Eyes

Glossary of Ocular Terms

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

A ,B | C,D,E | F,G,H | I | J,K, L | M,N,O | P,Q,R,S | T,U,V,W | X,Y, Z

T

Tear film
the thin layer of tear fluid coating the front surface of the cornea; composed of three layers: lipid layer (most anterior), lacrimal layer (middle), and mucoid layer

Tearing
the flooding of “reflex” tear fluid (usually lacrimal fluid from the lacrimal gland) into the eye, due to some type of irritation or from crying

Tears, artificial
manufactured eyedrops or ointments, approximating the pH, osmolarity, osmolality, and tonicity of normal tears; often used to treat dry eye conditions, although prolonged use can disrupt the eye’s natural production of tears and can wash away the infection fighting agents contained in the eye’s natural tear film

Tenon’s capsule
the bulbar sheath surrounding the posterior half of the eye

Thrombosis
the formation or presence of a blood clot within a blood vessel

Thrombus
a clot of blood formed within a blood vessel and remaining attached to its place of origin

Tonometer
an instrument, such as a Goldmann tonometer or an “air puff” tonometer, used to measure intraocular pressure in millimeters of mercury (mm Hg)

Tonometry
the measuring of intraocular pressure using a tonometer

Toxicity
the quality, relative degree, or specific degree of being toxic or poisonous

Toxocariasis
infection with or the disease caused by nematode worms of the genus Toxocara

Toxoplasmosis
infection with or the disease caused by a sporozoan of the genus Toxoplasma (T. gondii) that invades the tissues and may seriously damage the central nervous system, especially of infants

Trabecular meshwork
a network of fibers, located near the junction of the cornea and the sclera, responsible for the continual drainage of the aqueous humor from the eye and into the canal of Schlemm

Tritanopia
a type of anomalous dichromatism color deficiency marked by confusion of blue and yellow, due to a lack of (or lack of function of) “S-cone” photoreceptors sensitive to short (bluish) wavelengths of light

Trochlea
a ring-like tendon, functioning as a pulley, through which the superior oblique muscle passes before it attaches to the eye

Tuberculosis (TB)
a usully chronic, highly variable disease that is caused by the tubercle bacillus and rarely in the U.S. by a related mycobacterium (Mycobacterium bovis); usually is communicated by inhalation of the airborne causative agent; affects especially the lungs but may spread to other areas (as the kidneys or spinal column) from local lesions or by way of the lymph or blood vessels; characterized by fever, cough, difficulty in breathing, inflammatory infiltrations, formation of tubercles, caseation (necrosis with conversion of damaged tissue into a soft cheesy substance), pleural effusion, and fibrosis

Tunica fibrosa
the outermost, tough, protective layer of the eye, including the opaque sclera (posterior 5/6) and the clear cornea (anterior 1/6)

Tunica intima
the inner, mostly photosensitive layer of the eye, including the neural retina, pigmented epithelium, and posterior portion of the iris and ciliary body

Tunica vasculosa
uveal tract; the middle, heavily pigmented layer of the eye, including the anterior iris, the anterior ciliary body, and the choroid

Tunnel vision
constriction of the visual field, resulting in loss of peripheral vision

u

Uveitis
inflammation of any or all of the structures contained in the uvea (including the iris, ciliary body, and choroid)

Uonules of Zinn
suspensory ligaments; a ring of fibrous strands, composed mainly of elastin microfibrils, connecting the ciliary body with the crystalline lens of the eye; hold the lens in place, stretching and loosening (due to relaxation and contraction of the ciliary muscle) to change the shape of the lens to focus at far and at near (accommodation)

v

Virtual image
the image formed by a concave lens (or a convex mirror), on the same side of the lens on which the light is entering; the image is formed at the focal point of the lens, at a distance (focal length) away from the lens, in meters, equal to the inverse of the dioptric power of the lens

Visual acuity
the relative ability of the visual system to resolve detail, usually expressed as the reciprocal of the minimum angular separation, in minutes of arc, of two lines just resolvable as separate and that forms in the average human eye an angle of one minute of arc; often measured by a “Snellen test,” a test presenting letters of graduated sizes to determine the smallest size that can be read at a standard distance (a 20/20 letter located 20 feet away from an eye subtends an angle of 5 minutes of arc at the eye); “normal” acuity in the human eye is 20/20, although some eyes are capable of 20/15 or even 20/10 acuity

