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Schematic diagram of the human eye en.svg
Schematic diagram of the vertebrate eye.
Compound eye of Antarctic krill

Eyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[1] Image-resolving eyes are present in cnidaria, molluscs, chordates, annelids and arthropods.[2]

The simplest "eyes", such as those in unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.



Eye of the wisent,
the European bison

Complex eyes can distinguish shapes and colors. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception; in other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision.

The first proto-eyes evolved among animals 600 million years ago, about the time of the Cambrian explosion.[3] 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 thirty-plus[4] main phyla.[1] In most vertebrates and some molluscs, 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. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals 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; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye,[5] and reducing aberrations when there is enough light.[6]

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.[7]

Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, 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, 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 of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.

Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system.[8] 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.

In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which 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. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents - in this way the bearers can spot hot springs and avoid being boiled alive.[9]


Evolution of the eye

The common origin (monophyly) of all animal eyes is now widely accepted as fact based on shared anatomical and 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 540 million years ago.[10][11][12] 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".[13] 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.

Eyes in various animals show adaption 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 eye 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.[14] 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.[15]

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 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.[16]

The gap between tissue layers naturally formed a bioconvex 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. 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.[16]

Types of eye

There are ten different eye layouts — indeed every way of capturing an image known to man, with the exceptions of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface.[1] Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behavior 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, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures.[1] Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomic. These two groups are not monophyletic; the cnidaira also possess cilliated cells, [17] and some annelids possess both.[18]

Normal eyes

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustacea and cubozoa.[19]

Pit eyes

Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light.[1] Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm.[1] 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.[1]

Spherical lensed eye

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.[1] The most basic form, still 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.[1] 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.[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.[1]

Heterogeneous eyes have evolved at least eight times: four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.[1] No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".[1]

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.[1]

The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. 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.[20] 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.[20] 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).[20]


One weakness of this eye construction is that chromatic aberration is still quite high[1], although for organisms without color vision, this is a very minor concern.

A weakness of the vertebrate eye is the blind spot at the optic disc where the optic nerve is formed at the back of the eye; there are no light sensitive rods or cones to respond to a light stimulus at this point. By contrast, the cephalopod eye has no blind spot as the retina is in the opposite orientation.

Multiple lenses

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's eyes have two lenses, arranged like those in a telescope.[1] Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.[1]

Refractive cornea

In the eyes of most terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained.[1] As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material, or by flattening the lens.[1] Flattening the lens has a disadvantage: the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead.[1]

As mentioned above, a refractive cornea is only useful out of water; in water, there is no difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures which have returned to the water — penguins and seals, for example — lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong cornea.[1]

Reflector eyes

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.[1] 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.[1]

Many small organisms such as rotifers, copeopods and platyhelminths use such organs, but these are too small to produce usable images.[1] Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.[1]

There is at least one vertebrate, the 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 the 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 crystals.[21]

Compound eyes

An image of a house fly compound eye surface by using Scanning Electron Microscope at X457 magnification
Arthropods such as this carpenter bee have compound eyes

A compound eye may consist of thousands of individual photoreceptor units. 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 of light.[22] Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained. This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head.

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.[23] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.[24]

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.[25]

Structure of the ommatidia of appositon compound eyes

Apposition eyes

Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum.[1] 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.[1] (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. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels.[1]

Superposition eyes

The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. 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 kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also has a transparent gap but uses corner mirrors instead of lenses.

Parabolic superposition

This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.[9]


The compound eye of a dragonfly

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a 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.

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 specialized retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye.

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 chitons.

Nutrients of the eye

The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes.[1] It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor.[2] 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 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. 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 fibers with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. If need be, if a human were to go 57 days without food or water, it is proven if eaten the vitreous humour has enough nutrients to maintain the body for that period of time. Amazingly, with so little solid matter, it tautly holds the eye. The lens, on the other hand, is tightly packed with cells.[1] However, the vitreous has a viscosity two to four times that of pure water, giving it a gelatinous consistency. It also has a refractive index of 1.336[2].

Relationship to life requirements

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 African 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.[1]:23-4 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.[26] 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.[27] 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.[26] In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods 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.[26] 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.[9]

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.[26] 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.[26]

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.[26]

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.[9]

Visual acuity

A hawk's eye

Visual acuity is often measured in cycles per 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 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 gray block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD[28] (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.[29] 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 region.

Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.[30] 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.

Perception of color

All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm[31]. 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.[32]

The most sensitive pigment, rhodopsin, has a peak response at 500 nm.[33] Small changes to the genes coding for this protein can tweak the peak response by a few nm;[2] pigments in the lens can also "filter" incoming light, changing the peak response.[2] Many organisms are unable to discriminate between colors, 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 cells, from which the more sensitive rod cells evolved.[33] Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioral tests can this be deduced.[2]

Most organisms with colour vision are able to 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.[33]:309

Rods and cones

The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones.

Rods cannot distinguish colors, 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 and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.

