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The human eye is a
sense organ A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures dependi ...
that reacts to
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
and allows
vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), environment through photopic vision (daytime vision), color visio ...
. Rod and
cone A cone is a three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Greek wikt:πα ...

cone
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
in the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well ...

retina
are photoreceptive cells which are able to detect
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...

visible light
and convey this information to the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

brain
. Eyes signal information which is used by the brain to elicit the perception of colour, shape, depth, movement, and other features. The eye is part of the
sensory nervous system The sensory nervous system is a part of the nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense ...
. Similar to the eyes of other mammals, the human eye's non-image-forming
photosensitive ganglion cell Image:Overview of the retina photoreceptors (a).png, 400px, Overview of the retina photoreceptors. ipRGCs labelled at the top-right. Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRG ...
s in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone
melatonin Melatonin is a hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popula ...

melatonin
, and entrainment of the
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenou ...

circadian rhythm
.


Structure

Humans have two eyes, situated on the left and the right of the
face The face is the front of an animal's head that features the eyes Eyes are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

face
. The eyes sit in bony cavities called the
orbits In celestial mechanics Celestial mechanics is the branch of astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical obje ...
, in the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
. There are six
extraocular muscles The extraocular muscles are the six muscles that control Eye movement (sensory), movement of the eye and one muscle that controls eyelid elevation (Levator palpebrae superioris muscle, levator palpebrae). The actions of the six muscles responsible f ...

extraocular muscles
that control eye movements. The front visible part of the eye is made up of the whitish
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye The human eye is a organ (anatomy), sense organ that reacts to light and all ...
, a coloured iris, and the
pupil The pupil is a black hole located in the center of the iris of the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photore ...
. A thin layer called the
conjunctiva The conjunctiva is a tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...
sits on top of this. The front part is also called the
anterior Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient ...
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 in diameter, and 0.5 mm (500 μm) in thickness near its center. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about . The cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center 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 A sphincter is a circular muscle that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning. Sphincters are found in many animals. There are over 60 types in the huma ...
. 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
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 two millimetres. The eyeball is generally less tall than it is wide. The sagittal vertical (height) of a human adult eye is approximately , the transverse horizontal diameter (width) is and the axial anteroposterior size (depth) averages with no significant difference between sexes and age groups. Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88). The typical adult eye has an anterior to posterior diameter of , and a volume of . The eyeball grows rapidly, increasing from about diameter at birth to by three years of age. By age 12, the eye attains its full size.


Components

The eye is made up of three coats, or layers, enclosing various anatomical structures. The outermost layer, known as the
fibrous tunic The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth. The term "corneosclera" is also used to desc ...
, is composed of the
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

cornea
and
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye The human eye is a organ (anatomy), sense organ that reacts to light and all ...
, 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 The choroid, also known as the choroidea or choroid coat, is the Blood vessel, vascular layer of the eye, containing connective tissues, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye ...
,
ciliary body The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the cilia ...
, pigmented epithelium and iris. The innermost is the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well ...

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 The aqueous humour is a transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal us ...
anteriorly, between the cornea and lens, and the
vitreous body The vitreous body (vitreous meaning "glass-like", from Latin vitreus, equivalent to vitr(um) glass + -eus -ous) is the clear gel that fills the space between the Lens (vision), lens and the retina of the human eye, eyeball (the vitreous chamber) ...
, 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 The anterior chamber (Optometric Abbreviations#AC, AC) is the aqueous humor-filled space inside the human eye, eye between the iris (anatomy), iris and the cornea's innermost surface, the Corneal endothelium, endothelium. Hyphema, Uveitis, anterior ...
between the cornea and the iris, and the
posterior chamber The posterior chamber is a narrow space behind the peripheral part of the Iris (anatomy), iris, and in front of the suspensory ligament of the lens and the ciliary processes. The posterior chamber consists of small space directly posterior to the i ...
between the iris and the lens. The lens is suspended to the ciliary body by the suspensory ligament (
Zonule of Zinn The zonule of Zinn () (Zinn's membrane, ciliary zonule) (after Johann Gottfried Zinn) is a ring of fibrous strands forming a zonule (little band) that connects the ciliary body The ciliary body is a part of the eye that includes the ciliary musc ...
), 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.


