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 :

1. The iris ( the pigmented part )
2. The cornea ( a clear dome over the iris )
3. The pupil ( the black circular opening in the iris, which lets light in )
4. The sclera ( the white part )
5. 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, together 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.
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.
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.
Hyperopia ( farsightedness ) : Inability to see near objects clearly. The eye is “too short” for the lens, or certain eye muscles have weakened with age.
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.
Pterygium : A thickened conjunctival mass usually on the inner part of the eyeball. It may cover a part of the cornea, causing vision problems.
Scotoma : A blind or dark spot in the visual field.
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.
Cataract : A clouding of the lens, which hinders the passage of light through the lens.
Conjunctivitis : Also known as " pinkeye,” conjunctivitis is an infection or inflammation of the conjunctiva. It is usually caused by allergies, a virus, or a bacterial infection.
Glaucoma : Increased pressure inside the eye slowly reduces vision. Peripheral vision is lost first, often going undetected for years.
Diplopia ( double vision ) : Seeing double can be caused by many serious conditions. Diplopia requires immediate medical attention.
Retinal detachment : The retina comes loose from the back of the eye. Trauma and diabetes are common causes of this medical emergency.
Diabetic retinopathy : High blood sugar damages blood vessels in the eye. Eventually, weakened blood vessels may overgrow the retina or bleed, threatening vision.
Stye : Bacteria infect the skin on the edge of the eyelid, creating a tender red bump.
Chalazion : An oil-making gland gets blocked and swells into a bump. Often confused with styes, chalazions are not caused by infections.
Hyphema : Bleeding into the front of the eye, behind the cornea. Hyphema is usually caused by trauma.
Blepharitis : Inflammation of the eyelids near the eyelashes. Blepharitis is a common cause of itching or a feeling of grit in the eyes.
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.
Keratitis : Inflammation or infection of the cornea. Keratitis typically occurs after germs enter a corneal abrasion.
Retinitis : Inflammation or infection of the retina. Retinitis may be a long-term genetic condition or result from a viral infection.
Uveitis ( iritis ) : The colored part of the eye becomes inflamed or infected. An overactive immune system, bacteria, or viruses can be responsible.
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.
Optic neuritis : The optic nerve becomes inflamed, usually from an overactive immune system. Painful vision loss in one eye typically results.
Black eye : Swelling and discoloration around the eye as a result of injury to the face.