Eyes are organs of vision that detect light. Different kinds of light-sensitive organs are found in a variety of organisms. The simplest eyes do nothing but detect whether the surroundings are light or dark, while more complex eyes can distinguish shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth perception (binocular vision), as in humans; and others are placed so as to minimize the overlap, such as in rabbits and chameleons.

Varieties
In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells known as the retina at the rear of the eye, where the light is detected and converted into electrical signals. These are then 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. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.
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. Some arthropods, including many Strepsiptera, have compound eyes composed of a few facets each, with a retina capable of creating an image, which does provide multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing very wide-angle, high-resolution images.

Compound eye of Antarctic krill
Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system.[1] 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 varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
Some of the simplest eyes, called ocelli, can be found in animals like snails, who cannot actually "see" in the normal 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, 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 input to hunt and pounce on their prey. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image.

[edit] Evolution of eyes
Main article: Evolution of the eye

Diagram of major stages in the eye's evolution
The common origin (monophyly) of all animal eyes is established by shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye evolved some 540 million years ago.[2][3][4] The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an "arms race",[5] or rather, a phylogenetic radiation from the species with that first proto-eye, among the descendents of which, there may well have been an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.
Eyes in various animals show adaptation to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eyes in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry.
The earliest eyes, called "eyespots", were simple patches of photoreceptor cells, 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 lightsource.[6] This gradually changed as the eyespot depressed into a shallow "cup" shape, granting the ability to slightly discriminate directional brightness 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 slightly distinguishing dim shapes.[7]
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 specialize into a transparent humour that optimized color 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.[8]
The gap between tissue layers naturally formed a biconvex shape, an ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes.[8]

[edit] Anatomy of the mammalian eye

posterior compartment
ora serrata
ciliary muscle
ciliary zonules
canal of Schlemm
pupil
anterior chamber
cornea
iris
lens cortex
lens nucleus
ciliary process
conjuntiva
inferior oblique muscule
inferior rectus muscule
medial rectus muscle
retinal arteries and veins
optic disc
dura mater
central retinal artery
central retinal vein
optical nerve
vorticose vein
bulbar sheat
macula
fovea
sclera
choroid
superior rectus muscule
retina

[edit] Three layers
The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic.[9][10][11]
The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera.[12] The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape.[13]
The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized layer which includes the iris, ciliary body, and choroid.[12][14][15] The choroid contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye.
The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which includes the retina.[12][15] The retina contains the photosensitive rod and cone cells and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does have two points at which it is different; the fovea and optic disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading. The optic disc, sometimes referred to as the anatomical 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 whatsoever exist at this point, it is thus "blind". Squids and Octupi don't have this blind spot, however.

[edit] Anterior and posterior segments
The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment.[16]

[edit] Anterior segment
The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.[14][17] Within the anterior segment are two fluid-filled spaces: the anterior chamber and the posterior chamber. The anterior chamber is the space between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris, whereas the posterior chamber is between the iris and the front face of the vitreous.[14]
The cornea and lens help to converge light rays to focus onto the retina. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina. It is attached to the ciliary body via a ring of suspensory ligaments known as the Zonule of Zinn. To clearly see an object far away, the ciliary muscle is relaxed, which stretches the fibers connecting it with the lens, flattening the lens. When the ciliary muscle contracts, the tension of the fibers decrease (imagine that the distance between the tip of a triangle to its base, is less than the tip of the triangle to the other two tips.) which lets the lens bounce back a more convex and round shape. Humans gradually lose this flexibility with age, 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. The iris, between the lens and the first humour, is a pigmented ring of fibrovascular tissue and 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 makes sight difficult.
All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. Light enters the eye from an external medium such as air or water, passes through the cornea, and 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.

[edit] Posterior segment

Diagram of a human eye; note that not all eyes have the same anatomy as a human eye.
The posterior segment is the back two-thirds of the eye that includes the anterior hyaloid membrane and all structures behind it: the vitreous humor, retina, choroid, and optic nerve.[18] 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. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.

Light from a single point of a distant object and light from a single point of a near object being brought to a focus

[edit] Extraocular anatomy
In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets. This placement of the eyes helps to protect them from injury.
In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) 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—along with it the protective lipid layer—and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. This is made apparent when swimming in freshwater pools, as the osmotic gradient draws "pool water" into the corneal tissue (the pool water is hypotonic), causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye.
In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread tears 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 and below water. 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.
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.

[edit] Other articles regarding eye anatomy
Annulus of Zinn, Conjunctiva, Macula, Nictitating membrane, Schlemm's canal, Trabecular meshwork.

[edit] Cytology

This image clearly shows the pupil and iris of the human eye.
The structure of the mammalian eye owes itself completely to the task of focusing light 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.
The retina contains two forms of photosensitive cells important to vision—rods and cones. Though structurally and metabolically similar, their function is quite different. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions, however, they cannot detect color. 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). This is why the darker conditions become, the less color objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different wavelengths of light, which allows an organism to see color.
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" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing the individual to observe distant stars.
Rods and cones are both photosensitive, but respond 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 color-range. The process through which these proteins go is quite similar—upon being subjected to electromagnetic radiation of a particular wavelength and intensity, 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 hyperpolarization, this hyperpolarization of the cell leads to a release of transmitter molecules at the synapse.
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 by which information is relayed to the visual cortex. This is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. 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 hyperpolarize the connected bipolar cell. But because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapse of the bipolar cell to hyperpolarize it.
Furthermore, color 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 colors 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 red, green and blue are called primary colors and the colors obtained by mixing two of them, secondary colors. The secondary colors can be further complemented with primary colors to see tertiary colors