Pupil

pupilspupillaryeye pupilanatomical pupilbiological pupileye's pupilpupil disorderspupillary aperturepupillary defectsPupillary diameter
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.wikipedia
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Iris (anatomy)

irisirisesirides
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.
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.

Cornea

cornealcorneal diseasecorneal diseases
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.
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber.

Pupillary light reflex

pupillary reflexconstrict in response to lightconstrict when exposed to bright light
This is known as the pupillary light reflex.
The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness.

Human eye

eyeeyeseyeball
The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.
The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop.

Aperture

aperture stopapertureslens aperture
In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop.
The biological pupil of the eye is its aperture in optics nomenclature; the iris is the diaphragm that serves as the aperture stop.

Iris sphincter muscle

sphincter pupillaepupillary sphincteriris constrictor muscle
The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae.
It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

Mydriasis

dilated pupilsmydriaticpupil dilation
Dilation of the pupil is mydriasis.
Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response.

Atropine

atropine sulfateatropine eye dropsAtropine methonitrate
Other drugs, such as atropine, LSD, MDMA, mescaline, psilocybin mushrooms, cocaine and amphetamines may cause pupil dilation.
Topical atropine is used as a cycloplegic, to temporarily paralyze the accommodation reflex, and as a mydriatic, to dilate the pupils.

Miosis

mioticconstricted pupilsmiotics
Another term for the constriction of the pupil is miosis.
Anisocoria is the condition of one pupil being more dilated than the other.

Hippus

Pupillary athetosis
At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus.
It is particularly noticeable when pupil function is tested with a light, but is independent of eye movements or changes in illumination.

Mongoose

HerpestidaemongoosesHerpestinae
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.
Mongooses, much like goats, have narrow, ovular pupils.

Viperidae

vipervipersviperids
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.
The great majority have vertically elliptical, or slit-shaped, pupils that can open wide to cover most of the eye or close almost completely, which helps them to see in a wide range of light levels.

Tropicamide

MydriacylMydriafairTropicacyl
Dilation can be caused by mydriatic substances such as an eye drop solution containing tropicamide.
Tropicamide is an antimuscarinic drug that produces short acting mydriasis (dilation of the pupil) and cycloplegia when applied as eye drops.

Edinger–Westphal nucleus

Edinger-Westphal nucleusEdinger-Westphal
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.
The Edinger–Westphal nucleus supplies preganglionic parasympathetic fibers to the eye, constricting the pupil, accommodating the lens, and convergence of the eyes.

Cuttlefish

Sepiidacuttle-fishcuttle fish
Some skates and rays have crescent shaped pupils, gecko pupils range from circular, to a slit, to a series of pinholes, and the cuttlefish pupil is a smoothly curving W shape.
Cuttlefish have large, W-shaped pupils, eight arms, and two tentacles furnished with denticulated suckers, with which they secure their prey.

Goat

goatsdomestic goatmilk
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. Goats, toads and octopus pupils tend to be horizontal and rectangular with rounded corners.
Goats have horizontal, slit-shaped pupils.

Entrance pupil

entranceentrance apertureentrance-pupil
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.
The entrance pupil of the human eye, which is not quite the same as the physical pupil, is typically about 4 mm in diameter.

Octopus

octopusesOctopodaoctopodes
Goats, toads and octopus pupils tend to be horizontal and rectangular with rounded corners.
The cornea is formed from a translucent epidermal layer and the slit-shaped pupil forms a hole in the iris and lies just behind.

Oculomotor nerve

oculomotorIIIcranial nerve III
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.
The oculomotor nerve also controls the constriction of the pupils and thickening of the lens of the eye.

Pupillary response

pupil dilationpupillary dilationdilation
Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.

Cat

domestic catcatsFelis catus
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.
The domestic cat has slit pupils, which allow it to focus bright light without chromatic aberration.

Apple of my eye

Apple of the family's eye
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.
Originally, the phrase was simply an idiom referring to the pupil of the eye.

Anisocoria

unequal pupil sizepermanently dilated pupilpermanently enlarged
Anisocoria is a condition characterized by an unequal size of the eyes' pupils.

Eye examination

eye exameye testexamination
A full 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.

Argyll Robertson pupil

Argyll Robertson phenomenonArgyll Robertson pupilsDr Argyll Robertson
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).