Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.

The human eye is sometimes metaphorically called "the window to the soul."

An eye is an 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 vision. Many complex organisms including some mammals, birds, reptiles and fish have two eyes which may be placed on the same plane producing a single three-dimensional "image" (binocular vision), as in humans; or on different planes producing two separate "images" (monocular vision), as in rabbits and chameleons. Human eye cross-sectional view grayscale Copyright: public domain, credit to NIH National Eye Institute requested. ... Human eye cross-sectional view grayscale Copyright: public domain, credit to NIH National Eye Institute requested. ... Binomial name Homo sapiens Linnaeus, 1758 Subspecies Homo sapiens idaltu (extinct) Homo sapiens sapiens Human beings define themselves in biological, social, and spiritual terms. ... Anthere in a Cheche/Tagelmoust. ... Anthere in a Cheche/Tagelmoust. ... The soul according to many religious and philosophical traditions, is the ethereal substance — spirit (Hebrew:rooah or nefesh) — particular to a unique living being. ... In biology, an organ (Latin organum: instrument, tool) is a group of tissues, which perform a specific function or group of functions. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... Senses are the physiological methods of perception. ... Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. ... In biology and ecology, an organism (in Greek organon = instrument) is a living being. ... Orders Subclass Monotremata Monotremata Subclass Marsupialia Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Subclass Placentalia Xenarthra Dermoptera Desmostylia Scandentia Primates Rodentia Lagomorpha Insectivora Chiroptera Pholidota Carnivora Perissodactyla Artiodactyla Cetacea Afrosoricida Macroscelidea Tubulidentata Hyracoidea Proboscidea Sirenia The mammals are the class of vertebrate animals primarily characterized by the presence of mammary... Orders Many - see section below. ... Orders  Crocodylia - Crocodilians  Rhynchocephalia - Tuataras  Squamata   Suborder Sauria - Lizards   Suborder Serpentes - Snakes Testudines - Turtles Superorder Dinosauria  Saurischia  Ornithischia The reptiles are a group of vertebrate animals. ... Atlantic herring, Clupea harengus, the most abundant fish species in the world. ... 3-D or 3D abbreviates three dimensional and is often related to a stereoscopic display that exploits binocular vision. ... Binocular vision (also referred to as stereoscopic vision) is a type of visual system common in many kinds of animals where both the eyes produce only a single image in the brain. ... Binomial name Homo sapiens Linnaeus, 1758 Subspecies Homo sapiens idaltu (extinct) Homo sapiens sapiens Human beings define themselves in biological, social, and spiritual terms. ... Genera Pentalagus Bunolagus Nesolagus Romerolagus Brachylagus Sylvilagus Oryctolagus Poelagus Rabbits are small mammals in the family Leporidae, found in many parts of the world. ...

Varieties of eyes

Human eye

In most vertebrates and some mollusks the eye works by projecting images onto a light-sensitive retina, 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 transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the intensity of the light that enters the eye. Although they are quite similar in function and appearance once fully developed, vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells. Image:Menschliches auge. ... Image:Menschliches auge. ... Typical classes Petromyzontidae (lampreys) Placodermi - extinct Chondrichthyes (cartilaginous fish) Acanthodii - extinct Actinopterygii (ray-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Amphibia (amphibians) Reptilia (reptiles) Aves (birds) Mammalia (mammals) Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Human eye cross-sectional view. ... In the anatomy of animals, the brain, or encephalon, is the supervisory center of the nervous system. ... The optic nerve is the nerve that transmits visual information from the retina to the brain. ... In optics, transparency is the property of being transparent, or allowing light to pass. ... Vitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. ... The lens or crystalline lens is a component of the eye. ... The human iris The iris is the grey-brown area. ...

Compound eyes are found among the arthropods 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. Trilobites, 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. Compound eye of a dragonfly A compound eye is a visual organ found in certain arthropods (some insects and crustaceans). ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - Trilobites (extinct) Subphylum Chelicerata Arachnida - Spiders, Scorpions, etc. ... Orders Agnostida Redlichiida Corynexochida Lichida Nektaspida? Phacopida Proetida Asaphida Harpetida Ptychopariida Trilobites are extinct arthropods in the class Trilobita. ...

