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Eyes are organs that detect light. Different kinds of light-sensitive organs are found in a variety of animals. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms and may allow the organism to seek out or avoid light, but hardly can be called vision.

Overview

More complex eyes can distinguish shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth perception (binocular vision), as in humans; and others are placed so as to minimize the overlap, such as in rabbits and chameleons.
A human eye with a blue and green colored iris
A human eye with a blue and green colored iris

The first proto-eyes evolved among animals 540 million years ago, about the time of the so-called Cambrian explosion. Almost all animals have eyes, or descend from animals that did. In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, and reducing aberrations when there is enough light.

The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.

Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing very wide-angle, high-resolution images.

Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye. The largest eye ever to be reported measures 27 cm in diameter and belongs to a Colossal squid specimen.

In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight.

Evolution of eyes

Evolution of the eye

Biologists explain the origin and development of eyes, as well as of organs in general, by use of the principles of evolution.

The common origin (monophyly) of all animal eyes is established by shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye evolved some 540 million years ago.
Diagram of major stages in the eye's evolution
Diagram of major stages in the eye's evolution

The earliest "eyes", called eyespots, were light-sensitive proteins in unicellular organisms. In multicellular organisms, simple patches of photoreceptor cells are physically similar to the receptor patches for taste and smell. Eyespots and flat eye patches can only sense ambient brightness: they can distinguish light and dark, but not the direction of the lightsource. Thus, they are sufficient for synchronization of circadian rhythms and they enable a reaction such as turning toward or away from the light source, which from under water can mean the surface, for example. They are not sufficient for image-forming.

When the multicellular eyepatch depressed into a shallow "cup" shape, it achieved the ability to discriminate directional brightness by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of distinguishing dim shapes (for example in the nautilus).

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the photoreceptive cells, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.

The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an "arms race", or rather, a phylogenetic radiation from the species with that first proto-eye, among the descendents of which, there may well have been an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.

Vision in various animals shows adaptation to environmental requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eyes in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry.

Anatomy of the mammalian eye

posterior compartmentora serrataciliary muscleciliary zonulescanal of Schlemmpupilanterior chambercorneairislens cortexlens nucleusciliary processconjunctivainferior oblique musculeinferior rectus musculemedial rectus muscleretinal arteries and veinsoptic discdura matercentral retinal arterycentral retinal veinoptical nervevorticose veinbulbar sheathmaculafoveasclerachoroidsuperior rectus musculeretina

1. posterior compartment
2. ora serrata
3. ciliary muscle
4. ciliary zonules
5. canal of Schlemm
6. pupil
7. anterior chamber
8. cornea
9. iris
10. lens cortex
11. lens nucleus
12. ciliary process
13. conjunctiva
14. inferior oblique muscule
15. inferior rectus muscule
16. medial rectus muscle
17. retinal arteries and veins
18. optic disc
19. dura mater
20. central retinal artery
21. central retinal vein
22. optical nerve
23. vorticose vein
24. bulbar sheath
25. macula
26. fovea
27. sclera
28. choroid
29. superior rectus muscule
30. retina

Dimensions

Dimensions vary only 1–2 mm among humans. The vertical diameter is 24 mm; the transverse being larger. At birth it is generally 16–17 mm, enlarging to 22.5–23 mm by three years of age. Between then and age 13 the eye attains its mature size. It weighs 7.5 grams and its volume is roughly 6.5 milliliters.

Along a line through the nodal (central) point of the eye is the optic axis, which is slightly five degrees toward the nose from the visual axis (i.e., that going towards the focused point to the fovea.

Three layers

The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic.

* The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera. The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape.

* The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized layer which includes the iris, ciliary body, and choroid. The choroid contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye. The iris is seen rather than the cornea when looking straight in one's eye due to the latter's transparency, the pupil (central aperture of iris) is black because there is no light reflected out of the interior eye. If an ophthalmoscope is used, one can see the fundus, as well as vessels (which supply additional blood flow to the retina) especially those crossing the optic disk—the point where the optic nerve fibers depart from the eyeball—among others

* The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which includes the retina.
o Contributing to vision, the retina contains the photosensitive rod and cone cells and associated neurons. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It has two points at which it is different; the fovea and optic disc. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading. The optic disc, sometimes referred to as the anatomical blind spot, is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus "blind". Continuous with the retina are the ciliary epithelium and the posterior epithelium of the iris.
o In addition to the rods and cones, a small proportion (about 1-2% in humans) of the ganglion cells in the retina are themselves photosensitive through the pigment melanopsin. They are generally most excitable by blue light, about 470–485 nm. Their information is sent to the SCN (suprachiasmatic nuclei), not to the visual center, through the retinohypothalamic tract which is formed as melanopsin-sensitive axons exit the optic nerve. It is primarily these light signals which regulate circadian rhythms in mammals and several other animals. Many, but not all, totally blind individuals have their circadian rhythms adjusted daily in this way.

Anterior and posterior segments
Diagram of a human eye; note that not all eyes have the same anatomy as a human eye.
Diagram of a human eye; note that not all eyes have the same anatomy as a human eye.

The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment.

The human eye is not a plain sphere but is like two spheres combined, a smaller, sharper curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.

Within the anterior segment are two fluid-filled spaces:

* the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris.
* the posterior chamber between the iris and the front face of the vitreous.

Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures.

Some ophthalmologists specialize in the treatment and management of anterior segment disorders and diseases.

The posterior segment is the back five-sixths of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve.

The radii of the anterior and posterior sections are 8 mm and 12 mm, respectively. The point of junction is called the limbus.

On the other side of the lens is the second humour, the aqueous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.

Some ophthalmologists specialise in the treatment and management of posterior segment disorders and diseases.

Extraocular anatomy
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Lying over the sclera and the interior of the eyelids is a transparent membrane called the conjunctiva. It helps lubricate the eye by producing mucus and tears. It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.

In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread tears on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex.

In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.

In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets. This placement of the eyes helps to protect them from injury.

In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye.

Function of the mammalian eye
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The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.

In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness), cones (for color) and non-imaging ipRGC (intrinsincally photosensitive retinal ganglion cells) react to the light. They interact with each other and send messages to the brain. The rods and cones enable vision. The ipRGCs enable entrainment to the earth's 24-hour cycle, resizing of the pupil and acute suppression of the pineal hormone melatonin.

Retina

The retina contains one form of photosensitive cells important to vision—rods and cones—in addition to the photosensitive ganglion cells involved in circadian adjustment but probably not involved in vision. Though structurally and metabolically similar, the functions of rods and cones are quite different. Rod cells are highly sensitive to light, allowing them to respond in dim light and dark conditions; however, they cannot detect color differences. These are the cells that allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different wavelengths of light, which allows an organism to see color. The shift from cone vision to rod vision is why the darker conditions become, the less color objects seem to have.

