Eye: Difference between revisions

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{{Short description|Organ that detects light and converts it into electro-chemical impulses in neurons}}

: ''This article refers to the sight organ. See [[Eye (disambiguation)]] for other usages.''

{{About|the organ present in many organisms|the human organ specifically|Human eye|the pupil|Pupil|the region of a cyclone|Eye (cyclone)|other uses|Eye (disambiguation)}}

{{cleanup}}

{{Redirect-multi|3|Eyeball|Eyes|Ocular}}

{|align=right

{{pp-semi-indef|small=yes}}

|[[image:Human eye cross-sectional view grayscale.png|thumb|Diagram of a [[human]] eye. Note that not all eyes have the same anatomy as a human eye.]]

{{EngvarB|date=October 2016}}

|-

{{Infobox anatomy

|[[Image:Closeup of an blue-green human eye.jpeg|thumb|Closeup of an blue-green human eye]]

| Name = Eye

|-

| Latin = oculus

|[[Image:Timeyes.JPG|thumb|The human eyes are sometimes [[metaphor]]ically called "the windows to the [[soul]]."]]

| System = [[Nervous system|Nervous]]

|}

| Image = Krilleyekils.jpg

An '''eye''' is an [[organ (anatomy)|organ]] that detects [[light]]. Different kinds of light-sensitive organs are found in a variety of creatures. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the [[sense]] of [[visual perception|vision]]. Many complex [[organism]]s including some [[mammal]]s, [[bird]]s, [[reptile]]s and [[fish]] have two eyes which may be placed on the same plane to be interpreted as a single [[dimension|three-dimensional]] "image" ([[binocular vision]]), as in [[human]]s; or on different planes producing two separate "images" ([[monocular vision]]), such as in [[rabbit]]s and [[chameleon]]s.

| Caption = [[Compound eye]] of an [[Antarctic krill]]

| Image2 = Schematic diagram of the human eye en.svg

| Caption2 = Diagram of a [[human eye]]

}}

An '''eye''' is a [[sensory organ]] that allows an [[organism]] to perceive [[visual perception|visual]] information. It detects [[light]] and converts it into electro-chemical impulses in [[neurons]] (neurones). It is part of an organism's [[visual system]].

== Varieties of eyes ==

In higher organisms, the eye is a complex [[optics|optical]] system that collects light from the surrounding environment, regulates its intensity through a [[Iris (anatomy)|diaphragm]], [[Focus (optics)|focuses]] it through an adjustable assembly of [[Lens (anatomy)|lenses]] to form an [[image]], converts this image into a set of electrical signals, and transmits these signals to the [[brain]] through neural pathways that connect the eye via the [[optic nerve]] to the [[visual cortex]] and other areas of the brain.

[[Image:Menschliches_auge.jpg|thumb|200px|Human eye]]

Eyes with resolving power have come in ten fundamentally different forms, classified into [[compound eye]]s and non-compound eyes. Compound eyes are made up of multiple small visual units, and are common on [[insect]]s and [[crustacean]]s. Non-compound eyes have a single lens and focus light onto the retina to form a single image. This type of eye is common in mammals, including humans.

In most [[vertebrate]]s and some [[mollusk]]s the eye works by allowing light to enter it and project onto a light-sensitive panel of cells [[retina]] at the rear of the eye, where the light is detected and signals are transmitted to the [[brain]] via the [[optic nerve]]. Such eyes are typically roughly spherical, filled with a [[transparency (optics)|transparent]] gel-like substance called the [[vitreous humour]], with a focusing [[lens (vision)|lens]] and often an [[iris of the eye|iris]] which regulates the intensity of the light that enters the eye. The eyes of [[cephalopod]]s, [[fish]], [[amphibian]]s, and [[snake]]s usually have fixed lens shapes, and focusing vision is achieved by telescoping the eye (similar to how a [[camera]] focuses).

The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light.<ref name=Land1992>{{cite journal |first1=M.F. |last1=Land |first2=R.D. |last2=Fernald |year=1992 |doi=10.1146/annurev.ne.15.030192.000245 |title=The evolution of eyes |journal=[[Annual Review of Neuroscience]] |volume=15 |pages=1–29 |pmid=1575438|issn=0147-006X}}</ref>

Eyes of widely varied species may have evolved differently, but they tend to be similar in function and appearance once fully developed. [[Mollusk]]s' eyes seem to have evolved from different organs than vertebrate eyes, and may be an example of [[convergent evolution]]. Vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells. Vertebrate retina are layered with neurons in front of the [[photosensitive]] cells, mollusk retina are reverse with photosensitive cells in front of the neurons, this provides mollusk with no [[blind spot (anatomy)|blind spot]] and may provide clearer vision, but may also provide slower [[retinal]] recycling and thus slower vision refeshing and motion detection. Some cephalopods have no physical lense and use a [[pinhole lens]]. Some mollusks such as [[scallop]]s use [[concave]] mirror to focus light in conjunction with a lense.

Eyes enable several photo response functions that are independent of vision. In an organism that has more complex eyes, retinal [[photosensitive ganglion cell]]s send signals along the [[retinohypothalamic tract]] to the [[Suprachiasmatic nucleus|suprachiasmatic nuclei]] to effect circadian adjustment and to the [[pretectal area]] to control the [[pupillary light reflex]].

[[Compound eye]]s are found among the [[arthropod]]s and are composed of many simple facets which give a pixelated image (not multiple images as is often believed). Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360 degree field of vision. Compound eyes are very sensitive to motion. Some arthropodes (many [[Strepsiptera]]) have compound eye composed of a few facets each with a retina capable of creating an image, which does provide muliple image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain providing a very wide angle high resolution image.

==Overview==

[[Image:Krilleyekils.jpg|right|thumb|300px|[[Compound eye]] of [[Antarctic krill]]]]

[[File:Bison bonasus right eye close-up.jpg|thumb|upright|left|Eye of a [[European bison]]]]

[[File:Голубой глаз.jpg|thumb|[[Human eye]]]]

Complex eyes distinguish shapes and [[colour]]s. The [[Visual perception|visual]] fields of many organisms, especially predators, involve large areas of [[binocular vision]] for [[depth perception]]. In other organisms, particularly prey animals, eyes are located to maximise the field of view, such as in [[rabbit]]s and [[horse]]s, which have [[monocular vision]].

The first proto-eyes evolved among animals {{Ma|600}} about the time of the [[Cambrian explosion]].<ref>{{cite book |last=Breitmeyer|first=Bruno |title=Blindspots: The Many Ways We Cannot See |url=https://archive.org/details/blindspotsmanywa0000brei|url-access=registration|publisher=Oxford University Press |___location=New York |year=2010 |page=[https://archive.org/details/blindspotsmanywa0000brei/page/4 4] |isbn=978-0-19-539426-9}}</ref> The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the ~35{{efn|There is no universal consensus on the precise total number of phyla Animalia; the stated figure varies slightly from author to author.}} main [[Phylum|phyla]].<ref name=Land1992/> In most [[vertebrate]]s and some [[mollusc]]s, the eye allows light to enter and project onto a light-sensitive layer of [[cell (biology)|cells]] known as the [[retina]]. The [[cone cell]]s (for colour) and the [[rod cell]]s (for low-light contrasts) in the retina detect and convert light into neural signals which are transmitted to the [[brain]] via the [[optic nerve]] to produce vision. Such eyes are typically spheroid, filled with the [[transparency (optics)|transparent]] gel-like [[vitreous humour]], possess a focusing [[lens (anatomy)|lens]], and often an [[iris (anatomy)|iris]]. Muscles around the iris change the size of the [[pupil]], regulating the amount of light that enters the eye<ref>{{cite book | last=Nairne | first=James | title=Psychology | publisher=Wadsworth Publishing | ___location=Belmont | year=2005 | isbn=978-0-495-03150-5 | url=https://books.google.com/books?id=6MqkLT-Q0oUC&pg=PA146 | oclc=61361417 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104216/https://books.google.com/books?id=6MqkLT-Q0oUC&pg=PA146 | url-status=live }}</ref> and reducing aberrations when there is enough light.<ref>{{cite book | title=Visual Perception: Physiology, Psychology and Ecology | author1=Bruce, Vicki | author2=Green, Patrick R. | author3=Georgeson, Mark A. | publisher=Psychology Press | year=1996 | isbn=978-0-86377-450-8 | page=20 | url=https://books.google.com/books?id=ukvei0wge_8C&pg=PA20 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104216/https://books.google.com/books?id=ukvei0wge_8C&pg=PA20 | url-status=live }}</ref> The eyes of most [[cephalopod]]s, [[fish]], [[amphibian]]s and [[snake]]s have fixed lens shapes, and focusing is achieved by telescoping the lens in a similar manner to that of a [[camera]].<ref>{{cite web|last1=Kirk|first1=Molly|first2=David|last2=Denning|publisher=BioMedia Associates|date=2001|url=http://ebiomedia.com/gall/eyes/octopus-insect.html|title=What animal has a more sophisticated eye, Octopus or Insect?|archive-url=https://web.archive.org/web/20170226060919/http://www.ebiomedia.com/what-animal-has-a-more-sophisticated-eye-octopus-or-insect.html|archive-date=26 February 2017}}</ref>

[[Trilobite]]s, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varies, however: some trilobites had only one, and some had thousands of lenses in one eye.

