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Anterior chamber - Fluid-filled space located between the cornea and the iris.

Aqueous humor - Watery fluid that fills the space between the back surface of the cornea and the vitreous. Produced by the ciliary body, the aqueous humor bathes and nourishes the tissues of the eye.

Ciliary body - The ring of tissue that lies at the base of the iris. Its muscle fibers change the curvature of the lens and provide for fine focusing of light onto the retina. It produces the nutrient fluid called aqueous humor.

Chamber angle - The structure formed by the junction of the iris and the corneo-sclera. Normally the drainage of fluid from the eye occurs through the trabecular meshwork located in the chamber angle. If there is a malfunction in this area, fluid won't move through the meshwork like it should, and pressure in the eye builds up causing glaucoma.

Choroid - Spongy layer lying between the sclera and retina densely packed with blood vessels. It nourishes the sensory cells found in the retina with nutrients and oxygen.

Cones - Specialized cells in the retina called photoreceptors. Cones are concentrated in the macula. They provide sharp central vision and detect colors and fine details.

Conjunctiva - Thin membrane covering the sclera and the undersides of the eyelids. It is translucent. It assists the lacrimal glands in making tears.

Cornea - Clear, dome-shaped covering at the front of your eyeball. It is part of the same layer as the sclera, but is transparent. It helps focus light on the retina and provides two-thirds of the eye's focusing power. Although the cornea has no blood vessels, it is a very sensitive structure and is richly supplied with pain nerves.

Eyelids - Pieces of skin that keep the cornea of the eye moist by moving tears over the corneal surface with each blink. They prevent the evaporation of tears during sleep. They also protect the eye from injury, as well as from dust and particles in the air.

Fovea - Located in the center of the macula. It has the highest concentration of photoreceptors and is the place where the best vision occurs. There are no retinal blood vessels in the fovea.

Iris - Colored part of the eye located behind the cornea. The color, which tends to darken during infancy, varies from pale blue to dark brown. A muscular structure shaped like a doughnut, the iris controls the size of the pupil, thus controlling the amount of light entering the eye.

Lacrimal glands - Located in the outer part of the upper eyelid beneath the brow, they produce tears to moisten the eye.

Lens - Transparent biconvex structure made of protein located behind the iris. It works in conjunction with the cornea to focus light onto the retina. It changes shape to allow the eye to focus on objects. When the lens becomes opaque or cloudy, the condition is called a cataract.

Lens capsule - The thin membrane that surrounds the lens. It is connected to the muscular part of the ciliary body by a membrane called the zonule.

Macula - The highly developed central zone of the retina. It receives the most sharply formed images and is responsible for daylight and color vision. It provides critical vision for reading and discrimination of small objects.

Oblique muscles - Two of the six muscles of the eye. They pull the eye obliquely at an angle.

Optic disc - The place where the optic nerve leaves the eye. Since it has no photoreceptor cells, a blind spot is created in vision at this location.

Optic nerve - Cable-like structure composed of thousands of nerve fibers that carry impulses from the retina to the brain where visual perception occurs.

Orbit - Socket made of bone where the eye is located.

Posterior chamber - Space behind the iris that contains the lens.

Pupil - Dark circular opening in the iris. Involuntary circular muscles in the iris contract and relax to control the size of the pupil and the amount of light entering the eye. The pupil dilates in darkness to allow more light to enter the eye, and it constricts in bright light to reduce the amount of light entering the eye. The pupil may also change size in response to viewing things at close range, to various medications and eyedrops, and to changing emotional states.

Retina - The vital thin layer of tissue that lines the inside back two-thirds of the eye. Made up of layers, it is an extension of the optic nerve and is composed of millions of visual cells. The retina can be compared to film in a camera. It is responsible for converting light to electrical signals, amplifying them, and then sending them to the brain via the optic nerve.

Rectus muscles - Four of the six muscles of the eye. They are oriented straight ahead. They allow the eye to move up and down and from side to side.

Rods - Specialized cells in the retina called photoreceptors. Rods are located outside the macula and extend all the way to the edge of the retina. They provide peripheral vision, allow the eye to detect motion, and allow a person to see in dim light and at night.

Sclera - White, tough, protective outer wall of the eye that gives the eye its shape.

Vitreous humor - Gel that fills up the back two-thirds of the eye. It has the consistency of egg white, but is much clearer. It helps maintain the shape of the eye, holding the retina against the back wall of the eye. It also cushions the contents of the eye from shocks.

Zonules - Threadlike filaments that attach to both the lens and the ciliary body and hold the lens in place in the eye. When the ciliary body constricts, the pull on the zonules is relaxed and the lens changes shape so we can see nearby objects. When the ciliary body relaxes, tension is placed on the zonules and the lens changes shape so we can see distant objects.