Eyes

Glossary of Ocular Terms

Glossary of Terms for
Anatomy, Physiology & Pathology of the Human Eye

C

Canal of Schlemm
Schlemm’s canal; a circular canal lying in the substance of the sclero-corneal junction of the eye and draining the aqueous humor from the anterior chamber into the veins draining the eyeball

Capsule
a membrane or saclike structure enclosing a part or organ, such as the membrane around the crystalline lens

Cardinal positions of gaze
the six positions in which the eyes can be turned where each eye is controlled primarily by one muscle: up/right, right, down/right, down/left, left, and up/left

Cataract, primary
a clouding of the crystalline lens of the eye or its surrounding transparent membrane that hinders or obstructs the passage of light through the lens, resulting in blurry, hazy, or distorted vision

Cataract, secondary
a clouding of the posterior capsule of the crystalline lens of the eye, following primary cataract extraction, alleviated by penetration with a YAG laser

Cauterization
the burning, searing, or destruction of tissue, using a hot iron or caustic substance, such as to close a tear duct to keep more tears in the eye as a treatment for a dry eye condition

Central retinal artery
a branch of the ophthalmic artery that enters the retina from the middle of the optic nerve and branches to form the arterioles of the retina

Central retinal vein
a vein that is formed by union of the veins draining the retina and which passes into the middle of the optic nerve and empties into the superior ophthalmic vein

Central serous chorioretinopathy (CSCR)
painless disorder affecting the central retina (macula or para-macular area), usually only of one eye, where a collection of fluid under the retina causes a gradual blurring of vision, a distortion of objects, and sometimes a transitory increase in hyperopia; affects usually men 20-50, and often is associated with emotional or work-related stress

Chalazion
a small circumscribed tumor of the eyelid formed by retention of Meibomian gland secretions (oil) and sometimes accompanied by inflammation; removed through an incision made in the posterior portion of the affected eyelid

Choriocapilaris
the inner of the two vascular layers of the choroid of the eye that is composed largely of capillaries

Chorioretinitis
inflammation of the choroid and retina of the eye

Choroid
choroid coat; a vascular membrane containing large branched pigment cells that lies between the retina and the sclera of the eye

Ciliary body
an annular (ring-like) structure on the inner surface of the anterior wall of the eyeball, contained within the uveal tract and composed largely of the ciliary muscle and bearing the ciliary processes

Ciliary muscle
a circular band of smooth muscle fibers situated in the ciliary body and serving as the chief agent in accommodation when it contracts by drawing the ciliary processes centripetally and relaxing the suspensory ligament of the crystalline lens, permitting the lens to become more convex

Ciliary processes
vascular folds on the inner surface of the ciliary body that give attachment to the suspensory ligaments (zonules) of the crystalline lens

Ciliary spasm
an involuntary contraction of the ciliary muscle

Cloquet’s canal
an anterior-posterior canal through the middle of the vitreous humor in which the hyaloid artery was located prior to birth, the wall of which consist of multifenestrated sheaths, previously the basal lamina of the hyaloid artery

Collagen
an insoluble fibrous protein of vertebrates that is the chief constituent of the fibrils of connective tissue (as in skin, tendons, and vitreous humor) and of the organic substance of bones and yields gelatin and glue on prolonged heating with water

Color blindness
achromatopsia; very rare total color deficiency in which the colors of the spectrum are seen only as shades of white, gray and black

Color deficiency
inability to distinguish some colors and shades; occurs when the color-sensitive cone cells in the retina do not properly pick up or send normal color signals to the brain

Color vision
ability to distinguish colors and shades; occurs when the color-sensitive cone cells in the retina properly pick up and send normal color signals to the brain

Color, eye
the visible color of the iris of the eye, determined genetically

Concave
hollowed, arched, curved, or rounded inward like the inside of a bowl or the back surface of a spectacle or contact lens

Cones
the conical light-sensitive receptor cells of the retina that function in color vision and in the perception of fine detail; composed of an outer segment (containing the photoreceptor molecule iodopsin, which is composed of photopsin and retinene), inner segment, cell body, and synaptic region; about 6½ to 7 million per eye, much fewer than the number of rods; densest accumulation is in the macula and, especially, the fovea; absent in the optic disc (optic nerve head)