Cones are responsible for color vision. They require brighter light to function than rods require. 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 colors). The color seen is the combined effect of stimuli to, and 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 fibers of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain.[33]


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.[33]

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.[2]

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.[34] The eyes of vertebrates usually contain cilliary 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 ancestors used this pigment in vision, and that remnants survive in the eyes.[34] 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 resorbed into the brain on metamorphosis to the adult form.[34] 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.[34] Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group.[34] Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.[34]

See also



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  2. ^ a b c d e Frentiu, Francesca D.; Adriana D. Briscoe (2008). "A butterfly eye's view of birds". BioEssays 30 (11-12): 1151. doi:10.1002/bies.20828. PMID 18937365. 
  3. ^ Breitmeyer, Bruno (2010). Blindspots: The Many Ways We Cannot See. New York: Oxford University Press. p. 4. ISBN 9780195394269. 
  4. ^ The precise number depends on the author
  5. ^ Nairne, James (2005). Psychology. Belmont: Wadsworth Publishing. ISBN 049503150x. OCLC 61361417. 
  6. ^ Vicki Bruce, Patrick R. Green, and Mark A. Georgeson (1996). Visual Perception: Physiology, Psychology and Ecology. Psychology Press. pp. 20. ISBN 0863774504. 
  7. ^ BioMedia Associates Educational Biology Site: What animal has a more sophisticated eye, Octopus or Insect?
  8. ^ Who You Callin' "Shrimp"? – National Wildlife Magazine
  9. ^ a b c d Cronin, T. W.; Porter, M. L. (2008). "Exceptional Variation on a Common Theme: the Evolution of Crustacean Compound Eyes". Evolution Education and Outreach 1: 463–475. doi:10.1007/s12052-008-0085-0.  edit
  10. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "New perspectives on eye evolution." Curr. Opin. Genet. Dev. 5 (pp. 602 –609).
  11. ^ Halder, G., Callaerts, P. and Gehring, W.J. (1995). "Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila". Science 267 (pp. 1788–1792).
  12. ^ Tomarev, S.I., Callaerts, P., Kos, L., Zinovieva, R., Halder, G., Gehring, W., and Piatigorsky, J. (1997). "Squid Pax-6 and eye development." Proc. Natl. Acad. Sci. USA, 94 (pp. 2421–2426).
  13. ^ Conway-Morris, S. (1998). The Crucible of Creation. Oxford: Oxford University Press.
  14. ^ Land, M.F. and Fernald, Russell D. (1992). "The evolution of eyes." Annu Rev Neurosci 15 (pp. 1–29).
  15. ^ Eye-Evolution?
  16. ^ a b Fernald, Russell D. (2001). The Evolution of Eyes: Where Do Lenses Come From? Karger Gazette 64: "The Eye in Focus".
  17. ^ Kozmik, Zbynek; Ruzickova, Jana; Jonasova, Kristyna; Matsumoto, Yoshifumi; Vopalensky, Pavel; Kozmikova, Iryna; Strnad, Hynek; Kawamura, Shoji et al. (2008). "Assembly of the cnidarian camera-type eye from vertebrate-like components" (PDF). Proceedings of the National Academy of Sciences 105 (26): 8989–8993. doi:10.1073/pnas.0800388105. PMID 18577593. PMC 2449352. 
  18. ^ Fernald, Russell D. (September 2006). "Casting a Genetic Light on the Evolution of Eyes". Science 313 (5795): 1914–1918. doi:10.1126/science.1127889. PMID 17008522. 
  19. ^ "Vision Optics and Evolution". BioScience 39 (5): 298–307. 1 May 1989. doi:10.2307/1311112. ISSN 00063568.  edit
  20. ^ a b c Wilson, M. (1978). "The functional organisation of locust ocelli". Journal of Comparative Physiology (4): 297–316. 
  21. ^ Wagner, H.J., Douglas, R.H., Frank, T.M., Roberts, N.W., and Partridge, J.C. (Jan. 27, 2009). "A Novel Vertebrate Eye Using Both Refractive and Reflective Optics". Current Biology 19 (2): 108–114. doi:10.1016/j.cub.2008.11.061. PMID 19110427. 
  22. ^ Völkel, R; Eisner, M; Weible, K. J (June 2003). "Miniaturized imaging systems" (PDF). Microelectronic Engineering 67-68 (1): 461–472. doi:10.1016/S0167-9317(03)00102-3. 
  23. ^ Gaten, Edward (1998). "Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea)". Contributions to Zoology 67 (4): 223–236. 
  24. ^ Ritchie, Alexander (1985). "Ainiktozoon loganense Scourfield, a protochordate? from the Silurian of Scotland". Alcheringa 9: 137. doi:10.1080/03115518508618961. 
  25. ^ Mayer, G. (2006). "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?". Arthropod Structure and Development 35 (4): 231–245. doi:10.1016/j.asd.2006.06.003. PMID 18089073. 
  26. ^ a b c d e f Land, M. F. (1989). "The eyes of hyperiid amphipods: relations of optical structure to depth" (PDF). Journal of Comparative Physiology A: Sensory, Neural, and Behavioral Physiology 164 (6): 751–762. doi:10.1007/BF00616747. 
  27. ^ Zeil, J. (1996). "The variation of resolution and of ommatidial dimensions in the compound eyes of the fiddler crab Uca lactea annulipes (Ocypodidae, Brachyura, Decapoda)" (PDF). Journal of Experimental Biology 199 (7): 1569–1577. 
  28. ^ John C. Russ (2006). The Image Processing Handbook. CRC Press. ISBN 0849372542. OCLC 156223054. "The upper limit (finest detail) visible with the human eye is about 50 cycles per degree,… (Fifth Edition, 2007, Page 94)" 
  29. ^ Curtis D. Klaassen (2001). Casarett and Doull's Toxicology: The Basic Science of Poisons. McGraw-Hill Professional. ISBN 0071347216. OCLC 47965382. 
  30. ^ Robert E. Fischer; Biljana Tadic-Galeb. With contributions by Rick Plympton… (2000). Optical System Design. McGraw-Hill Professional. ISBN 0071349162. OCLC 247851267.,M1. 
  31. ^ Barlow, Horace Basil; Mollon, J. D (1982). The Senses. Cambridge: Univ. Pr.. pp. 98. ISBN 0521244749. 
  32. ^ Fernald, Russell D. (1997). "The Evolution of Eyes" (PDF). Brain, Behavior and Evolution 50 (4): 253–259. doi:10.1159/000113339. 
  33. ^ a b c d e Goldsmith, T. H. (1990). "Optimization, Constraint, and History in the Evolution of Eyes" (PDF). The Quarterly Review of Biology 65 (10000): 281. doi:10.1086/416840. PMID 2146698. 
  34. ^ a b c d e f Nilsson, E.; Arendt, D. (Dec 2008). "Eye Evolution: the Blurry Beginning". Current Biology 18 (23): R1096. doi:10.1016/j.cub.2008.10.025. ISSN 0960-9822. PMID 19081043.  edit


External links

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

Medical warning!
This article is from the 1911 Encyclopaedia Britannica. Medical science has made many leaps forward since it has been written. This is not a site for medical advice, when you need information on a medical condition, consult a professional instead.

EYE (0. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision.

Anatomy. - The eye consists of the eyeball, which is the true organ of sight, as well as of certain muscles which move it, and of the lachrymal apparatus which keeps the front of it in a moist condition. The eyeball is contained in the front of the orbit and is a sphere of about an inch (24 mm.) in diameter. From the front of this a segment of a lesser sphere projects slightly and forms the cornea (fig. 1, co). There are three coats Lens.

Vitreous body.

Zonule of Zinn, the ciliary process being removed to show it.

Canal of Petit.

Yellow spot.

The dotted line behind the cornea represents its posterior epithelium.

to the eyeball, an external (protective), a middle (vascular), and an internal (sensory). There are also three refracting media, the aqueous humour, the lens and the vitreous humour or body.

The protective coat consists of the sclerotic in the posterior five-sixths and the cornea in the anterior sixth. The sclerotic (fig. 1, Sc) is a firm fibrous coat, forming the " white of the eye," which posteriorly is pierced by the optic nerve and blends with the sheath of that nerve, while anteriorly it is continued into the cornea at the corneo-scleral junction. At this point a small canal, known as the canal of Schlemm, runs round the margin of the cornea in the substance of the sclerotic (see fig. 1). Between the sclerotic and the subjacent choroid coat is a lymph space traversed by some loose pigmented connective tissue, - the R 5c FIG. I. - Diagrammatic Section through the Eyeball.





Ciliary processes.

Ciliary muscle.

Optic nerve.


Iris. [humour. Anterior chamber of aqueous cj, co, Sc, ch, pc, mc, 0, R, I, aq, L, V, P, m, lamina fusca. The cornea is quite continuous with the sclerotic but has a greater convexity. Under the microscope it is seen to consist of five layers. Most anteriorly there is a layer of stratified epithelium, then an anterior elastic layer, then the substantia propria of the cornea which is fibrous with spaces in which the stellate corneal corpuscles lie, while behind this is the posterior elastic layer and then a delicate layer of endothelium. The transparency of the cornea is due to the fact that all these structures have the same refractive index.

The middle or vascular coat of the eye consists of the choroid, the ciliary processes and the iris. The choroid (fig. i, ch) does not come quite as far forward as the corneo-sclera] junction; it is composed of numerous blood-vessels and pigment cells bound together by connective tissue and, superficially, is lined by a delicate layer of pigmented connective tissue called the lamina suprachoroidea in contact with the already-mentioned perichoroidal lymph space. On the deep surface of the choroid is a structureless basal lamina.

The ciliary processes are some seventy triangular ridges, radially arranged, with their apices pointing backward (fig. i, pc), while their bases are level with the corneo-scleral junction. They are as vascular as the rest of the choroid, and contain in their interior the ciliary muscle, which consists of radiating and circular fibres. The radiating fibres (fig. r, mc) rise, close to the canal of Schlemm, from the margin of the posterior elastic lamina of the cornea, and pass backward and outward into the ciliary processes and anterior part of the choroid, which they pull forward when they contract. The circular fibres lie just internal to these and are few or wanting in short-sighted people.

The iris (fig. I, I) is the coloured diaphragm of the eye, the centre of which is pierced to form the pupil; it is composed of a connective tissue stroma containing blood-vessels, pigment cells and muscle fibres. In front of it is a reflection of the same layer of endothelium which lines the back of the cornea, while behind both it and the ciliary processes is a double layer of epithelium, deeply pigmented, which really belongs to the retina. The pigment in the substance of the iris is variously coloured in different individuals, and is often deposited after birth, so that, in newlyborn European children, the colour of the eyes is often slate-blue owing to the black pigment at the back of the iris showing through. White, yellow or reddish-brown pigment is deposited later in the substance of the iris, causing the appearance, with the black pigment behind, of grey, hazel or brown eyes. In blue-eyed people very little interstitial pigment is formed, while in Albinos the posterior pigment is also absent and the bloodvessels give the pink coloration. The muscle fibres of the iris are described as circular and radiating, though it is still uncertain whether the latter are really muscular rather than elastic. On to the front of the iris, at its margin, the posterior layer of the posterior elastic lamina is continued as a series of ridges called the ligamentum pectinatum iridis, while between these ridges are depressions known as the spaces of Fontana. The inner or sensory layer of the wall of the eyeball is the retina; it is a delicate transparent membrane which becomes thinner as the front of the eye is approached. A short distance behind the ciliary processes the nervous part of it stops and forms a scalloped border called the ora serrata, but the pigmented layer is continued on behind the ciliary processes and iris, as has been mentioned, and is known as the pars ciliaris retinae and pars iridica retinae. Under the microscope the posterior part of the retina is seen to consist of eight layers. In its passage from the lens and vitreous the light reaches these layers in the following order: - (r) Layer of nerve fibres; (2) Layer of ganglion cells; (3) Inner molecular layer; (4) Inner nuclear layer; (5) Outer molecular layer; (6) Outer nuclear layer; (7) Layer of rods and cones; (8) Pigmented layer.

