Eye

[Continued from above] . . . by nerves in the brain stem. The eyeball has a tough, outer coat called the sclera, or white part of the eye. The front, circular part is the cornea and is transparent. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the iris (colored part) with the pupil (center). The pupil is black and its diameter is changed by light intensity to control the amount of light that enters the eye. Immediately behind the iris and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms. The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels that lie just under it called the choroid plexus. The eyeball is sealed off from the outside by a flexible membrane called the conjunctiva, which is attached to the skin at the corners of the eye and forms the inner lining of the lids and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness.