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Eyes

Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.
Closeup of a blue-green human eye
The human eyes are sometimes metaphorically called "the windows to the soul."
This image clearly shows the pupil, iris, and blood vessels of the human eye.
An eye is an organ of vision that detects light. Different kinds of light-sensitive organs are found in a variety of creatures. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision. Many complex organisms, including some mammals, birds, reptiles and fish, have two eyes which may be placed on the same plane to be interpreted as a single three-dimensional "image" (binocular vision), as in humans; or on different planes producing two separate "images" (monocular vision), such as in rabbits and chameleons.

1 Varieties of eyes
2 Evolution of eyes
3 Anatomy
3.1 Other articles regarding eye anatomy
4 Cytology
5 Acuity
6 Dynamic range
7 Adnexa and related parts
7.1 The orbit
7.2 Eyebrows
7.3 Eyelids
7.4 Eyelashes
8 Eye movement
8.1 How we see an object
8.2 Colour vision
8.3 Extraocular muscles
8.4 Rapid eye movement
8.5 Saccades
8.6 Microsaccades
8.7 Vestibulo-ocular reflex
8.8 Smooth pursuit movement
8.9 Optokinetic reflex
8.10 Vergence movement
8.11 Accommodation
9 Diseases, disorders, and age-related changes
10 References

Varieties of eyes
Human eye
The compound eyes of a dragonfly
Compound eye of Antarctic krillIn 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, where the light is detected and converted into electrical signals, which 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 which regulates the intensity of the light that enters the eye. 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 give a pixelated image (not multiple images as is often believed). 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 (many Strepsiptera) have compound eye composed of a few facets each with a retina capable of creating an image, which does provide multiple image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain providing a very wide angle high resolution image.
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.
Some of the simplest eyes, called ocelli, can be found in animals like snails, who cannot actually "see" in the common 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 (day and night), but no more. This enables snails to keep out of direct sunlight. Jumping spiders have simple eyes that are so large, supported by an array of other smaller eyes, that they can get enough visual inputs to hunt and pounce on their prey. Some insect larvae like caterpillars have a different type of single eye (stemmata) which gives a rough image.

Evolution of eyes
How a complex structure like the projecting eye could have evolved is often said to be a difficult question for the theory of evolution. Darwin famously treated the subject of eye evolution in his Origin of Species:
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
The common origin of all animal eyes is now widely accepted as fact based on shared features of all eyes. All light-sensitive organs rely on photoreceptor systems employing a family of proteins called opsins, which, by structural and sequence homology can be shown to be of common origin. Indeed, the seven sub-families of opsins existed in the common animal ancestor.
Despite the precision and complexity of the eye, theoretical analysis of eye evolution, developed by Dan-Erik Nilsson and Susanne Pelger (Nilsson and Pelger, 1994, Proc Biol Sci), demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection. Pro-intelligent design mathematician David Berlinski [1] criticized these findings, including a criticism that the work contained no computer simulations (something assumed by a number of scientists but disclaimed by the original authors), and criticisms of the scientific establishment in general. [2] The original authors and other pro-evolution scientists subsequently challenged Berlinski's criticisms. [3]
Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans and some, like diurnal birds of prey, can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution. However, the development of the eye is considered by many experts to be monophyletic; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago (Mya).
As far as the vertebrate/mollusk eye is concerned, intermediate, functioning stages have existed in nature, which is also an illustration of the many varieties and peculiarities of eye construction. In the monophyletic model, these variations are less illustrative of non-vertebrate types such as the arthropod (compound) eye, but as those eyes are simpler to begin with, there are fewer intermediate stages to find.
Eye Spot- A simple patch of photosensitive cells, common among lower invertebrates. Can sense ambient brightness. Physically similar to receptor patches for taste and smell. Some organisms cover the spot in optically-transparent skin cells.
Pit Eye- The patch gradually depresses into a cup, which first grants the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepens. Pit eyes were seen in ancient snails, and are found in some invertebrates living today.
"Pinhole Camera" Eye- As the pit deepens into a cup, then a chamber, the opening of the chamber achieves true imaging, for fine directional sensing and some shape sensing. Currently found in the Nautilus.
Pinhole Camera with Protective Layer- An overgrowth of transparent cells prevents contamination and parasitic infestation. The chamber contents, now segregated, can slowly specialize into a transparent humour, for optimizations such as colour filtering, higher refractive index, blocking of ultraviolet, or the ability to operate in and out of water. The layer may in certain classes be related to the moulting of the organism's shell or skin.
Multiple Humours- The transparent cells over the aperture split into two layers, with liquid in between. The liquid originally serves as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles, greater imaging resolution, or both. Again, division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth. Note: this layout has not been found, nor is it expected to be found. Fossilization rarely preserves soft tissues. In case it does, the new humour would almost certainly close as the remains dessicate, or as sediment overburden forces the layers together. Then the fossilized eye would resemble the previous layout.
Crystalline Lens- It is biologically difficult to maintain a transparent layer of cells as sizes and thicknesses gradually increase. Deposition of transparent but nonliving material eases the need for nutrient supply and waste removal. In trilobites the material was calcite; in humans the material is a mixture of proteins called crystallins. A gap between tissue layers naturally forms a biconvex shape, which is optically and mechanically ideal for substances of normal refractive index. A biconvex lens confers not only optical resolution, but aperture and low-light ability, as resolution is now decoupled from hole size (which slowly increases again, free from the circulatory constraints).
Separate Cornea and Iris- Independently, a transparent layer and a nontransparent layer may split forward from the lens. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans.
"Backward" Illumination of Retina- The retina may revert on itself, forming a double layer. The nerves and blood vessels can migrate to the middle, where they do not block light, or form a blind spot on the retina. This type is seen in squids, which live in the dim oceans. In cats, which hunt at night, the retina does not revert. Instead a second, reflective layer (the tapetum) forms behind the retina. Light which is not absorbed by the retina on the first pass may bounce back and be detected. As a predator, the cat simply accommodates blind spots with head and eye motion.
At some point, color vision develops when receptor cells develop multiple pigments. As a chemical instead of mechanical adaptation, this may happen at any of the points described above, or not at all, and the capability may disappear and reappear, as organisms become predator or prey. Similarly, night and day vision emerge when receptors differentiate into rods and cones, respectively.

At some point, a focusing mechanism develops. Some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. Note that a focusing method is not a requirement. As photographers know, focal errors increase as f-number decreases. Thus, an organism with small eyes, active in direct sunlight, may survive with no focus mechanism at all. As the species grows larger, or transitions to dimmer environs, a means of focusing could appear gradually.

The majority of the process is believed to have taken only a few million years, as the first predator to gain true imaging would have touched off 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.