Conjunctiva
the mucous membrane that lines the inner surface of the eyelids and the sclera on the front of the eyeball

Conjunctiva, bulbar
the clear mucous membrane that covers the sclera on the front of the eyeball

Conjunctiva, palpebral
the mucous membrane that lines the inner surface of the eyelids

Conjunctivitis
“pink eye”; infection and inflammation of the conjunctiva, usually from an allergy, a virus, or a bacterium

Contact lens
a thin rigid or soft lens designed to fit on the front surface of the cornea and usually worn to correct defects in vision

Convergence insufficiency
difficulty or inability of the eyes to converge and work together at a near distance, such as when reading, writing, or drawing; can result in headaches, dizziness, and nausea/or, which can lead to irritability, low self-esteem and inability to concentrate; often can be treated with vision therapy or by using prismatic power in lenses

Convex
arched, curved, or rounded outward like the exterior of a sphere or circle or the front surface of a spectacle or contact lens

Cornea
the transparent, anterior, dome-shaped portion of the eyeball that covers the iris and pupil, acting like a window which admits light into the eye

Corneal ring implantation
surgical procedure in which a tiny plastic ring is inserted into the cornea, thus reshaping the cornea to refocus entering light to correct for myopia (nearsightedness), hyperopia (farsightedness), or astigmatism

Corneal transplantation
the transference of healthy corneal tissue from one individual to the eye of another individual after a portion the latter’s diseased cornea has been removed surgically

Cortex
the outer or superficial part of an organ or body structure, such as the layer of the crystalline lens peripheral to the nucleus or the outer layer of gray matter of the cerebrum and cerebellum

Cortex, lenticular
see lenticular cortex

Cortex, visual
see visual cortex

Corticosteroid
any of various adrenal-cortex steroids (as corticosterone, cortisone, and aldosterone) used medically, especially as anti-inflammatory agents

Cotton wool spots
small white areas in the retina caused by ischemic infarction of the nerve fiber layer of the superficial retina and due to occlusion of precapillary arterioles; do not threaten vision but can be indicative of a serious medical condition, such as diabetes, hypertension, cytomegalovirus infection, or HIV infection (AIDS)

Cranial nerve
any of the 12 paired nerves that arise from the lower surface of the brain with one of each pair on each side passing through openings in the skull to the periphery of the body; olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V)—including ophthalmic (V1), maxillary (V2), and mandibular (V3)—abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), and vagus (X)

Cryotherapy
the therapeutic use of cold to treat human tissue, as in a retinal tear to seal the retina to the wall of the eye again in an attempt to prevent further detachment

Crystalline lens
the highly transparent biconvex, lens-shaped or nearly spherical body in the eye, situated immediately behind the pupil, which focuses light rays entering the eye typically onto the retina

Cyclitis
inflammation of the ciliary body

Cytomegalovirus (CMV)
any of several herpes viruses (Herpesviridae) whose infection causes cellular enlargement and formation of eosinophilic inclusion bodies, especially in a cell’s nucleus; transmitted by sexual contact or exposure to infected body fluids; not highly contagious and occasionally produces mononucleosis-like symptoms in otherwise healthy adults, but acts as an opportunistic infectious agent in immunosuppressive conditions (such as AIDS)

D

Dellen
a small, indented region of the corneal surface resulting from constant dryness, usually due to the inability of the eye lid to make contact with that area of the cornea while blinking

Descemet’s membrane
posterior elastic lamina; fourth layer of the cornea; a transparent, highly elastic apparently structureless membrane that covers the posterior surface of the cornea and is lined with endothelium

Detachment, posterior vitreous (PVD)
a normal separation of the vitreous humor from the retina due to gradual liquification, with age, of the gel-like vitreous substance; often accompanied by floaters and flashes of light; infrequently leads to a retinal detachment

