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Wikipedia

Human eye

March 08, 2023

For eyes in general, see Eye. For other uses, see Eye (disambiguation).
This article uses anatomical terminology.

The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm.

Human eye
The human eye of the right side of the face, showing a white sclera with some blood vessels, a green iris, and the black pupil
Details
SystemVisual system
Identifiers
LatinOculi Hominum
Greekἀνθρώπινος ὀφθαλμός
MeSHD005123
TA98A01.1.00.007
A15.2.00.001
TA2113, 6734
FMA54448
Anatomical terminology
[edit on Wikidata]

The eye can be considered as a living optical device. It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye (the sclera) and one of its inner layers (the pigmented choroid) keeping the eye essentially light tight except on the eye's optic axis. In order, along the optic axis, the optical components consist of a first lens (the cornea—the clear part of the eye) that accomplishes most of the focussing of light from the outside world; then an aperture (the pupil) in a diaphragm (the iris—the coloured part of the eye) that controls the amount of light entering the interior of the eye; then another lens (the crystalline lens) that accomplishes the remaining focussing of light into images; then a light-sensitive part of the eye (the retina) where the images fall and are processed. The retina makes a connection to the brain via the optic nerve. The remaining components of the eye keep it in its required shape, nourish and maintain it, and protect it.

Three types of cells in the retina convert light energy into electrical energy used by the nervous system: rods respond to low intensity light and contribute to perception of low-resolution, black-and-white images; cones respond to high intensity light and contribute to perception of high-resolution, coloured images; and the recently discovered photosensitive ganglion cells respond to a full range of light intensities and contribute to adjusting the amount of light reaching the retina, to regulating and suppressing the hormone melatonin, and to entraining circadian rhythm.[1]

Structure

 
A detailed depiction of eye using a 3D medical illustration
 
MRI scan of the human eye

Humans have two eyes, situated on the left and the right of the face. The eyes sit in bony cavities called the orbits, in the skull. There are six extraocular muscles that control eye movements. The front visible part of the eye is made up of the whitish sclera, a coloured iris, and the pupil. A thin layer called the conjunctiva sits on top of this. The front part is also called the anterior segment of the eye.

The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of an anterior (front) segment and the posterior (back) segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5 mm (0.45 in) in diameter, and 0.5 mm (500 μm) in thickness near its centre. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about 24 mm (0.94 in). An area termed the limbus connects the cornea and sclera. The iris is the pigmented circular structure concentrically surrounding the centre of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles.

Light energy enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus (accommodation) and is controlled by the ciliary muscle. Photons of light falling on the light-sensitive cells of the retina (photoreceptor cones and rods) are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision.

Size

The size of the eye differs among adults by only one or 2 millimetres. The eyeball is generally less tall than it is wide. The sagittal vertical (height) of a human adult eye is approximately 23.7 mm (0.93 in), the transverse horizontal diameter (width) is 24.2 mm (0.95 in) and the axial anteroposterior size (depth) averages 22.0–24.8 mm (0.87–0.98 in) with no significant difference between sexes and age groups.[2] Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88).[2] The typical adult eye has an anterior to posterior diameter of 24 mm (0.94 in), and a volume of 6 cubic centimetres (0.37 cu in).[3]

The eyeball grows rapidly, increasing from about 16–17 mm (0.63–0.67 in) diameter at birth to 22.5–23 mm (0.89–0.91 in) by three years of age. By age 12, the eye attains its full size.

Components

 
Schematic diagram of the human eye. It shows a horizontal section through the right eye.

The eye is made up of three coats, or layers, enclosing various anatomical structures. The outermost layer, known as the fibrous tunic, is composed of the cornea and sclera, which provide shape to the eye and support the deeper structures. The middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly).

The spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, and the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid that is contained in two areas: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation (focusing). The vitreous body is a clear substance composed of water and proteins, which give it a jelly-like and sticky composition.[4]

Structures surrounding the eye

 
The outer parts of the eye

Extraocular muscles

Main article: Extraocular muscles

Each eye has seven extraocular muscles located in its orbit.[5] Six of these muscles control the eye movements, the seventh controls the movement of the upper eyelid. The six muscles are four recti muscles – the lateral rectus, the medial rectus, the inferior rectus, and the superior rectus, and two oblique muscles the inferior oblique, and the superior oblique. The seventh muscle is the levator palpebrae superioris muscle. 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.[6] Thus, the eye can be considered as undergoing rotations about a single point in the centre of the eye.

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    Eye and orbit anatomy with motor nerves

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    Image showing orbita with eye and nerves visible (periocular fat removed)

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    Image showing orbita with eye and periocular fat

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    Normal anatomy of the human eye and orbit, anterior view

Vision

See also: Visual acuity, Eye § Visual acuity, Fovea centralis § Angular size of foveal cones, and Color vision § Physiology of color perception

Field of view

 
Side-view of the human eye, viewed approximately 90° temporal, illustrating how the iris and pupil appear rotated towards the viewer due to the optical properties of the cornea and the aqueous humour

The approximate field of view of an individual human eye (measured from the fixation point, i.e., the point at which one's gaze is directed) varies by facial anatomy, but is typically 30° superior (up, limited by the brow), 45° nasal (limited by the nose), 70° inferior (down), and 100° temporal (towards the temple).[7][8][9] For both eyes combined (Binocular vision) visual field is approximately 100° vertical and a maximum 190° horizontal, approximately 120° of which makes up the binocular field of view (seen by both eyes) flanked by two uniocular fields (seen by only one eye) of approximately 40 degrees.[10][11] It is an area of 4.17 steradians or 13700 square degrees for binocular vision.[12] When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle.[13][14][15]

About 15° temporal and 1.5° below the horizontal is the blind spot created by the optic nerve nasally, which is roughly 7.5° high and 5.5° wide.[16]

Dynamic range

The retina has a static contrast ratio of around 100:1 (about 6.5 f-stops). As soon as the eye moves rapidly to acquire a target (saccades), it re-adjusts its exposure by adjusting the iris, which adjusts the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal rod photoreceptors is 80% complete in thirty minutes. The process is nonlinear and multifaceted, so an interruption by light exposure requires restarting the dark adaptation process over again.

