Anatomy & Refractive States of the Eye
Glossary

Accommodation: Focusing of the eye on a near object through relaxation of the ciliary muscle and thickening of the lens.

Amblyopia: The loss of vision without any apparent disease of the eye.

Ametropia: A refractive error in which the eye, when in a state of rest, does not focus the image of an object upon the retina; includes hyperopia, myopia, and astigmatism.

Aniseikonia: A condition in which the ocular image of an object as seen by one eye differs so much in size or shape from that seen by the other eye that the two images cannot be fused into a single impression.

Anisometropia: A condition in which the refractive error of one eye significantly differs from that of the other; each eye must have the same sign, i.e. both eyes are myopic or hyperopic.

Antimetropia: The refractive condition whereby the signs in each eye are opposite; one eye is hyperopic while the other is myopic.

Aphakia: An absence of the crystalline lens of the eye.

Aqueous humor: The clear watery fluid that fills the anterior and posterior chambers within the front part of the eye.

Astigmatism: A refractive error that prevents the light rays from coming to a single focus on the retina because of different degrees of refraction in the various meridians of the eye.

Cataract: A condition in which the crystalline lens of the eye, or its capsule, or both, become opaque, with consequent loss of visual acuity.

Choroid: The vascular intermediate coat that furnishes nourishment to other parts of the eyeball.

Ciliary body: That portion of the vascular coat between the iris and the choroid. It consists of ciliary processes and the ciliary muscle.

Compound hyperopic astigmatism: The refractive error which results in two points of focus falling behind the retina.

Compound myopic astigmatism: The refractive error which results in two points of focus falling in front of the retina.

Cornea: The clear transparent portion of the outer coat of the eyeball forming the covering of the aqueous chamber.

Crystalline Lens: A transparent colorless body suspended in the front part of the eyeball, between the aqueous and the vitreous, the function of which is to bring the rays of light to a focus on the retina.

Diplopia: The seeing of one object as two.

Emmetropia: The refractive condition of the normal eye when the eye is at rest, the image of distant objects is brought to a focus on the retina.

Fovea: A small depression in the retina at the back of the eye; the part of the macula adapted for most acute vision.

Fusion: The power of coordination by which the images received by the two eyes become a single image.

Glaucoma: An ocular disease having as its primary characteristic a sustained increase in intraocular pressure that the eye cannot withstand without damage to its structure or impairment of its function.

Hyperopia: A refractive error in which, because the eyeball is short or the refractive power of the lens is weak, the point of focus for rays of light from distant objects falls behind the retina; accommodation to increase the refractive power of the lens is necessary for distance vision as well as near vision.

Iris: The colored circular membrane suspended behind the cornea and immediately in front of the lens. The iris regulates the amount of light entering the eye by changing the size of the pupil.

Irregular Astigmatism: A refractive condition caused by a cornea which is damaged and irregular so that rays of light come to many focal points on the retina; is not correctable by cylinders.

Iseikonic Lenses: Spectacle lenses specially designed to manipulate image size when dissimilar sizes makes it difficult or impossible for fusion to occur; such as in aniseikonia.

Macula: The small area of the retina that surrounds the fovea and that with the fovea, comprises the area of the retina that gives distinct vision.

Mixed Astigmatism: The refractive condition in which light comes to two points of focus where one point is in front of the retina, and the other is behind the retina.