Anatomy

Schematic diagram of the human eye.
Light from a single point of a distant object and light from a single point of a near object being brought to a focus.The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. The cornea and lens help to converge light rays to focus 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.
Light enters the eye from an external medium such as air or water, passes through the cornea, and into the first of two humours, the aqueous humour. Most of the light refraction occurs at the cornea which has a fixed curvature. The first humour is a clear mass which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the convergence of light at the lens) and provides the corneal endothelium with nutrients. The iris, between the lens and the first humour, is a coloured ring of muscle fibres. Light must first pass though the centre of the iris, the pupil. The size of the pupil is actively adjusted by the circular and radial muscles to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina; too little light makes sight difficult. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina.
To clearly see an object far away, the circularly arranged ciliary muscles will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more convex, form. Humans gradually lose this flexibility with age, resulting in the inability to focus on nearby objects, which is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism.
On the other side of the lens is the second humour, the vitreous 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.
Three layers, or tunics, form the wall of the eyeball. The outermost is the sclera which gives the eye most of its white colour. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape. On the inner side of the sclera is the choroid, which contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. Within the eye, only the sclera and ciliary muscles contain blood vessels. The choroid gives the inner eye a dark colour, which prevents disruptive reflections within the eye. The inner most layer of the eye is the retina, containing the photosensitive rod and cone cells, and neurons.
To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does have 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 colour 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 whatsoever exist at this point, it is thus "blind".
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.
Other articles regarding eye anatomy
Aqueous humour, Anterior chamber, Ciliary body, Ciliary muscle, Cornea, Conjunctiva, Choroid, Fovea, Iris, Lens, Macula, Optic disc, Optic nerve, Ora serrata, Posterior chamber, Pupil, Retina, Schlemm's canal, Sclera, Suspensory ligament, Tapetum lucidum, Trabecular meshwork, Vitreous humour, Zonule of Zinn.

Cytology
The retina contains two forms of photosensitive cells - rods and cones. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, they do not distinguish between colours, and have low visual acuity (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colours (wavelengths) of light, which allows an organism to see colour.
The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires looking at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other 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 the individual to observe distant stars.
Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to electromagnetic radiation of a particular wavelength and intensity (ie. a colour visible light) 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 opsin in both opens ion channels on the cell membrane which leads to the generation of an action potential (an impulse which will eventually get to the visual cortex in the brain).
This 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 and information is relayed to the visual cortex. Whereas, a single cone cell is connected to a single bipolar cell. Thus, action potentials from rods share neurons, where those from cones are given their own. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential. Because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapse of the bipolar cell to attain the threshold level to generate an action potential.
Furthermore, colour is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents - resulting in different colours being seen. If, for example, the red and green cones are stimulated to the same extent, and no blue cones are stimulated, yellow is seen. For this reason red, green and blue are called primary colours and the colours obtained by mixing two of them, secondary colours. The secondary colours can be further complimented with primary colours to see tertiary colours.

Acuity
Main article: Visual acuity
Visual acuity can be measured with several different metrics.
Cycles per degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from angular resolution. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.73 millimeters at a distance of one meter. A horse can resolve about 14 CPD (0.62 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m).
A diopter is the unit of measure of focus.

Dynamic range
At any given instant, the retina can resolve a contrast ratio of around 100:1 (about 6 1/2 stops). As soon as your 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, over time, a contrast ratio of about 1,000,000:1 (about 20 stops) can be resolved. The process is nonlinear and multifaceted, so an interruption by light nearly 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.

Adnexa and related parts
The orbit
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.

Eyebrows
In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye. Water in the eye can alter the refractive 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. This is made apparent when swimming in freshwater pools, as the osmotic gradient draws 'pool water' into the corneal tissue, causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye with hypertonic saline.

Eyelids
In many animals, including humans, eyelids wipe the eye and prevent dehydration. They spread tear fluid 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.

Eyelashes
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.

Eye movement
Main article: Eye movements
Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body.
The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second (Westheimer and McKee, 1954). 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. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand. 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 vison 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.

How we see an object
The steps of how we see an object:
Enters the cornea/clear lens of the eye
Through the pupil
Through the Iris
Through the crystalline lens
Through the vitreous humor
Through the retina
Through the optic nerve
Through the visual pathway
Through the occipital cortex
Through the brain's processing
Colour vision
Main articles: Colour and Colour vision
What we see as color is essentially different combinations of certain ranges of wavelengths in the electromagnetic spectrum. In humans at least, there are three different kinds of cones for three ranges of wavelengths, roughly red, green and blue light. Each color of rod picks up the intensity of light in its range of wavelengths, and the combination is translated by the brain to a perceived color. Of course, some people lack the ability to be able to see in colour: they are referred to as being 'colour blind'.

Extraocular muscles
Main article: 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. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Rapid eye movement
Main article: Rapid eye movement
Rapid eye movement 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
Main article: Saccade
Saccades are rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900�/s in adult humans, and take roughly 250 milliseconds to be initiated by the neural network.

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

Vestibulo-ocular reflex
Main article: Vestibulo-ocular reflex
Many animals can look at something while turning their heads. The eyes are automatically rotated to remain fixed on the object, directed by input from the organs of balance near the ears.

Smooth pursuit movement
Main article: Pursuit movement
The eyes can also follow a moving object around. This 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.
While still, the eye can measure relative speed with high accuracy, however under movement relative speed is highly distorted. Take for example, when watching a plane while standing -- the plane has normal visual speed. However, if you watch the plane while moving in the opposite direction from the plane's movement, the plane will appear as if were standing still or moving very slowly.
When viewing an object in motion moving away or towards you, there is no eye movement occurring as in the examples above, however the ability to discern speed and speed difference is still present; although not as severe. The lack of visual input stimuli intensity (e.g. night vs. day) plays a major role in determining speed and speed difference. For example, no human can with reasonable accuracy, determine the speed of an approaching train in the evening as they could during the day. Similarly, while moving, the ability is further diminished unless there is another point of reference for determining speed; however the inaccuracy of speed or speed difference will always be present.

Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree 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 tree (through a saccade).

Vergence movement
Main article: Vergence
The two eyes converge to point to the same objectWhen 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 eachother' (divergence). Exaggerated convergence is called cross eyed viewing (focussing 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.

Accommodation
Main article: Accommodation (eye)
To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.
Diseases, disorders, and age-related changes
Main articles: List of eye diseases and disorders and List of systemic diseases with ocular manifestations
There are many diseases and disorders that may affect the eyes.
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. [1]
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.

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From Wikipedia, the free encyclopedia
LASIK
LASIK, an acronym for Laser-assisted In Situ Keratomileusis, is a form of refractive laser eye surgery procedure performed by ophthalmologists intended for correcting vision. The procedure is usually a preferred alternative to photorefractive keratectomy, PRK, as it requires less time for full recovery, and the patient experiences less pain overall.

