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Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent.〔Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0〕 At a young age, the vitreous is transparent, but as one ages, imperfections gradually develop. The common type of floater, which is present in most persons' eyes, is due to degenerative changes of the vitreous humour. The perception of floaters is known as ''myodesopsia'',〔From Greek (μυιώδης ) "fly-like" (Myiodes was also the name of a fly-deterring deity) and ὄψις "sight."〕 or less commonly as ''myodaeopsia'', ''myiodeopsia'', ''myiodesopsia''.〔 They are also called ''Muscae volitantes'' (Latin: "flying flies"), or ''mouches volantes'' (from the French). Floaters are visible because of the shadows they cast on the retina〔American Academy of Ophthalmology. "Floaters and Flashes: A Closer Look" (pamphlet) San Francisco: AAO, 2006. ISBN 1-56055-371-5〕 or refraction of the light that passes through them, and can appear alone or together with several others in one's visual field. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the observer's eyes.〔 As these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena. ==Signs and symptoms== Eye floaters are suspended in the vitreous humour, the thick fluid or gel that fills the eye. The vitreous humour, or vitreous body, is a jelly-like, transparent substance that fills a majority of the eye. It lies within the vitreous chamber behind the lens, and is one of the four optical components of the eye. Thus, floaters follow the rapid motions of the eye, while drifting slowly within the fluid. When they are first noticed, the natural reaction is to attempt to look directly at them. However, attempting to shift one's gaze toward them can be difficult as floaters follow the motion of the eye, remaining to the side of the direction of gaze. Floaters are, in fact, visible only because they do not remain perfectly fixed within the eye. Although the blood vessels of the eye also obstruct light, they are invisible under normal circumstances because they are fixed in location relative to the retina, and the brain "tunes out" stabilized images due to neural adaptation. This stabilization is often interrupted by floaters, especially when they tend to remain visible.〔 Floaters are particularly noticeable when looking at a blank surface or an open monochromatic space, such as blue sky. Despite the name "floaters", many of these specks have a tendency to sink toward the bottom of the eyeball, in whichever way the eyeball is oriented; the supine position (looking up or lying back) tends to concentrate them near the fovea, which is the center of gaze, while the textureless and evenly lit sky forms an ideal background against which to view them.〔 The brightness of the daytime sky also causes the eyes' pupils to contract, reducing the aperture, which makes floaters less blurry and easier to see. Floaters not present at birth may disappear within weeks or months.〔(Floaters may remain indefinitely )〕 They are not uncommon, and do not cause serious problems for most persons; they represent one of the most common presentations to hospital eye services. A survey of optometrists in 2002 suggested that an average of 14 patients per month per optometrist presented with symptoms of floaters in the UK. However, floaters are more than a nuisance and a distraction to those with severe cases, especially if the spots seem to constantly drift through the field of vision. The shapes are shadows projected onto the retina by tiny structures of protein or other cell debris discarded over the years and trapped in the vitreous humour. Floaters can even be seen when the eyes are closed on especially bright days, when sufficient light penetrates the eyelids to cast the shadows. It is not, however, only elderly persons who are troubled by floaters; they can also become a problem to younger people, especially if they are myopic. They are also common after cataract operations or after trauma. Floaters are able to catch and refract light in ways that somewhat blur vision temporarily until the floater moves to a different area. Often they trick persons who are troubled by floaters into thinking they see something out of the corner of their eye that really is not there. Most persons come to terms with the problem, after a time, and learn to ignore their floaters. For persons with severe floaters it is nearly impossible to completely ignore the large masses that constantly stay within almost direct view. Floaters have been reported in patients as young as 3. Floaters in teenage patients and young adults are usually harder to treat. For persons in this age group, the floater that is seen usually looks like a kind of translucent worm/web/cell. Very little is known about this region, and it only becomes distinct after the vitreous humour detaches from the retina at later stages of life. Due to their microscopic size they cannot be seen by doctors. They only appear as big as they do because of their proximity to the retina. This type of floater is still described occasionally in people in their 30s and very rarely occurs in people in their 40s.〔(【引用サイトリンク】 publisher =South Florida Eye Foundation )〕〔(【引用サイトリンク】 publisher =Vitreous Floater Solutions )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「floater」の詳細全文を読む スポンサード リンク
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