Visual axis
line of vision; a straight line joining the fovea of the eye with the eye’s fixation point

Visual cortex
the sensory area of the occipital lobe of the brain’s cerebral cortex receiving afferent projection fibers and concerned with the sense of sight

visual field
field of vision; the entire expanse of space visible at a given instant without moving the eyes

Visual pathway
the path of electrical impulses from the eye to the brain, resulting in the sense of vision; begins with a photochemical reaction, due to light stimulus, in the rods and cones, causing nerve impulses to be generated in the retinal ganglion cells, which then are transmitted through ganglion axon to the region of the optic disc; impulse continues along fibrous processes within the optic nerve, through the optic foramen and to the optic chiasm (or optic chiasma), where half of the fibers decussate (cross over) to the other side, becoming the optic tract, which terminates in the lateral geniculate body located in the thalamus; fibers then give rise to new fibers, which form the optic radiation fibers that extend out and back through the brain to the cortex (surface of the brain) in a region of the brain called the optical lobes; area of the cortex that receives the optic radiations surrounds the calcarine fissure and is called the striate area, striate cortex, visual area, or Brodmann’s Area 17; gross area of vision in the brain is called the occipital cortex.

Vitamin A
includes retinol (preformed vitamin A) and beta-carotene (provitamin A); can help in the prevention and treatment of night vision deficiency (“night blindness”), respiratory tract infection, and skin problems

Vitamin C
ascorbic acid (C6H8O6); a water-soluble vitamin found in plants, especially in fruits and in leafy vegetables, or made synthetically and used in the prevention and treatment of scurvy and as an antioxidant

Vitamin E
any of several fat-soluble vitamins that are chemically tocopherols, are essential in the nutrition of various vertebrates in which their absence is associated with infertility, degenerative changes in muscle, or vascular abnormalities; found especially in leaves and in seed germ oils; used chiefly as an antioxidant and also as an anticoagulant and a temperature regulator, as well as in boosting the immune system booster and in hindering heart disease

Vitreous humor
the transparent gelatinous mass occupying the posterior compartment (the space between the crystalline lens and the retina of the eye) which is enclosed by a delicate hyaloid membrane; composed of water (99%), collagen fibrils, highly hydrated hyaluronic acid, halocytes, inorganic salts, sugar, and ascorbic acid; produced by halocytes located peripherally in the vitreous body

Vorticose vein
a vein formed by branches from the back surface of the eye and the ciliary body; empties into the ophthalmic veins

w

Waardenburg syndrome
a condition in which there is a wide bridge of the nose (owing to a lateral displacement of the inner canthus of each eye), pigmentary disturbances (frontal white blaze of hair, heterochromia iridis, white eye lashes, leukoderma [white patches on the skin]), and often cochlear deafness

Wavelength
the distance in the line of advance of a wave of light from any one point to the next point of corresponding phase; usually measured in nanometers

X

Xantholasma
whitish fatty deposits on an eyelid or lids; may be indicative of hyperbetalipoproteinemia, a syndrome characterized by an increased level of low density lipoproteins and their associated lipid components

X-chrome lens
a reddish or maroon colored contact lens worn on one eye, enabling people with certain color deficiencies to estimate or determine the colors of objects

Xerophthalmia
a dry thickened lusterless condition of the eyeball resulting especially from a severe systemic deficiency of vitamin A

y

Yttrium aluminum garnet (YAG) laser
Neodymium: YAG laser; used to vaporize a secondary cataract (the clouded capsule of the crystalline lens behind the intraocular lens implant) in a procedure called a capsulotomy

z

Zonules of Zinn
suspensory ligaments; a ring of fibrous strands, composed mainly of elastin microfibrils, connecting the ciliary body with the crystalline lens of the eye; hold the lens in place, stretching and loosening (due to relaxation and contraction of the ciliary muscle) to change the shape of the lens to focus at far and at near (accommodation)