The compound eyes of the [[arthropod]]s are composed of many simple facets which, depending on anatomical detail, may give either a single pixelated image or multiple images per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors arranged hexagonally, which can give a full 360° field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many [[Strepsiptera]], have compound eyes of only a few facets, each with a retina capable of creating an image. With each eye producing a different image, a fused, high-resolution image is produced in the brain.

Some of the simplest eyes, called [[ocellus|ocelli]], can be found in animals like [[snail]]s, who can not actually 'see' in the common sense. They do have [[photosensitive]] cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark (day and night), but no more. This enables snails to keep out of direct sunlight.

Jumping spiders have simple eyes that are so large, supported by an array of other smaller eyes, that they can get enough visual inputs to hunt and pounce on their prey. Some insect larvae like caterpillars have a different type of single eye ([[stemmata]]) which gives a rough image.

[[Image:Odontodactylus scyllarus eyes.jpg|thumb|upright=1.1|The eyes of a mantis shrimp (here ''[[Odontodactylus scyllarus]]'') are considered the most complex in the whole animal kingdom.]]

== Evolution of eyes ==

The [[mantis shrimp]] has the world's most complex colour vision system. It has detailed [[hyperspectral]] colour vision.<ref>{{cite web|url=http://www.nwf.org/news-and-magazines/national-wildlife/animals/archives/2005/who-you-callin-shrimp.aspx|title=Who You Callin' "Shrimp"?|work=National Wildlife Magazine|publisher=[[National Wildlife Federation]]|date=1 October 2010|access-date=3 April 2014|archive-date=9 August 2010|archive-url=https://web.archive.org/web/20100809032945/http://www.nwf.org/News-and-Magazines/National-Wildlife/Animals/Archives/2005/Who-You-Callin-Shrimp.aspx|url-status=live}}</ref>

How a complex structure like the projecting eye could have evolved is often said to be a difficult question for the theory of [[evolution]]. Darwin famously treated the subject of eye evolution in his [[Origin of Species]]:

:''To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and [[chromatic aberration]], could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.''

Despite the precision and complexity of the eye, computer models of eye evolution, developed by Dan-Erik Nilsson and Susanne Pelger, demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection.

[[Trilobite]]s, now extinct, had unique compound eyes. Clear [[calcite]] crystals formed the lenses of their eyes. They differ in this from most other arthropods, which have soft eyes. The number of lenses in such an eye varied widely; some trilobites had only one while others had thousands of lenses per eye.

Eyes in various animals show adaption to their requirements. For example, [[bird of prey|birds of prey]] have much greater visual acuity than humans and some, like [[diurnal]] [[bird of prey|birds of prey]], can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of [[parallel evolution]], suggesting that the development of eyes through evolution might not be so improbable as it might seem. However, the development of the eye is considered to be [[monophyletic]]; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago (Mya).

In contrast to compound eyes, simple eyes have a single lens. [[Jumping spider]]s have one pair of large simple eyes with a narrow [[field of view]], augmented by an array of smaller eyes for [[peripheral vision]]. Some insect [[larva]]e, like [[caterpillar]]s, have a type of simple eye ([[stemmata]]) which usually provides only a rough image, but (as in [[sawfly]] larvae) can possess resolving powers of 4 degrees of arc, be polarization-sensitive, and capable of increasing its absolute sensitivity at night by a factor of 1,000 or more.<ref name="Meyer-Rochow 1974">{{cite journal|last=Meyer-Rochow|first=V.B.|title=Structure and function of the larval eye of the sawfly larva ''Perga''|doi=10.1016/0022-1910(74)90087-0|pmid=4854430|journal=Journal of Insect Physiology|date=1974|volume=20|issue=8|pages=1565–1591}}</ref> [[ocellus|Ocelli]], some of the simplest eyes, are found in animals such as some of the [[snail]]s. They have [[photosensitive]] cells but no lens or other means of projecting an image onto those cells. They can distinguish between light and dark but no more, enabling them to avoid direct [[sunlight]]. In organisms dwelling near [[Hydrothermal vent|deep-sea vents]], compound eyes are adapted to see the [[Infrared|infra-red light]] produced by the hot vents, allowing the creatures to avoid being boiled alive.<ref name=Cronin2008/>

== Anatomy ==

==Types==

The structure of the mammalian eye owes itself completely to the task of focusing [[light]] onto the [[retina]]. All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. The [[cornea]] and [[lens (vision)|lens]] help to focus ([[converge]]) light rays onto the retina. This light causes [[chemical]] changes in the [[photosensitive]] cells of the retina, the products of which trigger [[nerve impulse]]s which travel to the brain.

There are ten different eye layouts. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. "Simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution—the physics of [[compound eyes]] prevents them from achieving a resolution better than 1°. Also, [[eye#Superposition eyes|superposition eyes]] can achieve greater sensitivity than [[apposition eye]]s, so are better suited to dark-dwelling creatures.<ref name=Land1992/>

Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being ciliated (as in the vertebrates) or [[rhabdomeric]]. These two groups are not monophyletic; the [[Cnidaria]] also possess ciliated cells,<ref name="Kozmik2008">{{Cite journal| last3=Jonasova | first1=Z.| last2=Ruzickova| last7=Strnad | first2=J. | first3=K.| last6=Kozmikova| last5=Vopalensky | first4=Y.| last4=Matsumoto | first5=P. | first6=I.| last9=Piatigorsky | first7=H. | first9=J. |display-authors=9 <!-- This article has exactly 11 authors -->| title=Assembly of the cnidarian camera-type eye from vertebrate-like components | last1=Kozmik | first8=S.| last8=Kawamura | first11=C.| format=Free full text | journal=Proceedings of the National Academy of Sciences of the United States of America| last10=Paces | volume=105| last11=Vlcek | issue=26 | pages=8989–8993 | date=2008 | pmid=18577593 | doi=10.1073/pnas.0800388105 | pmc=2449352 | first10=V. |bibcode=2008PNAS..105.8989K | doi-access=free}}</ref> and some [[Gastropoda|gastropods]]<ref name="Zhukov2006">{{cite journal | last1=Zhukov|first1=ZH|last2=Borisseko|first2=SL|last3=Zieger|first3=MV|last4=Vakoliuk|first4=IA|last5=Meyer-Rochow|first5=VB|title=The eye of the freshwater prosobranch gastropod Viviparus viviparus: ultrastructure, electrophysiology and behaviour|journal=Acta Zoologica|year=2006|volume=87|pages=13–24|doi=10.1111/j.1463-6395.2006.00216.x}}</ref> and [[annelid]]s possess both.<ref name="Fernald2006">{{Cite journal| author=Fernald, Russell D.| year=2006| title=Casting a Genetic Light on the Evolution of Eyes| journal=Science| volume=313| issue=5795| pages=1914–1918| doi=10.1126/science.1127889| pmid=17008522| bibcode=2006Sci...313.1914F| s2cid=84439732}}</ref>

Light enters the eye from an external medium such as air or water, passes through the [[cornea]], into the first of two humours, the [[aqueous humour]]. Most of the light refraction occurs at the [[cornea]] which has a fixed curvature. The first humour is a clear mass which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the [[convergence]] of light at the lens) and provides the [[corneal endothelium]] with nutrients. The [[iris]], between the lens and the first humour, is a coloured ring of muscle fibres. Light must first pass though the centre of the iris, the [[pupil]]. The size of the pupil is actively adjusted by the [[circular muscle|circular]] and [[radial muscle]]s to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina, too little light would be blinding. The lens, behind the iris, is a [[convex]], springy disk which focuses light, through the second humour, onto the [[retina]].

