The eye is composed of two parts, one is the wall of the eye and the other is the contents of the eye. The wall of the eye is divided into three layers consisting of outer, middle and inner membranes. The outer membrane includes the cornea and the sclera, with the cornea making up the first 1/6 of the outer layer and the sclera the next 5/6. The middle membrane is the uvea, which consists of the iris, ciliary body, and choroid. The innermost layer is the retina, and the contents of the eye are generally composed of the crystal and vitreous humor, and the cloudy crystal is what people often call cataract. In general, people feel that there are flying mosquitoes and black shadows in front of their eyes, which are generally caused by vitreous turbidity.