Content
1 History of LASIK
2 Surgical procedure
2.1 Preoperative
2.2 The operation
3 Higher Order Aberrations
3.1 Wavefront-guided LASIK
4 Complications
4.1 Preoperative sources of complications
4.2 Intraoperative complications
4.3 Early postoperative complications
4.4 Late postoperative complications
4.5 Other
4.6 Factors affecting the surgery
5 Satisfaction
6 Safety and efficacy

History of LASIK
The LASIK technique was made possible by Dr Jose Barraquer (Colombia), who around 1970 developed the first microkeratome, used to cut thin flaps in the cornea and alter its shape, in a procedure called keratomileusis. This procedure was developed and pioneered by the world leading Barraquer Clinic, based in Bogota, Colombia.
LASIK surgery was developed in 1990 by Dr. Lucio Buratto (Italy) and Dr. Ioannis Pallikaris (Greece) as a melding of two prior techniques, keratomileusis and photorefractive keratectomy. It quickly became popular because of its greater precision and lower frequency of complications compared with those techniques.[1]
In 1991, LASIK was performed for the first time in the United States by Drs. Stephen Brint and Stephen Slade [2]. The same year, Drs. Thomas and Tobias Neuhann successfully treated the first German LASIK patients with an automated microkeratome.

Surgical procedure
Preoperative
Patients wearing soft contact lenses typically are instructed to stop wearing them approximately 7 to 10 days before surgery. One industry body recommends that patients wearing hard contact lenses should stop wearing them for a minimum of six weeks plus another six weeks for every three years the hard contacts had been worn. [3] Before the surgery, the surfaces of the patient's corneas are examined with a computer-controlled scanning device to determine their exact shape. Using low-power lasers, it creates a topographic map of the cornea. This process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and locations of corneal tissue to be removed during the operation. The patient typically is prescribed an antibiotic to start taking beforehand, to minimize the risk of infection after the procedure.

[edit]
The operation
The operation is performed with the patient awake and mobile; however, the patient typically is given a mild sedative (such as Valium or diazepam) and anesthetic eye drops.
Lasik is performed in two steps. The initial step is to create a flap of corneal tissue. This process is achieved with a mechanical microkeratome using a metal blade, or a femtosecond laser microkeratome that creates a series of tiny closely arranged bubbles within the cornea.[4] A hinge is left at one end of this flap. The flap is folded back, revealing the stroma, the middle section of the cornea.
The second step of the procedure is to use an excimer laser (193 nm) to remodel the corneal stroma. The laser vaporizes tissue in a finely controlled manner without damaging adjacent stroma by releasing the molecular bonds that hold the cells together. No burning with heat or actual cutting is required to ablate the tissue. The layers of tissue removed are tens of micrometers thick.
Currently manufactured excimer lasers use a computer system that tracks the patient's eye position up to 4,000 times per second, redirecting laser pulses for precise placement. After the laser has reshaped the cornea, the Lasik flap is repositioned over the treatment area by the surgeon. The flap remains in position by natural adhesion until healing is completed.
Performing the laser ablation in the deeper corneal stroma and under the Lasik flap fools the cornea into not knowing that it has had surgery. The wound response is muted, thus the patient is typically provided rapid visual recovery and virtually no pain.

Higher Order Aberrations
Higher order Aberrations are visual problems not captured in a traditional eye exam. In a young healthy eye, the level of higher order aberrations are typically low and insignificant. Concern has long plagued the tendency of refractive surgeries to induce higher order aberration not correctible by traditional contacts or glasses. The advancement of lasik technique and technologies has helped eliminate the risk of clinically significant visual impairment after the surgery. There has been controversy about the amount of higher order aberrations that would lead to significant vision impairment. In extreme cases, where proper policy was not followed and before key advances, some people could suffer rather debilitating symptoms including serious loss of contrast sensitivity in poor lighting situations.
Over time, most of the attention has been focused on spherical aberration. Lasik and PRK tend to induce spherical aberration, because of the tendency of the laser to undercorrect as it moves outward from the center of the treatment zone. This is really only a significant issue for large corrections. There is some thought if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would be induced. Hence, in eyes with little existing higher order aberrations, "wavefront optimized" lasik rather than wavefront guided Lasik may well be the future. Regardless, most patients with even the highest corrections remain highly satisfied even with conventional lasik.

Wavefront-guided LASIK
Wavefront-guided LASIK is a variation of LASIK surgery where, rather than apply a simple correction of focusing power to the cornea (as in traditional LASIK), an ophthalmologist applies a spatially varying correction, using a computer-controlled high-power UV laser guided by measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the final result still depends on the physician's success at predicting changes which occur during healing. Nor are wavefront aberrations the factor to degrade vision; especially in older patients, scattering from microscopic particles plays a major role. Hence, patients expecting so-called "super vision" from such procedures may be disappointed. However, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos", the visual artifact caused by spherical aberration induced in the eye by earlier methods.

Complications
A subconjunctival hemorrhage is a common and minor post-LASIK complication.The incidence of refractive surgery patients having unresolved complications six months after surgery has been estimated from 3%[1] to 6%[2]. The following are some of the more frequently reported complications of LASIK[1][5]:

Dry eyes
Overcorrection or undercorrection
Visual acuity fluctuation
Halos or starbursts around light sources at night
Light sensitivity
Ghosts or double vision
Wrinkles in flap (striae)
Decentered ablation
Debris or growth under flap
Thin or buttonhole flap
Induced astigmatism
Epithelium erosion
Posterior vitreous detachment[3]
Macular hole[4]
Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources[5]:

Preoperative sources of complications
Intraoperative complications
The incidence of flap complications has been estimated to be 0.244%[6]. Flap complications (such as displaced flaps or folds in the flaps that necessitate repositioning, diffuse lamellar keratitis, and epithelial ingrowth) are common in lamellar corneal surgeries [6] but rarely lead to permanent visual acuity loss; the incidence of these microkeratome-related complications decreases with increased physician experience [7] [8].
A slipped flap (a corneal flap that detaches from the rest of the cornea) is one of the most common complications. The chances of this are greatest immediately after surgery, so patients typically are advised to go home and sleep, to let the flap heal.
Flap interface particles are another finding whose clinical significance is undetermined[7]. A Finnish study found that particles of various sizes and reflectivity were clinically visible in 38.7% of eyes examined via slit lamp biomicroscopy, but apparent in 100% of eyes using confocal microscopy[7].

Early postoperative complications
The incidence of diffuse lamellar keratits (DLK)[9], also known as the Sands of Sahara syndrome, has been estimated at 2.3%[8]. When diagnosed and appropriately treated, DLK resolves with no lasting vision limitation.
The incidence of infection responsive to treatment has been estimated at 0.4%[8]. Infection under the corneal flap is possible. It is also possible that a patient has the genetic condition keratoconus that causes the cornea to thin after surgery. Although this condition is screened in the preoperative exam, it is possible in rare cases (about 1 in 5,000) for the condition to remain dormant until later in life (the mid-40s). If this occurs, the patient may need rigid gas permeable contact lenses, Intrastromal Corneal Ring Segments (Intacs)[10], Corneal Collagen Crosslinking with Riboflavin[11] or a corneal transplant.
The incidence of persistent dry eye has been estimated to be as high as 28% in Asian eyes and 5% in Caucasian eyes[2]. Nerve fibers in the cornea are important for stimulating tear production. A year after LASIK subbasal nerve fiber bundles remain reduced by more than half [9].
The incidence of subconjunctival hemorrhage has been estimated at 10.5%[8].

