The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.[1] 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.

In humans the pupil is round, but other species, such as some cats, have vertical slit pupils, goats have horizontally oriented pupils, and some catfish have annular types.[2] 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.

Eye iris
The pupil is the central transparent area, appearing as black. The grey/blue area surrounding it is the iris. The white outer area is the sclera, the central transparent part of which is the cornea.
Schematic diagram of the human eye en
Cross-section of the human eye, showing the position of the pupil.
Part ofEye
SystemVisual system
LatinPupilla. (Plural: Pupillae)
Anatomical terminology


The pupil is a hole located in the centre of the iris of the eye that allows light to strike the retina.[1] 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 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.

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.

Effect of light

The pupil gets wider in the dark and narrower in light. When narrow, the diameter is 2 to 4 millimeters. In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 3 to 8 mm. In any human age group there is however considerable variation in maximal pupil size. For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals. After 25 years of age the average pupil size decreases, though not at a steady rate.[3][4] 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.[5]

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.

Clinical significance

Effect of drugs

Iris dilated for retina examination

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.[6] Other drugs, such as atropine, LSD, MDMA, mescaline, psilocybin mushrooms, cocaine and amphetamines may cause pupil dilation.[7]

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.


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 afflicted with 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 of pupil

The size of the pupil can be a symptom of an underlying disease. Dilation of the pupil is known as mydriasis and contraction as miosis.

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.[8] The simple act of recollection can dilate the size of the pupil,[9] however when the brain is required to process at a rate above its maximum capacity, the pupils contract.[10] There is also evidence that pupil size is related to the extent of positive or negative emotional arousal experienced by a person.[11]

Other animals

The W-shaped pupil of the cuttlefish expanding when the lights are turned off.

Not all animals have circular pupils. Some have slits or ovals which may be oriented vertically, as in crocodiles, vipers, cats and foxes, or horizontally as in some rays, flying frogs, mongooses and artiodactyls such as sheep, elk, red deer, reindeer and hippopotamus, as well as the domestic horse. Goats, toads and octopus pupils tend to be horizontal and rectangular with rounded corners. Some skates and rays have crescent shaped pupils,[12] gecko pupils range from circular, to a slit, to a series of pinholes[13], and the cuttlefish pupil is a smoothly curving W shape. There are rare cases of humans with very unusually shaped pupils, such as peanut-shell-shaped pupils, as if the sides of the pupils are pinched together in the middle, like a vertical peanut shell, being wide at the top and bottom of the pupil, but narrow in the middle.

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 European red fox (Vulpes vulpes) has vertical slit pupils whereas their large relatives, the gray wolf (Canis lupus lupus) and domestic dogs (Canis lupus familiaris) have round pupils.

One explanation for the evolution of slit pupils is that they can exclude light more effectively than a circular pupil. 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.[14] For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans.[15] However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5 mm in the tarsier) and the rectangular pupils of many ungulates which do not close to a narrow slit in bright light.[16] 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.[2] It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, breaking up the circular outline of the eye.[17]

In a study of Australian snakes, pupil shapes correlated both with diel activity times and with foraging behaviour. Most snake species with vertical pupils were nocturnal and also ambush foragers, and most snakes with circular pupils were 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.[17]

A 2015 study[18] 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.

Closeup of goat eye

A goat with horizontal rectangular pupils

Taeniura grabata eye

A stingray with crescent pupils

Crocodylus siamensis closeup

A crocodile with thin vertical slit pupils

Cuttlefish eye

A cuttlefish with W-shaped pupils


A gecko with 'thin string of pearls' pupils


A cat with thick vertical slit pupils

Society and culture

An anomaly of etymology is that in a surprising number of unrelated languages the meaning of the term for pupil is little person.[19] This may be because the reflection of one's image in the pupil is a minuscule version of one's self.[20]