The layer of nerve fibres (fig. 2, 2) is composed of the axis-cylinders only of the fibres of the optic nerve which pierce the sclerotic, choroid and all the succeeding layers of the retina to radiate over its surface.

The ganglionic layer (fig. 2, 3) consists of a single stratum of large ganglion cells, each of which is continuous with a fibre of the preceding layer which forms its axon. Each also gives off a number of finer processes (dendrites) which arborize in the next layer.

The inner molecular layer (fig. 2, 4) is formed by the interlacement of the dendrites of the last layer with those of the cells of the inner nuclear layer which comes next.

The inner nuclear layer (fig. 2, 5) contains three different kinds of cells, but the most important and numerous are large bipolar cells, which send one process into the inner molecular layer, as has just been mentioned, and the other into the outer molecular layer, where they arborize with the ends of the rod and cone fibres.

The outer molecular layer (fig. 2, 6) is very narrow and is formed by the arborizations just described. The outer nuclear layer (fig. 2, 7), like the inner, consists of oval cells, which are of two kinds. The rod granules are transversely striped, and are connected externally with the rods, while internally processes pass into the outer molecular layer to end in a knob around which the arborizations of the inner nuclear cells lie. The cone granules are situated more externally, and are in close contact with the cones; internally their processes form a foot-plate in the outer molecular layer from which arborizations extend.

The layer of rods and cones (fig. 2, g) contains these structures, the rods being more numerous than the cones. The rods are spindleshaped bodies, of which the inner segment is thicker than the outer. The cones are thicker and shorter than the rods, and resemble Indian clubs, the handles of which are directed outward and are transversely striped. In the outer part of the rods the visual purple or rhodopsin is found.

The pigmented layer consists of a single layer of hexagonal cells containing pigment, which is capable of moving towards the rods and cones when the eye is exposed to light and away from them in the dark.

Supporting the delicate nervous structures of the retina are a series of connective tissue rods known as the fibres of Muller (fig. 2, Ct); these run through the thickness of the retina at 9876 J 4 3 2 FIG. 2. - Diagrammatic section through the retina to show the several layers, which are numbered as in the text. Ct, The radial fibres of the supporting connective tissue.

right angles to its surface, and are joined together on the inner side of the layer of nerve fibres to form the inner limiting membrane. More externally, at the bases of the rods and cones, they unite again to form the outer limiting membrane.

When the retina is looked at with the naked eye from in front two small marks are seen on it. One of these is an oval depression about 3 mm. across, which, owing to the presence of pigment, is of a yellow colour and is known as the yellow spot (macula lutea); it is situated directly in the antero-posterior axis of the eyeball, and at its margin the nerve fibre layer is thinned and the ganglionic layer thickened. At its centre, however, both these layers are wanting, and in the layer of rods and cones only the cones are present. This central part is called the fovea centralis and is the point of acutest vision. The second mark is sit uated a little below and to the inner side of the yellow spot; it is a circular disk with raised margins and a depressed centre and is called the optic disk; in structure it is a complete contrast to the yellow spot, for all the layers except that of the nerve fibres are wanting, and consequently, as light cannot be appreciated here, it is known as the " blind spot." It marks the point of entry of the optic nerve, and at its centre the retinal artery appears and divides into branches. An appreciation of the condition of the optic disk is one of the chief objects of the ophthalmoscope.

The crystalline lens (fig. r, I,) with its ligament separates the aqueous from the vitreous chamber of the eye; it is a biconvex lens the posterior surface of which is more curved than the anterior. Radiating from the anterior and posterior poles are three faint lines forming a Y, the posterior Y being erect and the anterior inverted. Running from these figures are a series of lamellae, like the layers of an onion, each of which is made u p of a number of fibrils called the lens fibres. On the anterior surface of the lens is a layer of epithelial cells, which, towards the margin or equator, gradually elongate into lens fibres. The whole lens is enclosed in an elastic structureless membrane, and, like the .41Wi tti; (40)'"  ?A` 1 111. ...4 ., >. ': ' '1111....4.,>. --?fi? - ? ?r cornea, its transparency is due to the fact that all its constituents have the same refractive index.

The ligament of the lens is the thickened anterior part of the hyaloid membrane which surrounds the vitreous body; it is closely connected to the iris at the ora serrata, and then splits into two layers, of which the anterior is the thicker and blends with the anterior part of the elastic capsule of the lens, so that, when its attachment to the ora serrata is drawn forward by the ciliary muscle, the lens, by its own elasticity, increases its convexity. Between the anterior and posterior splitting of the hyaloid membrane is a circular lymph space surrounding the margin of the lens known as the canal of Petit (fig. i, p). The aqueous humour (fig. i, aq) is contained between the lens and its ligament posteriorly and the cornea anteriorly. It is practically a very weak solution of common salt (chloride of sodium 1.4%). The space containing it is imperfectly divided into a large anterior and a small posterior chamber by a perforated diaphragm - the iris.

