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Eye

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   The human eye.
   Enlarge
   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 organisms. The
   simplest eyes do nothing but detect whether the surroundings are light
   or dark, while more complex eyes can distinguish shapes and colors.
   Many animals, 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.

Varieties of eyes

   The compound eyes of a dragonfly.
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   The compound eyes of a dragonfly.

   In most vertebrates and some mollusks, the eye works by allowing light
   to enter it and project onto a light-sensitive panel of cells known as
   the retina at the rear of the eye, where the light is detected and
   converted into electrical signals. These 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, including
   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.
   Compound eye of Antarctic krill.
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   Compound eye of Antarctic krill.

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

   Some of the simplest eyes, called ocelli, can be found in animals like
   snails, who cannot actually "see" in the normal sense. They do have
   photosensitive cells, but no lens and no other means of projecting an
   image onto these cells. They can distinguish between light and dark,
   but no more. This enables snails to keep out of direct sunlight.
   Jumping spiders have simple eyes that are so large, supported by an
   array of other, smaller eyes, that they can get enough visual input 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

   Diagram of major stages in the eye's evolution.
   Enlarge
   Diagram of major stages in the eye's evolution.

   The common origin ( monophyly) of all animal eyes is now widely
   accepted as fact based on shared anatomical and genetic features of all
   eyes; that is, all modern eyes, varied as they are, have their origins
   in a proto-eye evolved some 540 million years ago. The majority of the
   advancements in early eyes are believed to have taken only a few
   million years to develop, as the first predator to gain true imaging
   would have touched off an "arms race". 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.

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

   The earliest eyes, called "eyespots", were simple patches of
   photoreceptor cells, physically similar to the receptor patches for
   taste and smell. These eyespots could only sense ambient brightness:
   they could distinguish light and dark, but not the direction of the
   lightsource. This gradually changed as the eyespot depressed into a
   shallow "cup" shape, granting the ability to slightly discriminate
   directional brightness by using the angle at which the light hit
   certain cells to identify the source. The pit deepened over time, the
   opening diminished in size, and the number of photoreceptor cells
   increased, forming an effective pinhole camera that was capable of
   slightly distinguishing dim shapes.

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

   The gap between tissue layers naturally formed a bioconvex shape, an
   ideal structure for a normal refractive index. Independently, a
   transparent layer and a nontransparent layer split forward from the
   lens: the cornea and iris. 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.

Anatomy of the mammalian eye

Three layers

   The structure of the mammalian eye can be divided into three main
   layers or tunics whose names reflect their basic functions: the fibrous
   tunic, the vascular tunic, and the nervous tunic.
     * The fibrous tunic, also known as the tunica fibrosa oculi, is the
       outer layer of the eyeball consisting of the cornea and sclera. The
       sclera gives the eye most of its white 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.
     * The vascular tunic, also known as the tunica vasculosa oculi, is
       the middle vascularized layer which includes the iris, ciliary
       body, and choroid. The choroid contains blood vessels that supply
       the retinal cells with necessary oxygen and remove the waste
       products of respiration. The choroid gives the inner eye a dark
       colour, which prevents disruptive reflections within the eye.
     * The nervous tunic, also known as the tunica nervosa oculi, is the
       inner sensory which includes the retina. The retina contains the
       photosensitive rod and cone cells and associated neurons. To
       maximise vision and light absorption, the retina is a relatively
       smooth (but curved) layer. It 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".

Anterior and posterior segments

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

Anterior segment

   The anterior segment is the front third of the eye that includes the
   structures in front of the vitreous humour: the cornea, iris, ciliary
   body, and lens. Within the anterior segment are two fluid-filled
   spaces: the anterior chamber and the posterior chamber. The anterior
   chamber is the space between the posterior surface of the cornea (i.e.
   the corneal endothelium) and the iris, whereas the posterior chamber is
   between the iris and the front face of the vitreous.

   The cornea and lens help to converge light rays to focus onto the
   retina. The lens, behind the iris, is a convex, springy disk which
   focuses light, through the second humour, onto the retina. It is
   attached to the ciliary body via a ring of suspensory ligaments known
   as the Zonule of Zinn. To clearly see an object far away, the
   circularly arranged ciliary muscle will pull on the lens, flattening
   it. When the ciliary muscle contracts, 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. The iris, between the lens
   and the first humour, is a pigmented ring of fibrovascular tissue and
   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.