See also


  1. ^ a b Cassin, B. and Solomon, S. (1990) Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company.
  2. ^ a b Malmström T, Kröger RH (January 2006). "Pupil shapes and lens optics in the eyes of terrestrial vertebrates". J. Exp. Biol. 209 (Pt 1): 18–25. doi:10.1242/jeb.01959. PMID 16354774.
  3. ^ "Aging Eyes and Pupil Size". Amateurastronomy.org. Archived from the original on 2013-10-23. Retrieved 2013-08-28.
  4. ^ "Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects" (PDF). Retrieved 2013-08-28.
  5. ^ "Sensory Reception: Human Vision: Structure and Function of the Eye" Encyclopædia Brtiannicam Chicago, 1987
  6. ^ Larson, Merlin D. (2008-06-01). "Mechanism of opioid-induced pupillary effects". Clinical Neurophysiology. 119 (6): 1358–1364. doi:10.1016/j.clinph.2008.01.106. ISSN 1388-2457. PMID 18397839.
  7. ^ Johnson, Michael D. (October 1, 1999). "How to spot illicit drug abuse in your patients" (PDF). California Society of Addiction Medicine. Retrieved March 22, 2018.
  8. ^ Hess, Eckhard H.; Polt, James M. (1964-03-13). "Pupil Size in Relation to Mental Activity during Simple Problem-Solving". Science. 143 (3611): 1190–1192. doi:10.1126/science.143.3611.1190. ISSN 0036-8075. PMID 17833905.
  9. ^ L. Andreassi, John (2006). Psychophysiology: Human Behavior and Physiological Response (Psychophysiology: Human Behavior & Physiological Response); 5 edition. Psychology Press. ISBN 978-0805849516.
  10. ^ "My Brain is Overloaded". prezi.com. Retrieved 2017-02-28.
  11. ^ Partala, T. & Surakka, V. (2003). "Pupil size variation as an indication of affective processing". International Journal of Human-computer Studies. 59 (1–2): 185–198. doi:10.1016/S1071-5819(03)00017-X.
  12. ^ Murphy, C.J. & Howland, H.C. (1990). "The functional significance of crescent-shaped pupils and multiple pupillary apertures". Journal of Experimental Zoology. 256: 22. doi:10.1002/jez.1402560505.
  13. ^ Roth, Lina S. V.; Lundström, Linda; Kelber, Almut; Kröger, Ronald H. H.; Unsbo, Peter (2009-03-01). "The pupils and optical systems of gecko eyes". Journal of Vision. 9 (3): 27.1–11. doi:10.1167/9.3.27. ISSN 1534-7362. PMID 19757966.
  14. ^ Walls, G.L., (1942). The Vertebrate Eye and its Adaptive Radiation. The Cranbrook Institute of Science, Bloomington Hills, Michigan.
  15. ^ Hughes, A. (1977). "The topography of vision in mammals of contrasting life style: comparative optics and retinal organisation", pp. 613–756 in F. Crescitelli (Ed.), Handbook of Sensory Physiology VII/5., Springer-Verlag, Berlin.
  16. ^ Land, M.F. (2006). "Visual optics: the shapes of pupils". Current Biology. 16 (5): R167–8. doi:10.1016/j.cub.2006.02.046. PMID 16527734.
  17. ^ a b Brischoux, F., Pizzatto, L. and Shine, R. (2010). "Insights into the adaptive significance of vertical pupil shape in snakes". Journal of Evolutionary Biology. 23 (9): 1878–85. doi:10.1111/j.1420-9101.2010.02046.x. PMID 20629855.CS1 maint: Multiple names: authors list (link)
  18. ^ Banks et al. (2015) "Why do animal eyes have pupils of different shapes?" url=http://advances.sciencemag.org/content/1/7/e1500391/tab-article-info
  19. ^ Toward a Theory of Cultural Linguistics, by Gary B. Palmer, p102
  20. ^ Human Universals and Human Culture, p4

External links

Adie syndrome

Adie syndrome also known as the Holmes-Adie syndrome is a neurological disorder characterized by a tonically dilated pupil that reacts slowly to light but shows a more definite response to accommodation (i.e., light-near dissociation). It is frequently seen in females with absent knee or ankle jerks and impaired sweating. It is named after the British neurologists William John Adie and Gordon Morgan Holmes who independently described the same disease in 1931. It is caused by damage to the postganglionic fibers of the parasympathetic innervation of the eye, usually by a viral or bacterial infection which causes inflammation, and affects the pupil of the eye and the autonomic nervous system.


Anisocoria is a condition characterized by an unequal size of the eyes' pupils. Affecting 20% of the population, it can be an entirely harmless condition or a symptom of more serious medical problems.

Apt Pupil (film)

Apt Pupil is a 1998 American psychological thriller film directed and co-produced by Bryan Singer and written by Brandon Boyce. The film, which stars Ian McKellen and Brad Renfro, is based on the 1982 novella of the same name by Stephen King. Set in the 1980s in southern California, the film tells the fictional story of high school student Todd Bowden (Renfro), who discovers a fugitive Nazi war criminal, Kurt Dussander (McKellen), living in his neighborhood under a pseudonym. Bowden, obsessed with Nazism and the Holocaust, persuades Dussander to share his stories, and their relationship stirs malice in each of them. Singer has called Apt Pupil "a study in cruelty", with Nazism serving as a vehicle to demonstrate the capacity of evil.

The film was released in the United States and Canada in October 1998 to mixed reviews and made under $9 million. The main actors won several minor awards for their performances.

During the $14 million production, a lawsuit was filed by several extras who alleged that they were instructed to strip naked during a shower scene. The lawsuit was dismissed due to insufficient evidence.

Argyll Robertson pupil

Argyll Robertson pupils (AR pupils or, colloquially, "prostitute's pupils") are bilateral small pupils that reduce in size on a near object (i.e., they accommodate), but do not constrict when exposed to bright light (i.e., they do not react to light). They are a highly specific sign of neurosyphilis; however, Argyll Robertson pupils may also be a sign of diabetic neuropathy. In general, pupils that accommodate but do not react are said to show light-near dissociation (i.e., it is the absence of a miotic reaction to light, both direct and consensual, with the preservation of a miotic reaction to near stimulus (accommodation/convergence).AR pupils are extremely uncommon in the developed world. There is continued interest in the underlying pathophysiology, but the scarcity of cases makes ongoing research difficult.

Ciliary ganglion

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It measures 1–2 millimeters in diameter and in humans contains approximately 2,500 neurons. The oculomotor nerve coming into the ganglion contains preganglionic axons from the Edinger-Westphal nucleus (a part of the brainstem) which form synapses with the ciliary neurons. The postganglionic axons run in the short ciliary nerves and innervate two eye muscles:

the sphincter pupillae constricts the pupil, a movement known as Miosis. The opposite, Mydriasis, is the dilation of the pupil.

the ciliaris contracts, releasing tension on the Zonular Fibers, making the lens more convex, also known as accommodation.Both of these muscles are involuntary – they are controlled by the autonomic nervous system.

It is one of four parasympathetic ganglia of the head and neck. (The others are the submandibular ganglion, pterygopalatine ganglion, and otic ganglion).


Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in similar problems as those caused by presbyopia, in which the lens has lost elasticity and can also no longer focus on close-by objects. Cycloplegia with accompanying mydriasis (dilation of pupil) is usually due to topical application of muscarinic antagonists such as atropine and cyclopentolate.

Belladonna alkaloids are used for testing the error of refraction and examination of eye.

Elementary school

Elementary school is a school for students in their first school years, where they get primary education before they enter secondary education. The exact ages vary by country. In the United States, elementary schools usually have 6 grades with pupils aged between 6 and 13 years old, but the age can be up to 10 or 14 years old as well. In Japan, the age of pupils in elementary school ranges from 6 to 12, after which the pupils enter junior high school.

Elementary school is usually only one part of compulsory education, especially in Western countries.

Eretrian school

The Eretrian school of philosophy was originally the School of Elis where it had been founded by Phaedo of Elis; it was later transferred to Eretria by his pupil Menedemus. It can be referred to as the Elian–Eretrian School, on the assumption that the views of the two schools were similar. It died out after the time of Menedemus (3rd century BC), and, consequently, very little is known about its tenets. Phaedo had been a pupil of Socrates, and Plato named a dialogue, Phaedo, in his honor, but it is not possible to infer his doctrines from the dialogue. Menedemus was a pupil of Stilpo at Megara before becoming a pupil of Phaedo; in later times, the views of his school were often linked with those of the Megarian school. Menedemus' friend and colleague in the Eretrian school was Asclepiades of Phlius.

Like the Megarians they seem to have believed in the individuality of "the Good," the denial of the plurality of virtue, and of any real difference existing between the Good and the True. Cicero tells us that they placed all good in the mind, and in that acuteness of mind by which the truth is discerned. They denied that truth could be inferred by negative categorical propositions, and would only allow positive ones, and of these only simple ones.

Eye examination

An eye examination is a series of tests performed by an ophthalmologist (medical doctor), optometrist, or orthoptist, optician (UK), assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes.

Health care professionals often recommend that all people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are asymptomatic.