The vitreous body or humour is a jelly which fills all the contents of the eyeball behind the lens. It is surrounded by the hyaloid membrane, already noticed, and anteriorly is concave for the reception of the lens.

From the centre of the optic disk to the posterior pole of the lens a lymph canal formed by a tube of the hyaloid membrane stretches through the centre of the vitreous body; this is the canal of Stilling, which in the embryo transmitted the hyaloid artery to the lens. The composition of the vitreous is practically the same as that of the aqueous humour.

The arteries of the eyeball are all derived from the ophthalmic branch of the internal carotid, and consist of the retinal which enters the optic nerve far back in the orbit, the two long ciliaries, which run forward in the choroid and join the anterior ciliaries, from muscular branches of the ophthalmic, in the circulus iridis major round the margin of the iris, and the six to twelve short ciliaries which pierce the sclerotic round the optic nerve and supply the choroid and ciliary processes.

The veins of the eyeball emerge as four or five trunks rather behind the equator; these are called from their appearance venae vorticosae, and open into the superior ophthalmic vein. In addition to these there is a retinal vein which accompanies its artery.

Accessory Structures of the Eye. - The eyelids are composed of the following structures from in front backward: (I) Skin; (2) Superficial fascia; (3) Orbicularis palpebrarum muscle; (4) Tarsal plates of fibrous tissue attached to the orbital margin by the superior and inferior palpebral ligaments, and, at the junction of the eyelids, by the external and internal tarsal ligaments of which the latter is also known as the tendo oculi; (5) Meibomian glands, which are large modified sebaceous glands lubricating the edges of the lids and preventing them adhering, and Glands of Moll, large sweat glands which, when inflamed, cause a " sty "; (6) the conjunctiva, a layer of mucous membrane which lines the back of the eyelids and is reflected on to the front of the globe, the reflection forming the fornix: on the front of the cornea the conjunctiva is continuous with the layer of epithelial cells already mentioned.

The lachrymal gland is found in the upper and outer part of the front of the orbit. It is about the size of an almond and has an upper (orbital) and a lower (palpebral) part. Its six to twelve ducts open on to the superior fornix of the conjunctiva.

The lachrymal canals (canaliculi) (see fig. 3, 2 and 3) are superior and inferior, and open by minute orifices (puncta) on to the free margins of the two eyelids near their inner point of junction. They collect the tears, secreted by the lachrymal gland, which thus pass right across the front of the eyeball, continually moistening the conjunctiva. The two ducts are bent round a small pink tubercle called the caruncula lachrymalis (fig. 3, 4) at the inner angle of the eyelids, and open into the lachrymal sac (fig. 3, 5), which lies in a groove in the lachrymal bone. The sac is continued down into the nasal duct (fig. 3, 6), which is about 4 inch long and opens into the inferior meatus of the nose, its opening being guarded by a valve.

The orbit contains seven muscles, six of which rise close to the optic foramen. The levator palpebrae superioris is the highest, and passes forward to the superior tarsal plate and fornix of the conjunctiva. The superior and inferior recti are inserted into the upper and lower sur faces of the eyeball respectively; they make the eye look inward as well as up or down. The external and internal recti are inserted into the sides of the eyeball and make it look outward or inward. The superior oblique runs forward to a pulley in the inner and front part of the roof of the orbit, round which it turns to be inserted into the outer FIG. 3. - Lachrymal Canals and Duct. and back part of the i, Orbicular muscle. 5, Lachrymal sac. eyeball. It turns the 2, Lachrymal canal. 6, Lachrymal duct. glance downward and 3, Punctum. 7, Angular artery. outward. The inferior 4, Caruncula.

oblique rises from the inner and front part of the floor of the orbit, and is also inserted into the outer and back part of the eyeball. It directs the glance upward and outward. Of all these muscles the superior oblique is supplied by the fourth cranial nerve, the external rectus by the sixth and the rest by the third.

The posterior part of the eyeball and the anterior parts of the muscles are enveloped in a lymph space, known as the capsule of Tenon, which assists their movements.

Embryology. - As is pointed out in the article Brain, the optic vesicles grow out from the fore-brain, and the part nearest the brain becomes constricted and elongated to form the optic stalk (see figs. 4 and 5, 0). At the same time the ectoderm covering the side of the head thickens and becomes invaginated to form the lens vesicle (see figs. 4 and 5, 6), which later loses its connexion with the surface and approaches the optic vesicle, causing that structure to become cupped for its reception, so that what was the optic vesicle becomes the optic cup and consists of an external and an internal layer of cells (fig. 6 (3 and 6). Of these the outer cells become the retinal pigment, while the inner form the other layers of the retina. The invagination of the optic cup extends, as the choroidal fissure (not shown in the FIG. 4. Diagram of Developing Eye (1st stage).

a, Forebrain.

0, Optic vesicle.

y, Superficial ectoderm. 5, Thickening for lens.

diagrams), along the lower and back part of the optic stalk, and into this slit sinks some of the surrounding mesoderm to form the vitreous body and the hyaloid arteries, one of which persists.' When this has happened the fissure closes up. The anterior epithelium of the lens vesicle remains, but from the posterior the lens fibres are developed and these gradually fill up the cavity. The superficial layer of head ectoderm, from which the lens has been invaginated and separated, becomes the anterior 1 Some embryologists regard the vitreous body as formed from the ectoderm (see Quain's Anatomy, vol. i., 1908).