The human eye can detect a luminance from 10−6 cd/m2, or one millionth (0.000001) of a candela per square meter to 108 cd/m2 or one hundred million (100,000,000) candelas per square meter.[17][18][19] (that is it has a range of 1014 cd/m2, or one hundred trillion 100,000,000,000,000, about 46.5 f-stops). This range does not include looking at the midday sun (109 cd/m2)[20] or lightning discharge.

At the low end of the range is the absolute threshold of vision for a steady light across a wide field of view, about 10−6 cd/m2 (0.000001 candela per square meter).[21][22] The upper end of the range is given in terms of normal visual performance as 108 cd/m2 (100,000,000 or one hundred million candelas per square meter).[23]

The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics 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 mm in diameter, although it can range from 2 mm (f/8.3) in a brightly lit place to 8 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 in the dark, and may be as small as 1mm in the light.[24][25]

Eye movement

Main article: Eye movement
 
The light circle is the optic disc where the optic nerve exits the retina

The visual system in the human brain is too slow to process information if images are slipping across the retina at more than a few degrees per second.[26] Thus, to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Frontal-eyed animals have a small area of the retina with very high visual acuity, the fovea centralis. It 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. Any failure to make eye movements correctly can lead to serious visual degradation.

Having two eyes allows the brain to determine the depth and distance of an object, called stereovision, and gives the sense of three-dimensionality to the vision. Both eyes must point accurately enough that the object of regard falls on corresponding points of the two retinas to stimulate stereovision; otherwise, double vision might occur. Some persons with congenitally crossed eyes tend to ignore one eye's vision, thus do not suffer double vision, and do not have stereovision. The movements of the eye are controlled by six muscles attached to each eye, and allow the eye to elevate, depress, converge, diverge and roll. These muscles are both controlled voluntarily and involuntarily to track objects and correct for simultaneous head movements.

Rapid eye movement

Main article: Rapid eye movement sleep

Rapid eye movement, REM, typically refers to the sleep stage during which the most vivid dreams occur. During this stage, the eyes move rapidly.

Saccades

Main article: Saccade

Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain.

Fixational Eye Movements

Main article: Fixation (visual)

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.

Eye movements include drift, ocular tremor, and microsaccades. Some irregular drifts, movements smaller than a saccade and larger than a microsaccade, subtend up to one tenth of a degree. Researchers vary in their definition of microsaccades by amplitude. Martin Rolfs[27] states that 'the majority of microsaccades observed in a variety of tasks have amplitudes smaller than 30 min-arc'. However, others state that the "current consensus has largely consolidated around a definition of microsaccades that includes magnitudes up to 1°."[28]

Vestibulo-ocular reflexes

Main article: Vestibulo-ocular reflex

The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement in response to neural input from the vestibular system of the inner ear, thus maintaining the image in the centre of the visual field. For example, when the head moves to the right, the eyes move to the left. This applies for head movements up and down, left and right, and tilt to the right and left, all of which give input to the ocular muscles to maintain visual stability.

Smooth pursuit movement

Main article: Pursuit movement

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 saccades 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

Main article: Optokinetic response

The Optokinetic reflex (or optokinetic nystagmus) stabilizes the image on the retina through visual feedback. It is induced when the entire visual scene drifts across the retina, eliciting eye rotation in the same direction and at a velocity that minimizes the motion of the image on the retina. When the gaze direction deviates too far from the forward heading, a compensatory saccade is induced to reset the gaze to the centre of the visual field.[29]

For example, when looking out of the window at a moving train, the eyes can focus on a moving train for a short moment (by stabilizing it on the retina), until the train moves out of the field of vision. At this point, the eye is moved back to the point where it first saw the train (through a saccade).

Near response

The adjustment to close-range vision involves three processes to focus an image on the retina.

Vergence movement

Main article: Vergence
 
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 a nearby object, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from each other' (divergence).

Pupil constriction

Lenses cannot refract light rays at their edges as well as closer to the centre. The image produced by any lens is therefore somewhat blurry around the edges (spherical aberration). It can be minimized by screening out peripheral light rays and looking only at the better-focused centre. In the eye, the pupil serves this purpose by constricting while the eye is focused on nearby objects. Small apertures also give an increase in depth of field, allowing a broader range of "in focus" vision. In this way the pupil has a dual purpose for near vision: to reduce spherical aberration and increase depth of field.[30]

Accommodation of the lens

Main article: Lens (vertebrate anatomy)

Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens; this process is known as "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and also allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus.[30][31]

Eye care professionals

The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner. These specialists, or eye care professionals, serve different functions in different countries. Eye care professionals can have overlap in their patient care privileges. For example, both an ophthalmologist (M.D.) and optometrist (O.D.) are professionals who diagnoses eye disease and can prescribe lenses to correct vision. However, typically only ophthalmologists are licensed to perform surgical procedures. Ophthalmologists may also specialize within a surgical area, such as cornea, cataracts, laser, retina, or oculoplastics.

