Human Eye

The eye is a slightly asymmetrical globe, about an inch in diameter. The front part of the eye (the part you see in the mirror) includes:

• The iris (the pigmented part)
• The cornea (a clear dome over the iris)
• The pupil (the black circular opening in the iris that lets light in)
• The sclera (the white part)
• The conjunctiva (an invisible, clear layer of tissue covering the front of the eye, except the cornea)

Just behind the iris and pupil lies the lens, which helps to focus light on the back of the eye. Most of the eye is filled with a clear gel called the vitreous. Light projects through the pupil and the lens to the back of the eye. The inside lining of the eye is covered by special light-sensing cells that are collectively called the retina. The retina converts light into electrical impulses. Behind the eye, the optic nerve carries these impulses to the brain. The macula is a small sensitive area within the retina that gives central vision. It is located in the center of the retina and contains the fovea, a small depression or pit at the center of the macula that gives the clearest vision.
Eye color is created by the amount and type of pigment in the iris. Multiple genes inherited from each parent determine a person’s eye color.

Eye Conditions

• Age-related macular degeneration: A loss of central vision in both eyes.
• Amblyopia (lazy eye): One eye sees better than the other, a problem of childhood development. The weaker eye may or may not “wander.” The weaker eye is called the "lazy eye."
• Astigmatism: A defect that causes an inability to properly focus light onto the retina. Astigmatism causes blurry vision that can be corrected with glasses or contact lenses.
• Black eye: Swelling and discoloration around the eye as a result of injury to the face.
• Blepharitis: Inflammation of the eyelids near the eyelashes. Blepharitis is a common cause of itching or a feeling of grit in the eyes.
• Cataract: A clouding of the lens, which hinders the passage of light through the lens.
• Chalazion: An oil-making gland gets blocked and swells into a bump. Often confused with styes, chalazions are not caused by infections.
• Conjunctivitis: Also known as "pinkeye,” conjunctivitis is an infection or inflammation of the conjunctiva, the clear layer that covers the front of the eye. It is usually caused by allergies, a virus, or a bacterial infection.
• Corneal abrasion: A scratch on the clear part of the front of the eye. Pain, light sensitivity, or a feeling of grit in the eye are the usual symptoms.
• Diabetic retinopathy: High blood sugar damages blood vessels in the eye. Eventually, weakened blood vessels may overgrow the retina or bleed, threatening vision.
• Diplopia (double vision): Seeing double can be caused by many serious conditions. Diplopia requires immediate medical attention.
• Dry eye: Either the eyes don’t produce enough tears, or the tears are of poor quality. Dry eye can be caused by medical problems such as lupus, scleroderma, and Sjogren's syndrome.
• Glaucoma: Increased pressure inside the eye slowly reduces vision. Peripheral vision is lost first, often going undetected for years.
• Hyperopia (farsightedness): Inability to see near objects clearly. The eye is “too short” for the lens, or certain eye muscles have weakened with age.
• Hyphema: Bleeding into the front of the eye, behind the cornea. Hyphema is usually caused by trauma.
• Keratitis: Inflammation or infection of the cornea. Keratitis typically occurs after germs enter a corneal abrasion.
• Myopia (nearsightedness): Inability to see clearly at a distance. The eye is “too long” for the lens, so light isn’t focused properly on the retina.
• Optic neuritis: The optic nerve becomes inflamed, usually from an overactive immune system. Painful vision loss in one eye typically results.
• Pterygium: A thickened conjunctival mass usually on the inner part of the eyeball. It may cover a part of the cornea, causing vision problems.
• Retinal detachment: The retina comes loose from the back of the eye. Trauma and diabetes are common causes of this medical emergency.
• Retinitis: Inflammation or infection of the retina. Retinitis may be a long-term genetic condition or result from a viral infection.
• Scotoma: A blind or dark spot in the visual field.
• Strabismus: The eyes do not point in the same direction. The brain may then favor one eye, causing decreased vision (amblyopia) in the other eye.
• Stye: Bacteria infect the skin on the edge of the eyelid, creating a tender red bump.
• Uveitis (iritis): The colored part of the eye becomes inflamed or infected. An overactive immune system, bacteria, or viruses can be responsible.

Eye Tests

• Tonometry: A test that measures pressure in the eye, called intraocular pressure. Tonometry is used to check for glaucoma.
• Slit lamp examination: A physician or optometrist shines a vertical slit of light across your eye while examining through a magnifying glass. This general exam can detect many eye problems.
• Fundoscopic exam: Dilating drops first widen the pupil. By shining bright light in the back of the eye, the examiner can view the retina.
• Refraction: If vision is impaired, a series of lenses are placed before the eyes to determine the right corrective lens prescription.
• Visual acuity test: Reading ever-smaller-sized letters across the room identifies distance vision problems. Reading up-close can identify problems with near vision.
• Fluorescein angiography: A fluorescent dye is used to take a sequence of retinal images.
• Regular adult eye exam: This collection of tests may include the ones mentioned above plus others, such as eye movement.

Eye Treatments

• Contact lenses and glasses: Glasses or contact lenses correct refractive errors such as nearsightedness, farsightedness, and astigmatism.
• LASIK (laser assisted in situ keratomileusis): A doctor cuts a flap in the cornea with a tiny saw. A laser reshapes the cornea’s surface, improving nearsightedness. Used for correcting myopia, hyperopia, and astigmatism.
• Radial keratotomy (RK): A series of small incisions are made in the cornea to correct nearsightedness. Radial keratotomy is rarely used today.
• Photorefractive keratectomy (PRK): A doctor rubs off the surface of the cornea, then uses a laser to improve nearsightedness. The cornea then heals and grows back.
• LASEK (laser epithelial keratomileusis): Similar to LASIK, but no flap is cut in the cornea. Instead, the topmost layer of cornea cells is pulled off, allowing the laser to reshape the cornea.
• Artificial tears: Eyedrops with similar composition to natural tears, used to treat dry or irritated eyes.
• Cyclosporine eye drops (Restasis): When dry eye is from a condition called keratoconjunctivitis sicca, immune-suppressing eye drops could help.
• Laser photocoagulation: A doctor uses a laser to burn blood vessels in the retina that are leaking or growing abnormally. Laser photocoagulation is most often done for diabetic retinopathy.
• Cataract surgery: The cloudy cataract is removed from the lens and replaced by a manmade lens.

Sorce: http://www.webmd.com

 Image sorce: vissioneyes.com