V
v gauge A device used to measure the total diameter of a rigid contact lens. It consists of a channel cut into a long rectangle of plastic or metal. The channel increases in width from 6.0 to 12.50 mm and a scale is printed beside it. The lens is placed with its concave surface down at the widest end of the channel and that end of the gauge is raised so that the lens slides down the channel until it stops. The diameter is then read from the scale where the lens touches the side of the channel. Syn. v-channel gauge.
validity The extent to which a measurement correctly measures what it is supposed to measure or to which extent the findings of an investigation reflect the truth. In health sciences, validity is commonly assessed by determining the sensitivity and specificity factors.
See reliability; sensitivity; specificity.
value f See f number.
value, Munsell See Munsell colour system.
value, V- See constringence.
valve of Hasner A fold of mucous membrane at the lower end of the nasolacrimal duct. If well developed, it generally prevents air from being blown back from the nose into the lacrimal sac. Syn. plica lacrimalis; valve of Bianchi.
See lacrimal apparatus.
valve of Krause A fold of mucous membrane at the junction of the lacrimal sac and the nasolacrimal duct. Syn. valve of Beraud.
See lacrimal apparatus.
valve of Rosenmuller A fold of mucous membrane found at the junction between the common canaliculus and the lacrimal sac. It is not strictly a valve because fluids can be blown back to emerge at the puncta. It is not always fully developed.
See lacrimal apparatus.
van Herick, Shaffer and Schwartz method See method, van Herick, Shaffer and Schwartz.
vancomycin See antibiotic.
varicella-zoster virus See herpes zoster ophthalmicus; herpesvirus.
varifocal lens See lens, progressive.
vasa hyaloidea propria See artery, hyaloid.
vascularization See neovascularization; pannus.
vase, Rubin’s See Rubin’s vase.
vectogram A polarized stereogram consisting of two polarized images at right angles to each other. When viewed through polarizing filters it presents one image to one eye and another image to the other eye. The Vectograph is a chart based on this principle in which almost one half of a chart is seen by one eye and almost the other half by the other eye while some lines, letters or numbers are seen binocularly to lock fusion. The Vectograph is useful for balancing refraction and to detect suppression and fixation disparity. The Titmus stereotest (Fig. V1) consists of various vectograms, including one with a stereoscopic pattern representing a housefly, to establish whether the patient has gross stereopsis (it produces approximately 3000 seconds of arc of retinal disparity at 40 cm). Children are often tested by asking them to hold one of the wings of the fly, which they will do above the plate if it is seen stereoscopically. The other vectograms of the test provide finer tests for stereoscopic acuity.
See acuity, stereoscopic visual; disparity, retinal; stereogram, random-dot; suppression; test, balancing; test, two-dimensional.
VEGF (vascular endothelial growth factor) A major protein involved in regulating the differentiation and proliferation of vascular endothelial cells thus promoting the growth of new blood vessels (angiogenesis). VEGF is essential for normal embryonic development and contributes to the maintenance and repair of tissues. There are several VEGF proteins, depending on the number of amino acids that they contain (e.g. VEGF 121, VEGF 165, VEGF 189 and VEGF 206). However, under certain circumstances (e.g. higher than normal levels of VEGF as happens in hypoxia) it may participate in cancerous processes, inflammatory processes (e.g. rheumatoid arthritis) and ocular neovascularization as in exudative (wet) age-related macular degeneration and diabetic retinopathy. Anti-VEGF drugs are used to inhibit the action of VEGF.
See anti-VEGF drugs.
veiling glare See glare, veiling.
vein A tubular vessel that carries blood towards the heart.
