The eyeball has three major layers. The external layer consists of the sclera and cornea. The sclera is also known as “the white of the eye.” The cornea is the transparent, avascular, most anterior portion of the eyeball; it is the main refracting surface, which bends the light rays that enter the eye and helps focus them on the retina. To keep it healthy, the cornea obtains oxygen from the air dissolved in the tears.

The middle, vascular layer of the eye is the uvea, which consists of the iris, ciliary body, and choroid. The iris is the colored part of the eye. Its small round center is the pupil, through which light enters the eye. The pupil constricts when it is used for near vision or when bright light enters the eye; it dilates for far vision or in a dim environment. Behind the pupil and iris is the biconvex, transparent lens; it can change its shape to better focus light rays on the retina. (This kind of “accommodation” is controlled by the ciliary body.)

The third inner layer of the eye is the retina, which lines the inside of the eyeball, and extends and forms the optic nerve. The retina converts an image perceived by the eye into a form that the brain can process as vision. The macula, an area of the retina, receives light from the center of the visual field and provides the greatest visual acuity.

The aqueous humor is produced by the ciliary body. This watery fluid, which provides nutrients and oxygen to this area, is constantly formed and drained to maintain a relatively constant intraocular pressure (IOP). Increased IOP often results from inadequate drainage or absorption of the aqueous humor, potentially causing ischemia of the neurons of the eye and damage to the optic nerve.

The conjunctiva is a transparent mucous membrane that lines the inner surface of the eyelids and also covers the sclera. Thus, it forms a “pocket” under each eyelid.