[[Image:Focus in an eye2.png|thumb|left|Light from a single point of a distant object and light from a single point of a near object being brought to a focus.]]

To clearly see an object far away, the circularly arranged [[ciliary body|ciliary muscles]] will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more convex, form.

As we age we gradually lose this ability to spring back, resulting in the inability to focus on nearby objects, which is known as [[presbyopia]]. There are other [[refraction error]]s arising from the shape of the cornea and lens, and from the length of the eyeball. These include [[myopia]], [[hyperopia]], and [[astigmatism]].

Some organisms have [[Photosensitivity|photosensitive]] cells that do nothing but detect whether the surroundings are light or [[darkness|dark]], which is sufficient for the [[Entrainment (chronobiology)|entrainment]] of [[circadian rhythm]]s. These are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image.<ref>{{cite web|title=Circadian Rhythms Fact Sheet|work=National Institute of General Medical Sciences (NIGMS) |url=http://www.nigms.nih.gov/Education/Pages/Factsheet_CircadianRhythms.aspx|access-date=3 June 2015|publisher=National Institute of General Medical Sciences|archive-date=13 March 2020|archive-url=https://web.archive.org/web/20200313000520/https://www.nigms.nih.gov/education/pages/factsheet_circadianrhythms.aspx|url-status=dead}}</ref>

On the other side of the lens is the second humour, the [[vitreous humour]], which is bounded on all sides: by the [[lens]], [[ciliary body]], [[suspensory ligament]]s and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens.

Every technological method of capturing an optical image that humans commonly use occurs in nature, with the exception of [[Zoom lens|zoom]] and [[Fresnel lens]]es.<ref name=Land1992/>

Wrapped around these tissues are three layers of tissue surrounding the vitreous humour. The outermost is the [[sclera]] which gives most of they eye its white colour. It consists of [[fibrin]] [[connective tissue]] and both protects the inner components of the eye and maintains its shape. On the inner side of the sclera is the [[choroid]] which contains [[blood vessel]]s which supply the retinal cells with necessary [[oxygen]] and removes the waste products of [[cellular respiration|respiration]]. The sclera and ciliary muscles contain blood vessels, the rest of the eye does not. The choroid gives the inner eye a dark colour, which prevents disruptive reflections within the eye. The inner most layer of the eye is the retina, containing of the photosensitive [[rod cell|rod]] and [[cone cell]]s, and neurons.

===Non-compound eyes===

To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does however have two points at which it is different; the [[fovea]] and [[blind spot (anatomy)|blind spot]]. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for [[colour vision]] in humans, and enables high acuity, such as is necessary in [[reading]].

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in [[vertebrate]]s, [[cephalopod]]s, [[annelid]]s, [[crustacean]]s and [[Cubozoa]].<ref>{{cite journal |author=Nilsson, Dan-E. |year=1989 |title=Vision optics and evolution |journal=[[BioScience]] |volume=39 |issue=5 |pages=298–307 |doi=10.2307/1311112|jstor=1311112 }}</ref>{{Failed verification|date=June 2016|reason="seven times" doesn't appear; arthropods not included}}

The blind spot is a point on the [[retina]] where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus "blind".

In some animals, the retina contains a reflective layer (the [[tapetum lucidum]]) which increases the amount of light each photosensitive cell perceives, which allows the animal to see better under low light conditions.

====Pit eyes====

[[Image:Schematic_diagram_of_the_human_eye.png|thumb|500px|Schematic diagram of the human eye.]]

Pit eyes, also known as [[Simple eyes in invertebrates#Stemmata|stemmata]], are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eyes". They are small, comprising up to about 100 cells covering about 100&nbsp;μm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.<ref name=Land1992/>

* [[Aqueous humour]]

* [[Anterior chamber]]

* [[Blind spot (anatomy)|Blind spot]]

* [[Canal of Schlemm]]

* [[Ciliary muscle]] (or body)

* [[Cornea]]

* [[Conjunctiva]]

* [[Choroid]]

* [[Optic fovea|Fovea]]

* [[Iris of the eye|Iris]]

* [[Lens (vision)|Lens]]

* [[Macula]]

* [[Optic disc]]

* [[Optic nerve]]

* [[Ora serrata]]

* [[Posterior chamber]]

* [[Pupil]]

* [[Retina]]

* [[Sclera]]

* [[Suspensory ligament]]

* [[Tapetum lucidum]] (not in humans)

* [[Trabecular meshwork]]

* [[Vitreous humour]]

* [[Zonular fibers]]

[[Crotalinae|Pit vipers]] have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates (see [[infrared sensing in snakes]]). However, pit organs are fitted with receptors rather different from photoreceptors, namely a specific [[transient receptor potential channel]] (TRP channels) called [[TRPV1]]. The main difference is that photoreceptors are [[G protein-coupled receptor|G-protein coupled receptors]] but TRP are [[ion channel]]s.

== Acuity ==

{{main|Visual acuity}}

Visual acuity can be measured with several different metrics.

====Spherical lens eye====

Cycles per [[degree (angle)|degree]] (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from [[Angular_resolution|angular resolution]]. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.73 millimeters at a distance of one meter. A horse can resolve about 14 CPD (0.62 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m).

The resolution of pit eyes can be greatly improved by incorporating a material with a higher [[refractive index]] to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing [[spherical aberration]]. Such a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii.<ref name=Land1992/>

So-called under-focused lens eyes, found in gastropods and polychaete worms, have eyes that are intermediate between lens-less cup eyes and real camera eyes. Also [[box jellyfish]] have eyes with a spherical lens, cornea and retina, but the vision is blurry.<ref>[https://books.google.com/books?id=vQgWDAAAQBAJ&dq=Under-focused+lens+eyes+intermediate+cup+box+jellyfish&pg=PA76 Animal Eyes]</ref><ref>[https://books.google.com/books?id=A26JAgAAQBAJ&dq=Box+jellyfish+rhopalium+cornea+lens+pigment+retina&pg=PA306 Perceiving in Depth, Volume 1: Basic Mechanisms]</ref>

A [[diopter]] is the unit of measure of [[focus]].

Heterogeneous eyes have evolved at least nine times: four or more times in [[Sensory organs of gastropods|gastropods]], once in the [[copepod]]s, once in the [[annelid]]s, once in the [[cephalopod]]s,<ref name=Land1992/> and once in the [[chiton]]s, which have [[aragonite]] lenses.<ref name="Speiser2011">{{Cite journal | last1=Speiser | first1=D.I. | last2=Eernisse | first2=D.J. | last3=Johnsen | first3=S.N. | doi=10.1016/j.cub.2011.03.033 | title=A Chiton Uses Aragonite Lenses to Form Images | journal=Current Biology | volume=21 | issue=8 | pages=665–670 | year=2011 | pmid= 21497091| s2cid=10261602 | doi-access=free | bibcode=2011CBio...21..665S }}</ref> No extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".<ref name=Land1992/>

== Dynamic range ==

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles.<ref name=Land1992/>

At any given instant, the retina can resolve a [[Contrast_ratio|contrast ratio]] of around 100:1 (about 6 1/2 [[F_stop|stops]]). As soon as your eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Hence, over time, a contrast ratio of about 1,000,000:1 (about 20 [[F_stop|stops]]) can be resolved.

The [[ocellus|ocelli]] of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can not form a sharp image. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).<ref name="Wilson1978">{{Cite journal |last=Wilson |first=M. |year=1978 |title=The functional organisation of locust ocelli |journal=Journal of Comparative Physiology |volume=124 |issue=4 |pages=297–316 |doi=10.1007/BF00661380 |s2cid=572458}}</ref>

== Cytology ==

====Multiple lenses====

The retina contains two forms of photosensitive cells - [[rod cell|rods]] and [[cone cell|cones]]. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, they do not distinguish between colours, and have low [[visual acuity]] (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colours ([[wavelength]]s) of light, which allows an organism to see colour.

Some marine organisms bear more than one lens; for instance the [[copepod]] ''[[Pontella]]'' has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, ''[[Copilia]]'', has two lenses in each eye, arranged like those in a telescope.<ref name=Land1992/> Such arrangements are rare and poorly understood, but represent an alternative construction.