The differences between rods and cones are useful; apart from enabling sight in both dim and light conditions, they have further advantages. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. The fovea gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires staring at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other celestial objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing an individual to observe faint objects.

Rods and cones are both photosensitive, but respond differently to different frequencies of light. They contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each color-range. The process through which these proteins go is quite similar — upon being subjected to electromagnetic radiation of a particular wavelength and intensity, the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The breakdown results in the activation of Transducin and this activates cyclic GMP Phosphodiesterase, which lowers the number of open Cyclic nucleotide-gated ion channels on the cell membrane, which leads to hyperpolarization; this hyperpolarization of the cell leads to decreased release of transmitter molecules at the synapse.

Differences between the rhodopsin and the iodopsins is the reason why cones and rods enable organisms to see in dark and light conditions — each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell by which information is relayed to the visual cortex. This convergence is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. This divergence results in the high visual acuity, or the high ability to distinguish detail, of cone cells compared to rods. If a ray of light were to reach just one rod cell, the cell's response may not be enough to hyperpolarize the connected bipolar cell. But because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapses of the bipolar cell to hyperpolarize it.

Furthermore, color is distinguishable due to the different iodopsins of cone cells; there are three different kinds, in normal human vision, which is why we need three different primary colors to make a color space.

A small percentage of the ganglion cells in the retina contain melanopsin and, thus, are themselves photosensitive. The light information from these cells is not involved in vision and it reaches the brain not directly via the optic nerve but via the retinohypothalamic tract, the RHT. By way of this light information, the body clock's inherent approximate 24-hour cycling is adjusted daily to nature's light/dark cycle. Signals from these photosensitive ganglion cells have at least two other roles in addition. They exercise control over the size of the pupil, and they lead to acute suppression of melatonin secretion by the pineal gland.

Accommodation
Light from a single point of a distant object and light from a single point of a near object being brought to a focus on the retina
Light from a single point of a distant object and light from a single point of a near object being brought to a focus on the retina

Accommodation (eye)

The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina. Because of limited depth of field of the mammalian eye, an object at one distance from the eye might project a clear image, while an object either closer to or further from the eye will not. To make images clear for objects at different distances from the eye, its optical power needs to be changed. This is accomplished mainly by changing the curvature of the lens. For distant objects, the lens needs to be made flatter, for near objects the lens needs to be made thicker and more rounded.

Water in the eye can alter the optical properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. Osmotic effects are made apparent when swimming in freshwater pools, because the osmotic gradient draws water from the pool into the corneal tissue (the pool water is hypotonic), causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. The edema can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye.

The compound eye

Compound eye

The compound eye of an insect. Note the spines between individual lenses.
The compound eye of an insect. Note the spines between individual lenses.

Compound eyes may consist of thousands of ommatidia which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distinguish brightness and color. The image perceived by the arthropod is a combination of inputs from the numerous ommatidia, which are oriented to point in slightly different directions. Compared with single-aperture eyes, compound eyes have poor image resolution; however, they possess a very large view angle and the ability to detect fast movement and, in some cases, the polarization of light.

Acuity
A hawk's eye
A hawk's eye

Visual acuity is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white–black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white–black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution would be 50 CPD (1.2 minute of arc per line pair, or a 0.35 mm line pair, at 1 m). However, the eye can only resolve a contrast of 5%. Taking this into account, the eye can resolve a maximum resolution of 37 CPD, or 1.6 minute of arc per line pair (0.47 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.

Spectral response

visible spectrum

Rough plot of Earth's atmospheric opacity to various wavelengths of electromagnetic radiation. The human eye has evolved so as to be sensitive to a spectrum of low opacity (high transmittance), the "optical window".
Rough plot of Earth's atmospheric opacity to various wavelengths of electromagnetic radiation. The human eye has evolved so as to be sensitive to a spectrum of low opacity (high transmittance), the "optical window".

Human eyes respond to light with wavelength in the range of approximately 400 to 700 nm. Other animals have other ranges, with many such as some fish, turtles, and birds including a significant ultraviolet (shorter than 400 nm) response.

Dynamic range
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The retina has a static contrast ratio of around 100:1 (about 6 1/2 stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 stops) is possible. The process is nonlinear and multifaceted, so an interruption by light merely starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

Eye movement
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MRI scan of human eye
MRI scan of human eye

Eye movements

The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.

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

Extraocular muscles

Extraocular muscles

Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye. Once the human eye sustains damage to the optic nerve, the impulses will not be taken to the brain. Eye transplants can happen but the person receiving the transplant will not be able to see. As for the optic nerve, once it is damaged it cannot be fixed.

Rapid eye movement

Rapid eye movement

Rapid eye movement, or REM for short, typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Saccades

Saccade

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

Microsaccades

Microsaccade

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

Vestibulo-ocular reflex

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, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa.

Smooth pursuit movement

Pursuit movement

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

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

Optokinetic reflex

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

Vergence movement

Vergence

The two eyes converge to point to the same object.
The two eyes converge to point to the same object.

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

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

Diseases, disorders, and age-related changes
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Main articles: List of eye diseases and disorders and List of systemic diseases with ocular manifestations

The stye is a common irritating inflammation of the eyelid.
The stye is a common irritating inflammation of the eyelid.

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

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

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

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

Eye injury and safety
An example of eye trauma.
An example of eye trauma.

Accidents involving common household products cause 125,000 eye injuries each year in the U.S. More than 40,000 people a year suffer eye injuries while playing sports. Sports-related eye injuries occur most frequently in baseball, basketball and racquet sports.

Occupational eye injury

Each day about 2000 U.S. workers have a job-related eye injury that requires medical treatment. About one third of the injuries are treated in hospital emergency departments and more than 100 of these injuries result in one or more days of lost work. The majority of these injuries result from small particles or objects striking or abrading the eye. Examples include metal slivers, wood chips, dust, and cement chips that are ejected by tools, wind blown, or fall from above a worker. Some of these objects, such as nails, staples, or slivers of wood or metal penetrate the eyeball and result in a permanent loss of vision. Large objects may also strike the eye/face causing blunt force trauma to the eyeball or eye socket. Chemical burns to one or both eyes from splashes of industrial chemicals or cleaning products are common. Thermal burns to the eye occur as well. Among welders, their assistants, and nearby workers, UV radiation burns (welder’s flash) routinely damage workers’ eyes and surrounding tissue.

In addition to common eye injuries, health care workers, laboratory staff, janitorial workers, animal handlers, and other workers may be at risk of acquiring infectious diseases via ocular exposure.

