variations in size are indicative of disease however. In addition to dilation and contraction caused by light and darkness, it has been shown that solving simple multiplication problems affects the size of the pupil.<ref>{{Cite journal|last1=Hess|first1=Eckhard H.|last2=Polt|first2=James M.|date=1964-03-13|title=Pupil Size in Relation to Mental Activity during Simple Problem-Solving|journal=Science |volume=143|issue=3611|pages=1190–2|doi=10.1126/science.143.3611.1190 |pmid=17833905|bibcode=1964Sci...143.1190H|s2cid=27169110}}</ref> The simple act of recollection can dilate the size of the pupil,<ref>{{Cite book|title=Psychophysiology: Human Behavior and Physiological Response (Psychophysiology: Human Behavior & Physiological Response) |edition=5th |last=L. Andreassi|first=John|publisher=Psychology Press|year=2006|isbn=978-0-8058-4951-6}}</ref> however when the brain is required to process at a rate above its maximum capacity, the pupils contract.<ref>{{cite web|url=https://prezi.com/e00bcswh1fhp/my-brain-is-overloaded/|title=My Brain is Overloaded|website=prezi.com |access-date=2017-02-28}}</ref> There is also evidence that pupil size is related to the extent of positive or negative emotional arousal experienced by a person.<ref name="Partala and Surakka (2003)">{{cite journal|author1=Partala, T.  |author2=Surakka, V. |name-list-style=amp |title=Pupil size variation as an indication of affective processing|journal=International Journal of Human-Computer Studies|volume=59|issue=1–2|pages=185–198|doi=10.1016/S1071-5819(03)00017-X|year=2003|s2cid=7007209 }}</ref>

[[Near-sightedness|Myopic]] individuals have [[Mydriasis|larger]] resting and dark dilated pupils than [[Far-sightedness|hyperopic]] and [[Emmetropia|emmetropic]] individuals, likely due to requiring less accommodation (which results in pupil constriction).<ref>{{Cite journal |last1=Cakmak |first1=Hasan Basri |last2=Cagil |first2=Nurullah |last3=Simavlı |first3=Hüseyin |last4=Duzen |first4=Betul |last5=Simsek |first5=Saban |date=February 2010 |title=Refractive Error May Influence Mesopic Pupil Size |url=http://www.tandfonline.com/doi/full/10.3109/02713680903447892 |journal=Current Eye Research |language=en |volume=35 |issue=2 |pages=130–136 |doi=10.3109/02713680903447892 |pmid=20136423 |s2cid=27407880 |issn=0271-3683|url-access=subscription }}</ref>

Some humans are able to exert direct control over their [[Iris (anatomy)|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 (vertebrate eye)|accommodation]] state.<ref>{{Cite journal |last1=Eberhardt |first1=Lisa V. |last2=Grön |first2=Georg |last3=Ulrich |first3=Martin |last4=Huckauf |first4=Anke |last5=Strauch |first5=Christoph |date=2021-10-01 |title=Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI |url=https://www.sciencedirect.com/science/article/pii/S0167876021008448 |journal=International Journal of Psychophysiology |volume=168 |pages=33–42 |doi=10.1016/j.ijpsycho.2021.08.001 |pmid=34391820 |issn=0167-8760}}</ref> However, this ability is likely very rare and its purpose or advantages over those without it are unclear.