Some of the simplest eyes, called ocelli, can be found in animals like snails, who can't 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. Ocelli is one of the types of photoreceptor organs in animals. ... Giant African Snail (Achatina fulica) The name snail applies to most members of the molluscan class Gastropoda that have coiled shells. ... Photosensitivity is the amount to which an object reacts upon receiving photons of light. ...

Evolution of eyes

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: Charles Darwin, the father of modern evolutionary theory In the life sciences, evolution is a change in the traits of living organisms over generations, including the emergence of new species. ... The 1859 edition of On the Origin of Species First published in 1859, The Origin of Species (full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life) by British naturalist Charles Darwin is one of the pivotal...

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.

Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans and some, like diurnal 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). Orders Accipitriformes     Cathartidae     Pandionidae     Accipitridae     Sagittariidae Falconiformes     Falconidae A bird of prey or raptor is a bird that hunts its food, especially one that preys on mammals or other birds. ... 1. ... Orders Accipitriformes     Cathartidae     Pandionidae     Accipitridae     Sagittariidae Falconiformes     Falconidae A bird of prey or raptor is a bird that hunts its food, especially one that preys on mammals or other birds. ... In evolutionary biology, parallel evolution occurs when two independent species evolve together at the same time in the same ecospace and acquire similar characteristics. ...

Anatomy

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 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 impulses which travel to the brain. Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... Human eye cross-sectional view. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... The lens or crystalline lens is a component of the eye. ... Converge denotes Converge PL a programming language developed by Laurence Tratt Converge, a metalcore band from Massachusetts For the mathematical meaning of this term see Convergence. ... A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ... Photosensitivity is the amount to which an object reacts upon receiving photons of light. ... Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...

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 and radial muscles 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. The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... Convergence means approaching a definite value, as time goes on; or approaching a definite point, or a common view or opinion, or a fixed state of affairs. ... Corneal endothelium is the inner most layer of the cornea, the corneal endothelium is actually a monolayer of squamate epithelial cells lining the anterior chamber of the eye. ... Iris can mean: The sphincter around the pupil of the eye, see iris (anatomy) The equivalent device in a camera, see iris (camera) The messenger of the gods in Greek mythology, see Iris (mythology) The genus Iris of flowering plants, and their flowers: see iris (plant) The Institute for Research... The term pupil can also mean student. ... In mathematics, an object is convex if for any pair of points within the object, any point on the straight line segment that joins them is also within the object. ... Human eye cross-sectional view. ...

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 muscles will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more concave, 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 errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism. Redrawing of Image:Focus in an eye. ... Redrawing of Image:Focus in an eye. ... There are two sets of ciliary muscles in the eye. ... Contents // Source: http://www. ... Refraction error is an error in the focussing of light by the human eye. ... Normal vision for a achromatopsic colour-blind person. ... Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on... Aspherical cornea (top, exagerated) and normal eye below. ...

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 ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. Vitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. ... A lens is: a part of the eye an optical device that may be used in a camera or in a telescope; see lens (optics). ... There are two sets of ciliary muscles in the eye. ... Suspensory ligament describes any ligament that supports a body part, especially an organ. ...

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 vessels which supply the retinal cells with necessary oxygen and removes the waste products of 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 and cone cells, and neurons. The sclera is the white outer coating of the eye. ... Fibrin is a protein involved in the clotting of blood. ... Connective tissue is any type of biological tissue with an extensive extracellular matrix. ... The choroid is a collection of blood vessels in the rear of the eye that oxygenate the retina; it is located beneath the sclera. ... The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... General Name, Symbol, Number Oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16 (VIA), 2, p Density, Hardness 1. ... Cellular respiration is, in its broadest definition, the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. ... Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Cone cells, or cones, are cells in the retina which only function in relatively bright light. ...