FIG. 5.

Diagram of Developing Eye (2nd stage). 1 3, Optic cup.

(5, Invagination of lens. Other letters as in fig. 4.

epithelium of the cornea (fig. 6, E), and between it and the lens the mesoderm sinks in to form the cornea, iris and anterior chamber of the eye, while surrounding the optic cup the mesoderm forms the sclerotic and choroid coats (fig. 7, i and O. Up to the seventh month the pupil is closed by the membrana pupillaris, derived from the capsule of the lens which is part of the mesodermal ingrowth through the choroidal fissure already mentioned. The hyaloid artery remains, as a prolongation of the retinal artery to the lens, until just before birth, but after Diagram of Developing that its sheath forms the canal of Di Eye (3rd stage). Stilling. Most of the fibres of the d, Solid lens. optic nerve are centripetal and begin E, Corneal epithelium. as the axons of the ganglionic cells of Other letters as in the retina; a few, however, are centrifigs. 4 and 5.

fugal and come from the nerve cells in the brain.

The eyelids are developed as ectodermal folds, which blend with one another about the third month and separate again before birth in - Man (fig. 7, «). The lachrymal sac and duct are formed from solid ectodermal thickenings which later become canalized.

It will thus be seen that the optic nerve and retina are formed from the brain ectoderm; the lens, anterior epithelium of the cornea, skin of the eyelids, conjunctiva and lachrymal apparatus from the superficial ectoderm; while the sclerotic, choroid, vitreous and aqueous humours as well as the iris and cornea are derived from the mesoderm.

See Human Embryology, by C. S. Minot (New York); Quain's Anatomy, vol. i. (1908); " Entwickelung des Auges der Wirbeltiere," by A. Froriep, in Handbuch der vergleichenden and experimentellen Entwickelungslehre der Wirbeltiere (0. Hertwig, Jena, 1905).

Comparative Anatomy. - The Acrania, as represented by Amphioxus (the lancelet), have a patch of pigment in the fore part of the brain which is regarded as the remains of a degenerated eye. In the Cyclostomata the hag (Myxine) and larval lamprey (Ammocoetes) have ill-developed eyes lying beneath the skin and devoid of lens, iris, cornea and sclerotic as well as eye muscles. In the adult lamprey (Petromyzon) these structures are developed at the metamorphosis, and the skin becomes transparent, rendering sight possible. Ocular muscles are developed, but, unlike most vertebrates, the inferior rectus is supplied by the sixth nerve while all the others are supplied by the third. In all vertebrates the retina consists of a layer of senso-neural cells, the rods and cones, separated from the light by the other layers which together represent the optic ganglia of the invertebrates; in the latter animals, however, the senso-neural cells are nearer the light than the ganglia.

In fishes the eyeball is flattened in front, but the flat cornea is compensated by a spherical lens, which, unlike that of other vertebrates, is adapted for near vision when at rest. The iris in some bony fishes (Teleostei) is not contractile. In the Teleostei, too, there is a process of the choroid which projects into the vitreous chamber and runs forward to the lens; it is known as the processus falciformis, and, besides nourishing the lens, is concerned in accommodation. This specialized group of fishes is also remarkable for the possession of a so-called choroid gland, which is really a rete mirabile (see Arteries) between the choroid and sclerotic. The sclerotic in fishes is usually chondrified and sometimes calcified or ossified. In the retina the rods and cones are about equal in number, and the cones are very large. In the cartilaginous fishes (Elasmobranchs) there is a silvery layer, called the tapetum lucidum, on the retinal surface of the choroid.

In the Amphibia the cornea is more convex than in the fish, but the lens is circular and the sclerotic often chondrified. There is no processus falciformis or tapetum lucidum, but the class is interesting in that it shows the first rudiments of the ciliary muscle, although accommodation is brought about by shifting the lens. In the retina the rods outnumber the cones and these latter are smaller than in any other animals. In some Amphibians coloured oil globules are found in connexion with the cones, and sometimes two cones are joined, forming double or twin cones.

In Reptilia the eye is spherical and its anterior part is often protected by bony plates in the sclerotic (Lacertilia and Chelonia). The ciliary muscle is striated, and in most reptiles accommodation is effected by relaxing the ciliary ligament as in higher vertebrates, though in the snakes (Ophidia) the lens is shifted as it is in the lower forms. Many lizards have a vascular projection of the choroid into the vitreous, foreshadowing the pecten of birds and homologous with the processus falciformis of fishes. In the retina the rods are scarce or absent.

In birds the eye is tubular, especially in nocturnal and raptorial forms; this is due to a lengthening of the ciliary region, which is always protected by bony plates in the sclerotic. The pecten, already mentioned in lizards, is a pleated vascular projection from the optic disk towards the lens which in some cases it reaches. In Apteryx this structure disappears. In the retina the cones outnumber the rods, but are not as numerous as in the reptiles. The ciliary muscle is of the striped variety.