==Animals==
[[File:Sepia eyelid shape.theora.ogv|thumb|The W-shaped pupil of the cuttlefish expanding when the lights are turned off.]]
Not all [[animal]]s have circular pupils. Some have slits or ovals which may be oriented vertically, as in [[crocodile]]s, [[Viperidae|vipers]], [[Felidae|cats]] and [[fox]]es, or horizontally as in some [[Batoidea|rays]], [[flying frog]]s, [[mongoose]]s and [[artiodactyl]]s such as [[elk]], [[red deer]], [[reindeer]] and [[hippopotamus]], as well as the domestic [[horse]]. [[Goat]]s, [[sheep]], [[toad]]s and [[octopus]] pupils tend to be horizontal and rectangular with rounded corners. [[Gecko]] pupils are generally of the shape of a slit with multiple notches, such that when it is closed, the pupil becomes a series of pinholes, and when it is open, the pupil is roughly circular.<ref name=":2">{{Cite journal|last1=Roth|first1=Lina S. V.|last2=Lundström|first2=Linda|last3=Kelber|first3=Almut|last4=Kröger|first4=Ronald H. H.|last5=Unsbo|first5=Peter|date=2009-03-01|title=The pupils and optical systems of gecko eyes|journal=Journal of Vision|volume=9|issue=3|pages=27.1–11|doi=10.1167/9.3.27|pmid=19757966 |doi-access=free}}</ref> When a pupil has a slit form, it is sometimes called ''stenopaic''.<ref name=":7">{{Citation |last=Smythe |first=R. H. |title=The Eye of the Cat |date=1975 |work=Vision in the Animal World |pages=75–79 |editor-last=Smythe |editor-first=R. H. |place=London |publisher=Palgrave Macmillan UK |language=en |doi=10.1007/978-1-349-02533-6_5 |isbn=978-1-349-02533-6}}</ref> The [[cuttlefish]] pupil is a smoothly curving W shape.

[[Osteichthyes|Bony fishes]] generally have circular pupils with a [[Matthiessen's ratio|nearly spherical lens]] behind it. But in some, especially nocturnal ones, the pupil is elongated in the rostral end to create a [[Aphakia|lensless (aphakic)]] gap.<ref>{{Cite journal |last1=Schmitz |first1=Lars |last2=Wainwright |first2=Peter C |date=2011 |title=Nocturnality constrains morphological and functional diversity in the eyes of reef fishes |journal=BMC Evolutionary Biology |language=en |volume=11 |issue=1 |page=338 |doi=10.1186/1471-2148-11-338 |pmid=22098687 |doi-access=free |issn=1471-2148 |pmc=3240680 |bibcode=2011BMCEE..11..338S }}</ref><ref>{{Cite journal |last=Sivak |first=J. G. |date=March 1978 |title=The functional significance of the aphakic space of the fish eye |url=https://cdnsciencepub.com/doi/abs/10.1139/z78-072 |journal=Canadian Journal of Zoology |volume=56 |issue=3 |pages=513–516 |doi=10.1139/z78-072 |bibcode=1978CaJZ...56..513S |issn=0008-4301}}</ref>

Amphibian pupils can be circular (urodeles and aquatic anurans), horizontal oval (most anurans), horizontal diamond (''[[Hyperolius horstockii]]''), vertical diamond (''[[Corythomantis greeningi]]''), heart-shaped (''[[Bombina variegata|Bombinator pachypus]]''), vertically pear-shaped (''[[Pelobates fuscus]]'') or diagonally pear-shaped (''[[Calyptocephalella gayi|Caloptochephalus quoyi]]'').<ref>{{Cite journal |last=MAnn |first=Ida |date=1931 |title=Iris Pattern in the Vertebrates |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1096-3642.1931.tb00659.x |journal=The Transactions of the Zoological Society of London |language=en |volume=21 |issue=4 |pages=355–412 |doi=10.1111/j.1096-3642.1931.tb00659.x |issn=1469-7998|url-access=subscription }}</ref><ref name=":5" />{{Pg|page=223}} The [[king penguin]]'s pupil when contracting in bright light becomes square-shaped.<ref>{{Cite journal |last=Martin |first=Graham R. |date=1999 |title=Eye structure and foraging in King Penguins Aptenodytes patagonicus |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1474-919X.1999.tb04413.x |journal=Ibis |language=en |volume=141 |issue=3 |pages=444–450 |doi=10.1111/j.1474-919X.1999.tb04413.x |issn=1474-919X|url-access=subscription }}</ref> The pupil of some sharks is a diagonal slit, as in [[Ginglymostoma cirratum|nurse shark]], [[Tasselled wobbegong|tassled wobbegong]], and [[Squatina squatina|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.<ref name=":6">{{Citation |last=Williams |first=David |title=Ophthalmology of Cartilaginous Fish: Skates, Rays, and Sharks |date=2022 |work=Wild and Exotic Animal Ophthalmology: Volume 1: Invertebrates, Fishes, Amphibians, Reptiles, and Birds |pages=47–59 |editor-last=Montiani-Ferreira |editor-first=Fabiano |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-71302-7_4 |isbn=978-3-030-71302-7 |editor2-last=Moore |editor2-first=Bret A. |editor3-last=Ben-Shlomo |editor3-first=Gil}}</ref> Most seals have vertical slit pupils, but ''[[Bearded seal|Phoca barbata]]'' has diagonal slit pupils. Most [[prosimian]]s have vertical slit pupils, except the [[tarsier]]s with their circular pupils with an enormous [[dynamic range]].<ref name=":5" />{{Pg|page=227}}