Late postoperative complications
The incidence of epithelial ingrowth has been estimated at 0.1%[8].

Glare is another commonly reportedly complication of those who have had LASIK[10]. Halos or starbursts around bright lights at night are caused by the irregularity between the lasered part and the untouched part. It is not practical to perform the surgery so that it covers the width of the pupil at full dilation at night, and the pupil may expand so that light passes through the edge of the flap into the pupil. In daytime, the pupil is smaller than the edge. Newer equipment is available to properly treat those with large pupils, and responsible physicians will check for them during examination.

Although there have been a number of improvements in LASIK technology [12][13] [14] , a large body of conclusive evidence on the chances of long-term complications is not yet in place. Also, there is a small chance of complications, such as slipped flap, corneal infection, haziness, halo, or glare. The procedure is irreversible.

The incidence of macular hole has been estimated at 0.2%[4] to 0.3% [11].
The incidence of retinal detachment has been estimated at 0.36%[11].
The incidence of choroidal neovascularization has been estimated at 0.33%[11].
The incidence of uveitis has been estimated at 0.18%[12]

Although the cornea usually is thinner after LASIK because of the removal of part of the stroma, refractive surgeons strive to maintain a minimum thickness in order to not structurally weaken the cornea. Decreased atmospheric pressure at higher altitudes has not been shown to be extremely dangerous to the eyes of LASIK patients. However, some mountain climbers have experienced a myopic shift at extreme altitudes [15] [16]. Although there are no published reports documenting diving-related complications after LASIK [17], urban legends that describe eyes that have popped open during scuba diving still persist. There are also concerns about possible LASIK-related problems with night vision, to the exent that some armed forces around the world advise aspiring air force and special forces personnel not to have the surgery.
Laser in situ keratomileusis increases higher order wavefront aberrations of the cornea[13] [14]. Glasses do not correct higher order aberrations.
Microfolding has been reported as "an almost unavoidable complication of LASIK" whose "clinical significance appears negligible" [7].

Factors affecting the surgery
The cornea typically is avascular because it must be transparent to function normally. Its cells absorb oxygen from the tear film. Low oxygen-permeable contact lenses reduce the cornea's absorption of oxygen, which sometimes results in the growth of blood vessels into the cornea, a process known as corneal neovascularization. This can cause a mild increase in inflammation and healing time and some discomfort during the surgery because of augmented bleeding. Although some contact lenses, notably modern RGP and soft silicone hydrogel lenses, are made of materials with higher oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are cautioned to avoid overwearing their lenses. It is usually recommended that contact lens use be discontinued several days or weeks before the LASIK procedure.
A 2004 Wake Forest University study found that LASIK results are affected by heat and humidity, both during the procedure and in the two weeks before surgery[18].

Satisfaction
Various surveys have been performed to determine patient satisfaction with LASIK:

According to a 2005 survey, 92.2% of patients reported that they were satisfied or very satisfied with their surgery[10].
According to a 2004 survey, 97.8% declared themselves as satisfied[15].
According to a 2003 survey, 97% of subjects returning a questionnaire reported that they would recommend LASIK to a friend. The study found that those who would not recommend the procedure were more likely to have experienced glare, halos, and/or starbursts[16].
According to a 2000 survey, 97.9% of patients reported that they were satisfied[17].
Safety and efficacy
The reported figures for safety and efficacy are open to interpretation. In 2003, the Medical Defence Union (MDU), the largest insurer for doctors in the United Kingdom, reported a 166% increase in claims involving laser eye surgery; however, the MDU averred that these claims resulted primarily from patients' �unrealistic expectations� of LASIK rather than �faulty surgery� [19]. A 2003 study reported in the medical journal Ophthalmology found that nearly 18% of treated patients and 12% of treated eyes needed retreatment [20]. The authors concluded that �higher initial corrections, astigmatism, and older age are risk factors for LASIK retreatment.�
In 2004, the British National Health Service's National Institute for Health and Clinical Excellence (NICE) considered a systematic review of four randomized controlled trials [21] [22] before issuing guidance for the use of LASIK within the NHS[23]. Regarding the procedure's efficacy, NICE reported, "Current evidence on LASIK for the treatment of refractive errors suggests that it is effective in selected patients with mild or moderate short-sightedness" but that "evidence is weaker for its effectiveness in severe short-sightedness and long-sightedness." Regarding the procedure's safety, NICE reported that "there are concerns about the procedure's safety in the long term and current evidence does not appear adequate to support its use within the NHS without special arrangements for consent and for audit or research." Leading refractive surgeons in the United Kingdom and United States, including at least one author of a study cited in the report, believe NICE relied on information that is severely dated and weakly researched[24]

For more information on Lasik, please visit Wikipedia

From Wikipedia, the free encyclopedia
Surgery

A typical modern surgical operationFor other uses, see Surgery (disambiguation).
Surgery (from the Greek cheirourgia meaning "hand work") is the medical specialty that treats diseases or injuries by operative manual and instrumental treatment. Surgeons may be physicians, dentists, or veterinarians who specialize in surgery.
A surgery can also refer to the place where surgery is performed, or simply the office of a physician, dentist, or veterinarian.

Contents
1 History of surgery
2 Development of modern surgery
3 Diseases that can be treated by surgery
4 Common surgical procedures
5 Noted surgeons
6 See also (surgeries)