Structures surrounding the eye


Extraocular muscles

Each eye has six
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s that control its movements: the
lateral rectus The lateral rectus muscle is a muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (bi ...
, the
medial rectus The medial rectus muscle is a muscle in the orbit (anatomy), orbit. As with most of the muscles of the orbit, it is innervation, innervated by the inferior division of the oculomotor nerve (Cranial Nerve III). This muscle shares an origin with sev ...
, the
inferior rectus The inferior rectus muscle is a muscle in the orbit (anatomy), orbit. Structure Innervation As with most of the muscles of the orbit, it is innervation, innervated by the inferior division of oculomotor nerve (Cranial Nerve III). Function It D ...
, the
superior rectus The superior rectus muscle is a muscle in the orbit (anatomy), orbit. It is one of the extraocular muscles. It is innervation, innervated by the superior division of the oculomotor nerve (III). In the primary position (looking straight ahead), its ...
, the
inferior oblique The inferior oblique muscle or obliquus oculi inferior is a thin, narrow muscle placed near the anterior margin of the floor of the orbit (anatomy), orbit. The inferior oblique is an extraocular muscle, and is attached to the maxillary bone (origi ...
, and the
superior oblique The superior oblique muscle, or obliquus oculi superior, is a fusiform muscle originating in the upper, medial side of the orbit (anatomy), orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only e ...
. 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. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye. File:Eye orbit anterior.jpg, Eye and orbit anatomy with motor nerves File:Lateral orbit nerves.jpg, Image showing orbita with eye and nerves visible (periocular fat removed). File:Lateral orbit anatomy 2.jpg, Image showing orbita with eye and periocular fat. File:Eye orbit anatomy anterior2.jpg, Normal
anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things. It ...

anatomy
of the human eye and orbit, anterior view


Vision


Field of view

The approximate
field of view The field of view (FoV) is the extent of the observable world that is at any given moment. In the case of s or sensors it is a through which a detector is sensitive to . Humans and animals In the context of human and primate vision, th ...

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). For both eyes combined (
Binocular vision In , binocular vision is a type of in which an animal has two s capable of facing the same direction to perceive a single of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather t ...

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. It is an area of 4.17
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

steradian
s or 13700
square degree __NOTOC__ A square degree (deg2) is a non-SI Units of measurement, unit measure of solid angle. Other denotations include ''sq. deg.'' and (°)2. Just as degree (angle), degrees are used to measure parts of a circle, square degrees are used to meas ...
s for binocular vision. 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. 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.


Dynamic range

The retina has a static
contrast ratio The contrast ratio (CR) is a property of a display system, defined as the ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the rat ...
of around 100:1 (about 6.5 f-stops). As soon as the eye moves rapidly to acquire a target (
saccade A saccade ( , French French (french: français(e), link=no) may refer to: * Something of, from, or related to France France (), officially the French Republic (french: link=no, République française), is a country primarily located in ...
s), 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 range of 1014, or one hundred trillion (100,000,000,000,000) (about 46.5 f-stops), 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. This range does not include looking at the midday sun (109 cd/m2) or lightning discharge. At the low end of the range is the
absolute threshold In neuroscience and psychophysics, an absolute threshold was originally defined as the lowest level of a Stimulus (physiology), stimulus – light, sound, touch, etc. – that an organism could detect. Under the influence of Detection theory, signal ...
of vision for a steady light across a wide field of view, about 10−6 cd/m2 (0.000001 candela per square meter). 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). The eye includes a
lens A lens is a transmissive optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, s ...
similar to
lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simple ...

lenses
found in optical instruments such as cameras and the same physics principles can be applied. The
pupil The pupil is a black hole located in the center of the iris of the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photore ...

pupil
of the human
eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light L ...

eye
is its
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of ray (optics), rays that come to a focus (optics), focus ...

aperture
; the iris is the diaphragm that serves as the aperture stop. Refraction in the
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

cornea
causes the effective aperture (the
entrance pupil In an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the ...
) 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 () in a brightly lit place to 8 mm () 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.