Only one bird, [[black skimmer]], is known to have slit pupils.<ref name=":5" />{{Pg|page=226}}

[[Llama]]s and [[camel]]s 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''<ref>{{Cite journal |last1=Johnson |first1=George Lindsay |author-link1=George Lindsay Johnson |last2=Gadow |first2=Hans Friedrich |date=January 1997 |title=I. Contributions to the comparative anatomy of the mammalian eye, chiefly based on ophthalmoscopic examination |url=https://royalsocietypublishing.org/doi/abs/10.1098/rstb.1901.0001 |journal=Philosophical Transactions of the Royal Society of London. Series B, Containing Papers of a Biological Character |volume=194 |issue=194–206 |pages=1–82 |doi=10.1098/rstb.1901.0001|url-access=subscription }}</ref> or sometimes the ''granula iridica''.<ref>{{Cite journal |last=Crispin |first=Sheila M. |date=2000 |title=Developmental anomalies and abnormalities of the equine iris |url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1463-5224.2000.3230093.x |journal=Veterinary Ophthalmology |language=en |volume=3 |issue=2–3 |pages=93–98 |doi=10.1046/j.1463-5224.2000.3230093.x |issn=1463-5224|url-access=subscription }}</ref> 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.<ref name=":5" />{{Pg|page=227}} Many [[artiodactyl]]s have a similar pupil.<ref name="Murphy and Howland, (2005)" /> 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.<ref>{{Citation |last1=Freeman |first1=Katie |title=Ophthalmology of Afrotheria: Aardvarks, Hyraxes, Elephants, Manatees, and Relatives |date=2022 |work=Wild and Exotic Animal Ophthalmology: Volume 2: Mammals |pages=49–70 |editor-last=Montiani-Ferreira |editor-first=Fabiano |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-81273-7_5 |isbn=978-3-030-81273-7 |last2=Ben-Shlomo |first2=Gil |last3=McMullen |first3=Richard |last4=Moore |first4=Bret A. |editor2-last=Moore |editor2-first=Bret A. |editor3-last=Ben-Shlomo |editor3-first=Gil}}</ref> This allows them to look into the sun, leading to a Zulu legend that they are blind.<ref>{{cite book |last=Roberts |first=Austin |title=The mammals of South Africa |date=1951 |publisher=Trustees of "The Mammals of South Africa" Book Fund |location=South Africa |page=254 |ol=OL15016448M}}</ref> Similar structures appear in some [[Loricariidae|Loricariids]],<ref>{{Cite book |last=Roth |first=Willi |url=https://archive.org/details/die-iris-der-loricariiden |title=Die Iris der Loricariiden |date=1920 |language=de |trans-title=The iris of the Loricariids}}</ref> [[flatfish]]es,<ref name=":5" />{{Pg|page=222}} the [[bottlenose dolphin]],<ref>{{Cite journal |last1=Mass |first1=Alla M. |last2=Supin |first2=Alexander YA. |date=2007 |title=Adaptive features of aquatic mammals' eye |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ar.20529 |journal=The Anatomical Record |language=en |volume=290 |issue=6 |pages=701–715 |doi=10.1002/ar.20529 |pmid=17516421 |issn=1932-8494}}</ref> the [[beluga whale]].