Why do we sometimes see flying in the air colorless fly?

Many people see flying in the air colorless "flies", especially when looking at the bright lit surface, such as a clear sky. This effect is the scientific name - destruction of the vitreous tela.V Ideally vitreous our eyes - is transparent gelatinous substance, but due to illness, injury, increased eye strain or simply age-related changes in individual fibers thicken and it loses its transparency, which is perceived by us as mushki.

Usually destruction of a vitreous nothing dangerous and cause complications, but if flies appear very rapidly, accompanied by flashes of light, it can speak about the imminent retinal detachment, which leads to blindness.