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. 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. 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 which increases the amount of light each photosensitive cell perceives, which allows the animal to see better under low light conditions. The fovea, a part of the eye, is a spot located in the center of the macula. ... This article discusses the use of the term blind spot in anatomy. ... Color vision is a psychophysical phenomenon that exists only in our minds. ... Reading is an activity: Reading is an activity performed by a human. ... Human eye cross-sectional view. ...

Schematic diagram of the human eye.

• Aqueous humour
• Anterior chamber
• Blind spot
• Canal of Schlemm
• Ciliary muscle (or body)
• Cornea
• Conjunctiva
• Choroid
• Fovea
• Iris
• Lens
• Macula
• Optic disc
• Optic nerve
• Ora serrata
• Posterior chamber
• Pupil
• Rectus medialis
• Retina
• Sclera
• Suspensory ligament
• Tapetum lucidum (not in humans)
• Vitreous humour
• Zonular fibers

(created with Sodipodi) Schematic diagram of the human eye Aqueous humour Anterior chamber Choroid Cornea Ciliary muscle Iris Lens Optic disc Fovea Optic nerve Posterior chamber Pupil Retina Sclera Suspensory ligament Vitreous humor Zonular fibers File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev... (created with Sodipodi) Schematic diagram of the human eye Aqueous humour Anterior chamber Choroid Cornea Ciliary muscle Iris Lens Optic disc Fovea Optic nerve Posterior chamber Pupil Retina Sclera Suspensory ligament Vitreous humor Zonular fibers File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev... The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... This article discusses the use of the term blind spot in anatomy. ... The ciliary muscle is a muscle that affects zonules in the eye (fibers that suspend the lens in position during accommodation), enabling changes in lens shape for light focusing. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... The conjunctiva is a membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. ... The choroid is a collection of blood vessels in the rear of the eye that oxygenate the retina; it is located beneath the sclera. ... The fovea, a part of the eye, is a spot located in the center of the macula. ... The human iris The iris is the grey-brown area. ... The lens or crystalline lens is a component of the eye. ... Human eye cross-sectional view. ... The optic disc is the point in the eye where the optic nerve enters the retina; it is not sensitive to light. ... The optic nerve is the nerve that transmits visual information from the retina to the brain. ... Human eye cross-sectional view. ... The term pupil can also mean student. ... Human eye cross-sectional view. ... The sclera is the white outer coating of the eye. ... Suspensory ligament describes any ligament that supports a body part, especially an organ. ... This cats tapeta lucida make its eyes flash green in a photograph The tapetum lucidum (Latin: bright carpet) or tapetum is a reflecting layer behind the retina of the eye; it serves to reflect light back to the retina, increasing the quantity of light caught by the retina. ... Vitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. ...

Cytology

The retina contains two forms of photosensitive cells - rods and 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 (wavelengths) of light, which allows an organism to see colour. Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Cone cells, or cones, are cells in the retina which only function in relatively bright light. ... Visual acuity is the eyes ability to detect fine details and is the quantitative measure of the eyes ability to see an in-focus image at a certain distance. ... The wavelength is the distance between repeating units of a wave pattern. ...

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 astronomers, 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. An astronomer or astrophysicist is a scientist whose area of research is astronomy or astrophysics. ...

Rods and cones are both photosensitive, but differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. 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 channels 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). Photoreceptors are light-sensitive proteins involved in the function of photoreceptor cells. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Categories: Biochemistry stubs | G protein coupled receptors | Sensory receptors | Pigments ... Electromagnetic radiation or EM radiation is a combination (cross product) of oscillating electric and magnetic fields perpendicular to each other, moving through space as a wave, effectively transporting energy and momentum. ... A rhodopsin molecule in the cell membrane. ... The Retinenes (Retinene1 and Retinene2) are chemical derivatives of the dietary supplement vitamin A (see retinol) formed through oxidation reactions. ... Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed). ... The Retinenes (Retinene1 and Retinene2) are chemical derivatives of the dietary supplement vitamin A (see retinol) formed through oxidation reactions. ... Ion channels are present in the membranes that surround all biological cells. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that encapsulate the cell. ... Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...