Detachment, retinal (RD)
separation of the retina from the choroid beneath it, often initiated by a retinal tear resulting in the seepage of vitreous fluid underneath the retina; can be associated with the observation of a shower of floaters, flashes of light, and/or a “web” or “veil” in front of the eye or in the periphery; usually treated with a laser, freezing (“cryotherapy” or “cryopexy"), or a scleral buckle

Deuteranomaly
a type of anomalous trichromatism color deficiency in which there is a partial loss of green color vision, so that an increased intensity of this color is required in a mixture of red and green to match a given yellow

Deuteranopia
a type of anomalous dichromatism color deficiency marked by confusion of purplish red and green, due to a lack of (or lack of function of) “M-cone” photoreceptors sensitive to medium (greenish) wavelengths of light

Diabetes insipidus
a disorder of the pituitary gland characterized by intense thirst and by the excretion of large amounts of urine

Diabetes mellitus
a variable disorder of carbohydrate metabolism caused by a combination of hereditary and environmental factors and usually characterized by an inadequate secretion of insulin by the pancreas or proper utilization of the available insulin, by excessive urine production, by excessive amounts of sugar in the blood and urine, and by thirst, hunger, and loss of weight; type I (juvenile) diabetes mellitus: insulin-dependent diabetes; type II (adult-onset) diabetes mellitus: non-insulin-dependent diabetes; can cause neovascularization (new blood vessel growth) in the retina (retinitis proliferans), resulting in fragile blood vessels which can bleed into the vitreous humor and lead to severe visual impairment or blindness

Diabetic retinopathy, background
changes in the retina due to diabetes, eventually leading to proliferative diabetic retinopathy if the diabetes is not controlled; caused by adverse changes in the endothelial cells and pericyte cells in the retinal blood vessels, causing blood vessel weakening and leading to microaneurysms, venous beading, ischemic areas of the retina, cotton wool spots, intraretinal hemorrhages, and macular edema

Diabetic retinopathy, proliferative
neovascularization (growth of new blood vessels), usually out of the retina and along the surface of the vitreous or into the vitreous cavity, leading to the formation of scar tissue on the surface of the retina (which can cause retinal tears and detachment) and leaking of blood by the new fragile vessels into the eye (vitreous hemorrhage), all capable of causing severe vision loss or blindness

Diopter (D)
a unit of measure of the refractive power of a lens or of an eye, where the dioptric power is the inverse (reciprocal) of the focal length of the lens or of the eye’s optical system in meters; a +1.00 D convex lens focuses light, from an infinite source, 1 ÷ 1.00 D = 1 meter away from the lens (and corrects an eye which is 1.00 D hyperopic); a +2.00 D lens focuses infinite light 1 ÷ 2.00 D = ½ meter away (and corrects an eye which is 2.00 D hyperopic); a +0.25 D lens focuses infinite light 1 ÷ 0.25 D = 4 meters away (and corrects an eye which is 0.25 D hyperopic); etc.

Diplopia
double vision, usually caused by a strabismus

Dry eye
a chronic lack of sufficient lubrication and moisture in the eye(s), causing sensations of dryness, scratchiness, burning; can be caused by conditions such as rheumatoid arthritis, Sjogren’s syndrome, keratoconjunctivitis sicca, xerophthalmia, lupus erythematosis, Grave’s disease, diabetes, or scleroderma

Dura mater
the outer meningeal layer of the optic nerve; fuses with the sclera where the optic nerve enters the eye

Dystrophy, corneal
one of a number of inherited diseases—all potentially blinding disorders which frequently require corneal transplantation to improve visual acuity—characterized by the accumulation of abnormal material in the cornea, typically classified depending on which of the three levels of the cornea they primarily affect, including anterior corneal dystrophies (such as map-dot-fingerprint dystrophy and Reis-Bückler’s dystrophy), stromal dystrophies (such as granular, lattice, and Avellino dystrophy), and posterior corneal dystrophy (such as Fuch’s dystrophy and posterior polymorphous dystrophy or PPMD)

Dystrophy, macular
one of a number of progressive, genetically determined degenerations of the retina or choroid, typically occurring at an early age, primarily affecting the macular area by causing anything from minimal visual acuity loss and color vision abnormality to profound loss of reading ability with night vision difficulty; includes Stargardt’s disease, fundus flavimaculatus, Best’s vitelliform dystrophy, cone or cone/rod dystrophy, juvenile retinoschisis, and juvenile macular dystrophy