History of surgery
The earliest known surgical procedure is trepanation, also known as trephinning or trepanning, in which a hole is drilled or scraped into the skull, leaving the membrane around the brain intact. A trepanned cranium found near Kiev, Ukraine, is the oldest yet found, dating back to 7300-6220 BC. Trepanation attempts to address health problems that relate to abnormal intracranial pressure, and has been found in cultures around the world. Modern surgery has largely abandoned this practice, although it is still used in cases of acute subdural hematomas and acute epidural hematomas.
Researchers have also uncovered an Ancient Egyptian mandible, dated to approximately 2750 BC, having two perforations just below the root of the first molar, indicating the draining of an abscessed tooth. Recent excavations of the construction workers of the Egyptian pyramids also led to the discovery of evidence of brain surgery on a labourer, who continued living for two years afterwards.
Indian physician Sushrutha(600 BC) is an important figure in the history of surgery. He lived, taught and practiced his art of surgery on the banks of the Ganges in the area that corresponds to the present day city of Benares in North-West India. Because of his seminal and numerous contributions to the science and art of surgery he is also known by the title "Father of Surgery". Much of what is known about this inventive surgeon is contained in a series of volumes he authored, which are collectively known as the Susrutha Samhita. It is the oldest known surgical text and it describes in exquisite detail the examination, diagnosis, treatment, and prognosis of numerous ailments.
Surgeons are now considered to be specialised physicians, the profession of surgeon and that of physician have different historical roots and surgeons have now even subspecialised as have physicians. For Example, Greek tradition was against opening the body and the Hippocratic Oath warns physicians against the practice of surgery, specifically that cutting persons laboring under the stone, i.e. lithotomy, an operation to relieve kidney stones, was to be left to such persons as practice [it]. Of course, most knowledge of surgery comes from dissecting bodies, a science which was repulsive to many healers.
By the thirteenth century, many European towns were demanding that physicians have several years of study or training before they could practice. Montpellier, Padua and Bologna Universities were particularly interested in the academic side to Surgery, and by the fifteenth century at the latest, Surgery was a separate university subject to Physic. Surgery had a lower status than pure medicine, beginning as a craft tradition until Rogerius Salernitanus composed his Chirurgia, which laid the foundation for the species of the occidental surgical manuals, influencing them up to modern times.
Among the first modern surgeons were battlefield doctors in the Napoleonic Wars who were primarily concerned with amputation. Naval surgeons were often barber-surgeons, who combined surgery with their main jobs as barbers.
In London, an operating theatre or operating room from the day before modern anaesthesia or antiseptic surgery still exists, and is open to the public. It is found in the roof space of St Thomas Church, Southwark, London and is called the Old Operating Theatre.

Development of modern surgery
Before the advent of anaesthesia, surgery was a traumatically painful procedure and surgeons were encouraged to be as swift as possible to minimize patient suffering. This also meant that operations were largely restricted to amputations and external growth removals. In addition, the need for strict hygiene during procedures was little understood, which often resulted in life threatening post-operative infections in patients.
Beginning in the 1840s, surgery began to change dramatically in character with the discovery of effective and practical anaesthetic chemicals such as ether and chloroform. In addition to relieving patient suffering, anaesthesia allowed more intricate operations in the internal regions of the human body. In addition, the discovery of muscle relaxants such as curare allowed for safer applications.
However, the move to longer operations increased the danger of dangerous complications since the prolonged exposure of surgical wounds to the open air heightened the chance of infections. It was only in the late 19th century with the rise of microbiology with scientists like Louis Pasteur and innovative doctors who applied their findings like Joseph Lister did the idea of strict cleanliness and sterile settings during surgery arise. In the United Kingdom, surgeons are distinguished from physicians by being referred to as "Mister." This tradition has its origins in the 18th century, when surgeons were barber-surgeons and did not have a degree (or indeed any formal qualification), unlike physicians, who were doctors with a university medical degree.
By the beginning of the 19th century, surgeons had obtained high status, and in 1800, the Royal College of Surgeons (RCS) in London began to offer surgeons a formal status via RCS membership. The title Mister became a badge of honour, and today only surgeons who hold the Membership or Fellowship of one of the Royal Surgical Colleges are entitled to call themselves Mister, Miss, Mrs or Ms.
In contrast, North American physicians and surgeons are always addressed as "Doctor."

Diseases that can be treated by surgery
Intraoperative X-Ray of a Humerus fixated by Kirschner wiresTrauma
Anatomical Abnormalities
Disorders of function
Inflammation
Ischaemia and infarction
Metabolic disorders
Neoplasia
Other abnormalities of tissue growth, e.g. cysts, hyperplasia or hypertrophy
Common surgical procedures
Of the eight most common surgical procedures in the US, four are obstetric:
episiotomy,
repair of obstetric laceration,
cesarean section, and
artificial rupture of the amniotic membrane.
The most common non-obstetric surgeries include:
dental extraction
circumcision
According to 1996 data from the US National Center for Health Statistics, 40.3 million inpatient surgical procedures were performed in the United States in 1996, followed closely by 31.5 million outpatient surgeries.

For more information on Surgery, please visit Wikipedia

From Wikipedia, the free encyclopedia
Contact Lens
A contact lens (also known simply as a "contact") is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye.

Contact lenses usually serve the same corrective purpose as conventional glasses, but are lightweight and virtually invisible�many commercial lenses are tinted a faint blue for visibility. Cosmetic lenses are deliberately and clearly visible and are meant to alter the appearance of the eye.
It has been estimated that about 125 million people use contact lenses worldwide[1], including 28 to 38 million in the United States[2][1] and 13 million in Japan [3]. The types of lenses used and prescribed vary markedly between countries, with rigid lenses accounting for over 20% of currently-prescribed lenses in Japan, Netherlands and Germany but less than 5% in Scandinavia[1].
People choose to wear contact lenses for various reasons[4]. Many consider their appearance to be more attractive with contact lenses than with glasses. Contact lenses are less affected by wet weather, do not steam up, and provide a wider field of vision. They are more suitable for a number of sporting activities[5]. Additionally, ophthalmological conditions such as keratoconus and aniseikonia may not be accurately correctable with glasses.

A pair of contact lenses when not inserted in the eye. They are positioned with the concave side facing upward.Contents [hide]
1 History
2 Types of contact lenses
2.1 By function
2.2 By constructional material
2.3 By wear time
2.4 By disposability
2.5 By shape
2.6 By number of focal points
2.7 Implantation
2.8 Orthokeratology (ie Corneal Refractive Therapy)
3 Manufacturing of contact lenses
4 Prescribing contact lenses
5 Complications
6 Contact lens care: cleaning and disinfection products