<ref name=":3">{{Cite journal |last=Mass |first=A. M. |date=2001-11-01 |title=Visual Field Organization and Retinal Resolution in the Beluga Whale Delphinapterus leucas (Pallas) |journal=Doklady Biological Sciences |language=en |volume=381 |issue=1 |pages=555–558 |doi=10.1023/A:1013326521559 |pmid=12918432 |issn=1608-3105}}</ref> In these species, it is usually called the ''operculum'' or the ''operculum pupillare''.<ref>{{Cite journal |last=Millar |first=R. P. |date=August 1973 |title=An unusual light-shielding structure in the eye of the dassie, Procavia capensis Pallas (Mammalia: Hydracoidea) |url=https://journals.co.za/doi/abs/10.10520/AJA00411752_258 |journal=Annals of the Transvaal Museum |volume=28 |issue=11 |pages=203–205 |hdl=10520/AJA00411752_258}}</ref> In the bottlenose dolphin, it has the function of allowing equivalent visual acuity both in air and in water.<ref name=":4">{{Cite journal |last1=Herman |first1=Louis M. |last2=Peacock |first2=Michael F. |last3=Yunker |first3=Michael P. |last4=Madsen |first4=Carolyn J. |date=1975-08-22 |title=Bottle-Nosed Dolphin: Double-Slit Pupil Yields Equivalent Aerial and Underwater Diurnal Acuity |url=https://www.science.org/doi/abs/10.1126/science.1162351 |journal=Science |volume=189 |issue=4203 |pages=650–652 |doi=10.1126/science.1162351|bibcode=1975Sci...189..650H |url-access=subscription }}</ref> The operculum of [[Skate (fish)|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.<ref name=":6" /><ref name="Murphy and Howland, (2005)">{{cite journal |author1=Murphy, C.J. |author2=Howland, H.C. |name-list-style=amp |year=1990 |title=The functional significance of crescent-shaped pupils and multiple pupillary apertures |journal=Journal of Experimental Zoology |volume=256 |issue=S5 |page=22 |bibcode=1990JEZ...256S..22M |doi=10.1002/jez.1402560505}}</ref><ref name=":5">{{Cite book |last=Walls |first=Gordon Lynn |url=http://archive.org/details/vertebrateeyeits00wall |title=The vertebrate eye and its adaptive radiation |date=1942 |publisher=Bloomfield Hills, Mich., Cranbrook Institute of Science}}</ref>{{Pg|page=158}} 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.<ref>{{Cite journal |last=Van |first=Dijk D. Eduard |date=September 1966 |title=Systematic and field keys to the families, genera and described species of Southern African Anuran tadpoles |url=https://journals.co.za/doi/abs/10.10520/AJA03040798_751 |journal=Annals of the Natal Museum |volume=18 |issue=2 |pages=231–286 |hdl=10520/AJA03040798_751}}</ref><ref>{{Cite journal |last1=Kruger |first1=Donnavan J.D. |last2=Weldon |first2=Ché |last3=Minter |first3=Les R. |last4=Du Preez |first4=Louis H. |date=2013 |title=Morphology of the elygium and developing umbraculum in the eye of Amietia vertebralis tadpoles |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/jmor.20115 |journal=Journal of Morphology |language=en |volume=274 |issue=5 |pages=551–556 |doi=10.1002/jmor.20115 |pmid=23293087 |bibcode=2013JMorp.274..551K |issn=1097-4687|url-access=subscription }}</ref>