Cuisine

In some countries, stuffed cow's eyes are considered a delicacy. They are made by first removing the vitreous humor, lens, cornea, and iris, then are usually boiled. Cow eyes are often stuffed with varieties of coleslaw, beef, and even cream cheese.

Seal eyes are eaten by the Inuit, providing a source of zinc in their diet.

A delicacy in western Norwegian cuisine is the singed head of a sheep or lamb, smalahovud, where the eyes are also eaten.

Ophthalmology is the branch of medicine which deals with the diseases and surgery of the visual pathways, including the eye, brain, and areas surrounding the eye, such as the lacrimal system and eyelids. By convention the term ophthalmologist is more restricted and implies a medically trained surgical specialist. Since ophthalmologists perform operations on eyes, they are generally categorized as surgeons.

The word ophthalmology comes from the Greek roots ophthalmos meaning eye and logos meaning word, thought or discourse; ophthalmology literally means "The science of eyes." As a discipline it applies to animal eyes also, since the differences from human practice are surprisingly minor and are related mainly to differences in anatomy or prevalence, not differences in disease processes. However, veterinary medicine is regulated separately in many countries and states/provinces resulting in few ophthalmologists treating both humans and animals.

History

The eye, including its structure and mechanism, has fascinated scientists and the public in general since ancient times. The majority of all input to the brain comes from vision. Many of the expressions in the English language that mean to understand are equivalent vision terms. "I see", to mean I understand.

Many patients when told that they may have an eye problem will be more concerned about diseases that affect vision than other, more lethal diseases. Being deprived of sight can have a devastating effect on the psyche, as well as economic and social effects, as many blind individuals require significant assistance with activities of daily living and are often unable to continue gainful employment previously held while seeing.

The maintenance of ocular health and correction of eye problems that decrease vision contribute greatly to the ability to appreciate the longer lifespan that all of medicine continues to allow. Given the importance of vision to quality of life, many ophthalmologists consider their job to be rewarding, as they are often able to restore or improve a patient's sight. As detailed below, advances in diagnosis and treatment of disease, and improved surgical techniques have extended our abilities to restore vision like never before.

Sushruta

Sushruta wrote Sushruta Samhita in about fifth Century BCE in India. He described about 72 ocular diseases as well as several ophthalmological surgical instruments and techniques. Sushruta has been described as the first Indian cataract surgeon. Arab scientists are some of the earliest to have written about and drawn the anatomy of the eye—the earliest known diagram being in Hunain ibn Is-haq's Book of the Ten Treatises on the Eye. Earlier manuscripts exist which refer to diagrams which are not known to have survived. Current knowledge of the Gr?co-Roman understanding of the eye is limited, as many manuscripts lacked diagrams. In fact, there are very few Gr?co-Roman diagrams of the eye still in existence. Thus, it is not clear to which structures the texts refer, and what purpose they were thought to have.

An eye examination is a battery of tests performed by an optometrist or ophthalmologist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes. All people should have periodic and thorough eye examinations as part of routine primary care, especially since many eye diseases are silent or asymptomatic.

Eye examinations may detect potentially treatable blinding eye diseases, ocular manifestations of systemic disease, or signs of tumours or other anomalies of the brain.
Entrance tests

* External examination
* Visual acuity
* Amplitude of accommodation
* Color vision
* Cover test
* Stereopsis
* Near point of convergence
* Extraocular motilities
* Pupils
* Visual field screening
* Interpupillary distance

Refraction

* Lensometry
* Keratometry
* Retinoscopy
* Refraction

* Monocular
* Binocular balance

* Cycloplegic refraction

Functional tests

* Accommodative system

* Negative relative accommodation
* Positive relative accommodation

* Vergence system

Health assessment

* Slit lamp biomicroscopy
* Direct ophthalmoscopy
* Binocular indirect ophthalmoscopy
* Tonometry
* Amsler grid
* Visual field assessment
* Gonioscopy

Advanced techniques

* Corneal topography
* Corneal pachymetry
* Scheimpflug ocular imaging
* Retinal tomography
* Ocular computed tomography
* Scanning laser polarimetry

Corneal Pachymetry

Corneal pachymetry is a measurement of the thickness of the cornea using ultrasound

Setting

Ideally, the eye examination consists of an external examination, followed by specific tests for visual acuity, pupil function, extraocular muscle motility, visual fields, intraocular pressure and ophthalmoscopy through a dilated pupil.

A minimal eye examination consists of tests for visual acuity, pupil function, and extraocular muscle motility, as well as direct ophthalmoscopy through an undilated pupil.

Basic examination
Determining a prescription for eyeglasses
Determining a prescription for eyeglasses

External examination

External examination of eyes consists of inspection of the eyelids, surrounding tissues and palpebral fissure. Palpation of the orbital rim may also be desirable, depending on the presenting signs and symptoms. The conjunctiva and sclera can be inspected by having the individual look up, and shining a light while retracting the upper or lower eyelid. The cornea and iris may be similarly inspected.

Visual acuity

Visual acuity

Visual acuity is the eye's ability to detect fine details and is the quantitative measure of the eye's ability to see an in-focus image at a certain distance. The standard definition of normal visual acuity (20/20 or 6/6 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. The terms 20/20 and 6/6 are derived from standardized sized objects that can be seen by a "person of normal vision" at the specified distance. For example, if one can see at a distance of 20 ft an object that normally can be seen at 20 ft, then one has 20/20 vision. If one can see at 20 ft what a normal person can see at 40 ft, then one has 20/40 vision. Put another way, suppose you have trouble seeing objects at a distance and you can only see out to 20 ft what a person with normal vision can see out to 200 feet, then you have 20/200 vision. The 6/6 terminology is more commonly used in Europe and Australia, and represents the distance in metres.

This is often measured with a Snellen chart.

Pupil function

Pupil

An examination of pupilary function includes inspecting the pupils for equal size (1 mm or less of difference may be normal), regular shape, reactivity to light, and direct and consensual accommodation. These steps can be easily remembered with the mnemonic PERRLA (D+C): Pupils Equal and Regular; Reactive to Light and Accommodation (Direct and Consensual).

A swinging-flashlight test may also be desirable if neurologic damage is suspected. The swinging-flashlight test is the most useful clinical test available to a general physician for the assessment of optic nerve anomalies. This test detects the afferent pupil defect, also referred to as the Marcus Gunn pupil. In a normal reaction to the swinging-flashlight test, both pupils constrict when one is exposed to light. As the light is being moved from one eye to another, both eyes begin to dilate, but constrict again when light has reached the other eye.

