Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. At a young age, the vitreous is transparent, but as one ages, imperfections gradually develop.[citation needed] The common type of floater, which is present in most persons' eyes, is due to degenerative changes of the vitreous humour.[citation needed] The perception of floaters is known as myodesopsia, or less commonly as myodaeopsia, myiodeopsia, or 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 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. They are not to be confused with visual snow, although these two conditions may co-exist.