Any condition affecting the eye is classified as an eye disorder. Eye disorders can affect any part of the eye, including the eyelids, eyelashes, iris, pupil, lens, and the sclera (the "white" of the eye). Nerves, muscles, and blood vessels that surround the eye can also be affected by eye disorders.
Eye disorders may include any of the following: vision problems, including astigmatism (vision difficulties due to a football-shaped cornea), diabetic retinopathy, myopia (nearsightedness), or hyperopia (farsightedness); conjunctivitis (inflammation of the conjunctiva); uveitis (inflammation of the uvea); keratitis (inflammation of the cornea); xerophthalmia (dry eye); glaucoma (an increase in intraocular pressure); age-related macular degeneration; and cataract (an opacity that develops in the crystalline lens of the eye).
Please see individual Natural Standard condition monographs on glaucoma, cataracts, retinopathy, and macular degeneration for more information on these eye disorders.
Age-related macular degeneration, allergic conjunctivitis, aqueous humor, arcus senilis, astigmatism, bacterial conjunctivitis, calcific band keratopathy, chlamydial conjunctivitis, choroid, ciliary body, ciliary muscles, conjunctiva, conjunctivitis, cornea, corneal abrasion, dry eye, ectropion, eye inflammation, giant papillary conjunctivitis, gonococcal conjunctivitis, hyperopia, iris, keratitis, keratoconjunctivitis sicca, lacrimal glands, laser-assisted in situ keratomileusis, LASIK, Leber's disease, lupus, myopia, nasolacrimal duct, neonatal conjunctivitis, non-infectious conjunctivitis, ophthalmia neonatorum, ophthalmia neonatorum prevention, ophthalmologists, ophthalmoscopy, optometrists, photoreactive keratectomy, photoreceptors, photorefractive keratectomy, pinkeye, presbyopia, PRK, punctum, retinal examination, retinitis pigmentosa, rheumatoid arthritis, sclera, scotomas, Sjögren syndrome, slit-lamp examination, thimerosal, uveitis, viral conjunctivitis, vision problems, visual field loss, vitrectomy, xerophthalmia.
structure and function of the eye
The eye is a hollow, fluid-filled, three-layered ball. The outer layer is the sclera, the innermost is the retina (the thin light-gathering layer), and the middle layer is the uvea. The uvea is made up of the iris, the ciliary body, and the choroid.
The eyeball is divided into two sections, each of which is filled with fluid. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor. The pressure generated by these fluids fills out the eyeball and helps maintain its shape.
The anterior segment itself is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris; the back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.
Light and image transmission
Light enters the eye through the cornea, a transparent dome on the front surface of the eye. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the iris), which is actually a hole through the iris. The iris is the circular, colored area of the eye and controls the amount of light that enters the pupil. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle. These muscles contract to either dilate (enlarge) or constrict (shrink) the size of the pupil. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright.
Behind the iris sits the lens. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects. By changing its shape, the lens focuses light onto the retina.
The retina contains the photoreceptors, which are nerve cells, and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors. The density of photoreceptors in the macula allows for highly detailed visual images. All photoreceptors are connected to individual nerves, which bundle together to form the optic nerve. The optic nerve begins at the optic disk, a small round structure at the back of the eye. The photoreceptors in the retina convert the image into electrical impulses, which are carried to the brain by the optic nerve.
There are two main types of photoreceptors, cones and rods. Cones are clustered mainly in the macula and are responsible for color vision as well as sharp, detailed central vision. The rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color. Rods are grouped mainly in the peripheral areas of the retina and do not contribute to detailed central vision as do the cones.
The bony structures of the orbit protrude beyond the surface of the eye. They protect the eye while allowing it to move freely in a wide arc.
The eye has a relatively tough white outer layer (sclera or white of the eye). Near the front of the eye, the sclera is covered by a thin mucous membrane (conjunctiva), which runs to the edge of the cornea and also covers the moist back surface of the eyelids.