   All of the individual components through which light travels within the
   eye before reaching the retina are transparent, minimising dimming of
   the light. 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.

Posterior segment

   Diagram of a human eye. Note that not all eyes have the same anatomy as
   a human eye.
   Enlarge
   Diagram of a human eye. Note that not all eyes have the same anatomy as
   a human eye.

   The posterior segment is the back two-thirds of the eye that includes
   the anterior hyaloid membrane and all structures behind it: the
   vitreous humor, retina, choroid, and optic nerve. 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. 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.
   Light from a single point of a distant object and light from a single
   point of a near object being brought to a focus.
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   Light from a single point of a distant object and light from a single
   point of a near object being brought to a focus.

Extraocular anatomy

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

   In humans, the eyebrows redirect flowing substances (such as rainwater
   or sweat) away from the eye. 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 (the
   pool water is hypotonic), 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 which
   osmotically draws the excess water out of the eye.

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

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

Other articles regarding eye anatomy

   Annulus of Zinn, Conjunctiva, Macula, Nictitating membrane, Schlemm's
   canal, Trabecular meshwork.

Cytology

   This image clearly shows the pupil, iris, and blood vessels of the
   human eye.
   Enlarge
   This image clearly shows the pupil, iris, and blood vessels of the
   human eye.

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

   The retina contains two forms of photosensitive cells important to
   vision — rods and cones. Though structurally and metabolically similar,
   their function is quite different. 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). 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
   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, 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 colors
   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 colors and the
   colors obtained by mixing two of them, secondary colors. The secondary
   colors can be further complimented with primary colors to see tertiary
   colors.

Acuity

   Closeup of a hawk's eye.
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   Closeup of a hawk's eye.

   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 0.93 millimeters at
   a distance of one meter. A horse can resolve about 17 CPD (0.66 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 optical power.

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.

Eye movement

   MRI scan of human eye.
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   MRI scan of human eye.

   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.

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

How we see an object

   The steps of how we see an object:
    1. The light rays enter the eye through the cornea (transparent front
       portion of eye to focus the light rays)
    2. Then, light rays move through the pupil, which is surrounded by
       Iris to keep out extra light
    3. Then, light rays move through the crystalline lens (Clear lens to
       further focus the light rays )
    4. Then, light rays move through the vitreous humor (clear jelly like
       substance)
    5. Then, light rays fall on the retina, which processes and converts
       incident light to neuron signals using special pigments in rod and
       cone cells.
    6. These neuron signals are transmitted through the optic nerve,
    7. Then, the neuron signals move through the visual pathway: Optic
       nerve → Optic Chiasm → Optic Tract → Optic Radiations → Cortex
    8. Then, the neuron signals reach the occipital (visual) cortex and
       its radiations for the brain's processing.
    9. The visual cortex interprets the signals as images and along with
       other parts of the brain, interpret the images to extract form,
       meaning, memory and context of the images.

Colour vision

   What is seen as colour 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 cone
   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 see some or all of the colour spectrum:
   they are referred to as being 'colour blind'.

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. Thus, the eye can be considered as undergoing rotations
   about a single point in the centre of the eye.

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

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

Microsaccades

   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

Smooth 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 an observer watches the plane while moving in
   the same direction as the plane's movement, the plane will appear as if
   were standing still or moving very slowly.

   When an observer views an object in motion moving away or towards
   himself, 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 intensity of light (e.g. night vs.
   day) plays a major role in determining speed and speed difference. For
   example, no human can with reasonable accuracy, visually 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

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

   When a creature with binocular vision looks at an object, the eyes must
   rotate around a vertical axis so that the projection of the image is in
   the centre of the retina in both eyes. To look at an object closer by,
   the eyes rotate 'towards each other' ( convergence), while for an
   object farther away they rotate 'away from each other' ( divergence).
   Exaggerated convergence is called cross eyed viewing (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

   To see clearly, the lens will be pulled flatter or allowed to regain
   its thicker form.

Diseases, disorders, and age-related changes

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

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

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

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

   Various eye care professionals, including ophthalmologists,
   optometrists, and opticians, are involved in the treatment and
   management of ocular and vision disorders. A Snellen chart is one type
   of eye chart used to measure visual acuity. At the conclusion of an eye
   examination, an eye doctor may provide the patient with an eyeglass
   prescription for corrective lenses.
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