Although human pupils are normally circular, abnormalities like [[coloboma]]s 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,<ref name=":7" /> 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 [[dog]]s ''(Canis lupus familiaris)'' have round pupils.<ref>{{Cite journal |last=Kaminski |first=Juliane |last2=Waller |first2=Bridget M. |last3=Diogo |first3=Rui |last4=Hartstone-Rose |first4=Adam |last5=Burrows |first5=Anne M. |date=2019-07-16 |title=Evolution of facial muscle anatomy in dogs |url=https://www.pnas.org/doi/10.1073/pnas.1820653116 |journal=Proceedings of the National Academy of Sciences |volume=116 |issue=29 |pages=14677–14681 |doi=10.1073/pnas.1820653116 |pmc=6642381 |pmid=31209036}}</ref>

=== Evolution and adaptation ===
One explanation for the [[evolution]] of slit pupils is that they can exclude light more effectively than a circular pupil.<ref>{{Cite journal |last=Land |first=Michael F. |date=2006-03-07 |title=Visual Optics: The Shapes of Pupils |url=https://www.sciencedirect.com/science/article/pii/S0960982206011961 |journal=Current Biology |volume=16 |issue=5 |pages=R167–R168 |doi=10.1016/j.cub.2006.02.046 |issn=0960-9822 |quote=The explanation usually given for the use of a slit pupil is that it can exclude light more effectively than a circular pupil, and so slit pupils tend to be found in the eyes of animals with a crepuscular or nocturnal lifestyle that need to protect their eyes during daylight.}}</ref><ref>{{cite book | last = Walls | first = G.L. | title = The Vertebrate Eye and its Adaptive Radiation | publisher = The Cranbrook Institute of Science | location = Bloomfield Hills, Michigan | year = 1942 }}</ref> This would explain why slit pupils tend to be found in the eyes of animals with a [[crepuscular]] or [[nocturnal]] lifestyle that need to protect their eyes during daylight. Constriction of a circular pupil (by a ring-shaped muscle) is less complete than closure of a slit pupil, which uses two additional muscles that laterally compress the pupil.<ref name="Walls et al., (1942)">{{cite book |first=G.L. |last=Walls |title=The vertebrate eye and its adaptive radiation |publisher=Hafner |orig-date=1942 |year=1967 |oclc=10363617 |series=Cranbrook Institute of Science Bulletin |volume=19}}</ref> For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans.<ref name="Hughes, (1977)">{{cite book |last=Hughes |first=A. |orig-date=1977 |chapter=The topography of vision in mammals of contrasting life style: comparative optics and retinal organisation |editor-first=F. |editor-last=Crescitelli |title=The Visual System in Vertebrates |chapter-url=https://books.google.com/books?id=qCDqCAAAQBAJ&pg=PA613 |date=2013 |publisher=Springer |isbn=978-3-642-66468-7 |pages=613–756 |series=Handbook of Sensory Physiology |volume=7/5}}</ref> However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5&nbsp;mm in the [[tarsier]]) and the rectangular pupils of many [[ungulate]]s which do not close to a narrow slit in bright light.<ref name="Land, (2006)">{{cite journal|doi=10.1016/j.cub.2006.02.046|pmid=16527734|author=Land, M.F.|title= Visual optics: the shapes of pupils|journal=Current Biology|volume=16|issue=5|pages=R167–8|year=2006|doi-access=free|bibcode=2006CBio...16.R167L }}</ref> An alternative explanation is that a partially constricted circular pupil shades the peripheral zones of the lens which would lead to poorly focused images at relevant wavelengths. The vertical slit pupil allows for use of all wavelengths across the full diameter of the lens, even in bright light.<ref name="Malmstrom" /> It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, [[Disruptive coloration|breaking up]] the circular outline of the eye.<ref name="Brischoux et al., (2010)">{{cite journal|doi=10.1111/j.1420-9101.2010.02046.x|pmid=20629855|author=Brischoux, F., Pizzatto, L. and Shine, R.|title=Insights into the adaptive significance of vertical pupil shape in snakes|journal=Journal of Evolutionary Biology|volume=23|issue=9|pages=1878–85|year=2010|doi-access=free}}</ref>

