Eyes are organs that detect light.

The eyelid protects the eye, and helps keep the eye clean and moist by opening and shutting several times a minute. This is called "blinking." Glands of the eyelids secrete oil, water and other substances that coat the eye.  This coating is called the "tear film." If the glands are not healthy, the tear film does not have all of its components in the right amounts and the eye will feel uncomfortable. The resulting discomfort may cause excess tearing. Even with the added tears, the eye will feel dry and scratchy. This is a common condition that can be treated by using warm compresses. Artificial tears also will often help this condition. It is best to use artificial tears that are not labeled as "taking the red out."

Over time, the eyelids lose elasticity, partly because they blink many times a day. The eyelid skin sags the eyelid muscles may become loose, causing drooping eyelids that may block vision. An ocular plastic surgeon can surgically correct this problem.

The white part of the eyeball is called "sclera." The sclera covers most of the eyeball. Blood vessels on the sclera look like small, pink threads. They deliver blood to the sclera.

The cornea is the clear outer part of the eye located at the front of the eye. It does most of the eye's focusing. The cornea contains cells that continuously pump water. If those cells become damaged, the cornea will become swollen and cloudy. Corneal transplants will replace a damaged cornea. Because the cornea has no blood vessels, corneal transplants are among the most successful of all organ transplants. New drugs are making corneal transplants even more successful. However, there is a constant need for organ donors to provide the corneas to help those needing them.

The iris is the ring-shaped colored part of the eye behind the cornea. It regulates the amount of light entering the eye. The pupil is the opening at the center of the iris. The muscles of the iris change the width of the iris and thus change the size of the pupil. In this way, the iris controls how much light can enter the eye.

After light goes through the pupil, it hits the lens, which is behind the iris. The lens helps to focus light onto the retina. A normal lens is about the size and shape of an M&M candy and is clear. A cloudy lens is called a "cataract." The word "cataract" means "white water," as in a waterfall. Cataract surgery can be done when a person notices that his or her vision is inadequate. The surgeon makes a tiny incision and inserts a long, thin instrument that breaks up the cloudy lens with ultrasound waves and suctions the pieces out of the eye. A clear plastic lens is then placed in the eye. The incision is so small that stitches usually are not needed.

The vitreous gel is a clear gel that fills the inside of the eye. The retina is a light-sensitive tissue that covers the inside surface of the eye like wallpaper. It converts light into electrical impulses that are sent to the brain through the optic nerve, a bundle of more than one million nerve fibers.

Two common conditions that can damage the retina and thus cause vision loss are diabetes and age-related macular degeneration (AMD). Diabetes can cause growth of abnormal blood vessels on the surface of the retina. These blood vessels can leak and bleed, and over time they may contract and detach the retina. Diabetes also can cause swelling of the retina. A person with diabetes should have an annual eye exam to check for abnormal blood vessels and retinal swelling. These problems are treated most easily and most effectively when caught early. The best way to prevent vision problems from diabetes is to keep the blood sugar at a healthy level.

AMD commonly occurs among Caucasian people over 65 and affects the macula. The macula is a small area of the retina at the center of the back of the eye. It provides central vision. At the center of the macula is the fovea, which allows one to see details sharply. Because AMD affects the macula, it affects central vision. AMD can take a dry or wet form. In the wet form there is growth of abnormal blood vessels, and the dry form can convert to the wet form. Though not proven, a healthful lifestyle, including maintaining a healthy weight, good nutrition, regular exercise and no tobacco use, may be the best means to prevent AMD.

Another cause of vision loss is glaucoma. There are many types of glaucoma, and all can cause vision loss by damaging the optic nerve. Usually, peripheral vision is lost first and total vision loss can occur if the disease is not treated. The causes of the most common type of glaucoma is thought to be both poor blood flow to the optic nerve and eye pressure that is too high for the nerve. The eye constantly produces fluid inside, and if the fluid does not drain properly, the pressure in the eye builds up. Eye drops, lasers and surgery can reduce pressure, and often only a drop used once a day is all that is needed for treatment. A person will not notice a change in vision from glaucoma until the disease is very advanced. The best way to detect the disease early is to have regular eye exams that include eye drops to enlarge the pupil so that the eye doctor can examine the optic nerve.