If there is an efferent defect in the left eye, the left pupil will remain dilated regardless of where the light is shining, while the right pupil will respond normally. If there is an afferent defect in the left eye, both pupils will dilate when the light is shining on the left eye, but both will constrict when it is shining on the right eye.

If there is a unilateral small pupil with normal reactivity to light, it is unlikely that a neuropathy is present. However, if accompanied by ptosis of the upper eyelid, this may indicate Horner's syndrome.

If there is a small, irregular pupil that constricts poorly to light, but normally to accommodation, this is an Argyll Robertson pupil.

Ocular motility

Extraocular muscles

Ocular motility should always be tested, especially when patients complain of double vision or physicians suspect neurologic disease. First, the doctor should visually assess the eyes for deviations that could result from strabismus, extraocular muscle dysfunction, or palsy of the cranial nerves innervating the extraocular muscles. Saccades are assessed by having the patient move his or her eye quickly to a target at the far right, left, top and bottom. This tests for saccadic dysfunction whereupon poor ability of the eyes to "jump" from one place to another may impinge on reading ability and other skills.

Slow tracking, or "pursuits" are assessed by the 'follow my finger' test, in which the examiner's finger traces an imaginary "double-H", which touches upon the eight fields of gaze. These test the inferior, superior, lateral and medial rectus muscles of the eye, as well as the superior and inferior oblique muscles.

Visual field (confrontation) testing

Visual field
Visual field test

Evaluation of the visual fields should never be omitted from the basic eye examination. Testing the visual fields consists of confrontation field testing in which each eye is tested separately to assess the extent of the peripheral field. To perform the test, the individual occludes one eye while fixated on the examiner's eye with the non-occluded eye. The patient is then asked to count the number of fingers that are briefly flashed in each of the four quadrants. This method is preferred to the wiggly finger test that was historically used because it represents a rapid and efficient way of answering the same question: is the peripheral visual field affected?

Common problems of the visual field include scotoma (area of reduced vision), hemianopia (half of visual field lost), homonymous quadrantanopia (involving both eyes) and bitemporal hemianopia.

Intraocular pressure

Intraocular pressure can be measured by any of a series of devices designed to measure the outflow (and resistance to outflow) of the aqueous humour from the eye.

Ophthalmoscopy

Ophthalmoscopic examination may include visually magnified inspection of the internal eye structures and also assessment of the quality of the eye's red reflex.

Ophthalmoscopy allows the one to look directly at the retina and other tissue at the back of the eye. This is best done after the pupil has been dilated with eye drops. A limited view can be obtained through an undilated pupil, in which case best results are obtained with the room darkened and the patient looking towards the far corner.

The appearance of the optic disc and retinal vasculature are the main focus of examination during ophthalmoscopy. Anomalies in the appearance of these internal ocular structures may indicate eye disease or condition.

A red reflex can be seen when looking at a patient's pupil through a direct ophthalmoscope. This part of the examination is done from a distance of about 50 cm and is usually symmetrical between the two eyes. An opacity may indicate a cataract.

Slit-lamp

Close inspection of the anterior eye structures and ocular adnexa are often done with a slit lamp machine. A small beam of light that can be varied in width, height, incident angle, orientation and colour, is passed over the eye. Often, this light beam is narrowed into a vertical "slit", during slit-lamp examination. The examiner views the illuminated ocular structures, through an optical system that magnifies the image of the eye.

This allows inspection of all the ocular media, from cornea to vitreous, plus magnified view of eyelids, and other external ocular related structures. Fluorescein staining before slit lamp examination may reveal corneal abrasions or herpes simplex infection.

The binocular slit-lamp examination provides stereoscopic magnified view of the eye structures in striking detail, enabling exact anatomical diagnoses to be made for a variety of eye conditions.

Also ophthalmoscopy and gonioscopy examinations can also be performed through the slit lamp when combined with special lenses. These lenses include the Goldmann 3-mirror lens, gonioscopy single-mirror/ Zeiss 4-mirror lens for (ocular) anterior chamber angle structures and +90D lens, +78D lens, +66D lens & Hruby (-56D) lens, the examination of retinal structures is accomplished.
===Pre-Hippocrates (pre-Ahad Fazalat) The pre-Hippocratics largely based their anatomical conceptions of the eye on speculation, rather than empiricism. They recognized the sclera and transparent cornea running flushly as the outer coating of the eye, with an inner layer with pupil, and a fluid at the centre. It was believed, by Alcamaeon and others, that this fluid was the medium of vision and flowed from the eye to the brain via a tube. Aristotle advanced such ideas with empiricism. He dissected the eyes of animals, and discovering three layers (not two), found that the fluid was of a constant consistency with the lens forming (or congealing) after death, and the surrounding layers were seen to be juxtaposed. He, and his contemporaries, further put forth the existence of three tubes leading from the eye, not one. One tube from each eye met within the skull.

Alexandrian studies

Alexandrian studies extensively contributed to knowledge of the eye. A?tius tells us that Herophilus dedicated an entire study to the eye which no longer exists. In fact, no manuscripts from the region and time are known to have survived, leading us to rely on Celsius' account—which is seen as a confused account written by a man who did not know the subject matter. From Celsius it is known that the lens had been recognised, and they no longer saw a fluid flowing to the brain through some hollow tube, but likely a continuation of layers of tissue into the brain. Celsius failed to recognise the retina's role, and did not think it was the tissue that continued into the brain.

Rufus

Rufus recognised a more modern eye, with conjunctiva, extending as a fourth epithelial layer over the eye. Rufus was the first to recognise a two chambered eye - with one chamber from cornea to lens (filled with water), the other from lens to retina (filled with an egg-white-like substance). Galen remedied some mistakes including the curvature of the cornea and lens, the nature of the optic nerve, and the existence of a posterior chamber. Though this model was roughly a correct but simplistic modern model of the eye, it contained errors. Yet it was not advanced upon again until after Vesalius. A ciliary body was then discovered and the sclera, retina, choroid and cornea were seen to meet at the same point. The two chambers were seen to hold the same fluid as well as the lens being attached to the choroid. Galen continued the notion of a central canal, though he dissected the optic nerve, and saw it was solid, He mistakenly counted seven optical muscles, one too many. He also knew of the tear ducts.

After Galen

After Galen a period of speculation is again noted by Arab scientists - the lens modified Galen's model to place the lens in the middle of the eye, a notion which lasted until Vesalius reversed the era of speculation. However, Vesalius was not an ophthalmologist and taught that the eye was a more primitive notion than the notion of both Galen and the Arabian scientists - the cornea was not seen as being of greater curvature and the posterior side of the lens wasn't seen to be larger.