The eyelashes are short, tough hairs that grow from the edge of the eyelid. The upper lashes are longer than the lower lashes and turn upward. The lower lashes turn downward. Eyelashes keep insects and foreign particles away from the eye by acting as a physical barrier and by causing the person to blink reflexively at the slightest disturbance.
The upper and lower eyelids are thin flaps of skin that can cover the eye. They reflexively close quickly (blink) to form a mechanical barrier that protects the eye from foreign objects, wind, dust, insects, and very bright light. The reflex is triggered by the sight of an approaching object, the touch of an object on the surface of the eye, or the eyelashes being exposed to wind or small particles such as dust or insects. On the moist back surface of the eyelid, the conjunctiva loops around to cover the front surface of the eyeball, right up to the edge of the cornea. The conjunctiva protects the sensitive tissues underneath it.
When an individual blinks, the eyelids help spread tears over the surface of the eye. Tears consist of a salty fluid that continuously bathes the surface of the eye to keep it moist and transfers oxygen and nutrients to the cornea, which lacks blood vessels that normally supply these substances to other tissues. When closed, the eyelids help trap the moisture against the surface of the eye. Small glands at the edge of the upper and lower eyelids secrete an oily substance that contributes to the tear film and keeps tears from evaporating. Tears also trap and sweep away small particles that enter the eye. Moreover, tears are rich in antibodies that help prevent infection. The eyelids and tears protect the eye while allowing clear access to light rays entering the eye. If allowed to dry, the transparent cornea may become injured, infected, or opaque.
The lacrimal glands, located at the top outer edge of each eye, produce the watery portion of tears. Mucous glands in the conjunctiva produce mucus, which mixes with the watery portion of the tears to create a more protective tear film. Tears drain from each eye into the nose through one of the two nasolacrimal ducts; each of these ducts has openings at the edge of the upper and lower eyelids near the nose, called the punctum.
common eye disorders
Years of exposure to ultraviolet light, wind, and dust may cause changes to the sclera (the white of the eyes) late in life. These changes may include yellowing or browning, random splotches of pigment (more common in people with a dark complexion), or a bluish hue due to increased transparency of the sclera.
Dry eye: The number of mucous cells in the conjunctiva may decrease with age. Tear production may also decrease with age, so that fewer tears are available to keep the surface of the eye moist. Both of these changes explain why older people are more likely to have dry eyes.
Arcus senilis: Arcus senilis (a deposit of calcium and cholesterol salts) appears as a gray-white ring at the edge of the cornea. It is common in people older than 60. Arcus senilis does not affect vision.
Retinal disorders: Some diseases of the retina are more likely to occur in old age, including macular degeneration, diabetic retinopathy, and retinopathy. Other eye diseases, such as cataracts, also become common. Please see individual condition monographs for more information on these eye disorders.
Ectropion: The muscles that squeeze the eyelids shut decrease in strength with age; this, combined with gravity and age-related looseness of the eyelids, sometimes results in the lower eyelid falling away from the eyeball, a condition called ectropion.
Puffy eyelids: In some older people, the fat around the orbit shrinks, causing the eyeball to sink into the orbit. Because of lax tissues in the eyelids, the orbital fat can also bulge forward into the eyelids making them appear constantly puffy.
The muscles that work to regulate the size of the pupils weaken with age. The pupils become smaller, react more sluggishly to light, and dilate more slowly in the dark. Therefore, people older than 60 may find that objects are not as bright, that they are dazzled initially when going outdoors (or when facing oncoming cars during night driving), and that they have difficulty going from a brightly lit environment to a darker one. These changes may be particularly bothersome when combined with the effects of a cataract.
Other changes in eye function also occur as people age. The sharpness of vision (acuity) is reduced despite use of the best glasses, especially in people who have a cataract, macular degeneration, or advanced glaucoma. The amount of light that reaches the back of the retina is reduced, increasing the need for brighter illumination and for greater contrast between objects and the background.
20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If an individual has 20/20 vision, they can see clearly at 20 feet what should normally be seen at that distance. If the individual has 20/100 vision, it means that they must be as close as 20 feet to see what a person with normal vision can see at 100 feet.
