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The retina. Fromer explained that the retina is the innermost of three tissue layers that make up the eye. The outermost layer, called the sclera, is what gives most of the eyeball its white color. The cornea is also a part of the outer layer.The middle layer between the retina and sclera is called the choroid. The 7 Biggest Mysteries of the Human Body ]. Behind the cornea is a colored, ring-shaped membrane called the iris. The iris has an adjustable circular opening called the pupil, which can expand or contract to control the amount of light entering the eye, Fromer said. Ciliary muscles surround the lens.
Tears play an important role in keeping us healthy. Tears keep the surface of our eyeballs clean and moist, and help protect our eyes from damage. Although they make appear to be nothing more than water, our tears are actually quite complex. Tears are made of mucus, water and oil. Mucus coats the surface of the eye and helps bind the tear layer to the eye. Without a healthy mucus layer, dry spots may form on the cornea, the clear dome-like structure on the front of the eye.