Understanding of the eye had been so slow to develop because for a long time the lens was perceived to be the seat of vision, not as part of the pathway for vision. This mistake was corrected when Fabricius and his successors correctly placed the lens and developed the modern notion of the structure of the eye. They removed the idea of Galen's seventh muscle (the retractor bulbi) and reinstated the correct curvatures of the lens and cornea, as well as stating the ciliary body as a connective structure between the lens and the choroid.

Muslim ophthalmology

Ophthalmology in medieval Islam

Of all the branches of Islamic medicine, ophthalmology was considered one of the foremost. The specialized instruments used in their operations ran into scores. Innovations such as the “injection syringe”, invented by the Iraqi physician Ammar ibn Ali of Mosul, which was used for the extraction by suction of soft cataracts, were quite common. In cataract surgery, Ammar ibn Ali attempted the earliest extraction of cataracts using suction. He introduced a hollow metallic syringe hypodermic needle through the sclerotic and successfully extracted the cataracts through suction.

Ibn al-Haytham (Alhazen), the "father of optics", studied the anatomy of the eye extensively. He made important contributions to ophthalmology and eye surgery and posited the first correct explanations of the process of sight and visual perception in his Book of Optics (1021). He was also the first to hint at the retina being involved in the process of image formation.

Ibn al-Nafis, in The Polished Book on Experimental Ophthalmology, discovered that the muscle behind the eyeball does not support the ophthalmic nerve, that they do not get in contact with it, and that the optic nerves transect but do not get in touch with each other. He also discovered many new treatments for glaucoma and the weakness of vision in one eye when the other eye is affected by disease. Salah–ud-din bin Youssef al-Kalal bi Hama (i.e. the eye doctor of Hama) was a Syrian oculist who flourished in Hama in 1296. He wrote for his son a very elaborate treatise of ophthalmology entitled Nur al-Uyun wa Jami al-Funun (light of the eyes and collection of rules).

Seventeenth and eighteenth century

The seventeenth and eighteenth century saw the use of hand-lenses (by Malpighi), microscopes (van Leeuwenhoek), preparations for fixing the eye for study (Ruysch) and later the freezing of the eye (Petit). This allowed for detailed study of the eye and an advanced model. Some mistakes persisted such as: why the pupil changed size (seen to be vessels of the iris filling with blood), the existence of the posterior chamber, and of course the nature of the retina. In 1722 Leeuwenhoek noted the existence of rods and cones though they were not properly discovered until Gottfried Reinhold Treviranus in 1834 by use of a microscope.

Ophthalmic surgery in Great Britain

The first ophthalmic surgeon in Great Britain was John Freke, appointed to the position by the Governors of St Bartholomew's Hospital in 1727, but the establishment of the first dedicated ophthalmic hospital in 1805 - now called Moorfields Eye Hospital in London, England was a transforming event in modern ophthalmology. Clinical developments at Moorfields and the founding of the Institute of Ophthalmology by Sir Stewart Duke-Elder established the site as the largest eye hospital in the world and a nexus for ophthalmic research.

Professional requirements

Ophthalmologists are medical doctors (M.D.) or Doctors of Osteopathic Medicine (D.O.). who have completed a college degree, medical school, and an additional four years of post-graduate training in ophthalmology in many countries. Many ophthalmologists also undergo additional specialized training in one of the many subspecialities. Ophthalmology was the first branch of medicine to offer board certification, now a standard practice among all specialties.

United States

In the United States, 4 to 5 years of residency training after medical school are required, with the first year being an internship in surgery, internal medicine, pediatrics, or a general transition year. Most currently practicing ophthalmologists train in medical residency programs accredited by the Accreditation Council for Graduate Medical Education (ACGME) and are board certified by the American Board of Ophthalmology. Some physicians train in osteopathic medical schools may hold a Doctor of Osteopathy degree denoted as DO rather than MD. The same residency and certification requirements for ophthalmology training must be fulfilled by osteopathic physicians (DO degree). Completing the requirements of continuing medical education is mandatory for continuing licensure and re-certification. Professional bodies like the AAO and ASCRS organize conferences and help members through CME programs to maintain certification, in addition to political advocacy and peer support.

United Kingdom

In the United Kingdom, there are four colleges that grant postgraduate degrees in ophthalmology. The Royal College of Ophthalmologists grants MRCOphth and FRCOphth (postgraduate exams), the Royal College of Edinburgh grants MRCSEd, the Royal College of Glasgow grants FRCS and Royal College of Ireland grants FRCSI. Work experience as a specialist registrar and one of these degrees is required for specialisation in eye diseases.

Australia and New Zealand

In Australia and New Zealand, the FRACO/FRANZCO is the equivalent postgraduate specialist qualification. Overseas-trained Ophthalmologists are assessed using the pathway published on the RANZCO website. Those who have completed their formal training in the UK and have the CCST/CCT are usually deemed to be comparable.

India

In India, after completing MBBS degree, post-graduation in Ophthalmology is required. The degrees are Doctor of Medicine (MD), Master of Surgery (MS), Diploma in Ophthalmic Medicine and Surgery (DOMS) or Diplomate of National Board (DNB). The concurrent training and work experience is in the form of a Junior Residency at a Medical College, Eye Hospital or Institution under the supervision of experienced faculty. Further work experience in form of fellowship, registrar or senior resident refines the skills of these eye surgeons. All India Ophthalmological Society (AIOS) and various state level Ophthalmological Societies (like DOS) hold regular conferences and actively promote continuing medical education. Royal colleges of the united kingdom, mainly Royal college of surgeons of Edinburgh (RCSEd), Royal College of ophthalmologists (RCOphth) and Royal college of physicians and Surgeons of Glasgow (RCPSG) are conducting their fellowship and membership examinations since mid 1990s and awarding fellowships and memberships to the successful candidates.

Pakistan

In Pakistan, there is a structured residency program leading into FCPS. Further detail is at http://cpsp.edu.pk/

Canada

In Canada, an Ophthalmology residency after medical school is undertaken. The residency lasts a minimum of 5 years after the MD degree although subspecialty training is undertaken by about 30% of fellows (FRCSC). There are about 30 vacancies per year for ophthalmology training in all of Canada.

Finland

In Finland, physicians willing to become ophthalmologists must undergo a 5 year specialization which includes practical training and theoretical studies.

Distinction from Optometry

Optometrists treat patients with medications and optical aids, including ordering and interpreting examinations, prescribing optical aids such as eyeglasses, treatment of infectious and inflammatory conditions, and treatment of allergies. Optometrist are sometimes affiliated with chains of opticians but ophthalmologists generally are not.