20/20 does not necessarily mean perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance. There are other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability, and color vision that contribute to the individual's overall visual ability.
Some people can see well at a distance, but are unable to bring nearer objects into focus. This condition can be caused by hyperopia (farsightedness) or presbyopia (loss of focusing ability). Others can see items that are close, but cannot see those far away. This condition may be caused by myopia (nearsightedness).
There are many types of visual disturbances. These include blurred vision, halos, blind spots, floaters, and other symptoms. Blurred vision is the loss of sharpness of vision and the inability to see small details. Blind spots (scotomas) are dark "holes" in the visual field in which nothing can be seen.
Changes in vision, blurriness, blind spots, halos around lights, or dimness of vision should always be evaluated by a healthcare professional. Such changes may represent an eye disease, aging, eye or brain injury, or a condition like diabetes that affects many organs in the body. Whatever the cause, vision changes should never be ignored. Vision problems can get worse and significantly impact the quality of the individual's life.
Astigmatism: Astigmatism simply refers to a very common eye condition in which the eye curves more steeply on one side than the other, so that the cornea is oval or "football" shaped. Astigmatism causes light rays to focus at two different points in the eye which causes distant and near images to be blurred.
Myopia: Myopia, or nearsightedness, is probably the most common type of vision disturbance, affecting about one in four people in the United States. Individuals with myopia can see things up close (this may be as little as a few inches), but anything further away appears blurry or even indistinguishable. With myopia, the eyeball is too long, or the cornea is too steep, so images are focused in the vitreous inside the eye rather than on the retina at the back of the eye. This causes blurred vision.
Hyperopia, or farsightedness, results from an eye that is too short, meaning that light rays reach the retina before they have the opportunity to converge into a focused image. Individuals with this condition are able to see, with varying degrees of success, objects further away, but close-up items are blurry.
Presbyopia: Presbyopia, or double vision, is difficulty focusing on objects that are close. In middle age (usually starting in the middle 40s), the lens of the eye becomes less flexible and less able to thicken. The eyes become less able to focus on nearby objects, a condition called presbyopia. Reading glasses, or bifocal lenses, can help compensate for this problem.
Currently, an estimated 90 million people in the United States either have presbyopia or will develop it by 2014.
Floaters: Floaters are tiny particles drifting across the eye. Eye floaters look like black or gray specks, strings, or cobwebs that drift about when the individual moves their eyes. Most eye floaters are caused by age-related changes that occur as the jelly-like substance (vitreous) inside the eyes becomes more liquid. When this happens, microscopic fibers within the vitreous tend to clump together. These clumps of debris float around within the vitreous cavity, and they can cast tiny shadows on the retina, which the individual may see as floaters. Eye floaters are most noticeable when the individual looks at a plain bright background such as a blue sky or a plain white wall. Although often brief and harmless, they may be a sign of retinal detachment. Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Eye floaters tend to affect those over age 40. Other risk factors for eye floaters include nearsightedness, eye trauma, complications from cataract surgery, diabetic retinopathy, and eye inflammation.
Other vision problems: Other potential causes of vision problems include fatigue, overexposure to the outdoors (temporary and reversible blurring of vision), and many medications. Medications that can affect vision, including causing blurry vision and dry, irritated eyes, include: antihistamines (such as diphenhydramine or Benadryl®), anticholinergics (such as dicyclomine or Bentyl®), digitalis derivatives (such as digoxin or Lanoxin®), some high blood pressure pills (guanethidine, reserpine, and thiazide diuretics), indomethacin (Indocin®), phenothiazines (such as prochlorperazine or Compazine®), and medications for malaria (such as chloroquine or Aralen®) and tuberculosis (including ethambutol or Myambutol®).
Conjunctivitis, or pink eye, is an inflammation or infection of the transparent membrane (conjunctiva) that lines the eyelid and part of the eyeball. The cause of pink eye is usually a bacterial or viral infection, and it is often associated with colds. Both viral and bacterial types are very contagious. Conjunctivitis can also be caused by an allergic reaction. In newborns, an incompletely opened tear duct may lead to conjunctivitis. Blocked tear ducts may also lead to conjunctivitis.