Eye care professionals include:

Eye irritation

See also: Irritation § Eye irritation
 
Conjunctival injection, or redness of the sclera surrounding the iris and pupil

Eye irritation has been defined as "the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye".[32] It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are discomfort, dryness, excess tearing, itchiness, grating, foreign body sensation, ocular fatigue, pain, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from mild to severe. It has been suggested that these eye symptoms are related to different causal mechanisms, and symptoms are related to the particular ocular anatomy involved.[33]

Several suspected causal factors in our environment have been studied so far.[32] One hypothesis is that indoor air pollution may cause eye and airway irritation.[34][35] Eye irritation depends somewhat on destabilization of the outer-eye tear film, i.e. the formation of dry spots on the cornea, resulting in ocular discomfort.[34][36][37] Occupational factors are also likely to influence the perception of eye irritation. Some of these are lighting (glare and poor contrast), gaze position, reduced blink rate, limited number of breaks from visual tasking, and a constant combination of accommodation, musculoskeletal burden, and impairment of the visual nervous system.[38][39] Another factor that may be related is work stress.[40][41] In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users.[42][43] Other risk factors, such as chemical toxins/irritants (e.g. amines, formaldehyde, acetaldehyde, acrolein, N-decane, VOCs, ozone, pesticides and preservatives, allergens, etc.) might cause eye irritation as well.

Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation. Personal factors (e.g. use of contact lenses, eye make-up, and certain medications) may also affect destabilization of the tear film and possibly result in more eye symptoms.[33] Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high.[33] 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.[33]

There are two major measures of eye irritation. One is blink frequency which can be observed by human behavior. The other measures are break up time, tear flow, hyperemia (redness, swelling), tear fluid cytology, and epithelial damage (vital stains) etc., which are human beings' physiological reactions. Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation. Blink frequencies are individual with mean frequencies of < 2–3 to 20–30 blinks/minute, and they depend on environmental factors including the use of contact lenses. Dehydration, mental activities, work conditions, room temperature, relative humidity, and illumination all influence blink frequency. Break-up time (BUT) is another major measure of eye irritation and tear film stability.[44] It is defined as the time interval (in seconds) between blinking and rupture. BUT is considered to reflect the stability of the tear film as well. In normal persons, the break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained.[33] Studies have shown that blink frequency is correlated negatively with break-up time. This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch.[45][46] Other evaluating methods, such as hyperemia, cytology etc. have increasingly been used to assess eye irritation.

There are other factors that are related to eye irritation as well. Three major factors that influence the most are indoor air pollution, contact lenses and gender differences. Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparisons with random samples of the general population.[47][48][49][50] These research results might indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers.[51][52][53] Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms, dryness, redness, and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers.[53] Studies have shown that incidence of dry eyes increases with age,[54][55] especially among women.[56] Tear film stability (e.g. tear break-up time) is significantly lower among women than among men. In addition, women have a higher blink frequency while reading.[57] Several factors may contribute to gender differences. One is the use of eye make-up. Another reason could be that the women in the reported studies have done more VDU work than the men, including lower grade work. A third often-quoted explanation is related to the age-dependent decrease of tear secretion, particularly among women after 40 years of age.[56][58][59]

In a study conducted by UCLA, the frequency of reported symptoms in industrial buildings was investigated.[60] 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.[61] 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 (SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.[62]

Many of the symptoms described in SBS and multiple chemical sensitivity (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals.[63] 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.[64] 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.[64] 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.[64]

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.[64]

Several actions can be taken to prevent eye irritation—

  • trying to maintain normal blinking by avoiding room temperatures that are too high; avoiding relative humidities that are too high or too low, because they reduce blink frequency or may increase water evaporation.[33]
  • trying to maintain an intact film of tears by the following actions:
  1. Blinking and short breaks may be beneficial for VDU users.[65][66] Increasing these two actions might help maintain the tear film.
  2. Downward gazing is recommended to reduce ocular surface area and water evaporation.[67][68][69]
  3. The distance between the VDU and keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by a low direction of the gaze,[70] and
  4. Blink training can be beneficial.[71]

In addition, other measures are proper lid hygiene, avoidance of eye rubbing,[72] and proper use of personal products and medication. Eye make-up should be used with care.[73]

Eye disease

 
Diagram of a human eye (horizontal section of the right eye)
1. Lens, 2. Zonule of Zinn or Ciliary zonule, 3. Posterior chamber and 4. Anterior chamber with 5. Aqueous humour flow; 6. Pupil, 7. Corneosclera or Fibrous tunic with 8. Cornea, 9. Trabecular meshwork and Schlemm's canal. 10. Corneal limbus and 11. Sclera; 12. Conjunctiva, 13. Uvea with 14. Iris, 15. Ciliary body (with a: pars plicata and b: pars plana) and 16. Choroid); 17. Ora serrata, 18. Vitreous humor with 19. Hyaloid canal/(old artery), 20. Retina with 21. Macula or macula lutea, 22. Fovea and 23. Optic disc → blind spot; 24. Visual axis (line of sight). 25. Optical axis. 26. Optic nerve with 27. Dural sheath, 28. Tenon's capsule or bulbar sheath, 29. Tendon.
30. Anterior segment, 31. Posterior segment.
32. Ophthalmic artery, 33. Artery and central retinal vein → 36. Blood vessels of the retina; Ciliary arteries (34. Short posterior ones, 35. Long posterior ones and 37. Anterior ones), 38. Lacrimal artery, 39. Ophthalmic vein, 40. Vorticose vein.
41. Ethmoid bone, 42. Medial rectus muscle, 43. Lateral rectus muscle, 44. Sphenoid bone.