In the Mammalia the eye is largely enclosed in the orbit, and bony plates in the sclerotic are only found in the monotremes. The cornea is convex except in aquatic mammals, in which it is flattened. The lens is biconvex in diurnal mammals, but in nocturnal and aquatic it is spherical. There is no pecten, but the numerous hyaloid arteries which are found in the embryo represent it. The iris usually has a circular pupil, but in some ungulates and kangaroos it is a transverse slit. In the Cetacea this transverse opening is kidney-shaped, the hilum of the kidney being above. In many carnivores, especially nocturnal ones, the slit is vertical, and this form of opening seems adapted to a feeble light, for it is found in the owl, among birds. The tapetum lucidum is found in Ungulata, Cetacea and Carnivora. The ciliary muscle is unstriped. In the retina the rods are more numerous than the cones, while the macula lutea only appears in the Primates in connexion with binocular vision.

Among the accessory structures of the eye the retractor bulbi muscle is found in amphibians, reptiles, birds and many mammals; its nerve supply shows that it is probably a derivative of the external or posterior rectus. The nictitating membrane or third eyelid is well-developed in amphibians, reptiles, birds and some few sharks; it is less marked in mammals, and in Man is only represented by the little plica semilunaris. When functional it is drawn across the eye by special muscles derived from the retractor bulbi, called the bursalis and pyramidalis. In connexion with the nictitating membrane the Harderian gland is developed, while the lachrymal gland secretes fluid for the other eyelids to spread over the conjunctiva. These two glands are specialized parts of a row of glands which in the Urodela (tailed amphibians) are situated along the lower eyelid; the outer or posterior part of this row becomes the lachrymal gland, which in higher vertebrates shifts from the lower to the upper eyelid, while the inner or anterior part becomes the Harderian gland. Below the amphibians glands are not necessary, as the water keeps the eye moist.

The lachrymal duct first appears in the tailed amphibians; in snakes and gecko lizards, however, it opens into the mouth.

For literature up to 1900 see R. Wiedersheim's Vergleichende Anatomie der Wirbeltiere (Jena, 1902). Later literature is noticed in the catalogue of the Physiological Series of the R. College of Surgeons of England Museum, vol. iii. (London, 1906). (F. G. P.)

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also eye, and ẹyẹ


Proper noun




  1. (British, colloquial) the comedic magazine Private Eye.
  2. (British) The London Eye, a tourist attraction in London.


  • Anagrams of eey
  • yee

Bible wiki

Up to date as of January 23, 2010

From BibleWiki

(Heb. 'ain, meaning "flowing"), applied

(1) to a fountain, frequently;

(2) to colour (Num 11:7; R.V., "appearance," marg. "eye");

(3) the face (Ex 10:5; Num 22:5), in Num 14:14, "face to face" (R.V. marg., "eye to eye"). "Between the eyes", i.e., the forehead (Ex 13:9).

The expression (Prov 23:31), "when it giveth his colour in the cup," is literally, "when it giveth out [or showeth] its eye." The beads or bubbles of wine are thus spoken of.

"To set the eyes" on any one is to view him with favour (Gen 44:21; Job 24:23; Jer 39:12). This word is used figuratively in the expressions an "evil eye" (Mt 20:15), a "bountiful eye" (Prov 22:9), "haughty eyes" (Prov 6:17 marg.), "wanton eyes" (Isa 3:16), "eyes full of adultery" (2 Pet 2:14), "the lust of the eyes" (1Jn 2:16).

Christians are warned against "eye-service" (Eph 6:6; Col 3:22).

Men were sometimes punished by having their eyes put out (1Sam 11:2; Samson, Jdg 16:21; Zedekiah, 2Kg 25:7).

The custom of painting the eyes is alluded to in 2Kg 9:30, R.V.; Jer 4:30; Ezek 23:40, a custom which still prevails extensively among Eastern women.

This entry includes text from Easton's Bible Dictionary, 1897.

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Simple English

For the eye of a cyclone, see eye (cyclone).


File:Focus in an
Light from a single point of a distant object and light from a single point of a near object being brought to a focus.

The eye is an organ for sensing light. About 97 percent of animals have eyes.[1] Image-resolving eyes are present in cnidaria, molluscs, chordates, annelids and arthropods.[2][1]

The simplest 'eyes' are those found in unicellular organisms. They do nothing but detect if the surroundings are light or dark. Most animals have a biochemical 'clock' inside. These simple eye-spots are used to adjust this daily clock, which is called circadian rhythm. Some snails, for example, see no image (picture) at all, but they sense light, which helps them stay out of bright sunlight.

More complex eyes have not lost this function. A special type of cells in the eye senses light for a different purpose than seeing. These cells are called ganglion cells. They are located in the retina. They send their information about light to the brain along a different path (the retinohypothalamic tract). This information adjusts (synchronizes) the animal's circadian rhythm to nature's light/dark cycle of 24 hours. The system also works for some blind people who cannot see light at all.

Eyes that are a little bit better are shaped like cups, which lets the animal know where the light is coming from.

More complex eyes give the full sense of vision, including color, motion, and texture. These eyes have a round shape that makes light rays focus on the back part of the eye, called the retina.

In mammals, two kinds of cells, rods and cones, allow sight by sending signals through the optic nerve to the brain.

Some animals can see light that humans can not see. They can see ultraviolet or infrared light.

The lens on the front part of the eye is acts like a camera lens. It can be pulled flatter by muscles inside the eye, or allowed to become rounder. As some people get older, they may not be as able to do this perfectly. Many people are born with other small problems or get them later in life, and they may need eyeglasses (or contact lenses) to fix the problem.


Types of eye

Today, ten different types of eyes are known. Most ways of capturing an image have evolved at least once.