Multiple lenses are seen in some hunters such as eagles and jumping spiders, which have a refractive cornea: these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.<ref name=Land1992/>

The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires looking at things. Its requirement for high intensity light does cause problems for [[astronomer]]s, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" where rods also exist, and where the light ''is'' sufficient to stimulate cells, allowing the individual to observe distant stars.

====Refractive cornea====

Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented [[photoreceptor]] [[protein]]s. Rod cells contain the protein [[rhodopsin]] and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to [[electromagnetic radiation]] of a particular wavelength and intensity (ie. a colour visible light) the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into [[opsin]] and [[retinal]]; iodopsin of cones breaks down into [[photopsin]] and [[retinal]]. The opsin in both opens [[ion channel]]s on the [[cell membrane]] which leads to the generation of an [[action potential]] (an impulse which will eventually get to the visual cortex in the brain).

[[File:Human eye, lateral view.jpg|thumb|A refractive cornea type eye of a human. The cornea is the clear domed part covering the [[Anterior chamber of eyeball|anterior chamber of the eye]].]]

In the [[Mammalian eye|eyes of most mammals]], [[Bird vision#Anatomy of the eye|birds]], reptiles, and most other terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air. In general, the lens is not spherical. Spherical lenses produce spherical aberration. In refractive corneas, the lens tissue is corrected with inhomogeneous lens material (see [[Luneburg lens]]), or with an aspheric shape. Flattening the lens has a disadvantage; the quality of vision is diminished away from the main line of focus. Thus, animals that have evolved with a wide field-of-view often have eyes that make use of an inhomogeneous lens.<ref name=Land1992/>

As mentioned above, a refractive cornea is only useful out of water. In water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.<ref name=Land1992/>

This is the reason why cones and rods enable organisms to see in dark and light conditions - each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, [[synaptic convergence]] means that several rod cells are connected to a single [[bipolar cell]], which then connects to a single [[ganglion cell]] and information is relayed to the [[visual cortex]]. Whereas, a single cone cell is connected to a single bipolar cell. Thus, action potentials from rods share neurons, where those from cones are given their own. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential. Because several "converge" onto a bipolar cell, enough [[neurotransmitter|transmitter molecule]]s reach the [[synapse]] of the bipolar cell to attain the [[threshold level]] to generate an [[action potential]].

[[File:Closed human eye, superior view.jpg|thumb|[[Eyelid|Eyelids]] and [[Eyelash|eyelashes]] are a unique characteristic of most mammalian eyes, both of which are evolutionary features to protect the eye.]]

A unique feature of most mammal eyes is the presence of [[Eyelid|eyelids]] which wipe the eye and spread [[tears]] across the cornea to prevent dehydration. These eyelids are also supplemented by the presence of [[Eyelash|eyelashes]], multiple rows of highly innervated and sensitive hairs which grow from the eyelid margins to protect the eye from fine particles and small irritants such as insects.

====Reflector eyes====

Furthermore, colour is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see [[RGB color model|red, green and blue]] light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents - resulting in different colours being seen. If, for example, the red and green cones are stimulated to the same extent, and no blue cones are stimulated, [[yellow]] is seen. For this reason we call red, green and blue [[primary colour]]s and the products of mixing two [[secondary colour]]s. The secondary colours can be further complimented with primary colours to see [[tertiary colour]]s.

An alternative to a lens is to line the inside of the eye with "mirrors", and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.<ref name=Land1992/>

Many small organisms such as [[rotifer]]s, copepods and [[flatworm]]s use such organs, but these are too small to produce usable images. Some larger organisms, such as [[scallop]]s, also use reflector eyes. The scallop ''[[Pecten (bivalve)|Pecten]]'' has up to 100 millimetre-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.<ref name=Land1992/>

== Peripherals of the eye ==

There is at least one vertebrate, the [[Brownsnout spookfish|spookfish]], whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of [[guanine]] [[crystal]]s.<ref name="wagner et al">{{cite journal |author1=Wagner, H.J. |author2=Douglas, R.H. |author3=Frank, T.M. |author4=Roberts, N.W. |author5=Partridge, J.C. |name-list-style=amp|title=A Novel Vertebrate Eye Using Both Refractive and Reflective Optics |journal=Current Biology |volume=19 |pages=108–114 |date=Jan 27, 2009 | pmid=19110427 | doi=10.1016/j.cub.2008.11.061 |issue=2

===The orbit===

|s2cid=18680315 |doi-access=free |bibcode=2009CBio...19..108W }}</ref>

In many species, the eyes are inset in the portion of the skull known as the [[orbit (anatomy)|orbit]]s or eyesockets. This placement of the eyes helps to protect them from injury.

===Compound Reflexes eyes===

{{main|Compound eye}}

Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. [[blink|Blinking]] the eyes is, of course, also a reflex.

{{further|Arthropod eye}}

[[File:FLY EYE.jpg|thumb|right|An image of a house fly compound eye surface by using [[scanning electron microscope]]]]

[[File:Insect compound eye diagram.svg|thumb|upright=0.9|Anatomy of the compound eye of an insect]]

[[File:Calliphora vomitoria Portrait.jpg|thumb|Arthropods such as this [[Calliphora vomitoria|blue bottle fly]] have compound eyes.]]

A compound eye may consist of thousands of individual photoreceptor units or ommatidia ([[ommatidium]], singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the [[Polarization (waves)|polarisation]] of light.<ref>{{cite journal

=== Eyebrows ===

|url=http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf

In humans, the [[eyebrow]]s redirect flowing substances (usually rainwater) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid, and its beneficial effects, and can damage the cornea, due to [[osmosis|osmotic]] differences between tear fluid and freshwater.

|doi=10.1016/S0167-9317(03)00102-3

=== Eyelids ===

|title=Miniaturized imaging systems

In many animals, including humans, [[eyelid]]s wipe the eye and prevent the eyes from dehydration. They spread tear fluid on the eyes, which contains substances which help fight [[bacterial infection]] as part of the [[immune system]].

|date=June 2003

Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above water and below it.

|journal=Microelectronic Engineering

=== Eyelashes ===

|volume=67–68

In many animals, including humans, [[eyelash]]es prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.

|issue=1

|pages=461–472

|author1=Völkel, R

|author2=Eisner, M

|author3=Weible, KJ

|url-status=usurped

|archive-url=https://web.archive.org/web/20081001225326/http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf

|archive-date=2008-10-01

}}</ref> Because the individual lenses are so small, the effects of [[diffraction]] impose a limit on the possible resolution that can be obtained (assuming that they do not function as [[phased array]]s). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require very large compound eyes, around {{convert|11|m}} in radius.<ref>{{cite journal|last=Land|first=Michael|title=Visual Acuity in Insects|journal=Annual Review of Entomology|year=1997|volume=42|pages=147–177|url=http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|access-date=27 May 2013|doi=10.1146/annurev.ento.42.1.147|pmid=15012311|url-status=dead|archive-url=https://web.archive.org/web/20041123010008/http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|archive-date=23 November 2004}}</ref>

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.<ref>{{cite journal

== Eye movement ==

| last=Gaten

{{main|Eye movements}}

| first=Edward

| title=Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea)

| year=1998

| journal=Contributions to Zoology

| volume=67

| issue=4

| pages=223–236

| doi=10.1163/18759866-06704001

| doi-access=free

}}</ref> Compound eyes are common in arthropods, annelids and some bivalved molluscs.<ref>{{Cite journal

| last=Ritchie

| first=Alexander

| title=''Ainiktozoon loganense'' Scourfield, a protochordate from the Silurian of Scotland

| year=1985

| journal=Alcheringa

| volume=9

| page=137

| doi=10.1080/03115518508618961

| issue=2

| bibcode=1985Alch....9..117R

}}</ref> Compound eyes in arthropods grow at their margins by the addition of new ommatidia.<ref name=Mayer2006>

{{Cite journal

| last=Mayer

| first=G.

| year=2006

| title=Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?

| journal=Arthropod Structure and Development

| volume=35

| issue=4

| pages=231–245

| doi=10.1016/j.asd.2006.06.003

| pmid=18089073

| bibcode=2006ArtSD..35..231M

}}</ref>

====Apposition eyes====

Apposition eyes are the most common form of eyes and are presumably the ancestral form of compound eyes. They are found in all [[arthropod]] groups, although they may have evolved more than once within this phylum. Some [[annelids]] and [[bivalves]] also have apposition eyes. They are also possessed by ''[[Limulus]]'', the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.<ref name=Land1992/> (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the [[ommatidium]].

Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body.

====Superposition eyes====

The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second (Westheimer and McKee, 1954). Thus, to be able to see while we are moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one [[hertz]], the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand.

The second type is named the superposition eye. The superposition eye is divided into three types:

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, we would see double. Before dealing with this problem, we shall discuss the movements of one eye alone, and restrict our discussion to primates (monkeys, apes and humans). The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

* refracting,

* reflecting and

* parabolic superposition

The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This type of compound eye, for which a minimal size exists below which effective superposition cannot occur,<ref name="Meyer-Rochow 2004">{{cite journal|last1=Meyer-Rochow|first1=VB|last2=Gal|first2=J|title=Dimensional limits for arthropod eyes with superposition optics|journal=Vision Research|date=2004|volume=44|issue=19|pages=2213–2223|doi=10.1016/j.visres.2004.04.009|pmid=15208008|doi-access=free}}</ref> is normally found in nocturnal insects, because it can create images up to 1000 times brighter than equivalent apposition eyes, though at the cost of reduced resolution.<ref>{{cite thesis|type=PhD |last=Greiner |first=Birgit |title=Adaptations for nocturnal vision in insect apposition eyes |publisher=Lund University |date=16 December 2005 |url=http://www4.lu.se/upload/GreinerThesis.pdf |access-date=13 November 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130209164014/http://www4.lu.se/upload/GreinerThesis.pdf |archive-date=9 February 2013 }}</ref> In the parabolic superposition compound eye type, seen in arthropods such as [[mayfly|mayflies]], the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied [[Decapoda|decapod crustaceans]] such as [[shrimp]], [[prawn]]s, [[crayfish]] and [[lobster]]s are alone in having reflecting superposition eyes, which also have a transparent gap but use corner [[mirror]]s instead of lenses.

==== Extraocular muscles ====

{{main|Extraocular muscles]]

When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single joint in the center of the eye.

====Parabolic superposition====

Four of the extraocular muscles have their origin in the back of the orbit in a fibrous ring called the zonule of Zinn. They then course forward through the orbit and insert onto the globe on its anterior half (i.e., in front of the eye's equator). These muscles are named after their straight paths, and are called the four rectus muscles, or four recti. They insert on the globe at 12, 3, 6, and 9 o'clock, and are called the superior, lateral, inferior and medial rectus muscles. (Note that lateral and medial are relative to the subject, with lateral toward the side and medial toward the midline, thus the medial rectus is the muscle closest to the nose). The names are often abbreviated, so we shall call then the SR, LR, MR, and IR muscles, respectively. The other two extraocular muscles follow more complicated paths. The superior oblique (SO) muscle originates at the back of the orbit and courses forward to a rigid pulley, called the trochlea, on the upper, nasal wall of the orbit. The muscle passes through the pulley, turning sharply across the orbit, and inserts on the lateral, posterior part of the globe. Thus, the SO goes backward for the last part of its path, and even though it goes over the top of the eye, it pulls it downward and lateralward. The last muscle is the inferior oblique (IO), which originates at the lower front of the nasal orbital wall, and passes under the LR to insert on the lateral, posterior part of the globe. Thus, the IO pulls the eye upward and lateralward.

This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.<ref name=Cronin2008/>

{{ref|Carpenter}}{{ref|WestheimerMcKee}}

=== Rapid eye movement =Other====

Another kind of compound eye, found in males of Order [[Strepsiptera]], employs a series of simple eyes—eyes having one opening that provides light for an entire image-forming retina. Several of these ''eyelets'' together form the strepsipteran compound eye, which is similar to the 'schizochroal' compound eyes of some [[trilobites]].<ref>{{Cite journal|last1=Horváth|first1=Gábor|last2=Clarkson|first2=Euan N.K.|year=1997|title=Survey of modern counterparts of schizochroal trilobite eyes: Structural and functional similarities and differences|journal=Historical Biology|volume=12|issue=3–4|doi=10.1080/08912969709386565|pages=229–263|bibcode=1997HBio...12..229H }}</ref> Because each eyelet is a simple eye, it produces an inverted image; those images are combined in the brain to form one unified image. Because the aperture of an eyelet is larger than the facets of a compound eye, this arrangement allows vision under low light levels.<ref name=Land1992/>

{{main|Rapid eye movement}}

Rapid eye movement typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Good fliers such as flies or honey bees, or prey-catching insects such as [[praying mantis]] or [[dragonfly|dragonflies]], have specialised zones of [[ommatidium|ommatidia]] organised into a [[Fovea centralis|fovea]] area which gives acute vision. In the acute zone, the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. The black spot that can be seen on the compound eyes of such insects, which always seems to look directly at the observer, is called a [[pseudopupil]]. This occurs because the [[ommatidia]] which one observes "head-on" (along their [[optical axis|optical axes]]) absorb the [[incident light]], while those to one side reflect it.<ref name="Zeil">{{cite journal |author1=Jochen Zeil |author2=Maha M. Al-Mutairi |year=1996 |title=Variations in the optical properties of the compound eyes of ''Uca lactea annulipes'' |journal=[[The Journal of Experimental Biology]] |volume=199 |issue=7 |pages=1569–1577 |doi=10.1242/jeb.199.7.1569 |url=http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |pmid=9319471 |access-date=2008-09-15 |archive-date=2009-02-25 |archive-url=https://web.archive.org/web/20090225084203/http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |url-status=live }}</ref>

=== Saccades ===

{{main|Saccade}}

Saccades are rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900°/s in adult humans.

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the [[mysid]] shrimp, ''Dioptromysis paucispinosa''. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

=== Microsaccades ===

{{main|Microsaccade}}

Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

Another version is a compound eye often referred to as "pseudofaceted", as seen in ''[[Scutigera]]''.<ref name="Müller 2003">{{cite journal|last1=Müller|first1=CHG|last2=Rosenberg|first2=J|last3=Richter|first3=S|last4=Meyer-Rochow|first4=VB|title=The compound eye of Scutigera coleoptrata (Linnaeus, 1758) (Chilopoda; Notostigmophora): an ultrastructural re-investigation that adds support to the Mandibulata concept|journal=Zoomorphology|date=2003|volume=122|issue=4|pages=191–209|doi=10.1007/s00435-003-0085-0|s2cid=6466405}}</ref> This type of eye consists of a cluster of numerous [[ommatidia]] on each side of the head, organised in a way that resembles a true compound eye.

=== Vestibulo-ocular reflex ===

{{main|Vestibulo-ocular reflex}}

Many animals can look at something while turning their heads. The eyes are automatically rotated to remain fixed on the object, directed by input from the [[organ of balance|organs of balance]] near the [[ear]]s.

The body of ''[[Ophiocoma wendtii]]'', a type of [[brittle star]], is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many [[chiton]]s. The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.<ref name="Ullrich-Luter2011">{{Cite journal | last1=Ullrich-Luter | first1=E.M. | last2=Dupont | first2=S. | last3=Arboleda | first3=E. | last4=Hausen | first4=H. | last5=Arnone | first5=M.I. | title=Unique system of photoreceptors in sea urchin tube feet | doi=10.1073/pnas.1018495108 | journal=Proceedings of the National Academy of Sciences | volume=108 | issue=20 | pages=8367–8372 | year=2011 | pmid= 21536888| pmc=3100952| bibcode=2011PNAS..108.8367U | doi-access=free }}</ref>

=== Smooth pursuit movement ===

The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply [[feedback]]. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

=== Optokinetic reflex =Nutrients====

The '''ciliary body''' is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).

=== Vergence movement ===

[[Image:Stereogram_Tut_Eye_Convergence.png|thumb|120px|The two eyes converge to point to the same object]]

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' ([[convergence]]), while for an object farther away they rotate 'away from eachother' ([[divergence]]). Exaggerated convergence is called ''cross eyed viewing'' (focussing on the nose for example) <!--, while exaggerated divergence is called [[?]] (which is a rare ability in humans-->. When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge.

The '''vitreous''' is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.<ref name=Ali&Klyne1985>{{harvnb|Ali|Klyne|1985|page=8}}</ref> It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the [[hyaluronic acid]]), no blood vessels, and 98–99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the [[mucopolysaccharide]] hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.

Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.

=== Accommodation reflex ===

{{main|Accommodation reflex}}

To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.

== Aging changes Evolution==

{{main|Evolution of the eye}}

As the eye ages certain changes occur that can be attributed to solely the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability ([[presbyopia]]). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older person requires extra lighting. {{ref|AgingEyeTimes}}

[[File:Diagram of eye evolution.svg|thumb|upright=1.5|Evolution of the eye]]

Photoreception is [[Phylogenetics|phylogenetically]] very old, with various theories of phylogenesis.<ref name=Autrum1979>{{Cite book|author=Autrum, H|editor=H. Autrum|chapter=Introduction|year=1979|title=Comparative Physiology and Evolution of Vision in Invertebrates- A: Invertebrate Photoreceptors|___location=New York|series=Handbook of Sensory Physiology|volume=VII/6A|pages=4, 8–9|publisher=Springer-Verlag|isbn=978-3-540-08837-0}}</ref> The common origin ([[monophyly]]) of all animal eyes is now widely accepted as fact. This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 650-600 million years ago,<ref>{{cite journal | doi=10.1016/0959-437X(95)80029-8 | last1=Halder | first1=G. | last2=Callaerts | first2=P. | last3=Gehring | first3=W.J. | year=1995 | title=New perspectives on eye evolution | journal=Curr. Opin. Genet. Dev. | volume=5 | issue=5| pages=602–609 | pmid=8664548 }}</ref><ref>{{cite journal | doi=10.1126/science.7892602 | last1=Halder | first1=G. | last2=Callaerts | first2=P. | last3=Gehring | first3=W.J. | year=1995 | title=Induction of ectopic eyes by targeted expression of the ''eyeless'' gene in ''Drosophila''| journal=Science | volume=267 | issue=5205| pages=1788–1792 | pmid=7892602 | bibcode=1995Sci...267.1788H}}</ref><ref>{{cite journal | doi=10.1073/pnas.94.6.2421 | last1=Tomarev | first1=S.I. | last2=Callaerts | first2=P. | last3=Kos | first3=L. | last4=Zinovieva | first4=R. | last5=Halder | first5=G. | last6=Gehring | first6=W. | last7=Piatigorsky | first7=J. | year=1997 | title=Squid Pax-6 and eye development | journal=Proc. Natl. Acad. Sci. USA | volume=94 | issue=6| pages=2421–2426 | pmid=9122210 | pmc=20103 | bibcode=1997PNAS...94.2421T| doi-access=free }}</ref> and the [[PAX6]] gene is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race"<ref>Conway-Morris, S. (1998). ''The Crucible of Creation''. Oxford: Oxford University Press</ref> among all species that did not flee the photopic environment. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel (except those of groups, such as the vertebrates, that were only forced into the photopic environment at a late stage).

With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis,and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities - visible as floaters gradually increase in number.

Eyes in various animals show adaptation to their requirements. For example, the eye of a [[bird of prey]] has much greater visual acuity than a [[human eye]], and in some cases can detect [[ultraviolet]] radiation. The different forms of eye in, for example, vertebrates and molluscs are examples of [[parallel evolution]], despite their distant common ancestry. Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging [[cephalopod eye]], but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes<ref name="Lamb"/> and different lens crystallins show.<ref>{{cite journal |author1=Staaislav I. Tomarev |author2=Rina D. Zinovieva |year=1988 |title=Squid major lens polypeptides are homologous to glutathione S-transferases subunits |journal=[[Nature (journal)|Nature]] |volume=336 |issue=6194 |pages=86–88 |doi=10.1038/336086a0 |pmid=3185725 |bibcode=1988Natur.336...86T|s2cid=4319229 }}</ref>

== History of ophthalmology ==

The eye, including its structure and mechanism, has fascinated scientists and the public in general since ancient times. The discovery of the eye went through two cycles of limiting speculation and freeing observation, which led to a dark age between Galen and Vesalius.

The very earliest "eyes", called eye-spots, were simple patches of [[photoreceptor protein]] in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.<ref name=Land1992/>

Arabic scientists are some of the earliest to have written about and drawn the anatomy of the eye&mdash;the earliest known diagram being in [[Hunain ibn Is-hâq]]'s ''[[Book of the Ten Treatises on the Eye]]''. Earlier manuscripts exist which refer to diagrams which are not known to have survived. Current knowledge of the Græco-Roman understanding of the eye is limited, as many manuscripts lacked diagrams. In fact, there are very few extant diagrams of the eye. Thus, it is not clear to which structures the texts refer, and what purpose they were thought to have.

Through gradual change, the eye-spots of species living in well-lit environments depressed into a shallow "cup" shape. The ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective [[pinhole camera]] that was capable of dimly distinguishing shapes.<ref name="ee">{{cite web |url=http://library.thinkquest.org/28030/eyeevo.htm |title=Eye-Evolution? |publisher=Library.thinkquest.org |access-date=2012-09-01 |url-status=dead |archive-url=https://web.archive.org/web/20120915061324/http://library.thinkquest.org/28030/eyeevo.htm |archive-date=2012-09-15 }}</ref> However, the ancestors of modern [[hagfish]], thought to be the protovertebrate,<ref name="Lamb">{{cite journal |author1=Trevor D. Lamb |author2=Shaun P. Collin |author3=Edward N. Pugh Jr. |year=2007 |title=Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup |journal=[[Nature Reviews Neuroscience]] |volume=8 |issue=12 |pages=960–976 |pmc=3143066 |doi=10.1038/nrn2283 |pmid=18026166}}</ref> were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes.

The pre-[[Hippocrates|Hippocratics]] largely based their anatomical conceptions of the eye on speculation, rather than [[empiricism]]. They recognised the sclera and transparent cornea running flushly as the outer coating of the eye, with an inner layer with pupil, and a fluid at the centre. It was believed, by [[Alcamaeon]] and others, that this fluid was the medium of vision and flowed from the eye to the brain via a tube. [[Aristotle]] advanced such ideas with empiricism. He dissected the eyes of animals, and discovering three layers (not two), found that the fluid was of a constant consistency with the lens forming (or congealing) after death, and the surrounding layers were seen to be juxtaposed. He, and his contemporaries, further put forth the existence of three tubes leading from the eye, not one. One tube from each eye met within the skull.

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialise into a transparent humour that optimised colour filtering, blocked harmful radiation, improved the eye's [[refractive index]], and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent [[crystallin]] protein.<ref name="lenses come from">Fernald, Russell D. (2001). [http://www.karger.com/gazette/64/fernald/art_1_4.htm The Evolution of Eyes: Where Do Lenses Come From?] {{webarchive|url=https://web.archive.org/web/20060319050210/http://www.karger.com/gazette/64/fernald/art_1_4.htm |date=2006-03-19 }} ''Karger Gazette'' 64: "The Eye in Focus".</ref>

[[Alexandria]]n studies extensively contributed to knowledge of the eye. [[Aëtius_Amidenus|Aëtius]] tells us that [[Herophilus]] dedicated an entire study to they eye which no longer exists. In fact, no manuscripts from the region and time are known to have survived, leading us to rely on [[Celsius]]' account&mdash;which is seen as a confused account written by a man who did not know the subject matter. From Celsius we know that the lens had been recognised,and they no longer saw a fluid flowing to the brain through some hollow fluid, but likely a continuation of layers of tissue into the brain. Celsius failed to recognise the retina's role, and did not think it was the tissue that continued into the brain.

The gap between tissue layers naturally formed a biconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the [[cornea]] and [[iris (anatomy)|iris]]. Separation of the forward layer again formed a humour, the [[aqueous humour]]. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes.<ref name="lenses come from"/>

[[Rufus]] recognised a more modern eye, with [[conjunctiva]], extending as a fourth epithelial layer over the eye. Rufus was the first to recognise a two chambered eye - with one chamber from cornea to lens (filled with water), the other from lens to retina (filled with a egg-white-like substance). Galen remedied some mistakes including the curvature of the cornea and lens, the nature of the optic nerve, and the existence of a posterior chamber. Though this model was roughly a correct but simplistic modern model of the eye, it contained errors. Yet it was not advanced upon again until after [[Vesalius]]. A [[ciliary body]] was then discovered and the sclera, retina, choroid and cornea were seen to meet at the same point. The two chambers were seen to hold the same fluid as well as the lens being attached to the choroid. Galen continued the notion of a central canal, though he dissected the optic nerve, and saw it was solid, He mistakenly counted seven optical muscles, one too many. He also knew of the [[tear duct]]s.

