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Eye

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

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

An eye is an organ which has evolved for the purpose of detecting light. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision.

Compound eyes are found among the arthropods (insects and kin), and are composed of many simple facets which give a pixelated image (not multiple images as is often believed).

In most vertebrates and some mollusks (such as octopuses) the eye works by projecting images onto a light-sensitive retina, where the light is detected and transmitted to the brain via the optic nerve. The eye is typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often a muscle called the iris that controls how much light enters.

Although they are quite similar in function and appearance once fully developed, vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells. The two are a good example of parallel evolution.

Focusing

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

In order for light rays to be brought to a focus they must be refracted. The amount of refraction required depends on the distance of the object which is being viewed. A distant object will require less bending of light than a nearer one. Most of the refraction occurs at the cornea which has a fixed curvature. The remainder of the required refraction occurs at the lens. The lens can be pulled flatter or rounder by muscles, which adjust the power of the lens. As we age we lose this ability to adjust the focus. Such a condition 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.

Parts of the eye

• Aqueous humour
• Blind spot
• Cornea
• Iris
• Lens
• Macula
• Fovea
• Pupil
• Retina
• Sclera
• Tapetum lucidum (not in humans)
• Vitreous humour

Closeup of a blue-green eye

Problems

• Achromatopsia
• Age-related macular degeneration
• Aniridia
• Amblyopia
• Anisometropia
• Arc eye
• Astigmatism
• Blindness
• Cataracts
• Color blindness
• Conjunctivitis
• Corrective lenses
• Floaters
• Glaucoma
• Hypermetropia
• Myopia
• Nyctalopia
• Presbyopia
• Retinal detachment
• Retinopathy
• Scotoma
• Snow blindness
• Strabismus
• Uveitis

See also

The human eye is said to be the window to the soul.

• Adaptation
• Crystallin
• Eye contact
• Eyeglass prescription
• Macropsia
• Micropsia
• Nictating membrane
• Optometry
• Ophthalmology
• Persistence of vision
• Saccade
• Snellen chart
• Tears
• Visual acuity
• Visual perception

External links

• Evolution of the Eye (http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html)
• Diagram of the eye (http://webvision.med.utah.edu/anatomy.html)
• Uveitis - German Description (http://www.medizin.de/gesundheit/deutsch/827.htm)

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This article uses material from Wikipedia and is licensed under the GNU Free Documentation License.