Eye
From open-encyclopedia.com - the free encyclopedia.
- This article refers to the sight organ. See Eye (disambiguation) for other usages.
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.
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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.
How a complex structure like the eye could have evolved is a difficult question for the theory of evolution, since intermediate forms would presumably have been of little use, and light-sensitive organs are present in a variety of different creatures without any clear evolutionary link. 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. These are often claimed as examples of parallel evolution.
Focusing
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
Problems
- Achromatopsia
- Age-related macular degeneration
- Aniridia
- Amblyopia
- Anisometropia
- Arc eye
- Astigmatism
- Blindness
- Cataracts
- Color blindness
- Conjunctivitis
- Corrective lenses
- Floaters
- Glaucoma
- Hyperopia
- Myopia
- Nyctalopia
- Presbyopia
- Retinal detachment
- Retinopathy
- Scotoma
- Snow blindness
- Strabismus
- Uveitis
See also
- Adaptation
- Accommodation
- Crystallin
- Eye contact
- Eyeglass prescription
- Macropsia
- Micropsia
- Nictating membrane
- Optometry
- Ophthalmology
- Persistence of vision
- Saccade
- Snellen chart
- Tears
- Visual acuity
- Visual perception
External links
| Sensory system - Visual system - Eye | |
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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 |
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Eye - Optic nerve - Optic chiasm - Optic tract - Lateral geniculate nucleus - Optic radiations - Visual cortex |
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