Ophthalmologists are surgical and medical specialists who treat diseases affecting the eye, orbit, and visual system of the brain. Ophthalmologists may also assist other specialists in treating other ocular disorders. Ophthalmologists may treat patients with surgery or medication. Many ophthalmologists receive further training in one or several of the following sub-specialties: Vitreoretinal disease, Pediatric ophthalmology and Adult Strabismus, Cornea and External disease, Glaucoma, Neuro-Ophthalmology, Uveitis, Ocular Pathology and Oculoplastics.

Overlap areas between Ophthalmology and Optometry include:

* Both professions treat patients with medications and optical aids
* Both professions perform testing for common ocular problems affecting children (i.e., amblyopia and strabismus) and the adult population (cataract, glaucoma, and diabetic retinopathy). Optometrists usually refer to ophthalmologists for surgical assessment and treatment of ocular diseases if a consultation is required.
* Ophthalmologists may refer patients with poor vision to optometrists specializing in low vision for optical aids or low vision rehabilitation. Both ophthalmologists and optometrists are trained in refraction for glasses and contact lenses.

Sub-specialities
Extraocular muscle surgery for strabismus (Inferior rectus muscle here) in progress
Extraocular muscle surgery for strabismus (Inferior rectus muscle here) in progress

Ophthalmology includes sub-specialities which deal either with certain diseases or diseases of certain parts of the eye. Some of them are:

* Anterior segment surgery
* Cataract - not considered a subspecialty per se, since most general ophthalmologists do surgery for this.
* Cornea, ocular surface, and external disease

* Glaucoma
* Neuro-ophthalmology
* Ocular oncology
* Oculoplastics & Orbit surgery
* Ophthalmic pathology
* Pediatric ophthalmology/Strabismus (squint)
* Refractive surgery
* Medical retina, deals with treatment of retinal problems conservatively.
* Vitreoretinal Surgery, deals with surgical management of retinal and posterio segment diseases and disorders. Medical retina and vitreoretinal surgery sometimes together called posterior segment subspecialisation.
* Uveitis/Immunology
* Veterinary" Formal specialty training programs in veterinary ophthalmology now exist in some countries .

Ophthalmic surgery
For a comprehensive list of surgeries performed by ophthalmologists, see eye surgery.

Notable ophthalmologists

This is an incomplete list, which may never be able to satisfy certain standards for completeness.
Revisions and sourced additions are welcome.
See also: :Category:Ophthalmologists

Pre-18th century

* Marie Colinet, wife of Wilhelm Fabry, employs a magnet for removing a foreign body from the eye, 1627.

18th-19th century

* Sir William Adams (UK) Founder of Exeter's West of England Eye Infirmary.
* Carl Ferdinand von Arlt (1812-1887), the elder (Austrian) proved that myopia is largely due to an excessive axial length, published influential textbooks on eye disease, and ran annual eye clinics in needy areas long before the concept of volunteer eye camps became popular. His name is still attached to some disease signs, eg, von Arlt's line in trachoma. His son Ferdinand Ritter von Arlt, the younger, was also an ophthalmologist.
* Jacques Daviel (France) claimed to be the 'father' of modern cataract surgery in that he performed extracapsular extraction instead of needling the cataract or pushing it back into the vitreous. It is said that he carried out the technique on 206 patients in 1752-3, out of which 182 were reported to be successful. These figures are not very credible, given the total lack of both anaesthesia and aseptic technique at that time.
* Frans Cornelis Donders (1818-1889) (Dutch) published pioneering analyses of ocular biomechanics, intraocular pressure, glaucoma, and physiological optics. Made possible the prescribing of combinations of spherical and cylindrical lenses to treat astigmatism.
* Albrecht von Graefe (1828-1870) (Germany) Along with Helmholtz and Donders, one of the 'founding fathers' of ophthalmology as a specialty. A brilliant clinician and charismatic teacher who had an international influence on the development of ophthalmology. A pioneer in mapping visual field defects and diagnosis and treatment of glaucoma. Introduced a cataract extraction technique that remained the standard for over 100 years, and many other important surgical techniques such as iridectomy. Rationalised the use of many ophthalmically important drugs, including mydriatics & miotics. The founder of the one of the earliest ophthalmic societies (German Ophthalmological Society, 1857) and one of the earliest ophthalmic journals (Graefe's Archives of Ophthalmology). The most important ophthalmologist of the nineteenth century.
* Allvar Gullstrand (Sweden), Nobel Prize winner in 1911 for his research on the eye as a light-refracting apparatus. Described the schematic eye a mathematical model of the human eye based on his measurements known as the optical constants of the eye. His measurements are still used today.
* Hermann von Helmholtz, great German polymath, invented the ophthalmoscope (1851) and published important work on physiological optics, including colour vision (1850s).
* Hermann Snellen (Netherlands) introduced the Snellen chart to study visual acuity.

20th-21st century

* William Horatio Bates (1860-1931) (USA) Creator of the unorthodox Bates Method, credited for being the founder of the Natural Vision Improvement movement.
* Vladimir Petrovich Filatov (Ukraine) (1875-1956) His contributions to the medical world include the tube flap grafting method, corneal transplantation and preservation of grafts from cadaver eyes and tissue therapy. He founded The Filatov Institute of Eye Diseases & Tissue Therapy, Odessa, one of the leading eye care institutes in the world.
* Ignacio Barraquer (Spain, 1884-1965),invented in 1917 the first motorized vacuum instrument (erisophake) for intracapsular cataract extraction. Founder of the Barraquer Clinic (1941) and the Barraquer Institute (1947) in Barcelona, Spain.
* Tsutomu Sato (Japan), pioneer in incisional refractive surgery, including techniques for astigmatism and the invention of radial keratotomyfor myopia.
* Jules Gonin (Switzerland)"father of retinal detachment surgery"
* Sir Harold Ridley (UK) may have been the first to successfully implant an artificial intraocular lens 1949, after observing that plastic fragments in the eyes of wartime pilots were well tolerated. He fought for decades against strong reactionary opinions to have the concept accepted as feasible and useful.
* Charles Schepens (Belgium), "father of modern retinal surgery", developer of the Schepens indirect binocular ophthalmoscope whilst at Moorfields Eye Hospital, founder of the Schepens Eye Research Institute, Boston, USA. This premier research institute is associated with Harvard Medical School and Massachusetts Eye & Ear Infirmary.
* Marshall M. Parks, "father of pediatric ophthalmology".
* José Ignacio Barraquer (Spain, 1916-1998), "father of modern refractive surgery", developed in the 1960s lamellar techniques including keratomileusis and keratophakia, as well as the first microkeratome and corneal microlathe.
* Tadeusz Krwawicz (Poland), developed in 1961 the first cryoprobe for intracapsular cataract extraction.
* Svyatoslav Fyodorov (Russia) popularizer of radial keratotomy
* Charles Kelman (United States) developed the ultrasound and mechanized irrigation/aspiration system for phacoemulsification, first allowing cataract extraction through a small incision.
* Ioannis Pallikaris (Greece), performed the first laser assisted intrastromal keratomileusis or LASIK surgery.
* Fred Hollows (New Zealand/Australia) pioneered programs in Nepal, Eritrea, and Vietnam, and among Australian aborigines, including the establishment of cheap laboratory production of intraocular lenses in Nepal and Eritrea.
* Ian Constable (Australia) founded the Lions Eye Institute in Perth, Western Australia, the largest eye research institute in the southern hemisphere and home to 10 ophthalmologists.
* L. L. Zamenhof, Poland - Creator of the language Esperanto.
* Bashar al-Assad is the Syrian President. He did his ophthalmology residency in a London hospital.
* Syed Modasser Ali is an ophthalmic surgeon from Bangladesh who used to be the Director-General of Health Services for the Bangladesh government. He is the authour of the first book on Community Ophthalmology (public eye health).