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 transmitter molecules reach the synapse of the bipolar cell to attain the threshold level to generate an action potential. As a part of the retina, the bipolar cell exists between photoreceptors and ganglion cells. ... A ganglion cell (or sometimes called a gangliocyte) is a type of neuron located in the retina that receives visual information from photoreceptors via various intermediate cells such as bipolar cells, amacrine cells, and horizontal cells. ... The visual cortex is the part of the cerebral cortex that is responsible for processing visual stimuli. ... Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a presynaptic and a postsynaptic neuron. ... Synapses allow nerve cells to communicate with one another through axons and dendrites, converting electrical signals into chemical ones. ... Schematic of an electrophysiological recording of an action potential showing the various phases which occur as the wave passes a point on a cell membrane. ...

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 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 colours and the products of mixing two secondary colours. The secondary colours can be further complimented with primary colours to see tertiary colours. The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ... Yellow is the color of light whose wavelength is between 565 nm and 590 nm, or is a mixture of red and green light that appears to be the same color. ... This page is about the colors. ... A secondary color is a color made by mixing two primary colors in a given color space. ...

Peripherals of the eye

The eye is, for many animals (and humans), an important but delicate organ.

Eyesockets

In many animals, including humans, with binocular vision, the eyes are inset in the skull, in the eyesockets. This way, a stick or plate striking the head will not damage the eye, unless it breaks the skull.

Reflexes

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. Blinking the eyes is, of course, also a reflex.

Eyebrows

In humans, the eyebrows 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 osmotic differences between tear fluid and freshwater. Categories: Anatomy stubs | Human appearance ... Osmosis is the diffusion of a solvent through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration. ...

Eyelids

In many animals, including humans, eyelids 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. 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. An eyelid is a thin membrane of skin with the purpose of covering and protecting an eye. ... Infection is also the title of an episode of the television series Babylon 5; see Infection (Babylon 5). ... The immune system is the collection of organs and tissues involved in the adaptive defense of a body against foreign biological material. ...

Eyelashes

In many animals, including humans, eyelashes 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. A closed eye, displaying lashes The eye now open — lashes less visible An eyelash or simply lash is one of the hairs that grow at the edge of the eyelid. ...

Eye movement

This sections deals with the movement of projection eyes within their sockets, and with accommodation. 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. Compound eyes can't accommodate either.

Rapid eye movement is a term usually used to refer 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. Rapid eye movement (REM) is the stage of sleep during which the most vivid (though not all) dreams occur. ...

Saccades

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.

Microsaccades

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 will stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

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 organs of balance near the ears. For an alternative meaning, see ear (botany). ...

Smooth pursuit movement

The eyes can also follow a moving object around. This is less accurate than the vestibulo occular 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. In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. ...

Optokinetic reflex

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

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) . When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge. Image:Stereogram Tut Eye Convergence. ... Image:Stereogram Tut Eye Convergence. ... Convergence means approaching a definite value, as time goes on; or approaching a definite point, or a common view or opinion, or a fixed state of affairs. ... In vector calculus, the divergence is an operator that measures a vector fields tendency to originate from or converge upon a given point. ...

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

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

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.

Arabic scientists are some of the earliest to have written about and drawn the anatomy of the eye—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.

The pre-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. Hippocrates: a conventionalized image in a Roman portrait bust (19th century engraving) Hippocrates of Cos (c. ... Empiricism (greek εμπειρισμός, from empirical, latin experientia - the experience) is generally regarded as being at the heart of the modern scientific method, that our theories should be based on our observations of the world rather than on intuition or faith; that is, empirical research and a posteriori inductive reasoning rather... Aristotle (sculpture) Aristotle (Greek: Αριστοτέλης Aristotelēs) (384 BC – March 7, 322 BC) was an ancient Greek philosopher. ...