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 diseases of the aging eye. While there are many changes of significance in the non-diseased 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 decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. 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 the eye and area of infection occurs, the STD can be transmitted to the eye.[74]

With aging, a prominent white ring develops in the periphery of the cornea called arcus senilis. Aging causes laxity, 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. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities — visible as floaters — gradually increase in number.

Eye care professionals, including ophthalmologists and optometrists, 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 a complete eye examination, the eye doctor might provide the patient with an eyeglass prescription for corrective lenses. Some disorders of the eyes for which corrective lenses are prescribed include myopia (near-sightedness), hyperopia (far-sightedness), astigmatism, and presbyopia (the loss of focusing range during aging).

Macular degeneration

Main article: Macular degeneration

Macular degeneration is especially prevalent in the U.S. and affects roughly 1.75 million Americans each year.[75] Having lower levels of lutein and zeaxanthin within the macula may be associated with an increase in the risk of age-related macular degeneration.[76] < Lutein and zeaxanthin act as antioxidants that protect the retina and macula from oxidative damage from high-energy light waves.[77] As the light waves enter the eye they excite electrons that can cause harm to the cells in the eye, but they can cause oxidative damage that may lead to macular degeneration or cataracts. Lutein and zeaxanthin bind to the electron free radical and are reduced rendering the electron safe. There are many ways to ensure a diet rich in lutein and zeaxanthin, the best of which is to eat dark green vegetables including kale, spinach, broccoli and turnip greens. Nutrition is an important aspect of the ability to achieve and maintain proper eye health. Lutein and zeaxanthin are two major carotenoids, found in the macula of the eye, that are being researched to identify their role in the pathogenesis of eye disorders such as age-related macular degeneration and cataracts.[78]

Images of human eye

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    Right eye without labels (horizontal section)

  •  
  •  
  •  

    The structures of the eye labeled

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    Another view of the eye and the structures of the eye labeled

See also

References

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External links

  • 3D Interactive Human Eye
  • Interactive Tool to explore the Human Eye
  •   Media related to Human eyes at Wikimedia Commons
  •   Quotations related to eyes at Wikiquote