An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.

Types of eye care professionals

* Optometrist - A Doctor of Optometry (OD) trained to diagnose and treat common eye diseases and disorders as well as refractive vision correction. In the US, the standard education is four years of college, four years of Optometry school at an accredited Doctor of Optometry program. An additional one to two years of residency, internship, and/or fellowship and specialty training is required for specialty training.
* Ophthalmologist - A Doctor of Medicine (MD) who specializes in surgical eye care. In the US, this often requires four years of college, four years of medical school, and four to six more years of residency, internship, and/or fellowship and sub specialty training.
* Ophthalmic Medical Practitioner - A Doctor of Medicine (MD) who specialises in ophthalmic conditions but who has not completed a specialisation in Ophthalmology. (term used in the UK).
* Oculist - Older term for either an ophthalmologist or optometrist.
* Ocularist - Specialize in the fabrication and fitting of ocular prostheses for people who have lost eyes due to trauma or illness.
* Optician - Specializes in the fitting and fabrication of ophthalmic lenses, spectacles, contact lenses, low vision aids and ocular prosthetics. They may also be referred to as an Optical Dispenser, Dispensing Optician, Ophthalmic Dispenser. The prescription for the corrective lenses must be supplied by an ophthalmologist or optometrist. This is a regulated profession in most jurisdictions.
* Orthoptist - Specializes in ocular motility, which is the movement of the eye controlled by the extraocular muscles.
* Vision therapist - Works with patients that require vision therapy, such as low vision patients.
* Ophthalmic Medical Personnel - A collective term for allied health personnel in ophthalmology. It is often used to refer to specialized personnel (unlike ocularists or opticians). The Joint Commission on Allied Health Personnel in Ophthalmology administers OMP certifications.
* Optometric physician or medical optometrist - a term used by some optometrist that denotes expanded licensure.

The distinction between optometrist and ophthalmologist

An optometrist is defined by the World Council of Optometry (a member of the World Health Organisation) as follows:

Optometry is a healthcare profession that is autonomous, educated, and regulated (licensed/registered), and optometrists are the primary healthcare practitioners of the eye and visual system who provide comprehensive eye and vision care, which includes refraction and dispensing, detection/diagnosis and management of disease in the eye, and the rehabilitation of conditions of the visual system.

The American Academy of Ophthalmology describes an ophthalmologist as follows:

A medical doctor who specializes in all aspects of eye care including diagnosis, management, and surgery of ocular diseases and disorders.

Two important distinctions are evident in these definitions. First, ophthalmologists are medical doctors and have attended medical school and specialize in surgical care of the eye, while optometrists are doctors of optometry who have attended optometry school and specialize in the general care of the eye and vision. Second, ophthalmologists are responsible for surgical treatment or diseases and disorders. Optometrists "provide comprehensive eye and vision care, which includes refraction and detection/diagnosis and management of disease in the eye" with limited surgical involvement.

There are also important similarities. Both optometrists and ophthalmologists treat patients with medications and optical aids. Both perform screenings for common ocular problems affecting children (such as amblyopia and strabismus) and the adult population (such as cataracts, glaucoma, and diabetic retinopathy). Optometrists usually refer to ophthalmologists for further assessment for surgical treatment of ocular diseases. Both are required to participate in ongoing continuing education courses to maintain licensure and stay current on the latest standards of care.

Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye's iris. Humans and animals have many phenotypic variations in eye color. In humans, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris. The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.

Three main elements within the iris contribute to its color: the melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma. In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin. Color variations among different irises are typically attributed to the melanin content within the iris stroma. The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium.

Genetic determination of eye color

Eye colors can range from the most common color, brown, to the least common, gray. Rare genetic mutations can even lead to unusual eye colors: black, red, and violet. Eye color is an inherited trait influenced by more than one gene. These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is unknown at present, but there are a few likely candidates.

The gene OCA2 (OMIM: 203200), when in a variant form the gene causes the pink eye color and hypopigmentation common in human albinism, (the name of the gene is derived from the disorder it causes, oculocutaneous albinism type II). Different SNPs within OCA2 are strongly associated with brown and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are: SLC24A4 , TYR .

Blue eyes with a brown spot, green eyes and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 [of theHerc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots".

Eye color usually stabilizes when an infant is around 6 months old.

Synopsis
Schematic representation of different eye colors resulting from different conditions in the iris
Schematic representation of different eye colors resulting from different conditions in the iris

Classification of colors
The perception of color depends upon various factors. These are the same eyes; however, depending on the light and surrounding hues, the eye color can appear quite different.
The perception of color depends upon various factors. These are the same eyes; however, depending on the light and surrounding hues, the eye color can appear quite different.

Iris color can provide a large amount of information about an individual and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals. Various classification systems have ranged from a basic "light" or "dark" description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison.

As the perception of color is dependent on viewing conditions (e. g. the amount and type of illumination, as well as the hue of the surrounding environment), so is the perception of eye color.

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue. Seeing the need for a standardized classification system that was simple, yet detailed enough for research purposes, Seddon et. al developed a graded one based on the predominant iris color and the amount of brown or yellow pigment present. There are 3 true colors in the eyes that determine the outward appearance; brown, yellow, and blue. How much of each color one has determines the appearance of the eye color. The color of the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some blue, making them appear green. Brown eyes appear brown because most of the eye contains the brown color. The above is true for Homo sapiens; the iris color can vary in the animal world. Instead of blue in humans, autosomal recessive color in the species Corucia zebrata is black, whereas the autosomal dominant color is yellow-green.

Amber
Human amber eyes displaying the yellow pigments.
Human amber eyes displaying the yellow pigments.

