View source for Human eye

{{Infobox Anatomy |
 Name = The human eye|
 Image = Eye-diagram_no_circles_border.svg |
 Caption = 1. [[posterior chamber]] 2. [[ora serrata]] 3. [[ciliary muscle]] 4. [[ciliary zonules]] 5. [[canal of Schlemm]] 6. [[pupil]] 7. [[anterior chamber]] 8. [[cornea]] 9. [[iris (anatomy)|iris]] 10. [[lens cortex]] 11. [[lens nucleus]] 12. [[ciliary process]] 13. [[conjunctiva]] 14. [[inferior oblique muscle]] 15. [[inferior rectus muscle]] 16. [[medial rectus muscle]] 17. [[retinal arteries and veins]] 18. [[optic disc]] 19. [[dura mater]] 20. [[central retinal artery]] 21. [[central retinal vein]] 22. [[optic nerve]] 23. [[vorticose vein]] 24. [[bulbar sheath]] 25. [[macula]] 26. [[fovea]] 27. [[sclera]] 28. [[choroid]] 29. [[superior rectus muscle]] 30. [[retina]]
}}

The '''human eye''' is an organ which reacts to light for several purposes. 

As a conscious [[Sensory system|sense organ]], the eye allows [[Visual perception|vision]].  [[Rod cell|Rod]] and [[Cone cell|cone]] cells in the [[retina]] allow conscious light perception and vision including color differentiation and the perception of depth.  The human eye can distinguish about 10 million colors.<ref name="business">{{cite book|first=Deane B.|last=Judd|coauthors=Wyszecki, Günter|title=Color in Business, Science and Industry|publisher=[[Wiley-Interscience]]|series=Wiley Series in Pure and Applied Optics|edition=third edition|location=New York|year= 1975|page=388|isbn=0471452122}}</ref> 

In common with the [[Mammalian eye|eyes of other mammals]], the human eye's non-image-forming [[photosensitive ganglion cell]]s in the retina receive the light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone [[melatonin]] and [[Entrainment (chronobiology)|entrainment]] of the [[circadian rhythm|body clock]].

==General properties==
The eye is not properly a sphere, rather it is a fused two-piece unit.  The smaller frontal unit, more curved, called the cornea is linked to the larger unit called the sclera.  The corneal segment is typically about 8&nbsp;mm (0.3&nbsp;in) in radius.  The sclera constitutes the remaining five-sixths; its radius is typically about 12&nbsp;mm.  The cornea and sclera are connected by a ring called the limbus.  The iris – the color of the eye – and its black center, the pupil, are seen instead of the cornea due to the cornea's transparency.  To see inside the eye, an [[ophthalmoscope]] is needed, since light is not reflected out.  The fundus (area opposite the pupil) shows the characteristic pale optic disk (papilla), where vessels entering the eye pass across and optic nerve fibers depart the globe.

===Dimensions===
[[File:Blueye.JPG|thumb|right|125px|A human [[eye]]]]
The  dimensions differ among adults by only one or two millimeters. The vertical measure, generally less than the horizontal distance, is about 24&nbsp;mm among adults, at birth about 16–17&nbsp;mm. (about 0.65&nbsp;inch)  The eyeball grows rapidly, increasing to 22.5–23&nbsp;mm (approx. 0.89 in) by the age of three years.  From then to age 13, the eye attains its full size. The volume is 6.5 ml (0.4 cu. in.) and the weight is 7.5 g. (0.25 oz.)

===Components===
The eye is made up of three coats, enclosing three transparent structures.  The outermost layer is composed of the [[cornea]] and [[sclera]].  The middle layer consists of the [[choroid]], [[ciliary body]], and [[Iris (anatomy)|iris]].  The innermost is the [[retina]], which gets its circulation from the vessels of the choroid as well as the retinal vessels, which can be seen in an ophthalmoscope.

Within these coats are the aqueous humor, the vitreous body, and the flexible lens.  The aqueous humor is a clear fluid that is contained in two areas:  the [[anterior chamber]] between the cornea and the iris and exposed area of the lens; and the [[posterior chamber]], behind the iris and the rest.  The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than the aqueous humor, and is bordered by the sclera, zonule, and lens.  They are connected via the pupil.<ref>"eye, human."Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] 2009</ref>

==Dynamic range==
The retina has a static [[contrast ratio]] of around 100:1 (about 6 1/2 [[F_stop#Stops.2C_f-stop_conventions.2C_and_exposure|f-stops]]). As soon as the eye moves ([[saccades]]) it re-adjusts its exposure both chemically and geometrically by adjusting the iris which regulates the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the [[Purkinje effect]]) are mostly complete in thirty minutes. Hence, a dynamic [[contrast ratio]] of about 1,000,000:1 (about 20 f-stops) is possible.<ref>Barton, H. and Byrne, K. ''Introduction to Human Vision, Visual Defects & Eye Tests'' (March 2007), p. 22. [http://opticonsultinguk.com/downloads/Hugh_Barton-Kerry_Byrne_20070330.pdf PDF]</ref> The process is nonlinear and multifaceted, so an interruption by light merely starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