Alexandrian studies extensively contributed to knowledge of the eye. Aetius 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—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. Antiquity and modernity stand cheek-by-jowl in Egypts chief Mediterranean seaport Located on the Mediterranean Sea coast, Alexandria (in Arabic, الإسكندرية — al-Iskandariyah) is the chief seaport in Egypt, and that countrys second largest city, and the capital of the Al Iskandariyah governate. ... Flavius Aetius or simply Aetius, (circa 396 - 454), was a Roman general of the closing period of the Western Roman Empire. ... Herophilos, sometimes Latinized Herophilus (335-280 BC), was a Greek physician. ... link title Headline text Bold textItalic textItalic textInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text hereInsert non-formatted text here--220. ...

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 ducts. This article refers to the musical group, not the ancient Roman Publius Sulpicius Rufus or the French actor Rufus, or Rufus, from Kim Possible. ... The conjunctiva is a membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. ... Andreas Vesalius or Andreas Vesal, or Andras van Wesele (1514 - 1564) was a Belgian anatomist and the author of the first complete textbook on human anatomy: De Humanis Corporis Fabrica (On the workings of the Human Body) (Basel, 1543). ... There are two sets of ciliary muscles in the eye. ... Tears trickling down the cheeks Lacrimation is the bodys process of producing tears, which are a liquid to clean and lubricate the eyes. ...

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. Claudius Galenus of Pergamum (131-201 AD), better known as Galen, was an ancient Greek physician. ... There are three factors which may assist to varying degrees in determining whether someone is considered Arab or not: Political: whether they live in a country which is a member of the Arab League (or, more vaguely, the Arab world); this definition covers more than 300 million people. ...

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 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. There have been several people known by the name Fabricius: Fabricius (gens), a Roman gens Carel Fabricius (1622 - 1654), painter. ...

The seventeenth and eighteenth century saw the use of hand-lenses (by Malpighi), microscopes (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. A microscope (Greek: micron = small and scopos = aim) is an instrument for viewing objects that are too small to be seen by the naked or unaided eye. ... Events Abraham De Moivre states De Moivres theorem connecting trigonometric functions and complex numbers Publication of the first book of Bachs Well-Tempered Clavier Fall of Persias Safavid dynasty during a bloody revolt of the Afghani people. ... Anton von Leeuwenhoek Anton van Leeuwenhoek (October 24, 1632 - August 26, 1723) was a tradesman and scientist from Delft, in the Netherlands. ... 1834 was a common year starting on Wednesday (see link for calendar). ...

Eye related problems

• Achromotropsia aka Maskun — a low cone count or lack of function in cone cells
• Age-related macular degeneration — the photosensitive cells in the macula malfunction and over time cease to work
• Aniridia — a rare congenital eye condition leading to underdevelopment or even absence of the iris of the eye
• Amblyopia aka lazy eye — poor or blurry vision due to either no transmission or poor transmission of the visual image to the brain
• Anisometropia — the lenses of the two eyes have different focal lengths
• Arc eye aka snow blindness — a painful condition caused by exposure of unprotected eyes to bright light
• Astigmatism — the cornea or the lens of the eye is not perfectly spherical, resulting in different focal points in different planes.
• Blindness — the brain does not receive optical information, through various causes
• Cataracts — the lens becomes opaque
• Color blindness — the inability to perceive differences between some or all colors that other people can distinguish
• Conjunctivitis — inflammation of the conjunctiva (the outermost layer of the eye and the inner surface of the eyelids)
• Esotropia — the tendency for eyes to become cross-eyed
• Exotropia — the tendency for eyes to look outward
• Farsightedness aka Hyperopia —
• Floaters — shadow-like shapes which appear singly or together with several others in the field of vision
• Glaucoma — increased intraocular pressure causes damage to the optic nerve
• Hyperopia aka Farsightedness — the inability to focus on near objects, often caused by the eyeball being too short
• Keratoconus — the cornea thins and changes shape to be more like a cone than a parabola
• Maskun aka Achromotropsia — a low cone count or lack of function in cone cells
• Myopia — also known as nearsightedness. It is a refractive error in which the image focuses in front of the retina, leading to far away objects to become blurred
• Night blindness aka Nyctalopia
• Nyctalopia aka Nightblindness — a condition making it difficult or impossible to see in the dark
• Opthalmoplegia — the partial or total paralysis of the eye muscles
• Presbyopia — a condition that occurs with growing age and results in the inability of the human eye to focus on objects up close
• Retinal detachment Maskun aka the retina detaches from the choroid, leading to blurred and distorted vision
• Retinopathy — general term referring to non-inflammatory damage to the retina of the eye
• Scotoma aka Blind spot
• Snow blindness aka Arc eye — a painful condition caused by exposure of unprotected eyes to bright light
• Strabismus aka Crossed eye or Wandering eye — a disorder of the eyes involving a lack of coordination between the muscles of the eyes
• Uveitis — inflammatory process involving the interior of the eye.