===Relationship to life requirements===

After [[Galen]] a period of speculation is again noted by [[Arab]] scientists - the lens modified Galen's model to place the lens in the middle of the eye, a notion which lasted until Versalius reversed the era of speculation. He, however, was not an ophthalmologist and taught of the eye to a more primitive notion than both that of Galen and Arabian scientists - the cornea was not seen as being of greater curvature and the posterior side of the lens wasn't seen to be larger.

Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the [[Africa]]n plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.<ref name=Land1992/>{{Rp|23–24}} Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia.<ref name=Land1989/> Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example, if the animal is on a slope.<ref name="Zeil"/>

Understanding of the eye had been so slow to develop because for a long time the lens was perceived to be the seat of vision, not a tool of vision. This mistake was corrected when [[Hieronymus Fabricius|Fabricius]] and his successors correctly placed the lens and developed the modern notion of the structure of the eye. They removed the idea of Galen's seventh muscle (the ''retractor bulbi'') and reinstated the correct curvatures of the lens and cornea, as well as stating the ciliary body as a connective structure between the lens and the choroid.

An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey.<ref name=Land1989>{{Cite journal

The seventeenth and eighteenth century saw the use of hand-lenses (by Malpighi), [[microscope]]s (van Leeuwenhoek), preparations for fixing the eye for study (Ruysch) and later the freezing of the eye (Petit). This allowed for detailed study of the eye and an advanced model. Some mistakes persisted such as why the pupil changed size (seen to be vessels of the iris filling with blood), the existence of the posterior chamber, and of course the nature of the retina. In [[1722]] [[Leeuwenhoek]] noted the existence of rods and cones though they were not properly discovered until [[Treviranus]] in [[1834]] by use of a microscope.

| author=Land, M.F.

| year=1989

| title=The eyes of hyperiid amphipods: relations of optical structure to depth

| journal=Journal of Comparative Physiology A

| volume=164

| issue=6

| pages=751–762

| doi=10.1007/BF00616747| s2cid=23819801

}}</ref> In deep water organisms, it may not be the centre of the eye that is enlarged. The [[hyperiid]] [[amphipod]]s are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.<ref name=Land1989/> In the giant Antarctic isopod [[Glyptonotus antarcticus|Glyptonotus]] a small ventral compound eye is physically completely separated from the much larger dorsal compound eye.<ref name="Meyer-Rochow1982">{{cite journal|last=Meyer-Rochow|first=Victor Benno|title=The divided eye of the isopod Glyptonotus antarcticus: effects of unilateral dark adaptation and temperature elevation|journal=Proceedings of the Royal Society of London|date=1982| volume=B 215|issue=1201|pages=433–450|bibcode=1982RSPSB.215..433M|doi=10.1098/rspb.1982.0052|s2cid=85297324}}</ref> Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.<ref name=Cronin2008/>

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.<ref name=Land1989/>

== Eye diseases and disorders ==

{{main|List of eye diseases and disorders}}

There are many diseases and disorders that may affect the eyes.

It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.<ref name=Land1989/>

== See also ==

*[[Accommodation (eye)|Accommodation]]

*[[Adaptation (eye)|Adaptation]]

*[[Binocular vision]]

*[[Corrective lens]]

*[[Crystallin]]

*[[Evil eye]]

*[[Evolution of the Eye]]

*[[Eye color]]

*[[Eye contact]]

*[[Eyelid]]

*[[Eye tracking]]

*[[Eyeglass prescription]]

*[[Macropsia]]

*[[Micropsia]]

*[[Nictating membrane]]

*[[Ocular tremor]]

*[[Ophthalmologist|Ophthalmology]]

*[[Optician]]

*[[Optometry]]

*[[Persistence of vision]]

*[[Snellen chart]]

*[[Staring contest]]

*[[Tears]]

*[[Visual acuity]]

*[[Visual perception]]

Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.<ref name=Cronin2008>{{cite journal | first1=T.W.| first2=M.L.| last2=Porter| title=Exceptional Variation on a Common Theme: the Evolution of Crustacean Compound Eyes | last1=Cronin | journal=Evolution: Education and Outreach | volume=1 | issue=4| pages=463–475 | year=2008 | doi=10.1007/s12052-008-0085-0 | doi-access=free }}</ref>

==External links==

*[http://www.djo.harvard.edu/ DJO | Digital Journal of Ophthalmology]

==Physiology==

*[http://www.afb.org/eyeconditions.asp Glossary of Eye Conditions]

*[http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html Evolution of the Eye]

===Visual acuity===

*[http://www.eyetopics.com eye Topics]

[[File:Hawk eye.jpg|thumb|The eye of a [[red-tailed hawk]]]]

*[http://webvision.med.utah.edu/anatomy.html Diagram of the eye]

*[http://www.medizin.de/gesundheit/deutsch/827.htm Uveitis - German Description]

[[Visual acuity]], or resolving power, is "the ability to distinguish fine detail" and is the property of [[cone cells]].<ref name=Ali&Klyne1985p28>{{harvnb|Ali|Klyne|1985|page=28}}</ref> It is often measured in ''cycles per [[degree (angle)|degree]]'' (CPD), which measures an [[angular resolution]], or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75&nbsp;cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye.

*[http://www.bbc.co.uk/nature/animals/birds/weeklyfeature/whodareswins - BBC Science and Nature &#8211; Birds]

*[http://www.szgdocent.org/resource/ff/f-arth21.htm Why are arthropods so useful?]

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD<ref>{{cite book | title=The Image Processing Handbook | author=Russ, John C. | publisher=CRC Press | year=2006 | isbn=978-0-8493-7254-4 | url=https://books.google.com/books?id=Vs2AM2cWl1AC&pg=PT110 | quote=The upper limit (finest detail) visible with the human eye is about 50 cycles per degree,... (Fifth Edition, 2007, Page 94) | oclc=156223054 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104217/https://books.google.com/books?id=Vs2AM2cWl1AC&pg=PT110 | url-status=live }}</ref> (1.2 [[arcminute]] per line pair, or a 0.35&nbsp;mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.<ref>{{cite book | title=Casarett and Doull's Toxicology: The Basic Science of Poisons | author=Klaassen, Curtis D. | publisher=McGraw-Hill Professional | year=2001 | isbn=978-0-07-134721-1 | url=https://books.google.com/books?id=G16riRjvmykC&pg=PA574 | oclc=47965382 | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104218/https://books.google.com/books?id=G16riRjvmykC&pg=PA574 | url-status=live }}</ref> A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central [[Fovea centralis|fovea]] region.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html|title=The Retina of the Human Eye|website=hyperphysics.phy-astr.gsu.edu|access-date=2015-06-03|archive-date=2015-05-04|archive-url=https://web.archive.org/web/20150504053926/http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html|url-status=live}}</ref>

*[http://webvision.med.utah.edu/ Webvision. The organisation of the retina and visual system.]

Spherical aberration limits the resolution of a 7&nbsp;mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3&nbsp;mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.<ref>{{cite book | title=Optical System Design | publisher=McGraw-Hill Professional | year=2000 | isbn=978-0-07-134916-1 | url=https://books.google.com/books?id=byx2Ne9cD1IC&pg=PA164 | author1=Fischer, Robert E. | author2=Tadic-Galeb, Biljana | author3=Plympton, Rick | oclc=247851267 | editor=Steve Chapman | access-date=2020-10-19 | archive-date=2023-01-17 | archive-url=https://web.archive.org/web/20230117104217/https://books.google.com/books?id=byx2Ne9cD1IC&pg=PA164 | url-status=live }}</ref> A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an [[optotype]], corresponds to 20/20 ([[normal vision]]) in humans.