March 08, 2023
human, eyes, general, other, uses, disambiguation, this, article, uses, anatomical, terminology, human, sensory, organ, part, sensory, nervous, system, that, reacts, visible, light, allows, humans, visual, information, various, purposes, including, seeing, thi. For eyes in general see Eye For other uses see Eye disambiguation This article uses anatomical terminology The human eye is a sensory organ part of the sensory nervous system that reacts to visible light and allows humans to use visual information for various purposes including seeing things keeping balance and maintaining circadian rhythm Human eyeThe human eye of the right side of the face showing a white sclera with some blood vessels a green iris and the black pupil1 vitreous body 2 ora serrata 3 ciliary muscle 4 ciliary zonules 5 Schlemm s canal 6 pupil 7 anterior chamber 8 cornea 9 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 retinaDetailsSystemVisual systemIdentifiersLatinOculi HominumGreekἀn8rwpinos ὀf8almosMeSHD005123TA98A01 1 00 007 A15 2 00 001TA2113 6734FMA54448Anatomical terminology edit on Wikidata The eye can be considered as a living optical device It is approximately spherical in shape with its outer layers such as the outermost white part of the eye the sclera and one of its inner layers the pigmented choroid keeping the eye essentially light tight except on the eye s optic axis In order along the optic axis the optical components consist of a first lens the cornea the clear part of the eye that accomplishes most of the focussing of light from the outside world then an aperture the pupil in a diaphragm the iris the coloured part of the eye that controls the amount of light entering the interior of the eye then another lens the crystalline lens that accomplishes the remaining focussing of light into images then a light sensitive part of the eye the retina where the images fall and are processed The retina makes a connection to the brain via the optic nerve The remaining components of the eye keep it in its required shape nourish and maintain it and protect it Three types of cells in the retina convert light energy into electrical energy used by the nervous system rods respond to low intensity light and contribute to perception of low resolution black and white images cones respond to high intensity light and contribute to perception of high resolution coloured images and the recently discovered photosensitive ganglion cells respond to a full range of light intensities and contribute to adjusting the amount of light reaching the retina to regulating and suppressing the hormone melatonin and to entraining circadian rhythm 1 Contents 1 Structure 1 1 Size 1 2 Components 1 3 Structures surrounding the eye 1 4 Extraocular muscles 2 Vision 2 1 Field of view 2 2 Dynamic range 3 Eye movement 3 1 Rapid eye movement 3 2 Saccades 3 3 Fixational Eye Movements 3 4 Vestibulo ocular reflexes 3 5 Smooth pursuit movement 3 6 Optokinetic reflex 4 Near response 4 1 Vergence movement 4 2 Pupil constriction 4 3 Accommodation of the lens 5 Eye care professionals 6 Eye irritation 7 Eye disease 7 1 Macular degeneration 8 Images of human eye 9 See also 10 References 11 External linksStructure Edit A detailed depiction of eye using a 3D medical illustration MRI scan of the human eye Humans have two eyes situated on the left and the right of the face The eyes sit in bony cavities called the orbits in the skull There are six extraocular muscles that control eye movements The front visible part of the eye is made up of the whitish sclera a coloured iris and the pupil A thin layer called the conjunctiva sits on top of this The front part is also called the anterior segment of the eye The eye is not shaped like a perfect sphere rather it is a fused two piece unit composed of an anterior front segment and the posterior back segment The anterior segment is made up of the cornea iris and lens The cornea is transparent and more curved and is linked to the larger posterior segment composed of the vitreous retina choroid and the outer white shell called the sclera The cornea is typically about 11 5 mm 0 45 in in diameter and 0 5 mm 500 mm in thickness near its centre The posterior chamber constitutes the remaining five sixths its diameter is typically about 24 mm 0 94 in An area termed the limbus connects the cornea and sclera The iris is the pigmented circular structure concentrically surrounding the centre of the eye the pupil which appears to be black The size of the pupil which controls the amount of light entering the eye is adjusted by the iris dilator and sphincter muscles Light energy enters the eye through the cornea through the pupil and then through the lens The lens shape is changed for near focus accommodation and is controlled by the ciliary muscle Photons of light falling on the light sensitive cells of the retina photoreceptor cones and rods are converted into electrical signals that are transmitted to the brain by the optic nerve and interpreted as sight and vision Size Edit The size of the eye differs among adults by only one or 2 millimetres The eyeball is generally less tall than it is wide The sagittal vertical height of a human adult eye is approximately 23 7 mm 0 93 in the transverse horizontal diameter width is 24 2 mm 0 95 in and the axial anteroposterior size depth averages 22 0 24 8 mm 0 87 0 98 in with no significant difference between sexes and age groups 2 Strong correlation has been found between the transverse diameter and the width of the orbit r 0 88 2 The typical adult eye has an anterior to posterior diameter of 24 mm 0 94 in and a volume of 6 cubic centimetres 0 37 cu in 3 The eyeball grows rapidly increasing from about 16 17 mm 0 63 0 67 in diameter at birth to 22 5 23 mm 0 89 0 91 in by three years of age By age 12 the eye attains its full size Components Edit Schematic diagram of the human eye It shows a horizontal section through the right eye The eye is made up of three coats or layers enclosing various anatomical structures The outermost layer known as the fibrous tunic is composed of the cornea and sclera which provide shape to the eye and support the deeper structures The middle layer known as the vascular tunic or uvea consists of the choroid ciliary body pigmented epithelium and iris The innermost is the retina which gets its oxygenation from the blood vessels of the choroid posteriorly as well as the retinal vessels anteriorly The spaces of the eye are filled with the aqueous humour anteriorly between the cornea and lens and the vitreous body a jelly like substance behind the lens filling the entire posterior cavity The aqueous humour is a clear watery fluid that is contained in two areas the anterior chamber between the cornea and the iris and the posterior chamber between the iris and the lens The lens is suspended to the ciliary body by the suspensory ligament Zonule of Zinn made up of hundreds of fine transparent fibers which transmit muscular forces to change the shape of the lens for accommodation focusing The vitreous body is a clear substance composed of water and proteins which give it a jelly like and sticky composition 4 Structures surrounding the eye Edit The outer parts of the eye Extraocular muscles Edit Main article Extraocular muscles Each eye has seven extraocular muscles located in its orbit 5 Six of these muscles control the eye movements the seventh controls the movement of the upper eyelid The six muscles are four recti muscles the lateral rectus the medial rectus the inferior rectus and the superior rectus and two oblique muscles the inferior oblique and the superior oblique The seventh muscle is the levator palpebrae superioris muscle 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 6 Thus the eye can be considered as undergoing rotations about a single point in the centre of the eye Eye and