See artery.
anterior ciliary v. One of many veins that drains the ciliary body, the deep and superficial plexuses, the anterior conjunctival veins and the episcleral veins to empty into the vortex veins.
anterior facial v . Vein branching from the angular vein at the side of the nose and running obliquely downward and backward across the face. It crosses the mandible and joins the posterior facial vein to form the common facial vein, which opens into the internal jugular. The anterior facial vein drains the part of the eyelids anterior to the tarsus.
aqueous v . One of several veins serving as exit channels for the aqueous humour, which they carry from the canal of Schlemm to the episcleral, conjunctival and subconjunctival veins.
central retinal v. A vein formed by the junction of the superior and inferior retinal veins at about the level of the lamina cribrosa on the temporal side of the central retinal artery. After a short course within the optic nerve, it empties into the cavernous sinus, the superior ophthalmic vein and sometimes into the inferior ophthalmic vein.
See artery, central retinal; retinal vein occlusion.
conjunctival v. One of many veins that drains the tarsal conjunctiva, the fornix, and the major portion of the bulbar conjunctiva.
inferior ophthalmic v. Vein that commences as a plexus near the floor of the orbit, runs backward on the inferior rectus muscles and divides into two branches, one which runs to the pterygoid venous plexus and the other which joins the cavernous sinus, usually via the superior ophthalmic vein. The inferior ophthalmic vein receives tributaries from the lower and lateral ocular muscles, the conjunctiva, the lacrimal sac and the two inferior vortex veins.
palpebral v. One of the veins of the upper or lower eyelid that empties for the most part into the anterior facial vein as well as into the angular, supraorbital, superior and inferior ophthalmic, the lacrimal and the superficial temporal veins.
posterior ciliary v. See vein, vortex.
superior ophthalmic v. Vein that is formed near the root of the nose by a communication from the angular vein soon after it has been joined by the supraorbital vein. It passes into the orbit above the medial palpebral ligament, runs backward to the sphenoidal fissure where it usually meets the inferior ophthalmic vein, and drains into the cavernous sinus. It has many tributaries: the inferior ophthalmic vein, the anterior and posterior ethmoidal veins, the muscular vein, the lacrimal vein, the central retinal vein, the anterior ciliary vein and two of the posterior ciliary veins (the superior ones).
vortex v. One of usually four (two superior and two inferior) veins which pierce the sclera obliquely on either side of the superior and inferior recti muscles, some 6 mm behind the equator of the globe. The two superior ones open into the superior ophthalmic vein and the two inferior open into the inferior ophthalmic vein. These veins drain the posterior uveal tract. Syn. posterior ciliary vein; vena vorticosa.
See vein, anterior ciliary.
velocity of light See light, speed of.
velonoskiascopy A subjective method of detecting ametropia in which a thin rod held near the eye is moved across the pupil while the subject fixates a distant light source. The rod casts a shadow on the retina if the eye is ametropic. This shadow will appear to move with the rod in myopia and opposite to the movement of the rod in hyperopia. By moving the rod across the pupil in different meridians, astigmatism can be explored. No shadow is seen in emmetropia.
vena vorticosa See vein, vortex.
venous-stasis retinopathy See retinal vein occlusion.
ventral Relating to either the front (anterior), or to the bottom in brain orientation.
See dorsal; system, parvocellular visual.
vergence 1 . Denotes divergence of light travelling from, or convergence of light travelling from, or to an object or image. The object vergence at a refracting surface is equal to
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where n is the index of refraction of the first medium and l the distance between the object plane and the refracting surface in metres. The image vergence at a refracting surface is equal to
where n‘ is the index of refraction of the second medium and l‘ the distance between the image plane and the refracting surface in metres. The unit of vergence is the dioptre. 2. Disjunctive movements of the eyes such as convergence, divergence, cyclovergence, infravergence or supravergence.
See distance, image; distance, object; duction; paraxial equation, fundamental; power, refractive.
v. accommodation See accommodation, convergence.
accommodative v. See convergence, accommodative.
disparity v. See fusion, motor.
v. facility Ability of the eyes to make fusional vergence movements in a given period of time. Clinically, this is measured by introducing a relatively large prism in front of one or both eyes of a patient fixating a target until it appears single. The operation is repeated many times and the results are commonly presented in cycles per minute (one cycle indicates that single vision was reported both with the prism and after removing the prism).
