Anatomy of the eye

The eyes are complex sensory organs. They are designed to optimise vision under conditions of varying light. Their location, on the outside of the face, makes them susceptible to trauma, environmental chemicals and particles, and infectious agents. The eyelids are the major protective mechanism of the eye.

The eye consists of the eyeball, the upper and lower eyelids, and the conjunctivas. The eyeball consists of a clear, transparent dome at the front (the cornea) that is surrounded by the white of the eyeball (the sclera). The iris of the eye is the circular, coloured portion within the eyeball and behind the cornea, and the pupil is the central opening within the iris. The iris regulates the amount of light entering the eye by adjusting the size of the pupil. Behind the iris and the pupil is the eye's lens. The lens helps to focus light, or an image, on the retina. The retina is the light-sensitive tissue lining at the back of the eye. The retina converts light into electrical impulses that are sent to the brain through the optic nerve. The optic nerve is a bundle of more than one million nerve fibers that carrries visual messages from the retina to the brain. The vitreous gel is a clear gel that fills the inside of the eye. The inner sides of the eyelids, which touch the eyeball, are covered by a thin membrane (the palpebral conjunctiva) that produces mucus to lubricate the eye. This thin membrane folds back on itself and covers the visible sclera of the eyeball. This continuation of the palpebral conjunctiva is called the bulbar conjunctiva. The tissue between the skin of the eyelid and the palpebral conjunctiva is filled with glands that secrete sebum, an oily substance that provides additional lubrication for the eye.