Dictionary of Terms

Aqueous (AY-kwee-us)
The aqueous is the clear, watery fluid that fills the space between the back surface of the cornea and the front surface of the vitreous, bathing the lens. It nourishes the cornea, iris and lens and maintains intraocular pressure.

Choroid (KOR-oyd)
The choroid is the major blood vessel layer of the eye that lies between the retina and the sclera. It provides nourishment to the outer layers of the retina and forms part of the uvea, along with the ciliary body and the iris.

Conjunctiva (kahn-junk-TI-vuh)
The conjunctiva is the transparent mucous membrane that covers the outer surface of the eyeball except the cornea and lines the inner surfaces of the eyelids.

Cornea (KOR-nee-uh)
The cornea is a transparent membrane on the front of the eye. It begins the process of seeing by focusing the light that it receives. There are five layers to the cornea:
-Epithelium: The outermost layer of the cornea. It is what is lifted via a small flap during LASIK.
-Bowman's membrane: The second layer.
-Stroma: This middle layer forms 90 percent of the cornea.
-Descemet's membrane: Thin, elastic layer deep in the cornea composed of collagen and elastic fibrils.
-Endothelium: Innermost layer of the cornea between Descemet's membrane and the anterior chamber of the eye that acts of a pump to keep excess water out of the stroma.

Extraocular muscles (eks-truh-AHK-yu-lur)
The six tiny muscles that surround the eye and control its movements are the extraocular muscles. They are the lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus and inferior rectus muscles. The primary function of the four rectus muscles is to control the eye's movements from left to right and up and down. The two oblique muscles move the eye rotate the eyes inward and outward. All six muscles work in unison to move the eye.

Fovea (FOH-vee-uh)
The fovea is the central pit in the macula that produces the sharpest vision. It contains a high concentration of cones, the receptors aid in color discrimination.

Iris (EYE-ris)
The iris is the part of the eye that contains color. The function of the iris is to control the amount of light that passes through the cornea. The iris is able to do this by appropriately sizing the opening of the pupil, which lies in the center.

Lens (crystalline lens)
Housed behind the iris, the lens is the second component that focuses light rays. Light rays that pass through the lens travel through the vitreous (VI-tree-us, jelly like substance) onto the retina.

Macula (MAK-yu-luh)
The macula is the small central area of the retina surrounding the fovea that is responsible for acute central vision.

Optic Nerve
The optic nerve is the path from the eye to the brain. The electronic impulses that are developed by the retina are imbedded in the brain's visual cortex thus creating a visual image. It is the largest sensory nerve in the eye and is composed of retinal nerve fibers that exit the eyeball through the optic disc.

Punctum (PUNK-tum)
The punctum is the tiny hole in the upper and lower eyelids near the nose through which tears exit the eye. Plugs may be placed in it to help treat dry eyes.

Pupil
This black central part of the iris regulates the appropriate amount of light by reducing or enlarging in size.

Retina (RET-in-uh)
The retina is a thin lining that is the back inner part of the eye. Its light-sensitive cells convert the image that is focused through the cornea and lens to electronic impulses. These impulses are then passed through the optic nerve.

Sclera (SKLEH-ruh)
The sclera is the opaque protection outer layer of the eye ("the white of the eye") that is directly continuous with the cornea in front and with a sheath covering the optic nerve behind it.

Uvea (YU-vee-uh)
The colored layers of the eye (the iris, ciliary body and choroid) that contain most of intraocular blood vessels.

Vitreous (VIT-ree-us)
The vitreous is the transparent gelatinous mass that fills the rear two-thirds of the eyeball, between the lens and the retina.

The Eye
We are able to see by the light rays that are reflected off an object and through the eyeball. These light rays are focused upside-down on the retina where it is converted to electrical impulses and carried to the brain. The brain translates the image into its upright position.