The eye includes a [[Lens (anatomy)|lens]] not dissimilar to [[Lens (optics)|lenses]] found in optical instruments such as cameras and the same principles can be applied. The [[pupil]] of the human [[eye]] is its [[aperture]]; the iris is the diaphragm that serves as the aperture stop. Refraction in the [[cornea]] causes the effective aperture (the [[entrance pupil]]) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4&nbsp;mm in diameter, although it can range from 2&nbsp;mm ({{f/|8.3|}}) in a brightly lit place to 8&nbsp;mm ({{f/|2.1}}) in the dark. The latter value decreases slowly with age, older people's eyes sometimes dilate to not more than 5-6mm.

== Field of view ==
The approximate field of view of a human eye is 95° Out, 75° Down, 60° In, 60° Up.  About 12–15° temporal and 1.5° below the horizontal is the optic nerve or [[Blind spot (vision)|blind spot]] which is roughly 7.5° in height and 5.5° in width. <ref>MIL-STD-1472F, Military Standard, Human Engineering, Design Criteria For Military Systems, Equipment, And Facilities (23 Aug 1999) [http://www.everyspec.com/MIL-STD/MIL-STD+(1400+-+1499)/download.php?spec=MIL_STD_1472F.1208.pdf PDF]</ref>

==Eye irritation==
[[Image:Bloodshot.jpg|thumb|Bloodshot Eyeball]]
Eye irritation is a common problem experienced by people of all ages. There are numerous causes in which some can be prevented and treated properly. However, in order to take precaution it is important to have some basic knowledge regarding what eye irritants are and where they can be found in our environments. Eye irritation depends somewhat on destabilization of the outer-eye tear film. Certain [[volatile organic compounds]] that are both chemically reactive and airway irritants may cause eye irritation as well. Personal factors (eg, use of contact lenses, eye make-up, and certain medication) may also affect destabilization of the tear film and possibly result in more eye symptoms.<ref name="Wolkoff, P. 2003, pp. 411-430">Wolkoff, P., Skov, P., Franck, C., and Petersen, LN. “Eye irritation and environmental factors in the office environment-hypotheses, causes and a physiological model” Scandinavian Journal of Work, Environment & Health [Scand. J. Work Environ. Health]. Dec 2003, pp. 411–430. Vol. 29, No. 6.</ref> Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high.<ref name="Wolkoff, P. 2003, pp. 411-430"/> An integrated physiological risk model with blink frequency, destabilization, and break-up of the eye tear film as inseparable phenomena may explain eye irritation among office workers in terms of occupational, climate, and eye-related physiological risk factors.<ref name="Wolkoff, P. 2003, pp. 411-430"/> 

In a study conducted by [[UCLA]], the frequency of reported symptoms in industrial buildings was investigated.<ref>Wallingford K.M and Carpenter, J ''Proc. IAQ '86: Managing Indoor Air for Health and Energy Conserv.,'' American Society for Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, 448, 1986.</ref> The study's results were that eye irritation was the most frequent symptom in industrial building spaces, at 81%. Modern office work with use of office equipment has raised concerns about possible adverse health effects.<ref>Jaakkola, Maritta S. and Jouni J. K. Jaakkola. "Office Equipment and Supplies: A Modern Occupational Health Concern?", [[American Journal of Epidemiology]], 1999, pp. 1223, Vol. 150, No. 11</ref> Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying paper. Emission of various particulate and volatile substances has been suggested as specific causes. These symptoms have been related to [[Sick Building Syndrome]], which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.<ref>Nordström K., D. Norbäck, and R. Akselsson. “Influence of indoor air quality and personal factors on the sick building syndrome (SBS) in Swedish geriatric hospitals.”, Occupational and Environmental Medicine, 1995, pp. 170–176, Vol. 52.</ref>

Many of the symptoms described in [[Sick Building Syndrome]] (SBS) and [[multiple chemical sensitivity]] (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals.<ref>Anderson, Crosalind C. and Julius H. Anderson. “Sensory irritation and multiple chemical sensitivity.” Toxicology and Industrial Health,  1999, pp. 339–345, Vol. 15, No. 3-4</ref> A repeated measurement design was employed in the study of acute symptoms of eye and respiratory tract irritation resulting from occupational exposure to sodium borate dusts.<ref name="H Wegman 1992, pp. 706-713">X Hu, D H Wegman, E A Eisen, S R Woskie and R G Smith. “Dose related acute irritant symptom responses to occupational exposure to sodium borate dusts.” British Journal of Industrial Medicine 1992, pp. 706–713, Vol. 49.</ref> The symptom assessment of the 79 exposed and 27 unexposed subjects comprised interviews before the shift began and then at regular hourly intervals for the next six hours of the shift, four days in a row.<ref name="H Wegman 1992, pp. 706-713"/> Exposures were monitored concurrently with a personal real time aerosol monitor. Two different exposure profiles, a daily average and short term (15 minute) average, were used in the analysis. Exposure-response relations were evaluated by linking incidence rates for each symptom with categories of exposure.<ref name="H Wegman 1992, pp. 706-713"/>