Bacterial conjunctivitis: The most common types of bacteria that cause bacterial conjunctivitis include Staphylococcus aureus, Haemophilus influenzae, Streptococcus pneumoniae, and Pseudomonas aeruginosa. Bacterial conjunctivitis is usually associated with a thick discharge or pus and can affect one or both eyes. Bacterial conjunctivitis often produces a thick, yellow-green discharge and may be associated with a respiratory infection or with a sore throat. Adults and children alike can develop both of these types of pink eye. However, bacterial conjunctivitis is more common in children than it is in adults.
Viral conjunctivitis: Viral conjunctivitis is a common type of conjunctivitis caused by a viral infection, such as the common cold or influenza viruses. Viral conjunctivitis is highly contagious, because airborne viruses can be spread through sneezing and coughing. Viral conjunctivitis also can accompany common viral upper respiratory infections such as measles, the flu, or the common cold. Viral conjunctivitis is usually associated with a watery discharge, and may affect both eyes. However, the infection typically starts in one eye and quickly spreads to the other eye.
Gonococcal and chlamydial conjunctivitis: Gonococcal or chlamydial conjunctivitis are caused by bacteria commonly associated with sexually transmitted diseases, including gonorrhea and chlamydia. Newborn babies may be exposed when they pass through the birth canal of an infected mother. Chlamydial conjunctivitis typically affects sexually active teens and young adults and is the most frequent infectious cause of neonatal conjunctivitis in the United States. The Centers for Disease Control (CDC) recognizes chlamydia as one of the major sexually transmitted pathogens, estimating approximately three million new cases per year. Women seem to be more susceptible than men, possibly due to differences in reproductive anatomy. The incidence of infection seems to be directly related to sexual activity and geography, with urban populations having higher incidences. The incidence in pregnant women overall is 4-10%. Trachoma is a form of chlamydial infection that causes the eye's surface to become scarred. Trachoma is the world's leading cause of preventable blindness.
Neonatal conjunctivitis: Most cases of neonatal conjunctivitis are caused by viruses. In newborns, conjunctivitis may also result from an incompletely opened tear duct. Neonatal conjunctivitis in newborn babies can cause blindness when left untreated.
Neonatal conjunctivitis can also be caused by herpes viruses, which are also associated with sexually transmitted diseases. Herpes simplex virus type 2 is most often found in the genital area and can infect the eyes of infants during birth. Herpes simplex virus type 1, more commonly associated with cold sores on the mouth, also can cause genital herpes and may thus also result in neonatal conjunctivitis.
Allergic conjunctivitis: Allergies can cause lead to a very common form of conjunctivitis, called allergic conjunctivitis. Allergens that can trigger allergic conjunctivitis (as well as other types of allergies) include pollen, animal dander, and dust mites. The most common symptom of allergic conjunctivitis is itching of the eyes. Itchy eyes may be relieved with special eye drops containing antihistamines, which block the histamines that cause allergic reactions. These eye drops are available by prescription or over-the-counter.
Non-infectious conjunctivitis: Conjunctivitis may be caused by non-infectious sources, smoke, diesel exhaust, perfumes, and chemical splashes. Discharge tends to be mucus rather than pus. Sometimes, flushing and cleaning the eye to rid it of the chemical or object causes redness and irritation. Signs and symptoms usually clear up on their own within about a day.
Certain forms of non-infectious conjunctivitis, including giant papillary conjunctivitis (GPC), can be caused by the eye's immune responses to foreign objects such as contact lenses or ocular prosthetics (artificial eyes). Toxic conjunctivitis can be caused by allergic reactions to certain ingredients in eye drops or ointments, such as preservatives or other chemicals.
The eyes of newborns are susceptible to bacteria normally present in the mother's birth canal. These bacteria cause no symptoms in the mother, but in rare cases, these bacteria can cause infants to develop a serious form of conjunctivitis known as ophthalmia neonatorum. This condition requires immediate treatment in order to preserve sight. For this reason, doctors or healthcare workers apply antibiotics to every newborn's eyes as a preventative measure.