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called "lipochrome" in the iris (which is also found in green and violet eyes). They are nicknamed "Wolf eyes" due to the high rate of the amber eye color in wolves. Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise of many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples; while amber eyes are of a solid gold hue.

The eyes of some pigeons contain yellow fluorescing pigments known as pteridines. The bright yellow eyes of the Great Horned Owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome.

Blue
A blue eye
A blue eye

Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present is eumelanin. The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait, however it is a polygenic trait (meaning that it is controlled by the interactions of several genes, not just one). Eiberg and colleagues showed in a study published in Human Genetics that a mutation in the 86th intron of the HERC2 gene, which is hypothesized to interact with the OCA2 gene promoter, reduced expression of OCA2 with subsequent reduction in melanin production.

The authors concluded that the mutation may have arisen in a single individual around the Black Sea region 6,000-10,000 years ago, perhaps suggesting that all people with true blue eyes are more closely related. However, blue eyes with brown spots around the pupil are not related to this mutation.

Blue eyes are most common in Poland , Ireland , Netherlands , Iceland , Austria , Sweden , Norway , Denmark , Russia , Finland , Germany , France , Estonia , and the United Kingdom They are also present in Southern Europe, Spain , Italy and the Balkans , the Middle East (especially in Israel and Lebanon), India and are also found in Afghanistan. A 2002 study found the prevalence of blue eye color among Whites in the United States to be 33.8% for those born between 1936 and 1951 compared to 57.4% for those born between 1899 and 1905.

Brown
Brown human iris
Brown human iris
Light brown human iris
Light brown human iris

Brown eyes are predominant in humans and, in many populations, it is (with few exceptions) the only iris color present. It is less common in countries around the Baltic Sea, such as Finland and Estonia. Germany has also shown signs that many people have dark brown eyes.

In humans, brown eyes contain large amounts of melanin (eumelanin) within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Very dark brown irises may appear at a glance to be black.

Gray
A steel blue-gray eye
A steel blue-gray eye

Gray eyes have less melanin than blue eyes, even though they are considered a darker shade of blue (like blue-green). They are most common in European Russia, Finland and the Baltic States. Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

A gray iris may indicate the presence of a uveitis. However, other visual signs make a uveitis obvious.

Visually, gray eyes often tend to appear to change between the shades of blue, green and gray. This is said to be influenced by the lighting and the surroundings (such as clothes, makeup, etc.).

The Greek goddess Athena was renowned for having "owl-gray" (in Greek, γλαυκ?πι? – glaukōpis) or "sea-gray" eyes.

Green
Green eyes
Green eyes

The most unusual color, only 1-2% of the the world population have true green eyes. Green eyes are the product of moderate amounts of melanin. According to some researchers, green eyes are the result of mutations that change the melanin structure Green eyes are most common in Europe and to a lesser extent in the Middle East, Northern parts of India, Pakistan, and Afghanistan. 88% of the Icelandic population have either green or blue eye color .

Hazel
This eye shows a mixture of brown, green and amber colors.
This eye shows a mixture of brown, green and amber colors.
Some eye colors are too mixed to identify properly, and are identified as hazel for simplicity's sake.
Some eye colors are too mixed to identify properly, and are identified as hazel for simplicity's sake.

Hazel eyes are due to a combination of a Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from a light brown to a medium golden-green. A number of studies using three-point scales have assigned "hazel" to be the medium-color between light blue and dark green. This can sometimes produce a multicolored iris, i.e., an eye that is light brown near the pupil and charcoal or amber/dark green on the outer part of the iris (and vice versa) when observed in sunlight. Hazel is mostly found in some regions of North America, Europe, the Middle East, parts of Central Asia, parts of North India, Northern Pakistan and in Afghanistan. Rarely, hazel eyes can be found in people of sub-saharan African and East Asian descent.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light-brown, gold, or other times with green or dark green. In North America, "hazel" is often used to describe eyes that appear to change color, ranging from light brown to green and even gray, depending on current lighting in the environment.

Red

The eyes of an albinistic person may appear red under certain lighting conditions due the very low quantities of melanin. "True" red eyes also exist in albinistic populations, but are very rare.

The red-eye effect in flash photographs makes the pupils (rather than the irises) appear red. It is not related to eye color.

Violet

The appearance of "violet" eyes is thought to occur from the mixing of red and blue reflections. Some albinos have eyes that appear violet. Violet eyes are either a form of blue eyes or a mutation.

Medical implications

Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. An increased risk of uveal melanoma has been found in those with blue, green or gray iris color.

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites of the eyes is associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

Anomalous conditions

Aniridia

Aniridia

Aniridia: Eyes wherein the irises are not present; the eyes appear to be two large pupils.
Aniridia: Eyes wherein the irises are not present; the eyes appear to be two large pupils.

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.

Ocular albinism and eye color

Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism, the color of the irises is typically blue, but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much brighter in the albino, and this can increase the red eye effect in photographs. Edgar Winter's eyes are an example of this trait.

Heterochromia

Heterochromia

Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury. This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics such as chimerism and Waardenburg syndrome. Trauma and certain medications, such as some prostaglandin analogues can also cause increased or decreased pigmentation in one eye. On occasion, the condition of having two different colored eyes is caused by blood staining the iris after sustaining injury.

British singer David Bowie is a famous person often wrongly attributed with heterochromia. His apparent condition is due to a teenage injury. (One eye appears darker because the pupil is permanently dilated.) American actress Kate Bosworth has sectoral heterochromia, resulting in a hazel section at the bottom of her right blue eye, while the left is completely blue. American Actress Elizabeth Berkley has sectoral heterochromia; her right eye is half green and half brown, and her entire left eye is green. So does actor Anthony Head - he has a patch of hazel in his left eye where both eyes are blue-green overall. The lead vocalist of American band Rise Against, Tim McIlrath, has heterochromia; his left eye is blue while his right is brown. American actress Mila Kunis also has heterochromia, resulting in one blue eye and one brown-green eye. American Actress Demi Moore also has heterochromia, by having one green eye, and the other hazel.

Eye color change

Often, paler newborns have blue eyes, which change to green, hazel, light brown or dark brown. This is possibly the origin of the idiom "being blue-eyed" (i. e. na?ve; gullible).

It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to indicate their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes are seen, most often as a result of exposure to the sun. Sunlight triggers melanin production in the eye, as it does to the skin.

Eyedrops containing a prostaglandin analogue (such as latanoprost) may result in a permanently darkened iris; these eyedrops are commonly used to treat open-angle glaucoma.

Eye color and red-eye effect

The photographic red-eye effect is more prominent in people with pale (blue or grey) eyes, and a similar effect (possibly the same) is observed in the eyeshine of blue-eyed cats and dogs.

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