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Encyclopedia > Eyes
This article refers to the sight organ. See Eye (disambiguation) for other usages.
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Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.

An eye is an organ that detects light. Different kinds of light-sensitive organ are found in a variety of creature. 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.

Contents

Varieties of eye

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). Trilobites (now extinct) had a unique form of eye (usually compound) formed from crystals of calcite, incorporating a doublet structure that gave a good field of view despite the rigid lens.


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.


How a complex structure like the eye could have evolved is often said to be a difficult question for the theory of evolution, on the basis that intermediate forms of an eye would presumably have been of little use, and light-sensitive organs are present in a variety of different creatures without any clear evolutionary link. However, eyes in different levels various animals show adaption to their requirements (for example, birds of prey have much greater visual acuity than humans), and the different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution.


Focusing

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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

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A staring eye

Problems

See also

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Some believe that the human eye is the window to the soul.

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)


Sensory system - Visual system - Eye Edit (http://en.wikipedia.org/w/wiki.phtml?title=MediaWiki:Eye&action=edit)

Retina - Cornea - Iris - Pupil - Lens - Macula - Sclera - Optic fovea - Blind spot - Vitreous humour - Aqueous humour - Choroid - Ciliary body - Conjunctiva - Angle structure - Tapetum lucidum


Sensory system - Visual system

Eye - Optic nerve - Optic chiasm - Optic tract - Lateral geniculate nucleus - Optic radiations - Visual cortex


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Eye (170 words)
Eye is the official journal of The Royal College of Ophthalmologists.
Authors are recommended to read Eye's editorial "Publishing in Eye: The current status" (Eye 2007; 21: 1) and Instructions to Authors before submitting a manuscript for publication in the journal.
March was Age-related Macular Degeneration (AMD) awareness month, and in recognition of this Eye brought together its most recent content in the area and made it freely available to all.
Eye Allergies - ACAAI (1376 words)
The allergen causes certain cells in the eye (called mast cells) to release histamine and other substances or chemicals that cause blood vessels in the eyes to swell, and the eyes to become itchy, red and watery.
Eye allergies can be extremely annoying and uncomfortable, and they may disrupt your day-to-day activities, but they cannot harm your eyes.
Prescription eye drops provide both short- and long-term targeted relief of eye allergy symptoms, and they can be used to manage eye allergy symptoms in conjunction with an oral antihistamine that might be taken to manage nasal allergy symptoms.
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