Acute incidence rates for nasal, eye, and throat irritation, and coughing and breathlessness were found to be associated with increased exposure levels of both exposure indices. Steeper exposure-response slopes were seen when short term exposure concentrations were used. Results from multivariate logistic regression analysis suggest that current smokers tended to be less sensitive to the exposure to airborne sodium borate dust.<ref name="H Wegman 1992, pp. 706-713"/>

==Eye movement==
[[Image:MRI of human eye.jpg|thumb|right|[[Magnetic resonance imaging|MRI]] scan of human eye]]
{{Main|Eye movements}}

The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second.<ref>Westheimer, Gerald & McKee, Suzanne P.; "Visual acuity in the presence of retinal-image motion". ''Journal of the Optical Society of America'' 1975 '''65'''(7), 847–50.</ref> Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

===Extraocular muscles===
{{Main|Extraocular muscles}}

Each eye has six [[muscle]]s that control its movements: the [[Lateral rectus muscle|lateral rectus]], the [[Medial rectus muscle|medial rectus]], the [[Inferior rectus muscle|inferior rectus]], the [[Superior rectus muscle|superior rectus]], the [[Inferior oblique muscle|inferior oblique]], and the [[Superior oblique muscle|superior oblique]]. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation.<ref>Roger H.S. Carpenter (1988); ''Movements of the eyes (2nd ed.)''. Pion Ltd, London. ISBN 0-85086-109-8.</ref> Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

===Rapid eye movement===
{{Main|Rapid eye movement (sleep)}}
Rapid eye movement, or REM for short, typically refers to the [[sleep]] stage during  which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

===Saccades===
{{Main|Saccade}}
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain. Some irregular drifts, movements, smaller than a saccade and larger than a microsaccade, subtend up to six minutes of arc.

===Microsaccades===
{{Main|Microsaccade}}

Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

===Vestibulo-ocular reflex===
{{Main|Vestibulo-ocular reflex}}

The [[vestibulo-ocular reflex]] is [[a]] [[reflex]] [[Eye movement (sensory)|eye movement]] that stabilizes images on the [[retina]] during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa.

===Smooth pursuit movement===
{{Main|Pursuit movement}}

The eyes can also follow a moving object around. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply [[feedback]]. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.

===Optokinetic reflex===
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window at a moving train, the eyes can focus on a 'moving' train for a short moment (through smooth pursuit), until the train moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the train (through a saccade).

===Vergence movement===
{{Main|Vergence}}
[[Image:Stereogram Tut Eye Convergence.png|thumb|The two eyes converge to point to the same object.]]

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' ([[convergence (eye)|convergence]]), while for an object farther away they rotate 'away from each other' ([[divergence (eye)|divergence]]). Exaggerated convergence is called ''cross eyed viewing'' (focusing on the [[nose]] for example)<!--, while exaggerated divergence is called [[?]] (which is a rare ability in humans-->. When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge.

Vergence movements are closely connected to [[Accommodation (eye)|accommodation]] of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.

There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.

As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability ([[presbyopia]]). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of infection occurs, the STD can be transmitted to the eye.<ref>[http://www.agingeye.net/ AgingEye Times]</ref>

With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as [[ectropion]], [[entropion]], [[dermatochalasis]], and [[ptosis (eyelid)|ptosis]]. The vitreous gel undergoes liquefaction ([[posterior vitreous detachment]] or PVD) and its opacities — visible as [[floater]]s — gradually increase in number.

Various [[eye care professional]]s, including [[ophthalmologist]]s, [[optometrist]]s, and [[optician]]s, are involved in the treatment and management of ocular and vision disorders. A [[Snellen chart]] is one type of [[eye chart]] used to measure [[visual acuity]]. At the conclusion of an [[eye examination]], an eye doctor may provide the patient with an [[eyeglass prescription]] for [[corrective lens]]es. Some disorders of the eyes for which corrective lenses are prescribed include [[myopia]] (near-sightedness) which affects one-third of the population, [[hyperopia]] (far-sightedness) which affects one quarter of the population, and [[presbyopia]], a loss of focusing range due to aging.

==See also==
* [[Eye color]]
* [[Mammalian eye]]
* [[Optometry]]
* [[Ophthalmology]]

== References ==
{{reflist}}

{{Refimprove|date=June 2008}}

==External links==
{{commonscat-inline|human eyes}}

{{human anatomical features}}
{{Eye}}
{{Accessory organs of the eye}}
{{Muscles of orbit}}

{{DEFAULTSORT:Human Eye}}
[[Category:Eye|Eye]]
[[Category:Sensory organs]]
[[Category:Visual system]]
[[Category:Head and neck]]
[[Category:Ophthalmology]]
[[Category:Facial features]]
[[Category:Occupational safety and health]]
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