However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia. Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than vertebrate eyes.<ref>{{cite journal|author=Barlow, H.B.|year=1952|url=http://jeb.biologists.org/content/29/4/667.full.pdf+html|title=The size of ommatidia in apposition eyes|journal=J Exp Biol|volume=29|pages=667–674|issue=4|doi=10.1242/jeb.29.4.667|bibcode=1952JExpB..29..667B |access-date=2012-01-01|archive-date=2016-08-31|archive-url=https://web.archive.org/web/20160831132021/http://jeb.biologists.org/content/29/4/667.full.pdf+html|url-status=live|url-access=subscription}}</ref>

===Colour perception===

{{main|Colour vision}}

"Colour vision is the faculty of the organism to distinguish lights of different spectral qualities."<ref name=Ali&Klyne1985p161>{{harvnb|Ali|Klyne|1985|page=161}}</ref> All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between wavelengths of 400 and 700&nbsp;nm.<ref name=Fernald1982>{{Cite book

| year=1982

| title=The Senses

| page=[https://archive.org/details/senses0000barl/page/98 98]

| isbn=978-0-521-24474-9

| url=https://archive.org/details/senses0000barl

| url-access=registration

| publisher=Cambridge University Press

| ___location=Cambridge

|author1=Barlow, Horace Basil |author2=Mollon, J.D.

}}</ref> This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.<ref name=Fernald1997>{{Cite journal

| author=Fernald, Russell D.

| year=1997

| title=The Evolution of Eyes

| journal=Brain, Behavior and Evolution

| volume=50

| issue=4

| pages=253–259

| doi=10.1159/000113339

| url=http://www.stanford.edu/group/fernaldlab/pubs/1997%20Fernald.pdf

| pmid=9310200

| access-date=2008-09-16

| archive-date=2012-11-20

| archive-url=https://web.archive.org/web/20121120200307/http://www.stanford.edu/group/fernaldlab/pubs/1997

| url-status=live

}}</ref>

The most sensitive pigment, [[rhodopsin]], has a peak response at 500&nbsp;nm.<ref name=Goldsmith1990/> Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also filter incoming light, changing the peak response.<ref name="Frentiu2008">{{Cite journal |author=Frentiu, Francesca D. |author2=Adriana D. Briscoe |year=2008 |title=A butterfly eye's view of birds |journal=BioEssays |volume=30 |issue=11–12 |pages=1151–1162 |doi=10.1002/bies.20828 |pmid=18937365 |s2cid=34409725}}</ref> Many organisms are unable to discriminate between colours, seeing instead in shades of grey; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of [[cone cell]]s, from which the more sensitive [[rod cell]]s evolved.<ref name=Goldsmith1990>{{Cite journal

| author=Goldsmith, T.H.

| year=1990

| title=Optimization, Constraint, and History in the Evolution of Eyes

| journal=[[The Quarterly Review of Biology]]

| volume=65

| issue=3

| pages=281–322

| doi=10.1086/416840

| jstor=2832368

| pmid=2146698| s2cid=24535762

}}</ref> Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced.<ref name=Frentiu2008/>

Most organisms with colour vision can detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light—this precludes the possibility of any UV light being detected, as it does not even reach the retina.<ref name=Goldsmith1990/>

===Rods and cones===

The retina contains two major types of light-sensitive [[photoreceptor cell]]s used for vision: the [[rod cell|rods]] and the [[cone cell|cones]].

Rods cannot distinguish colours, but are responsible for low-light ([[scotopic]]) monochrome ([[black-and-white]]) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher ([[photopic]]) intensities. Rods are distributed throughout the retina but there are none at the [[Fovea centralis|fovea]] and none at the [[Blind spot (vision)|blind spot]]. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for [[color vision|colour vision]]. They require brighter light to function than rods require. In humans, there are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colours). The colour seen is the combined effect of [[Stimulus (physiology)|stimuli]] to, and [[stimulus–response model|responses]] from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on the fovea, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the [[optic nerve]]. When rods and cones are stimulated by light, they connect through adjoining cells within the retina to send an electrical signal to the optic nerve fibres. The optic nerves send off impulses through these fibres to the brain.<ref name=Goldsmith1990/>

==Pigmentation==

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related—although problems of convergence do exist.<ref name=Goldsmith1990/>

[[Opsin|Opsins]] are the pigments involved in photoreception. Other pigments, such as [[melanin]], are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.<ref name=Frentiu2008/>

There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.<ref name=Nilsson2008>{{cite journal | pages=R1096–R1098 | issue=23 | volume=18 | date=Dec 2008 | doi=10.1016/j.cub.2008.10.025| pmid=19081043| journal=Current Biology| last1=Nilsson| last2=Arendt | title=Eye Evolution: the Blurry Beginning | first2=D. | first1=E. | s2cid=11554469 | doi-access=free | bibcode=2008CBio...18R1096N }}</ref> The eyes of vertebrates usually contain ciliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ''ganglion'' cells of vertebrates express r-opsins, suggesting that their [[Ancestor|ancestors]] used this pigment in vision, and that remnants survive in the eyes.<ref name=Nilsson2008/> Likewise, c-opsins have been found to be expressed in the ''brain'' of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently [[:wikt:resorb|resorbed]] into the brain on metamorphosis to the adult form.<ref name=Nilsson2008/> C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins.<ref name=Nilsson2008/> [[Cnidaria]], which are an outgroup to the taxa mentioned above, express c-opsins—but r-opsins are yet to be found in this group.<ref name=Nilsson2008/> Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.<ref name=Nilsson2008/>

==Additional images==

<gallery>

File:Three Main Layers of the Eye.png|The structures of the eye labelled

File:Three Internal chambers of the Eye.svg|Another view of the eye and the structures of the eye labelled

</gallery>

==See also==

* [[Accommodation (vertebrate eye)]] (focusing)

* [[Adaptation (eye)]] (night vision)

* [[Capsule of lens]]

* [[Emission theory (vision)]]

* [[Eye color]]

* [[Eye development]]

* [[Eye disease]]

* [[Eye injury]]

* [[Eye movement]]

* [[Lens (vertebrate anatomy)]]

* [[Nictitating membrane]]

* [[Ophthalmology]]

* [[Orbit (anatomy)]]

* [[Simple eye in invertebrates]]

* ''[[Tapetum lucidum]]''

* [[Tears]]

==Notes==

{{notelist}}

==References==

===Citations===

*{{Web reference | URL=http://www.mrcophth.com/Historyofophthalmology/anatomy.htm | title=Anatomy | work=History of Ophthalmology | date=23 April | year=2005}}

{{Reflist}}

*{{Book reference | Author=Kandel, Schwartz, Jessell | Title=Principles of Neural Science, 4/e | Publisher=McGraw-Hill: Health Professions Division | Year=2000 | ID=ISBN 0-07-112000-9}}

*[http://soma.npa.uiuc.edu/courses/bio303/Ch11b.html Internet lecture on eye types in animal kindom]

===Bibliography===

# {{note|Carpenter}} Roger H.S. Carpenter (1988); ''Movements of the Eyes (2nd ed.)''. Pion Ltd, London. ISBN: 0850861098.

* {{cite book |last1=Ali |first1=Mohamed Ather |last2=Klyne |first2=M.A. |title=Vision in Vertebrates |place=New York |publisher=[[Plenum Press]] |year=1985 |isbn=978-0-306-42065-8 }}

#{{note|WestheimerMcKee}} Westheimer Gerald, McKee Suzanne P (1975); "Visual acuity in the presence of retinal-image motion". Journal of the Optical Society of America 65(7), 847-50.

#{{note|AgingEyeTimes}} [http://www.agingeye.net/ AgingEye Times]

==Further reading==

* {{cite journal

|author=Yong, Ed

|author-link=Ed Yong

|date=14 January 2016

|title=Inside the Eye: Nature's Most Exquisite Creation

|journal=National Geographic

|url=http://ngm.nationalgeographic.com/2016/02/evolution-of-eyes-text

|archive-url=https://web.archive.org/web/20160114205934/http://ngm.nationalgeographic.com/2016/02/evolution-of-eyes-text

|url-status=dead

|archive-date=January 14, 2016

}}

==External links==

{{Commons category|Eyes}}

* [https://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html Evolution of the eye]

* [http://www.lensshopper.com/eye-anatomy.asp Anatomy of the eye – flash animated interactive.] ([[Adobe Flash]])

* [http://webvision.med.utah.edu/ Webvision. The organisation of the retina and visual system.] An in-depth treatment of retinal function, open to all but geared most towards graduate students.

* [http://www.berkeley.edu/news/media/releases/2001/03/28_wers1.html Eye strips images of all but bare essentials before sending visual information to the brain, UC Berkeley research shows]

{{Eye anatomy}}

{{Visual_system}}

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