The eye is a magical organ that allows us to see and experience the world. But how does it function?

The human eye works like a living camera. Like a camera, the eye processes light and takes mental snapshots of images, which are then developed in the brain. In order to create vision, all parts of the eye must work together as a team. The cornea, the clear, film-like part of the eye, is the leader. When light enters the dark pupil of the eye, the cornea bends it. The refracted light travels to the lens behind the pupil. The lens changes its shape to adjust the focus on both distant and near objects. This process is known as accommodation.

The lens projects light from the outside world as an inverted image into the retina, which then acts like film in a camera and captures the image. The image is sent to the brain in the form of electric impulses to be developed.

The Anatomy of the Eye

View of an iris and pupil	View of tear ducts

Diagram of the eye muscles

View of the left eye muscles

View of the eye muscles

Sclera:

The outer, white part of the eye that gives protection to the delicate inner structures.

Iris:

The colored part of the eye that contains very fine muscles that control the size of the pupil.

Pupil:

The little black spot in the center of the iris that controls the amount of light reaching the retina by expanding in the dark to allow more light in, and contracting in the light to prevent excess light from entering.

Aqueous Humor:

A fluid produced by the ciliary body that circulates through the front and rear of the eye to maintain eye pressure and provide nourishment to the cornea and the lens.

Macula:

A tiny part of the retina that focuses and produces color vision.

Ciliary Body:

Contains a gland that secretes aqueous humor and a muscle that contracts and expands to control the curvature of the lens.

Optic disc:

The point where the retina meets the optic nerve, which is often referred to as the blind spot because of its insensitivity to light.

Vitreous gel:

A clear gel that fills the eye and provides it with a spherical shape.

Choroid:

Located between the sclera and the retina, it is made up of blood vessels that provide nourishment to the eye.

Zonulae:

Threadlike structures that attach the lens to the ciliary muscle and help it to change its curvature.

Perfect Eyesight

System Failure: Defects in Vision

So Close, So Far
NEARSIGHTEDNESS (myopia)

• A person can clearly see objects that are near, but objects that are far appear blurred.
• Caused by a refractive problem of the cornea or lens, or the eyeball is too long.
• The cornea focuses the image in front of the retina, rather than on the retina's surface.

Far and Away
FARSIGHTEDNESS (hyperopia)

• A person can clearly see objects that are far away, but objects that are near appear blurred.
• Caused by malfunction in the cornea or lens, or the eyeball is too short.
• The cornea focuses the image behind the retina, rather than on the retina's surface.

ASTIGMATISM

• The cornea is unevenly shaped, and bends and scatters light in different directions, which causes objects to look distorted and appear blurry.

Cataract

Formed when the lens becomes clouded, often resulting in blurry vision, poor night vision, or sensitivity to light.

Although commonly a result of aging (senile cataract), some people are born with them (congenital cataract). Trauma, medication, and long-term inflammation can also cause cataracts.

Treatment can involve extraction and replacement of the lens.

Sunglasses help to prevent cataracts.

Glaucoma

A condition resulting from too much aqueous humor present in the eye, causing the pressure to rise and push against the optic nerve, resulting in nerve damage and vision loss.

Age, family history, those who are nearsighted, and people of African descent are all highest risk factors for glaucoma.

Glaucoma is detected based on the above named risk factors, intraocular pressure testing, and examination of the aqueous drainage system, but because there are no early warning signs and vision loss is so gradual, a person may not realize that they are becoming blind.

Early detection is the key to successful treatment.

Macular Degeneration

Central vision is lost due to the deterioration of the macula, the part of the retina where most of the light focuses.

Age related macular degeneration, which is hereditary, is the most common form, and tends to progress slowly. While a patient loses central vision, peripheral vision remains.

Macular degeneration may be detected and diagnosed during annual comprehensive eye examinations, and depending on the form, treatment may be available.

Sunglasses help to prevent macular degeneration.

Amblyopia

This condition, also known as "lazy eye," occurs when when one eye is weaker than the other. Both eyes cannot blend an image.

Lazy eye usually starts at childhood, and can be corrected if detected and treated by the age of five.

By patching the strong eye, the weaker eye is forced to properly network to the brain, allowing it to become stronger.

Vitamin A Deficiency (xerophthalmia)

An eye disease caused by a deficiency in Vitamin A, xerophthalmia is a range of disorders that can eventually lead to blindness.

Symptoms include:

• Bitot’s spots: Foamy, soapy, and whitish patches on the white part of the eye that can affect sight during nighttime.
• Conjunctival xerosis: Dryness and wrinkling of the white part of the eye, which is usually wet, smooth, and shiny.
• Corneal lesions: Lesions that cover the clear part of the eye, which causes blindness.
• Night blindness and difficulty in seeing things in dim light.

Trachoma

An eye infection that scars the upper eyelid and causes it to go inward, resulting in the eyelashes scratching the cornea.

Unless properly treated by surgery or antibiotics, this can be a very slow, painfully blinding condition.

River Blindness (Onchocerciasis)

Usually rampant in African countries, River Blindness is caused by a parasite spread by black flies that breed in rivers and deposit the larvae of the worm onto the person they bite.

Causes severe itching and eye lesions, eventually leading to blindness.

Conjunctivitis (Pink Eye)

An eye infection that targets the conjunctiva, the outermost eye layer that covers the sclera.

The following are the three types of infection:

• Viral- Caused by an upper respiratory infection, cold, or sore throat. Symptoms include watery discharge and red eyes.
• Allergic - Caused by intolerance to chemical substances such as cosmetics or drugs. Symptoms include itching, tearing, and swollen eyelids
• Bacterial - Caused by a bacteria infection. Symptoms include eyelids sticking together, swelling, redness, and tearing.