E

Eccentric fixation
eye fixation with a point on the retina other than the fovea or macula; often found in a strabismic eye or in an eye with longstanding macular pathology; causes the visual acuity in that eye to be less than it would be if a healthy macula were being used, since visual acuity drops markedly the further away from the fovea or macula the image is located

Edema
an abnormal infiltration and excess accumulation of serous fluid in body tissue or in a serous cavity, causing swelling; dropsy or hydrops

Edema, corneal
infusion of fluid (such as aqueous humor or tears) into the cornea, causing swelling, haziness, and decreased vision

Edema, cystoid macular (CME)
painless disorder affecting the macula or central retina where multiple cyst-like (cystoid) formations appear in the macula and cause retinal swelling or edema with resultant blurred or decreased central vision; may accompany a variety of diseases such as retinal vein occlusion, uveitis, or diabetes, or can occur following cataract surgery

Edema, macular
infusion into the macula of serum leaked from blood vessels underneath the macula, causing macular swelling and decreased vision

Embolism
the sudden obstruction of a blood vessel by an embolus

Embolus
an abnormal particle (such as cholesterol, platelet clump, blood clot, calcium fleck, an air bubble, fat, or some combination of these) circulating in the blood

Emmetropia
no refractive error, where light coming from a distant object and entering the eye focuses upon the retina while the crystalline lens in its most relaxed (thinnest) state

Encephalomyelitis
concurrent inflammation of the brain and spinal cord

Endothelium, corneal
a single layer of cells of mesoblastic origin which line the posterior surface of the cornea

Endothelium, iridial
a single layer of cells of mesoblastic origin which line the anterior surface of the iris

Epiphora
a watering of the eyes, usually with spillage over the lower eyelids, due to excessive secretion of tears or to obstruction of the lacrimal passages

Epiretinal membrane (ERM)
a membrane of scar tissue that forms over the retina following irritation of that area by a posterior vitreous detachment; commonly known as a “macular pucker” if the macula is involved

Episcleritis
an inflammation (usually localized) of the episclera, a thin layer of tissue covering the sclera and containing many blood vessels that nourish the sclera; cause is unknown, but can be associated with certain diseases such as rheumatoid arthritis, Sjogren’s syndrome, syphilis, herpes zoster, and tuberculosis

Epithelium, corneal
a membranous cellular tissue that covers the anterior surface of the cornea

Epithelium, iridial
a membranous cellular tissue that covers the posterior surface of the iris

Epithelium, pigmented
supporting cells for the neural portion of the retina (photopigment regeneration, blood); dark with melanin which decreases light scatter within the eye

Esotropia
a strabismus where the deviating eye turns inward (toward the nose), as compared to the other eye which remains pointing straight ahead

Exotropia
a strabismus where the deviating eye turns outward (away from the nose), as compared to the other eye which remains pointing straight ahead

External limiting membrane
retinal layer separating the layer of outer and inner segments of cone and rod photoreceptors from the layer of photoreceptor cell bodies; composed of junctional complexes of rods and cones with Müller cells

Extracapsular cataract extraction (ECCE)
extraction of a cataract, usually by “phacoemulsification” (disintegration of the lens by ultrasonic vibrations and subsequent suction of the particles out of the eye) of the nucleus and cortex of the crystalline lens, leaving behind the posterior portion of the capsule into which an artificial implant lens is placed

Extraction, extracapsular cataract
see extracapsular cataract extraction (ECCE)

Extraocular muscles
six muscles (medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique) which serve to move an eye by rotating it about its vertical, horizontal, and antero-posterior axes

Eyeglasses
spectacles; lenses ground to the specifications of a prescription which, when mounted into frames and worn on the face, aid in refocusing the light entering each eye onto the retina at the back of the eye, providing clear vision

Eyelid
palpebra; either of the movable lids of skin and muscle that can be closed over the eyeball, providing eye protection and distribution of tears over the cornea while blinking