V
“V” selections in the Glossary of Terms for
Anatomy, Physiology & Pathology of the Human Eye
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- vasoconstrictor
- an agent (such as a sympathetic nerve fiber or a drug) that induces or initiates constriction of blood vessels
- vascular tunic
- see tunica vasculosa
- vasculosa, tunica
- see tunica vasculosa
- vein, central retinal
- see central retinal vein
- 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
- vision, color
- see color vision
- vision, tunnel
- see tunnel vision
- 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 detachment, posterior
- see detachment, posterior vitreous (PVD)
- 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
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