Glossary of Vision Terms

ACCOMMODATION
The ability of the eye to change its focus from distant to near objects; process achieved by the lens changing its shape.

ANTERIOR CHAMBER
The space in front of the iris and behind the cornea.

AQUEOUS HUMOR, AQUEOUS FLUID (A-kwe-us)
Clear, watery fluid that flows between and nourishes the lens and the cornea; secreted by the ciliary processes.

ASTIGMATISM (uh-STIG-muh-tizm)
A condition in which the surface of the cornea is not spherical; causes a blurred image to be received at the retina.

BINOCULAR VISION
The blending of the separate images seen by each eye into a single image; allows images to be seen with depth.

BLIND SPOT
(1) A small area of the retina where the optic nerve enters the eye; occurs normally in all eyes.
(2) Any gap in the visual field corresponding to an area of the retina where no visual cells are present; associated with eye disease.

CENTRAL RETINAL ARTERY
The blood vessel that carries blood into eye; supplies nutrition to the retina.

CENTRAL RETINAL VEIN
The blood vessel that carries blood from the retina.

CENTRAL VISION
See VISUAL ACUITY.

CHOROID (KOR-oyd)
The layer filled with blood vessels that nourishes the retina; part of the uvea.

CILIARY MUSCLES
The muscles that relax the zonules to enable the lens to change shape for focusing.

CILIARY PROCESSES
The extensions or projections of the ciliary body that secrete aqueous humor.

CONES, CONE CELLS
One type of specialized light-sensitive cells (photoreceptors) in the retina that provide sharp central vision and color vision. Also see RODS.

CONJUNCTIVA (KAHN-junk-TY-vuh)
The thin, moist tissue (membrane) that lines the inner surfaces of the eyelids and the outer surface of the sclera.

CONTRAST SENSITIVITY
The ability to perceive differences between an object and its background.

CORNEA (KOR-nee-uh)
The outer, transparent, dome-like structure that covers the iris, pupil, and anterior chamber; part of eye’s focusing system.

DILATION
A process by which the pupil is temporarily enlarged with special eye drops (mydriatic); allows the eye care specialist to better view the inside of the eye.

DRUSEN
Tiny yellow or white deposits in the retina or optic nerve head.

FLUORESCEIN ANGIOGRAPHY (FLOR-uh-seen an-jee-AHG-ruh-fee)
A test to examine blood vessels in the retina, choroid, and iris. A special dye is injected into a vein in the arm and pictures are taken as the dye passes through blood vessels in the eye.

FOVEA (FOH-vee-uh)
The central part of the macula that provides the sharpest vision.

FUNDUS
The interior lining of the eyeball, including the retina, optic disc, and macula; portion of the inner eye that can be seen during an eye examination by looking through the pupil.

HYPEROPIA (hy-pur-OH-pee-uh)
Farsightedness; ability to see distant objects more clearly than close objects; may be corrected with glasses or contact lenses.

INTRAOCULAR PRESSURE (IOP)
Pressure of the fluid inside the eye; normal IOP varies among individuals.

IRIS
The colored ring of tissue suspended behind the cornea and immediately in front of the lens; regulates the amount of light entering the eye by adjusting the size of the pupil.

LACRIMAL GLAND (LAK-rih-mul)
The small almond-shaped structure that produces tears; located just above the outer corner of the eye.

LEGAL BLINDNESS
In the U.S., (1) visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person must be at 20 feet from an eye chart to see what a person with normal vision can see at 200 feet) or (2) visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye. NOTE: These criteria are used to determine eligibility for government disability benefits and do not necessarily indicate a person’s ability to function.

LENS
The transparent, double convex (outward curve on both sides) structure suspended between the aqueous and vitreous; helps to focus light on the retina.

LOW VISION
Visual loss that cannot be corrected with eyeglasses or contact lenses and interferes with daily living activities.

MACULA (MAK-yoo-luh)
The small, sensitive area of the central retina; provides vision for fine work and reading.

MYOPIA (my-OH-pee-uh)
Nearsightedness; ability to see close objects more clearly than distant objects; may be corrected with glasses or contact lenses.

OPTIC CUP
The white, cup-like area in the center of the optic disc.

OPTIC DISC/OPTIC NERVE HEAD
The circular area (disc) where the optic nerve connects to the retina.

OPTIC NERVE
The bundle of over one million nerve fibers that carry visual messages from the retina to the brain.

PERIPHERAL VISION (per-IF-ur-al)
Side vision; ability to see objects and movement outside of the direct line of vision.

POSTERIOR CHAMBER
The space between the back of the iris and the front face of the vitreous; filled with aqueous fluid.

PRESBYOPIA (prez-bee-OH-pee-uh)
The gradual loss of the eye’s ability to change focus (accommodation) for seeing near objects caused by the lens becoming less elastic; associated with aging; occurs in almost all people over age 45.

PUPIL
The adjustable opening at the center of the iris that allows varying amounts of light to enter the eye.

REFRACTION
A test to determine the best eyeglasses or contact lenses to correct a refractive error (myopia, hyperopia, or astigmatism).

RETINA (RET-in-nuh)
The light-sensitive layer of tissue that lines the back of the eyeball; sends visual messages through the optic nerve to the brain.

RETINAL PIGMENT EPITHELIUM (RPE) (ep-ih-THEE-lee-um)
The pigment cell layer that nourishes the retinal cells; located just outside the retina and attached to the choroid.

RODS, ROD CELLS
One type of specialized light-sensitive cells (photoreceptors) in the retina that provide side vision and the ability to see objects in dim light (night vision). Also see CONES.

SCHLEMM’S CANAL
The passageway for the aqueous fluid to leave the eye.

SCLERA (SKLEH-ruh) The tough, white, outer layer (coat) of the eyeball; with the cornea, it protects the entire eyeball.

TONOMETRY
The standard to determine the fluid pressure inside the eye (intraocular pressure).

TRABECULAR MESHWORK (truh-BEC-yoo-lur)
The spongy, mesh-like tissue near the front of the eye that allows the aqueous fluid (humor) to flow to Schlemm’s canal then out of the eye through ocular veins.

UVEA, UVEAL TRACT (YOO-vee-uh)
The middle coat of the eyeball, consisting of the choroid in the back of the eye and the ciliary body and iris in the front of the eye.

VISUAL ACUITY
The ability to distinguish details and shapes of objects; also called central vision.

VISUAL FIELD The entire area that can be seen when the eye is forward, including peripheral vision.

VITREOUS (VIT-ree-us)
The transparent, colorless mass of gel that lies behind lens and in front of retina.

ZONULES (ZAHN-yoolz)
The fibers that hold the lens suspended in position and enable it to change shape during accommodation.

December 1998