Maskun is a medical condition (also called achromatopsia) characterized by a low cone count or lack of function in cone cells; these are the light receptors responsible for colour perception. ... This article or section should be merged with macular degeneration Treatment Those with AMD can sometimes benefit from the treatment tested in the Age-Related Eye Disease Study. ... Aniridia is a rare congenital eye condition leading to the absence of the iris of the eye. ... Amblyopia or lazy eye is a disorder of the eyes. ... Anisometropia is a condition in which the lenses of the two eyes have different focal lengths; that is, are in different states of myopia (nearsightedness) and hyperopia (farsightedness). ... Arc eye is a painful condition sometimes experienced by welders who have failed to use adequate eye protection. ... Aspherical cornea (top, exagerated) and normal eye below. ... Blindness can be defined physiologically as the condition of lacking sight. ... Cataract is also used to mean a waterfall or where the flow of a river changes dramatically. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... Esotropia is a form of strabismus where one or both of the eyes turn inward (often called lazy eye). The most common type of esotropia occurs in approximately one to two percent of the population. ... Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on... Artists impression of appearance of some floaters, as seen against the sky Floaters, or muscae volitantes (Latin: flying flies), are entoptic phenomena characterized by shadow-like shapes which appear singly or together with several others in ones field of vision. ... Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on... Keratoconus, or cone eye, is an eye condition in which the cornea thins and changes shape to be more like a cone than a parabola. ... Maskun is a medical condition (also called achromatopsia) characterized by a low cone count or lack of function in cone cells; these are the light receptors responsible for colour perception. ... Normal vision for a achromatopsic colour-blind person. ... Nyctalopia (literally night blindness) is a condition making it difficult or impossible to see in the dark. ... Nyctalopia (literally night blindness) is a condition making it difficult or impossible to see in the dark. ... Opthalmoplegia (or ophthalmoplegia) is the partial or total paralysis of the eye muscles. ... Contents // Source: http://www. ... Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. ... Maskun is a medical condition (also called achromatopsia) characterized by a low cone count or lack of function in cone cells; these are the light receptors responsible for colour perception. ... Retinopathy is a general term that refers to some form of non-inflammatory damage to the retina of the eye. ... The word scotoma is derived from the Greek word for darkness. ... Snow blindness is a painful condition caused by exposure of unprotected eyes to sunlight reflected from snow. ... Arc eye is a painful condition sometimes experienced by welders who have failed to use adequate eye protection. ... This person is ugly Strabismus (sometimes called crossed or wandering eye) is a disorder of the eyes involving a lack of coordination between the muscles of the eyes. ... Uveitis specifically refers to inflammation of the middle layer of the eye, termed the uvea but in common usage may refer to any inflammatory process involving the interior of the eye. ...

See also

A staring eye

• Adaptation
• Accommodation
• Binocular vision
• Corrective lens
• Crystallin
• Evil eye
• Evolution of the Eye
• Eye color
• Eye contact
• Eye tracking
• Eyeglass prescription
• Macropsia
• Micropsia
• Nictating membrane
• Ocular tremor
• Optometry
• Ophthalmology
• Persistence of vision
• Snellen chart
• Staring contest
• Tears
• Visual acuity
• Visual perception