orbit anatomy with motor nerves Image showing orbita with eye and nerves visible periocular fat removed Image showing orbita with eye and periocular fat Normal anatomy of the human eye and orbit anterior viewVision EditSee also Visual acuity Eye Visual acuity Fovea centralis Angular size of foveal cones and Color vision Physiology of color perception Field of view Edit Side view of the human eye viewed approximately 90 temporal illustrating how the iris and pupil appear rotated towards the viewer due to the optical properties of the cornea and the aqueous humour The approximate field of view of an individual human eye measured from the fixation point i e the point at which one s gaze is directed varies by facial anatomy but is typically 30 superior up limited by the brow 45 nasal limited by the nose 70 inferior down and 100 temporal towards the temple 7 8 9 For both eyes combined Binocular vision visual field is approximately 100 vertical and a maximum 190 horizontal approximately 120 of which makes up the binocular field of view seen by both eyes flanked by two uniocular fields seen by only one eye of approximately 40 degrees 10 11 It is an area of 4 17 steradians or 13700 square degrees for binocular vision 12 When viewed at large angles from the side the iris and pupil may still be visible by the viewer indicating the person has peripheral vision possible at that angle 13 14 15 About 15 temporal and 1 5 below the horizontal is the blind spot created by the optic nerve nasally which is roughly 7 5 high and 5 5 wide 16 Dynamic range Edit The retina has a static contrast ratio of around 100 1 about 6 5 f stops As soon as the eye moves rapidly to acquire a target saccades it re adjusts its exposure by adjusting the iris which adjusts the size of the pupil Initial dark adaptation takes place in approximately four seconds of profound uninterrupted darkness full adaptation through adjustments in retinal rod photoreceptors is 80 complete in thirty minutes The process is nonlinear and multifaceted so an interruption by light exposure requires restarting the dark adaptation process over again The human eye can detect a luminance from 10 6 cd m2 or one millionth 0 000001 of a candela per square meter to 108 cd m2 or one hundred million 100 000 000 candelas per square meter 17 18 19 that is it has a range of 1014 cd m2 or one hundred trillion 100 000 000 000 000 about 46 5 f stops This range does not include looking at the midday sun 109 cd m2 20 or lightning discharge At the low end of the range is the absolute threshold of vision for a steady light across a wide field of view about 10 6 cd m2 0 000001 candela per square meter 21 22 The upper end of the range is given in terms of normal visual performance as 108 cd m2 100 000 000 or one hundred million candelas per square meter 23 The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics 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 mm in diameter although it can range from 2 mm f 8 3 in a brightly lit place to 8 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 in the dark and may be as small as 1mm in the light 24 25 Eye movement EditMain article Eye movement The light circle is the optic disc where the optic nerve exits the retina The visual system in the human brain is too slow to process information if images are slipping across the retina at more than a few degrees per second 26 Thus to be able to see while moving the brain must compensate for the motion of the head by turning the eyes Frontal eyed animals have a small area of the retina with very high visual acuity the fovea centralis It 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 Any failure to make eye movements correctly can lead to serious visual degradation Having two eyes allows the brain to determine the depth and distance of an object called stereovision and gives the sense of three dimensionality to the vision Both eyes must point accurately enough that the object of regard falls on corresponding points of the two retinas to stimulate stereovision otherwise double vision might occur Some persons with congenitally crossed eyes tend to ignore one eye s vision thus do not suffer double vision and do not have stereovision The movements of the eye are controlled by six muscles attached to each eye and allow the eye to elevate depress converge diverge and roll These muscles are both controlled voluntarily and involuntarily to track objects and correct for simultaneous head movements Rapid eye movement Edit Main article Rapid eye movement sleep Rapid eye movement REM typically refers to the sleep stage during which the most vivid dreams occur During this stage the eyes move rapidly Saccades Edit Main article Saccade Saccades are quick simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain Fixational Eye Movements Edit Main article Fixation visual 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 Eye movements include drift ocular tremor and microsaccades Some irregular drifts movements smaller than a saccade and larger than a microsaccade subtend up to one tenth of a degree Researchers vary in their definition of microsaccades by amplitude Martin Rolfs 27 states that the majority of microsaccades observed in a variety of tasks have amplitudes smaller than 30 min arc However others state that the current consensus has largely consolidated around a definition of microsaccades that includes magnitudes up to 1 28 Vestibulo ocular reflexes Edit Main article Vestibulo ocular reflex The vestibulo ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement in response to neural input from the vestibular system of the inner ear thus maintaining the image in the centre of the visual field For example when the head moves to the right the eyes move to the left This applies for head movements up and down left and right and tilt to the right and left all of which give input to the ocular muscles to maintain visual stability Smooth pursuit movement Edit Main article Pursuit movement 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 saccades 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 Edit Main article Optokinetic response The Optokinetic reflex or optokinetic nystagmus stabilizes the image on the retina through visual feedback It is induced when the entire visual scene drifts across the retina eliciting eye rotation in the same direction and at a velocity that minimizes the motion of the image on the retina When the gaze direction deviates too far from the forward heading a compensatory saccade is induced to reset the gaze to the centre of the visual field 29 For example when looking out of the window at a moving train the eyes can focus on a moving train for a short moment by stabilizing it on the retina until the train moves out of the field of vision At this point the eye is moved back to the point where it first saw the train through a saccade Near response EditThe adjustment to close range vision involves three processes to focus an image on the retina Vergence movement Edit Main article Vergence 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 a nearby object the eyes rotate towards each other convergence while for an object farther away they rotate away from each other divergence Pupil constriction Edit Lenses cannot refract light rays at their edges as well as closer to the centre The image produced by any lens is therefore somewhat blurry around the edges spherical aberration It can be minimized by screening out peripheral light rays and looking only at the better focused centre In the eye the pupil serves this purpose by constricting while the eye is focused on nearby objects Small apertures also give an increase in depth of field allowing a broader range of in focus vision In this way the pupil has a dual purpose for near vision to reduce spherical aberration and increase depth