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.<ref>{{Cite journal |last1=Mäthger |first1=Lydia M. |last2=Hanlon |first2=Roger T. |last3=Håkansson |first3=Jonas |last4=Nilsson |first4=Dan-Eric |date=2013-05-03 |title=The W-shaped pupil in cuttlefish (Sepia officinalis): Functions for improving horizontal vision |url=https://www.sciencedirect.com/science/article/pii/S0042698913000539 |journal=Vision Research |volume=83 |pages=19–24 |doi=10.1016/j.visres.2013.02.016 |pmid=23474299 |issn=0042-6989|url-access=subscription }}</ref>

Many pupils, at maximal constriction, would become two or more pinholes. The pinholes are vertically aligned in some geckos and the cat.<ref name=":2" /> 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 [[artiodactyl]]s.<ref name="Murphy and Howland, (2005)" /> The pinholes are U-shaped in some skates.<ref name="Murphy and Howland, (2005)" /><ref name=":5" />{{Pg|page=158}} 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.<ref name=":3" /><ref name=":4" /> 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.<ref>{{Citation |last=Miller |first=William H. |title=Ocular Optical Filtering |date=1979 |work=Comparative Physiology and Evolution of Vision in Invertebrates |series=Handbook of Sensory Physiology |volume=7 / 6 / 6 A |pages=69–143 |editor-last=Autrum |editor-first=H. |url=http://link.springer.com/10.1007/978-3-642-66999-6_3 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-66999-6_3 |isbn=978-3-642-67001-5|url-access=subscription }}</ref> 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.<ref>{{Cite journal |last1=Murphy |first1=Christopher J. |last2=Howland |first2=Howard C. |date=1986-01-01 |title=On the gekko pupil and scheiner's disc |journal=Vision Research |volume=26 |issue=5 |pages=815–817 |doi=10.1016/0042-6989(86)90097-0 |issn=0042-6989}}</ref> Generally, animals that form multiple pinholes under bright light have rod-rich retinas, have no fovea, and are neither strictly nocturnal nor strictly diurnal.<ref name="Murphy and Howland, (2005)" />

=== Activity pattern and behavior ===
In a study of [[Australia]]n [[snake]]s, pupil shapes correlated both with [[diel]] activity times and with foraging behavior. Most snake species with vertical pupils were [[nocturnal]] and also ambush foragers, and most snakes with circular pupils were [[diurnality|diurnal]] and active foragers. Overall, foraging behaviour predicted pupil shape accurately in more cases than did diel time of activity, because many active-foraging snakes with circular pupils were not diurnal. It has been suggested that there may be a similar link between foraging behaviour and pupil shape amongst the felidae and canidae discussed above.<ref name="Brischoux et al., (2010)" />

A 2015 study<ref>{{cite journal |last1=Banks |first1=Martin S.  |last2=Sprague |first2=William W.  |last3=Schmoll |first3=Jürgen |last4=Parnell |first4=Jared A. Q.  |last5=Love |first5=Gordon D. |title=Why do animal eyes have pupils of different shapes? |journal=Science Advances |volume=1 |issue=7 |article-number=e1500391 |year=2015 |doi=10.1126/sciadv.1500391 |pmid=26601232 |pmc=4643806 |bibcode=2015SciA....1E0391B |doi-access=free }}</ref> 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.