One way to categorize eyes is to look at the number of "chambers". Simple eyes are made of only one concave chamber, perhaps with a lens. Compound eyes have many such chambers with their lenses on a convex surface.[1]

Eyes also can be grouped according to how the photoreceptor is made. Photoreceptors are either cillated, or rhabdomic.[3] and some annelids possess both.[4]

Simple eyes

Pit eyes

Pit eyes are set in a depression in the skin. This reduces the angles at which light can enter. It allows the organism to say where the light is coming from.[1]

Such eyes can be found in about 85% of phyla. They probably came before the development of more complex eyes. Pit eyes are small. They are made of up to about hundred cells, covering about 100 µm.[1] The directionality can be improved by reducing the size of the opening, and by putting a reflective layer behind the receptor cells.[1]

Pinhole eye

The pinhole eye is an advanced form of pit eye. It has several improvements, most notably a small aperture and deep pit. Sometimes, the aperture can be changed. It is only found in the Nautilus.[1] Without a lens to focus the image, it produces a blurry image. Consequently, nautiloids can not discriminate between objects with a separation of less than 11°.[1] Shrinking the aperture would produce a sharper image, but let in less light.[1]

Spherical lensed eye

The resolution of pit eyes can be improved a lot by adding a material to make a lens. This will reduce the radius of the blurring, and increase the resolution that can be achieved.[1] The most basic form can still be seen in some gastropods and annelids. These eyes have a lens of one refractive index. It is possible to get a better image with materials that have a high refractive index which decreases towards the edges. This decreases the focal length and allows a sharp image to form on the retina.[1]

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.[1]

The ocelli of insects have a simple lens, but their focal point always lies behind the retina.They can never form a sharp image. This limits the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina. They are very good at responding to rapid changes in light intensity across the whole visual field — this fast response is accelerated even more by the large nerve bundles which rush the information to the brain.[5] Focusing the image would also cause the sun's image to be focused on a few receptors. These could possibly be damaged by the intense light; shielding the receptors would block out some light and reduce their sensitivity.[5]

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).[5]

Refractive cornea

The eyes of most land-living vertebrates (as well as those of some spiders, and [[insect larvae) contain a fluid that has a higher refractive index than the air. That way, the lens does not have to reduce the focal length, because this is done by the fluid. That way, the lens can adjust the focus more easily. That way, a very high resolution can be obtained.[1]

Reflector eyes

Instead of using a lens it is also possible to have cells inside the eye that act like mirrors. The image can then be reflected to focus at a central point. This design also means that someone looking into such an eye will see the same image as the organism which has them.[1]

Many small organisms such as rotifers, copeopods and platyhelminthes use such this design, but their eyes are too small to produce usable images.[1] Some larger organisms, such as scallops, also use reflector eyes. The scallop Pecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.[1]

Compound eyes

File:Carpenter bee head and compound
Arthropods such as this carpenter bee have compound eyes

Compound eyes are different from simple eyes. Instead of having one organ that can sense light, they put together many such organs. Some compound eyes have thousands of them. The resulting image is put together in the brain, based on the signals of the many eye units. Each such unit is called ommatidium, several are called ommatidia. The ommatidia are located on a convex surface, each of them points in a slighly different direction. Unlike simple eyes, compound eyes have a very large angle of view. They can detect fast movement, and sometimes the polarization of light.[6]

Compound eyes are common in arthropods, annelids, and some bivalved molluscs[7]


File:Dragonfly compound
The compound eye of a dragonfly

Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives sharp vision. In this zone the eyes are flattened and the facets are larger. The flattening allows more ommatidia to receive light from a spot. This gives a higher resolution.

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 chitons.


  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Land, M.F.; Fernald, R.D. (1992). [Expression error: Unexpected < operator "The evolution of eyes"]. Annual Review of Neuroscience 15: 1–29. doi:10.1146/ 
  2. Frentiu, Francesca D.; Adriana D. Briscoe (2008), [Expression error: Unexpected < operator "A butterfly eye's view of birds"], BioEssays 30: 1151, doi:10.1002/bies.20828 
  3. Kozmik, Zbynek; Ruzickova, Jana; Jonasova, Kristyna; Matsumoto, Yoshifumi; Vopalensky, Pavel; Kozmikova, Iryna; Strnad, Hynek; Kawamura, Shoji; Piatigorsky, Joram; Paces, Vaclav; Vlcek, Cestmir (2008), "Assembly of the cnidarian camera-type eye from vertebrate-like components" (PDF), Proceedings of the National Academy of Sciences 105 (26): 8989–8993, doi:10.1073/pnas.0800388105, PMID 18577593, 
  4. Fernald, Russell D. (September 2006), [Expression error: Unexpected < operator "Casting a genetic light on the evolution of eyes"], Science 313 (5795): 1914–1918, doi:10.1126/science.1127889, PMID 17008522 
  5. 5.0 5.1 5.2 Wilson, M. (1978), [Expression error: Unexpected < operator "The functional organisation of locust ocelli"], Journal of Comparative Physiology (4): 297–316 
  6. Völkel R.; Eisner M.; Weible K. J. (June 2003). "Miniaturized imaging systems" (PDF). Microelectronic Engineering 67-68 (1): 461–472. doi:10.1016/S0167-9317(03)00102-3. 
  7. Ritchie, Alexander (1985). [Expression error: Unexpected < operator "Ainiktozoon loganense Scourfield, a protochordate? from the Silurian of Scotland"]. Alcheringa 9: 137. 
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