See convergence, fusional; fusion, motor; lens flippers.
v. formula See paraxial equation, fundamental.
fusional v. See convergence, relative.
v. power See power, refractive.
proximal v. See convergence, proximal.
v. reflex See reflex, vergence.
relative v. See convergence, relative.
tonic v. The passive state of vergence of the eyes in the absence of a stimulus, i.e. when the eyes are in total darkness or when looking at a bright empty field. This position is maintained by the tonus of the extraocular muscles. Only at death or when paralysed do the eyes return to their anatomical position of rest and tonic vergence disappears. Syn. dark vergence; tonic convergence.
See accommodation, resting state of; position of rest, physiological; tonus.
vertical fusional v . Movement of the eyes upward until an object that was imaged on slightly disparate vertical parts of the retina falls on corresponding retinal points.
Verhoeff phi phenomenon test See movement, phi.
Verhoeff’s circles Two black concentric circles designed for use with the duochrome test and as a target for the cross-cylinder method. The thickness and overall diameter of the inner ring are equivalent to a 6/6 (20/20) Snellen letter while the thickness and overall diameter of the outer ring are equivalent to a 6/15 (20/50) Snellen letter. Syn. Verhoeff’s rings.
See chart, Snellen; test for astigmatism, cross-cylinder; test, duochrome.
Verhoeff’s rings See Verhoeff’s circles.
vernal catarrh; conjunctivitis See conjunctivitis, vernal.
vernier visual acuity See acuity, vernier visual.
version Conjugate movements of the two eyes in the same direction, such as dextroversion, both eyes rotate to the right; laevoversion (levoversion), both eyes rotate to the left; supraversion (sursumversion), both eyes rotate upward; infraversion (deorsumversion), both eyes rotate downward: these versions bring the eyes into the secondary positions of gaze. Movements of the eyes up and to the right are called dextroelevation, up and to the left, laevoelevation, down and to the right, dextrodepression and down and to the left, laevodepression: these versions bring the eye into the tertiary positions of gaze. Version eye movements are performed by yoke muscles (Fig. V2). Syn. conjugate eye movements.
See deviation, conjugate; positions of gaze, cardinal; test, motility.
version prisms See prisms, yoke.
vertex The point where the optical axis intersects a reflecting or refracting surface. In a spectacle lens the back vertex is the point of intersection of the optical axis with the surface nearest to the eye, the other being the front vertex. Plural: vertices.
v. depth Distance between the posterior pole of a spectacle lens and the plane containing the posterior edge of the lens. The vertex depth s is given by the following formula
where r is the radius of curvature of the surface of the spectacle lens and y is the semi-diameter at the edge of the surface (Fig. V3). Syn. sag.
See clearance, apical; lens measure.
v. distance Distance along the line of sight between the apex of the cornea and the posterior surface of a spectacle lens. This distance normally varies between 11 mm and 15 mm.
See clearance, apical; plane, spectacle.
v. focal length The linear distance separating the principal focal point (focus) of an optical system or lens from the front or back vertices. They are called the front vertex focal length (fv) and the back vertex
focal length (f’v), respectively. In the case of a biconcave or biconvex lens the front and back vertex focal lengths are equal. In the case of a positive meniscus lens, the back vertex focal length is shorter than the front vertex focal length and vice versa in the case of a negative meniscus lens.
See power, back vertex; power, front vertex.
v. power See power, back vertex; power, front vertex.
vertexometer 1. Synonym of focimeter. 2. Synonym of distometer.
See distometer; focimeter.
vertical fusional vergence See vergence, vertical fusional.
vertigo The sensation of irregular movement in space of either oneself or of external objects. It can be experienced after vestibular stimulation.
vesicle 1. A small bladder or sac containing liquid. 2. A small elevation on the skin containing fluid, usually serous fluid. 3. Any structure that has the appearance of 1 or 2 above.
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