<gallery widths="240" heights="180" caption="Animals with non-circular pupils">
File:Horizontal Rectangular Pupil of a Goat.jpg|A [[goat]] with horizontal rectangular pupils
File:Taeniura grabata eye.jpg|A [[stingray]] with crescent pupils
File:Crocodylus siamensis closeup.jpg|A [[crocodile]] with thin vertical slit pupils
File:Cuttlefish eye.jpg|A [[cuttlefish]] with W-shaped pupils
File:Gecko-oeil.jpg|A [[gecko]] with 'thin string of pearls' pupils
File:Catpupil03042006.jpg|A [[cat]] with thick vertical slit pupils
File:Johnson 1901 Contributions to the Comparative Anatomy of the Mammalian Eye 55.jpg|Horse, ass, gazelle, goat, hyrax, camel, llama.
</gallery>

==Society and culture==
The pupil plays a role in [[Oculesics|eye contact]] and [[nonverbal communication]]. The voluntary or involuntary enlargement or [[Pupillary response|dilation of the pupils]] indicates cognitive [[arousal]], [[Interest (emotion)|interest]] in the subject of attention, and/or [[sexual arousal]]. On the other hand, when the pupil is voluntarily or involuntarily [[Miosis|contracted]], it could indicate the opposite - disinterest or disgust. Exceptionally large or [[Mydriasis|dilated pupils]] are also perceived to be an attractive feature in body language.<ref>{{Cite journal |last1=Tombs |first1=Selina |last2=Silverman |first2=Irwin |date=2004-07-01 |title=Pupillometry: A sexual selection approach |url=https://www.sciencedirect.com/science/article/pii/S1090513804000261 |journal=Evolution and Human Behavior |language=en |volume=25 |issue=4 |pages=221–228 |doi=10.1016/j.evolhumbehav.2004.05.001 |bibcode=2004EHumB..25..221T |issn=1090-5138|url-access=subscription }}</ref>

In a surprising number of unrelated languages, the [[etymology|etymological meaning]] of the term for pupil is "little person".<ref name="Brown2004" /><ref>{{Cite journal |last1=Brown |first1=Cecil H. |last2=Witkowski |first2=Stanley R. |date=1981 |title=Figurative Language in a Universalist Perspective |jstor=644304 |journal=American Ethnologist |volume=8 |issue=3 |pages=596–615|doi=10.1525/ae.1981.8.3.02a00110 }}</ref> 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".)<ref>"[http://www.oed.com/view/Entry/154768 pupil, n.2.]", ''[[Oxford English Dictionary]] Online'', 3rd. edn (Oxford University Press, 2007).</ref> This may be because the reflection of one's image in the pupil is a minuscule version of one's self.<ref name="Brown2004">{{Cite journal |last=Brown |first=Donald E. |date=2004 |title=Human Universals, Human Nature & Human Culture |journal=Daedalus |volume=133 |issue=4 |page=49|doi=10.1162/0011526042365645 |jstor=20027944 |s2cid=8522764 |doi-access=free }}</ref> 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.<ref name=":0">[http://www.oed.com/view/Entry/9678 apple, n.]", ''[[Oxford English Dictionary]] Online'', 3rd ed. (Oxford University Press, 2008), § 6 B.</ref>

==See also==
{{Portal|Biology}}
* [[Pupillary response]]
* [[Eye examination#Pupil function|Pupil function]]
* [[Dilated fundus examination]]
* [[Eye contact]]
* [[Horner's syndrome]]
* [[Mydriasis]]
* [[Synechia (eye)]]
* [[Anisocoria]]
* [[Adie's pupil]]
* [[Argyll Robertson pupil]]
* [[Light-near dissociation]]
* [[Marcus Gunn Pupil]]
* [[Pupillary distance]]

==References==
{{Reflist}}

==External links==
* {{UMichAtlas|eye_1}}&nbsp;— "Sagittal Section Through the Eyeball"
* {{UMichAtlas|eye_2}}&nbsp;— "Sagittal Section Through the Eyeball"
* [https://web.archive.org/web/20110309235856/http://cim.ucdavis.edu/EyeRelease/Interface/TopFrame.htm A pupil examination simulator], demonstrating the changes in pupil reactions for various nerve lesions.

{{Eye anatomy}}
{{Visual pathways}}
{{Authority control}}

[[Category:Pupil| ]]
[[Category:Ethology]]
[[Category:Articles containing video clips]]