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Human eye decorated withcosmeticsLight from a single point of a distant object and light from a single point of a near object being brought to a focusThe eye of a hawk
Inmammals, two kinds of cells,rods andcones, allow sight by sending signals through theoptic nerve to the brain.
Some animals can see light that humans cannot see. They can seeultraviolet orinfrared light.
The lens on the front part of the acts like acamera lens. It can be pulled flatter bymuscles inside the eye, or allowed to become rounder. As some people get older, they may not be able to do this perfectly. Many people are born with other small problems or get them later in life.Eyeglasses (orcontact lenses) may fix the problem.
Like different cameras, different eyes have different abilities. They may have higher or lower resolution, the ability to detect small details. They may have different performance in low light;nocturnal animals can see better at night than daytime animals. They may have different ability to distinguish colours.
The human eye is composed of several different parts. These parts may or may not be the same in other animals. They are:
Cornea: the outermost, transparent layer that protects the iris and pupil.
Pupil: the black circle in the middle of the eye, through which light passes.
Iris: the colorful circle of the eye around the pupil. It can be brown, blue, green, etc. Its main function is to regulate the amount of light coming into the eye.
Sclera: the large, white field around the iris that keeps the eyeball in shape.
Lens: behind the cornea there is a transparent bioconvex lens of very short focal length.This lens is held in the centre of eye ball with the help of ciliary muscles.
Retina: has the cells which turn light into nerve impulses.
Optic nerve: the nerve that connects the eye to the brain. Optical information is taken to the rear of the brain for processing: seecerebral cortex.
Today, ten different types of eyes are known. Most ways of capturing an image haveevolved at least once.
One way to categorize eyes is to look at the number of "chambers".Simple eyes are made of only oneconcave chamber, perhaps with a lens.Compound eyes have many such chambers with their lenses on aconvexsurface.[1]
Eyes also can be grouped according to how the photoreceptor is made. Photoreceptors are either ciliated, orrhabdomic.[4] and some annelids possess both.[5]
Pit eyes are set in a depression in the skin. This reduces theangles at which light can enter. It allows theorganism to say where the light is coming from.[1]
Such eyes can be found in about 85% ofphyla. 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 100µm.[1] The directionality can be improved by reducing the size of the opening, and by putting areflective layer behind the receptor cells.[1]
The pinhole eye is an advanced form of pit eye. It has several bits, most notably a smallaperture and deep pit. Sometimes, the aperture can be changed. It is only found in theNautilus.[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]
The resolution of pit eyes can be improved a lot by adding a material to make alens. 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 somegastropods andannelids. These eyes have a lens of onerefractive index. It is possible to get a better image with materials that have a high refractive index which decreases towards the edges. This decreases thefocal 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.[6] 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.[6]
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).[6]
The eyes of most land-livingvertebrates (as well as those of somespiders, andinsectlarvae) contain afluid that has a higher refractive index than theair. Thecornea is sharply curved and refracts light towards the focus. The lens need not do all of the refracting. This lets the lens adjust the focus more easily, for much higher resolution.[1]
Instead of using a lens it is also possible to have cells inside the eye that act likemirrors. 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 asrotifers, copeopods andplatyhelminthes use such this design, but their eyes are too small to produce usable images.[1] Some larger organisms, such asscallops, also use reflector eyes. The scallopPecten has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.[1]
Arthropods such as thiscarpenter 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 thebrain, based on the signals of the many eye units. Each such unit is calledommatidium, several are calledommatidia. 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 thepolarization of light.[7]
Theevolution of eyes started with simplest light-sensitive patches inunicellular organisms. These eye-spots do nothing but detect if the surroundings are light ordark. Most animals have a biochemical 'clock' inside. These simple eye-spots are used to adjust this daily clock, which is calledcircadian rhythm. Somesnails, for example, see no image (picture) at all, but they sense light, which helps them stay out of bright sunlight.[9][10]
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 calledganglion cells. They are located in theretina. They send their information about light to thebrain along a different path (theretinohypothalamic tract). This information adjusts (synchronizes) the animal'scircadian rhythm to nature's light/darkcycle of 24 hours. The system also works for someblind people who cannot see light at all.
Eyes that are a little bit better are shaped likecups, which lets the animal know where the light is coming from.
More complex eyes give the fullsense ofvision, includingcolor,motion, andtexture. These eyes have a round shape that makes lightraysfocus on the back part of the eye, called theretina.
Good fliers like flies or honey bees, or prey-catching insects likepraying mantis ordragonflies, have specialized zones ofommatidia organized into afovea 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 ofOphiocoma wendtii, a type ofbrittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of manychitons.
↑Kozmik, Zbyneket al 2008 (2008), "Assembly of the cnidarian camera-type eye from vertebrate-like components",Proceedings of the National Academy of Sciences,105 (26):8989–8993,doi:10.1073/pnas.0800388105,PMC2449352,PMID18577593{{citation}}: CS1 maint: numeric names: authors list (link)
123Wilson M. 1978, "The functional organisation of locust ocelli",Journal of Comparative Physiology (4):297–316{{citation}}: CS1 maint: numeric names: authors list (link)
↑Ritchie, Alexander (1985). "Ainiktozoon loganense Scourfield, a protochordate? from the Silurian of Scotland".Alcheringa.9 (2): 137.doi:10.1080/03115518508618961.
↑Autrum H. Introduction in Autrum H.Comparative physiology and evolution of vision in invertebrates. A: Invertebrate photoreceptors. Handbook of Sensory Physiology. VII/6A. New York: Springer-Verlag. pp. 4, 8–9.ISBN3-540-08837-7
↑Halder G; Callaerts P. & Gehring W.J. 1995. New perspectives on eye evolution.Curr. Opin. Genet. Dev.5 (5): 602–609.
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