This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... The term adaptation when used within the context of the biology of the eye refers to its ability to adjust to various levels of darkness and light. ... The Accommodation Reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). ... Binocular vision (also referred to as stereoscopic vision) is a type of visual system common in many kinds of animals where both the eyes produce only a single image in the brain. ... A corrective lens is a prosthetic lens worn on or before the eye, used to treat myopia, hypermetropia, presbyopia and astigmatism. ... In biology, a crystallin is a water-soluble structural protein in the lens of the eye, which accounts for the transparency of the structure. ... The evil eye is a widely distributed element of folklore or superstition. ... Human beings have many variations in eye color. ... Eye contact is an intense nonverbal, visual connection made as one person gazes into the eyes of another. ... Eye tracking is a technique used in cognitive science, psychology, human-computer interaction (HCI), advertising, medical research, and other areas. ... This article describes the optics of an ordinary eyeglass prescription, which is used to correct small refractive errors in the optical system of the eye. ... Macropsia is a neurological condition affecting human perception, in which the world appears larger than normal, and the sufferer, smaller. ... Alice in Wonderland syndrome (AIWS), or micropsia, is a disorienting neurological condition which affects human visual perception. ... Many species of land animals have a nictitating membrane, sometimes (but incorrectly) spelled nicitating membrane, which can move across the eyeball to give the sensitive eye structures additional protection in particular circumstances. ... Ocular tremor is eye tremor that assists vision. ... Optometrists are primary care practitioners for vision and ocular health concerns. ... Ophthalmology is the branch of medicine which deals with the diseases of the eye and their treatment. ... According to the theory of persistence of vision, the perceptual processes of the brain or the retina of the human eye retains an image for a split second. ... Traditional Snellen chart. ... An illustration depicting a typical staring contest A staring contest is a competitive game in which two people stare into each others eyes and attempt to maintain eye contact for a longer period of time than their opponent. ... Tears trickling down the cheeks Lacrimation is the bodys process of producing tears, which are a liquid to clean and lubricate the eyes. ... Visual acuity is the eyes ability to detect fine details and is the quantitative measure of the eyes ability to see an in-focus image at a certain distance. ... Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. ...

External links

• Evolution of the Eye (http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html)
• eye Topics (http://www.eyetopics.com)
• Diagram of the eye (http://webvision.med.utah.edu/anatomy.html)
• Uveitis - German Description (http://www.medizin.de/gesundheit/deutsch/827.htm)
• - BBC Science and Nature – Birds (http://www.bbc.co.uk/nature/animals/birds/weeklyfeature/whodareswins)
• Why are arthropods so useful? (http://www.szgdocent.org/resource/ff/f-arth21.htm)

References

• "Anatomy (http://www.mrcophth.com/Historyofophthalmology/anatomy.htm)." History of Ophthalmology. Accessed on 23 April 2005.
• Kandel, Schwartz, Jessell (2000). Principles of Neural Science, 4/e. McGraw-Hill: Health Professions Division. ISBN 0-07-112000-9.

April 23 is the 113th day of the year in the Gregorian Calendar (114th in leap years). ... 2005 is a common year starting on Saturday of the Gregorian calendar. ...

(See also sense) A sensory system is a part of the nervous system that consists of sensory receptors, neural pathways, and those parts of the brain responsible for processing the information. ... The visual system is what allows us to see. ... The optic disc is the point in the eye where the optic nerve enters the retina; it is not sensitive to light. ... Human eye cross-sectional view. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... The human iris The iris is the grey-brown area. ... The term pupil can also mean student. ... The lens or crystalline lens is a component of the eye. ... Human eye cross-sectional view. ... The sclera is the white outer coating of the eye. ... The fovea, a part of the eye, is a spot located in the center of the macula. ... This article discusses the use of the term blind spot in anatomy. ... Vitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. ... The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... The choroid is a collection of blood vessels in the rear of the eye that oxygenate the retina; it is located beneath the sclera. ... There are two sets of ciliary muscles in the eye. ... The conjunctiva is a membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. ... This cats tapeta lucida make its eyes flash green in a photograph The tapetum lucidum (Latin: bright carpet) or tapetum is a reflecting layer behind the retina of the eye; it serves to reflect light back to the retina, increasing the quantity of light caught by the retina. ...