History

In 1887, Adolf Fick was apparently the first to successfully fit contact lenses, which were made from brown glassLeonardo da Vinci is frequently credited with introducing the general principle of contact lenses in his 1508 Codex of the eye, Manual D, where he described a method of directly altering corneal power by submerging the eye in a bowl of water. Da Vinci, however, did not suggest his idea be used for correcting vision--he was more interested in learning about the mechanisms of accommodation of the eye.[6]
Ren� Descartes proposed another idea in 1636, in which a glass-tube filled with liquid is placed in direct contact with the cornea. The protruding end was to be composed of clear glass, shaped to correct vision; however the idea was unworkable, since it would make blinking impossible.
In 1801, while conducting experiments concerning the mechanisms of accommodation, scientist Thomas Young constructed a liquid-filled "eyecup" which could be considered a predecessor to the contact lens. On the eyecup's base, Young fitted a microscope eyepiece. However, like da Vinci's, Young's device was not intended to correct refraction errors.
Sir John Herschel, in a footnote of the 1845 edition of the Encyclopedia Metropolitana, posed two ideas for the visual correction: the first "a spherical capsule of glass filled with animal jelly", and "a mould of the cornea" which could be impressed on "some sort of transparent medium". Though Herschel reportedly never tested these ideas, they were both later advanced by several independent inventors, seemingly unaware of Herschel's suggestion.
It was not until 1887 that the German physiologist Adolf Eugen Fick constructed and fitted the first successful contact lens. While working in Z�rich, he described fabricating afocal scleral contact shells, which rested on the less sensitive rim of tissue around the cornea, and experimentally fitting them: initially on rabbits, then on himself, and lastly on a small group of volunteers. These lenses were made from heavy brown glass and were 18-21mm in diameter. Fick filled the empty space between cornea/callosity and glass with a grape sugar solution. He published his work, "Contactbrille", in the journal Archiv f�r Augenheilkunde in March 1888.
Fick's lens was large, unwieldy, and could only be worn for a few hours at a time. August Muller in Kiel, Germany, corrected his own severe myopia with a more convenient glass-blown scleral contact lens of his own manufacture in 1888.
Glass-blown scleral lenses remained the only form of contact lens until the 1930s when polymethyl methacrylate (PMMA or Perspex/Plexiglas) was developed, allowing plastic scleral lenses to be manufactured for the first time. In 1936 an optometrist, Dr. William Feinbloom introduced plastic lenses, making them lighter and more convenient. These lenses were a combination, however, of both plastic and glass.
In the 1950s, the first 'corneal' lenses were developed--these were much smaller than the original scleral lenses, as they sat only on the cornea rather than across all of the visible ocular surface. PMMA corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing (lathe) technology.
One important disadvantage of PMMA lenses is that no oxygen is transmitted through the lens to the cornea, which can cause a number of adverse clinical events. By the end of the 1970s, and through the 1980s and 1990s, a range of oxygen-permeable but rigid materials were developed to overcome this problem. Collectively, these polymers are referred to as 'rigid gas permeable' or 'RGP' materials or lenses. Although all the above lens types--sclerals, PMMA lenses and RGPs--could be correctly referred to as being 'hard' or 'rigid', the term 'hard' is now used to refer to the original PMMA lenses which are still occasionally fitted and worn, whereas 'rigid' is a generic term which can be used for all these lens types. That is, 'hard' lenses (PMMA lenses) are a sub-set of 'rigid' lenses. Occasionally, the term 'gas permeable' is used to describe RGP lenses, but this is potentially misleading, as soft lenses are also 'gas permeable' in that they allow oxygen to move through the lens to the ocular surface.
The principal breakthrough in soft lenses was made by the Czech chemist Otto Wichterle who published his work "Hydrophilic gels for biological use" in the journal Nature in 1959[7]. This led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the first approval of the 'Soflens' material by the United States Food and Drug Administration in 1971. These lenses were soon prescribed more often than rigid lenses, mainly due to the immediate comfort of soft lenses; by comparison, rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability by varying the ingredients making up the polymers.
In 1999, an important development was the launch of the first 'silicone hydrogels' onto the market. These new materials encapsulated the benefits of silicone�which has extremely high oxygen permeability�with the comfort and clinical performance of the conventional hydrogels which had been used for the previous 30 years. These lenses were initially advocated primarily for extended (overnight) wear although more recently, daily (no overnight) wear silicone hydrogels have been launched.

Types of contact lenses
Contact lenses are classified in many different manners[8] [9].

By function
Corrective contact lenses - A corrective contact lens is a lens designed to improve vision. In many people, there is a mismatch between the refractive power of the eye and the length of the eye, leading to a refraction error. A contact lens neutralizes this mismatch and allows for correct focusing of light onto the retina. Conditions correctable with contact lenses include near (or short) sightedness (myopia), far (or long) sightedness (hypermetropia), astigmatism and presbyopia. Recently there has been renewed interest in orthokeratology, the correction of myopia by deliberate overnight flattening of the cornea, leaving the eye without contact lens or eyeglasses correction during the day.
For those with certain color deficiencies, a red-tinted "X-Chrom" contact lens may be used. Although the lens does not restore normal color vision, it allows some colorblind individuals to distinguish colors better[10][11]. Other tinted lenses have been used with limited success[12].

A person seen wearing two different styles of cosmetic contact lensesCosmetic contact lenses - A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the color or design. In the United States, the FDA frequently calls non-corrective cosmetic contact lenses decorative contact lenses[13][14][15][16].
Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the entertainment industry to make the eye appear unusual or unnatural in appearance[17]. These lenses have been used by Wes Borland, Marilyn Manson, Twiztid, and Ray Park as Darth Maul in Star Wars Episode I: The Phantom Menace. Scleral lenses cover the white part of the eye (i.e. sclera) and are used in many theatrical lenses.
Similar lenses have more direct medical applications. For example, some lenses can give the iris an enlarged appearance, or mask defects such as absence (aniridia) or damage (dyscoria) to the iris.
Although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change the color of the eye are far less common, accounting for only 3% of contact lens fits in 2004[18].
Therapeutic contact lenses - Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lens protects an injured or diseased cornea to protect it from the constant rubbing of blinking eyelids thereby allowing it to heal[19]. They are used in the treatment of conditions including bullous keratopathy, dry eyes, corneal ulcers and erosion, keratitis, corneal edema, descemetocele, corneal ectasis, Mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis[20]. Contact lenses to deliver drugs to the eye have also been developed[21].

By constructional material
Contact lenses, once inserted in the eye, become almost invisible (except cosmetic contact lenses).The first contact lenses were made of glass, which caused eye irritation and it was not possible to wear these lenses very long. But when Dr. William Feinbloom introduced lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas), contacts become much more convenient. These PMMA lenses are commonly referred to as "hard" lenses (this term is not used for other types of contacts).
However, PMMA lenses have their own side effects: no oxygen is transmitted through the lens to the cornea, which can cause a number of adverse clinical events. In the late 1970s, and through the 1980s and 1990s, improved rigid materials � which were also oxygen-permeable � were developed. Collectively, these polymers are referred to as 'rigid gas permeable' or 'RGP' materials or lenses.
Rigid lenses offer a number of unique properties. In effect, the lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a regular (spherical) rigid contact lens can provide good level of vision in people who have astigmatism or distorted corneal shapes as with keratoconus.
Whilst rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in soft lenses made by Otto Wichterle led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the approval of the 'Soflens' material (polymacon) by the United States Food and Drug Administration in 1971. Soft lenses are immediately comfortable, while rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability by varying the ingredients making up the polymers.
A small number of hybrid rigid/soft lenses exist. An alternative technique is piggybacking of contact lenses, a smaller, rigid lens being mounted atop a larger, soft lens. This is done for a variety of clinical situations where a single lens will not provide the optical power, fitting characteristics, or comfort required.
In 1999, 'silicone hydrogels' became available. Silicone hydrogels have both the extremely high oxygen permeability of silicone and the comfort and clinical performance of the conventional hydrogels. These lenses were initially advocated primarily for extended (overnight) wear, although more recently daily (no overnight) wear silicone hydrogels have been launched.
While it provides the oxygen permeability, the silicone also makes the lens surface highly hydrophobic and less "wettable." This frequently results in discomfort and dryness during lens wear. In order to compensate for the hydrophobicity, hydrogels are added (hence the name "silicone hydrogels") to make the lenses more hydrophilic. However the lens surface may still remain hydrophobic. Hence some of the lenses undergo surface modification processes which cover the hydrophobic sites of silicone. Some other lens types incorporate internal rewetting agents to make the lens surface hydrophilic.