When any part of the uvea becomes inflamed, then it is called uveitis. The most common type of uveitis is an iritis (anterior uveitis, or inflammation of the iris). Uveitis can be quite serious and may lead to permanent vision loss. Early diagnosis and treatment are important. Uveitis is most common in people ages 20-50. However, in some people, uveitis is associated with autoimmune disorders, such as rheumatoid arthritis or ankylosing spondylitis; inflammatory disorders, such as Crohn's disease or ulcerative colitis; infections, such as syphilis, toxoplasmosis, or tuberculosis; eye injury, and certain cancers, such as lymphoma.
Xerophthalmia (Dry Eye)
Xerophthalmia,also known as keratoconjunctivitis sicca or commonly as dry eye, is a medical condition in which the eye fails to produce a sufficient amount of tears for lubrication. The more modern term is dysfunctional tear syndrome, as that covers the many causes of this very complex problem. Anything that disturbs either the production of tears and/or the quality of the tears leads to dry eye syndrome or dysfunctional tear syndrome.
Dry eye problems are extremely common with increasing age, although they are more common in women than men. About 10-14 million Americans have some form of this problem. With age, tear production and quality may decrease. There are several conditions and medications that can lead to dry eye syndrome or make it worse. These include autoimmune conditions such as rheumatoid arthritis or Sjögren syndrome; diabetes; thyroid disease; some ocular surgical procedures including cataract surgery, refractive surgery for vision correction, and glaucoma surgery; ocular allergies and allergies in general; certain oral medications such as diuretics, anti-allergy medications, beta blockers, sleeping pills, pain relievers, and anti-anxiety medications; wearing of contact lenses, especially excessive wear, low water contact lenses, and ill-fitting contact lenses; preservatives in many eye drops, such as thimerosal (mercury); high wind and low humidity conditions, not only outside but wind drafts from heaters, fans, or air conditioners aimed toward the eyes; prolonged reading, computer use, or viewing of television or movies; use of a CPAP (Continuous Positive Airway Pressure) device for treatment of sleep apnea; diseases that affect the ability of the eyelids to close fully such as Bell palsy or facial paralysis; a deficiency in vitamin A; surgery of the eyelids that causes greater exposure of the eye; bone marrow transplant; inflammatory diseases; and degenerations of the cornea (outer covering of the eye).
Keratitis is an inflammation of the cornea, the transparent membrane that covers the colored part of the eye (iris) and pupil of the eye. Keratitis is generally the result of a corneal infection injury or severe dryness. Risk factors that increase the likelihood of developing this condition include: poor contact lens care; overuse of contact lenses; illnesses or other factors that reduce the body's ability to overcome infection; cold sores, genital herpes, and other viral infections; crowded, dirty living conditions; poor hygiene; and poor nutrition (especially a deficiency of vitamin A, which is essential for normal vision).
Herpes simplex keratitis: Herpes simplex keratitis is caused by an infection by the herpes simplex virus types 1 and 2 and is a major cause of adult eye disease. Herpes simplex keratitis may lead to chronic inflammation of the cornea; development of tiny blood vessels in the eye; scarring; loss of vision; and glaucoma. Herpes simplex keratitis generally begins with infection and inflammation of the membrane lining the eyelid (conjunctiva) and the portion of the eyeball that comes into contact with it. It usually occurs in one eye. Subsequent infections are characterized by a pattern of lesions that resemble the veins of a leaf. These infections are called dendritic keratitis and aid in the diagnosis.
Recurrences may be brought on by stress, fatigue, or ultraviolet light (UV) exposure (such as outdoor activities especially on water, which reflects UV rays off of the surface). Repeated episodes of dendritic keratitis can cause sores, permanent scarring, and numbness of the cornea. Recurrent dendritic keratitis is often followed by disciform keratitis. This condition is characterized by clouding and deep, disc-shaped swelling of the cornea and by inflammation of the iris. It is very important not to use topical corticosteroids with herpes simplex keratitis as it can make it much worse, possibly leading to blindness.