Eye movement

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. 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 The fovea centralis is a small, central pit composed of closely packed Cone cell, cones in the human eye, eye. It is located in the center of the macula lutea of the retina. The fovea is responsible for sharp central visual perception, vision (al ...
. 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

Rapid eye movement, REM, typically refers to the
sleep Sleep is a naturally recurring state of mind and body, characterized by altered consciousness Consciousness, at its simplest, is or of internal and external existence. Despite millennia of analyses, definitions, explanations and deba ...

sleep
stage during which the most vivid dreams occur. During this stage, the eyes move rapidly.


Saccades

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


Fixational Eye Movements

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 Ocular microtremor (OMT) is a constant, physiological, high frequency (peak 80 Hz), low amplitude (estimated circa 150-2500 nm (1)) eye tremor A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving ...
, 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
microsaccade Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary Eye movement (sensory), eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (visual) ...
s by amplitude. Martin Rolfs 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°."


Vestibulo-ocular reflexes

The vestibulo-ocular reflex is a
reflex In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

reflex
eye movement that stabilizes images on the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well ...

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 center 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

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 Feedback occurs when outputs of a system are routed back as inputs as part of a chain A chain is a assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a in that it is flexib ...

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

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. 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

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 Convergence may refer to: Arts and media Literature *Convergence (book series), ''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-par ...
), while for an object farther away they rotate 'away from each other' (
divergence In vector calculus Vector calculus, or vector analysis, is concerned with derivative, differentiation and integral, integration of vector fields, primarily in 3-dimensional Euclidean space \mathbb^3. The term "vector calculus" is sometimes ...
).


Pupil constriction

Lenses cannot refract light rays at their edges as well as closer to the center. The image produced by any lens is therefore somewhat blurry around the edges (
spherical aberration Spherical aberration (SA) is a type of aberration found in optical systems that use elements with spherical surfaces. Lenses and curved mirrors are most often made with surfaces that are spherical, because this shape is easier to form than non ...

spherical aberration
). It can be minimized by screening out peripheral light rays and looking only at the better-focused center. 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 For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

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.


Accommodation of the lens

Changing the curvature of the lens is carried out by the
ciliary muscle The ciliary muscle is a ring of smooth muscleSchachar, Ronald A. (2012). "Anatomy and Physiology." (Chapter 4) . in the eye's middle layer ( vascular layer) that controls accommodation for viewing objects at varying distances and regulates the ...
s 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.


Eye care professionals

The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a
general practitioner In the medical profession, a general practitioner (GP) is a medical doctor A physician (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varietie ...
. These specialists, or
eye care professional An eye care professional (ECP) is an individual who provides a service related to the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina con ...
s, serve different functions in different countries. Eye care professionals can have overlap in their patient care privileges. For example, both an
ophthalmologist Ophthalmology () is a branch of medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as fa ...

ophthalmologist
(M.D.) and
optometrist Optometry is a health care profession that involves examining the eyes and applicable visual systems for defects or abnormalities as well as prescribing the correction of refractive error with glasses Glasses, also known as eyeglasses or spec ...
(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 The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...
,
cataracts A cataract is a cloudy area in the lens A lens is a transmissive optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natur ...