of field 30 Accommodation of the lens Edit Main article Lens vertebrate anatomy Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens this process is known as accommodation Accommodation narrows the inner diameter of the ciliary body which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens and also allows the lens to relax into a more convex or globular shape A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina allowing closer objects to be brought into better focus 30 31 Eye care professionals EditThe human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner These specialists or eye care professionals serve different functions in different countries Eye care professionals can have overlap in their patient care privileges For example both an ophthalmologist M D and optometrist O D are professionals who diagnoses eye disease and can prescribe lenses to correct vision However typically only ophthalmologists are licensed to perform surgical procedures Ophthalmologists may also specialize within a surgical area such as cornea cataracts laser retina or oculoplastics Eye care professionals include Ocularists Ophthalmologists Optometrists Opticians Orthoptists and Vision TherapistsEye irritation EditSee also Irritation Eye irritation Conjunctival injection or redness of the sclera surrounding the iris and pupil Eye irritation has been defined as the magnitude of any stinging scratching burning or other irritating sensation from the eye 32 It is a common problem experienced by people of all ages Related eye symptoms and signs of irritation are discomfort dryness excess tearing itchiness grating foreign body sensation ocular fatigue pain soreness redness swollen eyelids and tiredness etc These eye symptoms are reported with intensities from mild to severe It has been suggested that these eye symptoms are related to different causal mechanisms and symptoms are related to the particular ocular anatomy involved 33 Several suspected causal factors in our environment have been studied so far 32 One hypothesis is that indoor air pollution may cause eye and airway irritation 34 35 Eye irritation depends somewhat on destabilization of the outer eye tear film i e the formation of dry spots on the cornea resulting in ocular discomfort 34 36 37 Occupational factors are also likely to influence the perception of eye irritation Some of these are lighting glare and poor contrast gaze position reduced blink rate limited number of breaks from visual tasking and a constant combination of accommodation musculoskeletal burden and impairment of the visual nervous system 38 39 Another factor that may be related is work stress 40 41 In addition psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users 42 43 Other risk factors such as chemical toxins irritants e g amines formaldehyde acetaldehyde acrolein N decane VOCs ozone pesticides and preservatives allergens etc might cause eye irritation as well Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation Personal factors e g use of contact lenses eye make up and certain medications may also affect destabilization of the tear film and possibly result in more eye symptoms 33 Nevertheless if airborne particles alone should destabilize the tear film and cause eye irritation their content of surface active compounds must be high 33 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 33 There are two major measures of eye irritation One is blink frequency which can be observed by human behavior The other measures are break up time tear flow hyperemia redness swelling tear fluid cytology and epithelial damage vital stains etc which are human beings physiological reactions Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation Blink frequencies are individual with mean frequencies of lt 2 3 to 20 30 blinks minute and they depend on environmental factors including the use of contact lenses Dehydration mental activities work conditions room temperature relative humidity and illumination all influence blink frequency Break up time BUT is another major measure of eye irritation and tear film stability 44 It is defined as the time interval in seconds between blinking and rupture BUT is considered to reflect the stability of the tear film as well In normal persons the break up time exceeds the interval between blinks and therefore the tear film is maintained 33 Studies have shown that blink frequency is correlated negatively with break up time This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch 45 46 Other evaluating methods such as hyperemia cytology etc have increasingly been used to assess eye irritation There are other factors that are related to eye irritation as well Three major factors that influence the most are indoor air pollution contact lenses and gender differences Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparisons with random samples of the general population 47 48 49 50 These research results might indicate that indoor air pollution has played an important role in causing eye irritation There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers 51 52 53 Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms dryness redness and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers 53 Studies have shown that incidence of dry eyes increases with age 54 55 especially among women 56 Tear film stability e g tear break up time is significantly lower among women than among men In addition women have a higher blink frequency while reading 57 Several factors may contribute to gender differences One is the use of eye make up Another reason could be that the women in the reported studies have done more VDU work than the men including lower grade work A third often quoted explanation is related to the age dependent decrease of tear secretion particularly among women after 40 years of age 56 58 59 In a study conducted by UCLA the frequency of reported symptoms in industrial buildings was investigated 60 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 61 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 SBS which involves symptoms such as irritation to the eyes skin and upper airways headache and fatigue 62 Many of the symptoms described in SBS and multiple chemical sensitivity MCS resemble the symptoms known to be elicited by airborne irritant chemicals 63 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 64 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 64 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 64 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 64 Several actions can be taken to prevent eye irritation trying to maintain normal blinking by avoiding room temperatures that are too high avoiding relative humidities that are too high or too low because they reduce blink frequency or may increase water evaporation 33 trying to maintain an intact film of tears by the following actions Blinking and short breaks may be beneficial for VDU users 65 66 Increasing these two actions might help maintain the tear film Downward gazing is recommended to reduce ocular surface area and water evaporation 67 68 69 The distance between the VDU and keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by a low direction of the gaze 70 and Blink training can be beneficial 71 In addition other measures are proper lid hygiene avoidance of eye rubbing 72 and proper use of personal products and medication Eye make up should be used with care 73 Eye disease Edit Diagram of a human eye horizontal section of the right eye 1 Lens 2 Zonule of Zinn or Ciliary zonule 3 Posterior chamber and 4 Anterior chamber with 5 Aqueous humour flow 6 Pupil 7 Corneosclera or Fibrous tunic with 8 Cornea 9 Trabecular meshwork and Schlemm s canal 10 Corneal limbus and 11 Sclera 12 Conjunctiva 13 Uvea with 14 Iris 15 Ciliary body with a pars plicata and b pars plana and 16 Choroid 