Eye examinations may detect potentially treatable blinding eye diseases, ocular manifestations of systemic disease, or signs of tumours or other anomalies of the brain.

Ideally, the eye examination consists of an external examination, followed by specific tests for visual acuity, pupil function, extraocular muscle motility, visual fields, intraocular pressure and ophthalmoscopy through a dilated pupil.

A minimal eye examination consists of tests for visual acuity, pupil function, and extraocular muscle motility, as well as direct ophthalmoscopy through an undilated pupil.

Human eye

The human eye is an organ which reacts to light and pressure. As a sense organ, the mammalian eye allows vision. Human eyes help to provide a three dimensional, moving image, normally coloured in daylight. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can differentiate between about 10 million colors and is possibly capable of detecting a single photon.Similar to the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

Iris (anatomy)

In humans and most mammals and birds, the iris (plural: irides or irises) is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.

Marcus Gunn pupil

Relative afferent pupillary defect (RAPD) or Marcus Gunn pupil is a medical sign observed during the swinging-flashlight test whereupon the patient's pupils constrict less (therefore appearing to dilate) when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.

The most common cause of Marcus Gunn pupil is a lesion of the optic nerve (between the retina and the optic chiasm) or severe retinal disease. It is named after Scottish ophthalmologist Robert Marcus Gunn.

A second common cause of Marcus Gunn pupil is a contralateral optic tract lesion, due to the different contributions of the intact nasal and temporal hemifields.

Middle school

A middle school (also known as intermediate school or junior high school) is an educational stage which exists in some countries, providing education between primary school and secondary school. The concept, regulation and classification of middle schools, as well as the ages covered, vary between, and sometimes within, countries.


Miosis or myosis is excessive constriction of the pupil. The term is from Ancient Greek μύειν, mūein, "to close the eyes".

The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.


Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of drugs.

Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A mydriatic pupil will remain excessively large even in a bright environment. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis. More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation.

An informal term for mydriasis is blown pupil, and is used by medical providers. It is usually used to refer to a fixed, unilateral mydriasis, which could be a symptom of raised intracranial pressure.

The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological. Anisocoria is the condition of one pupil being more dilated than the other.


Ophthalmoscopy, also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part of a routine physical examination. It is crucial in determining the health of the retina, optic disc, and vitreous humor.

The pupil is a hole through which the eye's interior will be viewed. Opening the pupil wider (dilating it) is a simple and effective way to better see the structures behind it. Therefore, dilation of the pupil (mydriasis) is often accomplished with medicated eye drops before funduscopy. However, although dilated fundus examination is ideal, undilated examination is more convenient and is also helpful (albeit not as comprehensive), and it is the most common type in primary care.

An alternative or complement to ophthalmoscopy is to perform a fundus photography, where the image can be analysed later by a professional.

Primary school

A primary school (or elementary school in American English and often in Canadian English) is a school in which children receive primary or elementary education from the age of about five to eleven, coming after preschool, infant school and before secondary school. (In some countries there is an intermediate stage of middle school between primary and secondary education.)

Secondary school

A secondary school is both an organization that provides secondary education and the building where this takes place. Some secondary schools can provide both lower secondary education and upper secondary education (levels 2 and 3 of the ISCED scale), but these can also be provided in separate schools, as in the American middle school- high school system.

Secondary schools typically follow on from primary schools and lead into vocational and tertiary education. Attendance is compulsory in most countries for students between the ages of 11 and 16. The organisations, buildings, and terminology are more or less unique in each country.

Student–teacher ratio

Student–teacher ratio or student–faculty ratio is the number of students who attend a school or university divided by the number of teachers in the institution. For example, a student–teacher ratio of 10:1 indicates that there are 10 students for every one teacher. The term can also be reversed to create a teacher–student ratio.

The ratio is often used as a proxy for class size, although various factors can lead to class size varying independently of student–teacher ratio (and vice versa). In most cases, the student–teacher ratio will be significantly lower than the average class size.Student–teacher ratios vary widely among developed countries. In primary education, the average student–teacher ratio among members of the Organisation for Economic Co-operation and Development is just below 16, but ranges from 40 in Brazil to 28 in Mexico to 11 in Hungary and Luxembourg.

Anatomy of the globe of the human eye
Fibrous tunic (outer)
Uvea/vascular tunic (middle)
Retina (inner)
Anatomical regions of the eye
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