By wear time
A daily wear contact lens is designed to be removed prior to sleeping. An extended wear (EW) contact lens is designed for continuous overnight wear, typically for 6 or more consecutive nights. Newer materials, such as silicone hydrogels, allow for even longer wear periods of up to 30 consecutive nights; these longer-wear lenses are often referred to as continuous wear (CW). Generally, extended wear lenses are discarded after the specified length of time. These are increasing in popularity, due to their obvious convenience. Extended- and continuous-wear contact lenses can be worn for such long periods of time because of their high oxygen permeability (typically 5-6 times greater than conventional soft lenses), which allows the eye to remain remarkably healthy.
Extended lens wearers may have an increased risk for corneal infections and corneal ulcers, primarily due to poor care and cleaning of the lenses, tear film instability, and bacterial stagnation. Corneal neovascularization is also a common complication of extended lens wear, however the most common complication of extended lens use is conjunctivitis usually allergic or giant papillary conjunctivitis (GPC) associated with a poorly fitting contact lens.

By disposability
The various soft contact lenses available are often categorized by their replacement schedule. The shortest replacement sechedule is single use lenses, which are disposed of each night. These may be best for patients with ocular allergies or other conditions because it limits deposits of antigens and protein. Single use lenses are also useful for people who use contacts infrequently. More commonly, contact lenses are prescribed to be disposed of on a two-week or monthly basis. Quarterly or annual lenses, which used to be very common, have lost favor because a more frequent disposal schedule allows for thinner lenses and limits deposits. Rigid gas permeable lenses are very durable and may last for several years without the need for replacement.

By shape
A spherical contact lens is one in which both the inner and outer optical surfaces are portions of a sphere. A toric lens is one in which either or both of the optical surfaces have the effect of a cylindrical lens, usually in combination with the effect of a spherical lens. Myopic (nearsighted) and hypermetropic (farsighted) people who also have astigmatism and who have been told they are not suitable for regular contact lenses may be able to use toric lenses. If one eye has astigmatism and the other doesn't, the patient may be told to use a spherical lens in one eye and a toric lens in the other. Toric lenses are made from the same materials as regular contact lenses but have a couple of extra characteristics:
They correct for both spherical and cylindrical aberration.
They have a specific top and bottom, as they are not symmetrical around their centre and must not be rotated. Lenses must be designed to maintain their orientation regardless of eye movement. Often lenses are weighted more at the bottom and are marked by tiny striations so the wearer can insert them in the correct orientation, or they are designed so that the correct orientation is restored when the user blinks. Some do both.

By number of focal points
Similarly to eyeglasses, contact lenses can have one (single vision) or more (multifocal) focal points.
For correction of presbyopia or accommodative insufficiency multifocal contact lenses are almost always used; however, single vision lenses may also be used in a process known as monovision[22]: single vision lenses are used to correct one eye's far vision and the other eye's near vision. Alternatively, a person may wear single vision contact lenses to improve distance vision and reading glasses to improve near vision.
Multifocal contact lenses are more complex to manufacture and require more skill to fit. All soft bifocal contact lenses are considered "simultaneous vision" because both far and near vision are corrected simultaneously, regardless of the position of the eye. Commonly these are designed with distance correction in the center of the lens and near correction in the periphery, or viceversa. Rigid gas permeable contact lenses most commonly have a small lens on the bottom for the near correction: when the eyes are lowered to read, this lens comes into the optical path.

Implantation
Intraocular contact lenses, also known as an implantable contact lenses, are special small corrective lenses surgically implanted in the eye's posterior chamber behind the iris and in front of the lens to correct higher amounts of myopia and hyperopia.

Orthokeratology (ie Corneal Refractive Therapy)
See main page: Orthokeratology

Manufacturing of contact lenses
Most contact lenses are mass produced.
Spin-cast lenses - A spin cast lens is a soft contact lens manufactured by whirling liquid plastic in a revolving mold at high speed[23].
Lathe cut - A lathe cut contact lens is cut and ground on a lathe [23].
Molded
Hybrids
Although many companies make contact lenses, there are four major manufacturers: Vistakon, Ciba, Bausch & Lomb, and CooperVision[24].

Prescribing contact lenses
The prescribing of contact lenses is usually restricted to appropriately qualified eye care practitioners. In countries such as the United States (where all contact lenses are deemed to be medical devices by the Food and Drug Administration), the United Kingdom and Australia, optometrists are usually responsible. In France and eastern European countries, ophthalmologists play the major role. In other parts of the world, opticians usually prescribe contact lenses. Prescriptions for contact lenses and glasses may be similar, but are not interchangeable.
The practitioner or contact lens fitter typically determines an individual's suitability for contact lenses during an eye examination. Corneal health is verified; ocular allergies or dry eyes may affect a person's ability to successfully wear contact lenses.
The parameters specified in a contact lenses prescription may include:

Contact lens wear are relatively common, affecting roughly 5% of contact lens wearers each year [25]. Excessive wear of contact lenses, particularly overnight wear, is associated with the most serious safety concerns[2]. Problems associated with contact lens wear may affect the eyelid, the conjunctiva, the various layers of the cornea, and even the tear film that covers the outer surface of the eye[25].
Eyelid:
Ptosis
Conjunctiva:
Contact dermatitis
Giant papillary conjunctivitis
Superior limbic keratoconjunctivitis
Cornea:
Epithelium
Corneal abrasion
Corneal erosion
Corneal ulcer
Hypoxia
Stroma
Infection and keratitis
Bacteria
Protozoa: Acanthamoeba
Fungal: Fusarium [1]
Contact lens acute red eye (CLARE)
Keratoconus
Endothelium