cataracts
,
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...
,
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well ...
, or
oculoplastics Oculoplastics, or oculoplastic surgery, includes a wide variety of surgical procedures that deal with the orbit (eye socket), eyelids, tear ducts, and the face. It also deals with the reconstruction of the eye and associated structures. Training A ...
. Eye care professionals include: *
Ocularist{{unsourced, date=July 2016 An ocularist is someone who specializes in the fabrication and fitting of ocular prosthetic, ocular prostheses for people who have lost an eye or human eye, eyes due to trauma or illness. The fabrication process for a cust ...
s *
Ophthalmologists Ophthalmology () is a branch of medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment or Palliative ...
*
Opticians An optician, or ''dispensing optician'', is a technical practitioner who designs, fits and dispenses lenses for the correction of a person's vision. Opticians determine the specifications of various Ophthalmology, ophthalmic appliances that will ...
*
Optometrists Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. This often involves prescribing corrective lenses and providing medical eye care. Optometrists (Doctors of Op ...
*
Orthoptists Vision therapy (VT) is an umbrella term for a variety of treatments based around eye exercises. The treatments aim to cure convergence insufficiency and a range of neurological, educational, and spatial difficulties, but lack supporting evidence. ...
and Vision Therapists


Eye irritation

Eye irritation has been defined as "the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye". 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. Several suspected causal factors in our environment have been studied so far. One hypothesis is that
indoor air pollution Indoor air quality (IAQ) is the air quality Air pollution is the presence of substances in the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or ...
may cause eye and airway irritation. 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. 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. Another factor that may be related is work stress. In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users. 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 compound Volatile organic compounds (VOC) are organic chemicals , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's abilit ...
s 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. Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high. An integrated physiological risk model with
blink Blinking is a bodily function; it is a autonomic nervous system, semi-autonomic rapid closing of the eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrae superioris and the activation ...

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. 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 lens One-day disposable contact lenses with blue handling tint in blister-pack packaging Contact lenses, or simply contacts, are thin lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of ...

contact lens
es. 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. 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. 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.Sibony PA, Evinger C. "Anatomy and physiology of normal and abnormal eyelid position and movement". In: Miller NR, Newman NJ, editors. Walsh & Hoyt's ''Clinical Neuro-ophthalmology''. Baltimore, MD: Williams and Wilkins; 1998. pp. 1509–1592 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. 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. 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. Studies have shown that incidence of dry eyes increases with age,Seal, D. V., and Mackie, I. A. (1986). "The questionable dry eye as a clinical and biochemical entity". In F. J. Holly (Ed.) ''The preocular tear film – In health, disease, and contact lens wear''. Dry Eye Institute, Lubbock, TX, pp. 41–51. especially among women. 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. 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. In a study conducted by
UCLA The University of California, Los Angeles (UCLA) is a public In public relations Public relations (PR) is the practice of managing and disseminating information from an individual or an organization An organization, or organ ...
, the frequency of reported symptoms in industrial buildings was investigated. 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. 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 Sick building syndrome (SBS) is a condition in which people in a building suffer from symptoms of illness or become infected with chronic disease from the building in which they work or reside. The outbreaks may or may not be a direct result of ina ...
(SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue. Many of the symptoms described in SBS and multiple chemical sensitivity (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals. 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. 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. 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. Acute incidence rates for nasal, eye, and
throat irritation Throat irritation can refer to a dry cough A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes A microorganism, or microbe,, ''mikros'', "sma ...
, 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. 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. * trying to maintain an intact film of tears by the following actions: :# Blinking and short breaks may be beneficial for VDU users. Increasing these two actions might help maintain the tear film. :# Downward gazing is recommended to reduce ocular surface area and water evaporation. :# 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, and :#Blink training can be beneficial. In addition, other measures are proper lid hygiene, avoidance of eye rubbing, and proper use of personal products and medication. Eye make-up should be used with care.


Eye disease

There are many
diseases A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not due to any immediate external injury. Diseases are often known to be medical ...
, 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 Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold re ...