17 Ora serrata 18 Vitreous humor with 19 Hyaloid canal old artery 20 Retina with 21 Macula or macula lutea 22 Fovea and 23 Optic disc blind spot 24 Visual axis line of sight 25 Optical axis 26 Optic nerve with 27 Dural sheath 28 Tenon s capsule or bulbar sheath 29 Tendon 30 Anterior segment 31 Posterior segment 32 Ophthalmic artery 33 Artery and central retinal vein 36 Blood vessels of the retina Ciliary arteries 34 Short posterior ones 35 Long posterior ones and 37 Anterior ones 38 Lacrimal artery 39 Ophthalmic vein 40 Vorticose vein 41 Ethmoid bone 42 Medial rectus muscle 43 Lateral rectus muscle 44 Sphenoid bone 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 diseases of the aging eye While there are many changes of significance in the non diseased 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 decreases with age leading to a substantial decrease in light received at the retina In comparison to younger people it is as though older persons are constantly wearing medium density sunglasses 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 the eye and area of infection occurs the STD can be transmitted to the eye 74 With aging a prominent white ring develops in the periphery of the cornea called arcus senilis Aging causes laxity 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 The vitreous gel undergoes liquefaction posterior vitreous detachment or PVD and its opacities visible as floaters gradually increase in number Eye care professionals including ophthalmologists and optometrists 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 a complete eye examination the eye doctor might provide the patient with an eyeglass prescription for corrective lenses Some disorders of the eyes for which corrective lenses are prescribed include myopia near sightedness hyperopia far sightedness astigmatism and presbyopia the loss of focusing range during aging Macular degeneration Edit Main article Macular degeneration Macular degeneration is especially prevalent in the U S and affects roughly 1 75 million Americans each year 75 Having lower levels of lutein and zeaxanthin within the macula may be associated with an increase in the risk of age related macular degeneration 76 lt Lutein and zeaxanthin act as antioxidants that protect the retina and macula from oxidative damage from high energy light waves 77 As the light waves enter the eye they excite electrons that can cause harm to the cells in the eye but they can cause oxidative damage that may lead to macular degeneration or cataracts Lutein and zeaxanthin bind to the electron free radical and are reduced rendering the electron safe There are many ways to ensure a diet rich in lutein and zeaxanthin the best of which is to eat dark green vegetables including kale spinach broccoli and turnip greens Nutrition is an important aspect of the ability to achieve and maintain proper eye health Lutein and zeaxanthin are two major carotenoids found in the macula of the eye that are being researched to identify their role in the pathogenesis of eye disorders such as age related macular degeneration and cataracts 78 Images of human eye Edit Right eye without labels horizontal section The structures of the eye labeled Another view of the eye and the structures of the eye labeledSee also Edit Anatomy portalEye colour Eye development Eye disease Eye strain Hyaloid canal Iris recognition Knobloch syndrome Lacrimal caruncle Rheum Spectral sensitivityReferences Edit Zimmer Carl February 2012 Our Strange Important Subconscious Light Detectors Discover Magazine Retrieved 2012 05 05 a b Variations in eyeball diameters of the healthy adults Cunningham edited by Paul Riordan Eva Emmett T 2011 05 17 Vaughan amp Asbury s General Ophthalmology 18th ed New York McGraw Hill Medical ISBN 978 0 07 163420 5 a href Template Cite book html title Template Cite book cite book a first has generic name help eye human Encyclopaedia Britannica from Encyclopaedia Britannica Ultimate Reference Suite 2009 Haladaj R 2019 Normal Anatomy and Anomalies of the Rectus Extraocular Muscles in Human A Review of the Recent Data and Findings BioMed Research International 2019 8909162 doi 10 1155 2019 8909162 PMC 6954479 PMID 31976329 Carpenter Roger H S 1988 Movements of the eyes 2nd ed London Pion Ltd ISBN 0 85086 109 8 Savino Peter J Danesh Meyer Helen V 2012 Colour Atlas and Synopsis of Clinical Ophthalmology Wills Eye Institute Neuro Ophthalmology 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Industrial Medicine 49 10 706 713 doi 10 1136 oem 49 10 706 PMC 1012146 PMID 1419859 Carney LG Hill RM June 1982 The nature of normal blinking patterns Acta Ophthalmologica 60 3 427 433 doi 10 1111 j 1755 3768 1982 tb03034 x PMID 7136554 S2CID 22362219 Henning R A Jacques P Kissel G V Sullivan A B Alteras Webb S M January 1997 Frequent short rest breaks from computer work effects on productivity and well being at two field sites Ergonomics 40 1 78 91 doi 10 1080 001401397188396 PMID 8995049 Nakamori K Odawara M Nakajima T Mizutani T Tsubota K July 1997 Blinking is controlled primarily by ocular surface conditions American Journal of Ophthalmology 124 1 24 30 doi 10 1016 s0002 9394 14 71639 3 PMID 9222228 Barbato G Ficca G Muscettola G Fichele M Beatrice M Rinaldi F Mar 6 2000 Diurnal variation in spontaneous eye blink rate Psychiatry Research 93 2 145 151 doi 10 1016 S0165 1781 00 00108 6 PMID 10725531 S2CID 35982831 Sotoyama M Villanueva MB Jonai H Saito S 1995 Ocular surface area as an informative index of visual ergonomics Industrial Health 33 2 43 55 doi 10 2486 indhealth 33 43 PMID 7493821 Sotoyama Midori Jonai H Saito S Villanueva MB June 1996 Analysis of ocular surface area for comfortable VDT workstation layout Ergonomics 39 6 877 884 doi 10 1080 00140139608964508 PMID 8681929 Collins M Heron H Larsen R Lindner R February 1987 Blinking patterns in soft contact lens wearers can be altered with training PDF American Journal of Optometry and Physiological Optics 64 2 100 103 doi 10 1097 00006324 198702000 00004 PMID 3826282 S2CID 11828508 Piccoli B Assini R Gambaro S Pastoni F D Orso M Franceschin S Zampollo F De Vito G May 15 2001 Microbiological pollution and ocular infection in CAD operators an on site investigation Ergonomics 44 6 658 667 doi 10 1080 00140130117916 PMID 11373026 S2CID 37127979 Lozato PA Pisella PJ Baudouin C June 2001 The lipid layer of the lacrimal tear film physiology and pathology Journal Francais d Ophtalmologie 24 6 643 658 PMID 11460063 Barber Laurie Gray Gudgel Dan T March 2 2018 How Sexual Activity Can Affect Your Vision American Academy of Ophthalmology Retrieved November 28 2020 Friedman D S O Colmain B J Munoz B Tomany S C McCarty C De Jong P T Nemesure B Mitchell P Kempen J Eye Diseases Prevalence Research Group 2004 Prevalence of Age Related Macular Degeneration in the United States Archives of Ophthalmology 122 4 564 572 doi 10 1001 archopht 122 4 564 PMID 15078675 Bone R A Landrum J T Dixon Z Chen Y Llerena C M 2000 Lutein and zeaxanthin in the eyes serum and diet of human subjects Experimental Eye Research 71 3 239 245 doi 10 1006 exer 2000 0870 PMID 10973733 Johnson E J Hammond B R Yeum K J Qin J Wang X D Castaneda C Snodderly D M Russell R M 2000 Relation among serum and tissue concentrations of lutein and zeaxanthin and macular pigment density The American Journal of Clinical Nutrition 71 6 1555 1562 doi 10 1093 ajcn 71 6 1555 PMID 10837298 American Optometric Association 2013 Lutein and zeaxanthin External links Edit3D Interactive Human Eye Eye Hilzbook Retina Hilzbook Interactive Tool to explore the Human Eye Media related to Human eyes at Wikimedia Commons Quotations related to eyes at Wikiquote Retrieved from https en wikipedia org w index php title Human eye amp oldid 1142634892, wikipedia, wiki, book, books, library,

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