Bacterial keratitis: Bacterial keratitis is keratitis caused by bacteria including Streptococcus, Pseudomonas, Enterobacteriaceae (including Klebsiella, Enterobacter, Serratia, and Proteus), and Staphylococcus species. In bacterial keratitis, individuals
wake up with their eyelids stuck together. There may be pain, sensitivity to light, redness, tearing, and a decrease in vision. This condition, which is usually aggressive, can result from improper use of contact lenses. Overnight wear of contact lenses can increase the risk of bacterial keratitis by 10-15 times more than if wearing daily wear contact lenses. Improper lens care, such as not cleaning and storing properly, is also a factor. Contaminated makeup can also contain bacteria. Bacterial keratitis makes the cornea cloudy due to infection and inflammation. It may also cause abscesses to develop in the stroma, which is located beneath the outer layer of the cornea.
Fungal keratitis is a serious and painful corneal disease that is caused by fungal infection.
Fungal keratitis is usually a consequence of injuring the cornea in a farm-like setting or in a place where plant material contaminated with fungi is present. Fungal keratitis often develops slowly. This condition usually affects people with weakened immune systems and often results in infection within the eyeball.
Peripheral ulcerative keratitis:
Peripheral ulcerative keratitis is also called marginal keratolysis or peripheral rheumatoid ulceration. This condition is often associated with active or chronic rheumatoid arthritis, relapsing polychondritis (connective-tissue inflammation), and Wegener's granulomatosis (a rare condition characterized by kidney disease and development of nodules in the respiratory tract).
Superficial punctate keratitis: Superficial punctuate keratitis is often associated with the type of viruses that cause upper respiratory infection (adenoviruses). It is characterized by destruction of pinpoint areas in the outer layer of the cornea (epithelium). One or both eyes may be affected.
Acanthamoeba keratitis is an eye infection caused by Acanthamoeba. Acanthamoeba keratitis is a pus-producing condition that is very painful. It is a common source of infection in people who wear soft or rigid contact lenses. It can be found in tap water, soil, and swimming pools.
Photokeratitis or snowblindness is caused by excess exposure to UV light and a resulting burn of the cornea. This can occur with sunlight, sun tanning lamps, or a welding arc. It is called snow blindness because the sunlight is reflected off of the snow. It therefore can occur in water sports as well, because of the reflection of light off of the water. It is very painful and may occur several hours after exposure. It may last one to two days.
Also called parenchymatous keratitis, interstitial keratitis is a chronic inflammation of tissue deep within the cornea. Interstitial keratitis is rare in the United States. Interstitial keratitis affects both eyes and usually occurs as a complication of congenital or acquired syphilis. In congenital syphilis, it can occur between age two and puberty. It may also occur in people with tuberculosis, leprosy, or other diseases.
Other eye disorders
Corneal abrasion: A corneal abrasion is a painful scrape or scratch of the surface of the clear part of the eye known as the cornea. This transparent window covers the iris, the circular colored portion of the eye. The cornea has many nerve endings just under the surface, so that any disruption of the surface may be painful. A corneal abrasion may occur when an object, such as a piece of dirt, hits the eye. In addition to causing corneal injury, high-speed particles may penetrate the eye and injure deeper structures. Examples include pieces of metal fragments thrown by a grinding wheel or ashes from a hot cigarette. Corneal abrasions may also occur when the eyes are irritated or rubbed excessively. Wearing contact lenses longer than recommended may injure the corneal surface and cause a corneal abrasion.
Retinitis pigmentosa: Retinitis pigmentosa is actually the name given to a group of hereditary eye disorders, all of which involve the retina, the light-sensitive nerve layer that lines the back of the eye. All diseases classified as retinitis pigmentosa cause a progressive vision loss. The retinal defect may be found in the rod cells, which are found outside of the central retina. Rods help to transmit dim light and allow for peripheral vision. The retina's cone cells may also be affected and/or the connection between the cells that compose the retina. Cone cells are a type of retinal cell found inside the center of the retina that help to transmit the color and detail of images.