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 disease Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) and the older term venereal disease, are infection An infection is the invasion of an organism's body by , their multiplication, and the r ...
s such as herpes and genital warts. If contact between the eye and area of infection occurs, the STD can be transmitted to the eye. With aging, a prominent white ring develops in the periphery of the cornea called
arcus senilis Arcus senilis (AS), also known as gerontoxon, arcus lipoides, arcus cornae, corneal arcus, arcus adiposus, or arcus cornealis, is a deposition of phospholipid and cholesterol Cholesterol (from the Ancient Greek Ancient Greek includes t ...
. 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 Ectropion is a medical condition in which the lower eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. Th ...
,
entropion Entropion is a medical condition in which the eyelid (usually the lower lid) folds inward. It is very uncomfortable, as the eyelashes continuously rub against the cornea causing irritation. Entropion is usually caused by genetic factors. This is ...
, , and ptosis. The vitreous gel undergoes liquefaction (
posterior vitreous detachment A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. It refers to the separation of the posterior hyaloid membrane from the retina anywhere posterior to the vitreous base (a 3 ...
or PVD) and its opacities — visible as
floater Floaters or eye floaters are sometimes visible deposits within the eye's vitreous humour ("the vitreous"), which is normally transparent, or between the vitreous and retina.Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4 ...

floater
s — gradually increase in number. Various
eye care professional An eye care professional (ECP) is an individual who provides a service related to the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina con ...
s, including
ophthalmologist Ophthalmology () is a branch of medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as fa ...

ophthalmologist
s (eye doctors/surgeons),
optometrist Optometry is a health care profession that involves examining the eyes and applicable visual systems for defects or abnormalities as well as prescribing the correction of refractive error with glasses Glasses, also known as eyeglasses or spec ...
s, and
optician An optician, or ''dispensing optician'', is a technical practitioner who designs, fits and dispenses lenses for the correction of a person's vision. Opticians determine the specifications of various Ophthalmology, ophthalmic appliances that will ...

optician
s, are involved in the treatment and management of ocular and vision disorders. A
Snellen chart A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch Ophthalmology, ophthalmologist Herman Snellen, who developed the chart in 1862. Many ophthalmologists and vision scientists now us ...

Snellen chart
is one type of
eye chart __NOTOC__ An eye chart, or optotype, is a chart used to measure visual acuity Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual pe ...
used to measure
visual acuity Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), ...
. At the conclusion of a complete
eye examination An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ...

eye examination
, the eye doctor might provide the patient with an
eyeglass prescription An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist Optometry is a health care profession that involves examining the eyes and applicable visual systems for defects or abnormalities as well as prescri ...
for
corrective lens A corrective lens is a lens (optics), lens (i.e. a transmissive optical device) that is typically worn in front of the Human eye, eye to improve daily Visual perception, vision. The most common use is to treat refractive errors: myopia, hyperme ...
es. Some disorders of the eyes for which corrective lenses are prescribed include myopia (
near-sightedness Near-sightedness, also known as short-sightedness and myopia, is an eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo ...
), hyperopia (far-sightedness), astigmatism, and
presbyopia Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Symptoms include difficulty reading small print, having to hold re ...

presbyopia
(the loss of focusing range during aging).


Macular degeneration

Macular degeneration is especially prevalent in the U.S. and affects roughly 1.75 million Americans each year. Having lower levels of lutein and zeaxanthin within the macula may be associated with an increase in the risk of age-related macular degeneration. < Lutein and zeaxanthin act as antioxidants that protect the retina and macula from oxidative damage from high-energy light waves. 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.American Optometric Association (2013)
"Lutein and zeaxanthin"
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Images of human eye

File:Diagram of human eye without labels.svg, Right eye without labels (horizontal section) File:Blausen 0388 EyeAnatomy 01.png File:Blausen 0389 EyeAnatomy 02.png File:Three Main Layers of the Eye.png, The structures of the eye labeled File:Three Internal chambers of the Eye.svg, Another view of the eye and the structures of the eye labeled


See also

* Eye color * Eye development * Eye disease * Eye strain * Hyaloid canal * Iris recognition * Knobloch syndrome * Lacrimal caruncle * Rheum * Spectral sensitivity


References


External links


3D Interactive Human Eye

Eye – Hilzbook

Retina – Hilzbook

Interactive Tool to explore the Human Eye
* * {{Authority control Human eye, Facial features Sensory organs Ophthalmology Vision by taxon Visual system