Contact lens care: cleaning and disinfection products
While daily disposable lenses require no cleaning, other types require regular cleaning and disinfecting in order to retain clear vision and prevent discomfort and infections by various microorganisms including bacteria, fungi, and Acanthamoeba, that form a biofilm on the lens surface. There are a number of products that can be used to perform these tasks:
Multipurpose solution - The most popular cleaning solution for contact lenses. Used for rinsing, disinfecting, cleaning and storing the lenses. Using this product eliminates the need for multiple solutions.
Saline solution - Used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect the lenses.
Daily cleaner - Used to clean lenses on a daily basis. A few drops of cleaner are applied to the lens while it rests in the palm of the hand, then the lens is rubbed for about 20 seconds with a fingertip (check the cleaner's directions) on each side. Long fingernails can damage the lens, so care should be taken.
Hydrogen peroxide solution - Used for disinfecting the lenses, and available as 'two-step' or 'one-step' systems. If using a 'two-step' product, one must ensure that the lens taken out of the hydrogen peroxide is neutralized before it is worn, or else wear will be extremely painful. Saline must not be used to rinse away the peroxide. Some peroxide solutions, such as CIBA Vision's Clear Care, come with a special storage case that contains a catalyzing disk. If soaked in the solution with the disk for at least six hours, the hydrogen peroxide decomposes and the remaining solution is a saline that will not harm the eye. People with extremely sensitive, irritable eyes often use these types of cleaning solutions.
Enzymatic cleaner - Used for cleaning protein deposits off lenses, usually weekly, if the daily cleaner is not sufficient. Typically, this cleaner is in tablet form. Protein deposits make use of contact lenses uncomfortable, and may lead to various eye problems.
Some products must only be used with certain types of contact lenses: it is important to check the product label to make sure that it can be used for a given type of lens. It is also important to follow the product's directions carefully to reduce risk of eye infection or eye irritation.
It is important to ensure that the product does not become contaminated with microorganisms: the tips of the containers for these solutions should never touch any surface, and the container should be kept closed when not in use. To counteract minor contamination of the product and kill microorganisms on the contact lens, some products may contain preservatives such as thimerosal, benzalkonium chloride, benzyl alcohol, and other compounds. In 1989, thimerosal was responsible for about 10% of problems related to contact lenses[26]: because of this, many products no longer contain thimerosal. If a person has problems with a solution and suspects the preservative, it is worth trying a solution with a different preservative. If they suspect they are sensitive to most preservatives, try completely preservative-free products, described as "for sensitive eyes" or "preservative-free". However, preservative-free products usually have shorter shelf life. For example, non-aerosol preservative-free saline solutions can typically be used for only two weeks once opened.
Before touching the contact lens or one's eyes, it is important to thoroughly wash & rinse hands with a soap that does not contain moisturizers or allergens such as fragrances.

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From Wikipedia, the free encyclopedia
Ophthalmology
An optical refractor in use.Ophthalmology is the branch of medicine which deals with the diseases of the eye and their treatment. The word ophthalmology comes from the Greek roots ophthalmos meaning eye and logos meaning word; 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. By convention the term ophthalmologist is more restricted and implies a medically trained specialist. Since ophthalmologists perform operations on eyes, they are generally categorized as surgeons.

Content
1 History of ophthalmology
2 Professional requirements
3 Sub-specialities
4 Ophthalmic surgery
5 Famous ophthalmologists

History of ophthalmology
The eye, including its structure and mechanism, has fascinated scientists and the public in general since ancient times. The discovery of the eye went through two cycles of limiting speculation and freeing observation, which led to a dark age between Galen and Vesalius.
Arabic 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-h�q'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 extent diagrams of the eye. Thus, it is not clear to which structures the texts refer, and what purpose they were thought to have.
The pre-Hippocratics largely based their anatomical conceptions of the eye on speculation, rather than empiricism. They recognised 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 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 fluid, 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 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 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 Versalius reversed the era of speculation. He, however, 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 a tool of 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.
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 Treviranus in 1834 by use of a microscope.
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. Clincal 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 who have completed medical school and embark on a training schedule that generally lasts three years after medical school in most 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.
In the United States, four years of training after medical school are required, with the first year being an internship in surgery, internal medicine, pediatrics, or a general transition year. The scope of a physician's licensure is such that he or she need not be board certified in ophthalmology to practice as an ophthalmologist. The American Academy of Ophthalmology (AAO) promotes the use of the phrase "Eye MD" to distinguish ophthalmologists from optometrists who hold the degree OD (Doctor of Optometry). (This, however, sometimes leads to confusion among patients, since a few ophthalmologists' primary medical degree is a D.O., or Doctor of Osteopathy, rather than an M.D. In both cases, the same residency and certification requirements must be fulfilled.)
In the United Kingdom, FRCS / MRCOpth and FRCOpth (postgraduate exams) are required for specialisation in eye diseases.
In Australia and New Zealand, the FRACO/FRANZCO is the equivalent postgraduate specialist qualification. They do not generally accept outsiders with equivalent qualifications and require repeat training on case by case basis.
In India, a Junior Residency at a Medical College or Institution leading to degree of Doctor of Medicine (M.D.) or Master of Surgery (M.S.), or Diplomate of National Board (D.N.B.) degree, or a diploma course leading to (Diploma in Ophthalmic Medicine and Surgery (D.O.M.S.) in Ophthalmology is necessary before one can expertly deal with various problems of the eye.
In Canada, an Ophthalmology residency and FRCSC is the requirement for becoming a licenced Ophthalmologist. There are about 10 seats per year in whole of Canada for Ophthalmology residency.
Formal specialty training programs in veterinary ophthalmology now exist in some countries [1] [2] [3].

Sub-specialities
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
Cornea, ocular surface, and external disease
Eye trauma
Glaucoma
Neuro-ophthalmology
Ocular oncology
Oculo-plastic surgery
Ophthalmic pathology
Pediatric ophthalmology/Strabismus (squint)
Refractive surgery
Retina and Vitreous (sometimes labelled as a posterior segment specialization)
Uveitis/Immunology

Ophthalmic surgery
See eye surgery for a comprehensive list of surgeries performed by ophthalmologists.

Famous ophthalmologists
See also: Category:Ophthalmologists.
Sir William Adams (UK) Founder of Exeter's West of England Eye Infirmary.
Hermengildo Arruga (Spain)
Ignacio Barraquer (Spain) carried out the first intracapsular lens extraction using enzymatic zonulolysis.
Alan C. Bird (U.K.) pioneer in medical retina and ophthalmic genetics in the second half of the 20th century. Based at Moorfields Eye Hospital and the Institute of Ophthalmology at University College London.
Ramon Castroviejo (Spain) pioneer in corneal transplantation surgery.
Marie Colinet, wife of Wilhelm Fabry, employs a magnet for removing a foreign body from the eye, 1627.
Florent Cunier (Belgium) founded the world's first ophthalmologic journal, Annales d'Oculistique, 1838.
Jacques Daviel (Normandy) claimed to be the 'father' of modern cataract surgery in that he performed intracapsular 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 (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.
Sir Stewart Duke-Elder (U.K.) Author of System of Ophthalmology, an immensely influential mid-20th century multivolume compendium of ophthalmic history, embryology, comparative ophthalmology, refraction, ocular basic sciences, medical ophthalmology and therapeutics, but avoiding discussion of surgical techniques (which he viewed as ephemera). Consultant at Moorfields Eye Hospital and founder of the Institute of Ophthalmology (now an integral part of University College London)
Svyatoslav Fyodorov (Russia) - creator of radial keratotomy
Jules Gonin (Switzerland)
Albrecht von Graefe (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 19th 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 mathamatical 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).
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.
Charles Kelman (United States) developed the cryo-probe used in intracapsular cataract extraction; introduced extracapsular cataract extraction and Kelman phacoemulsification.
P. Siva Reddy (India) holds the world record for the highest number of cataract operations by an individual doctor.
Sir Harold Ridley (U.K.) 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.
Hermann Snellen (Netherlands) introduced the Snellen chart to study visual acuity.
Carl Ferdinand Ritter von Arlt, 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.

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