Floaters are eposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. They may be of embryonic origin or acquired due to degenerative changes of the vitreous humour or retina. The perception of floaters is known as myodesopsia, or less commonly as myiodeopsia, myiodesopsia, or myodeopsia. Floaters appear as shadow-like shapes that appear alone or together with several others in one's field of vision. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the sufferer's eyes. Since these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena.