Eye Anatomy & Structure

The eye is a complex structure that sends nerve impulses to the brain when stimulated by light rays reflected from an object. Then, the brain processes these impulses to create the perception of vision.

The eye is made up of numerous parts that work together to make vision possible. The iris, the colored circle in the middle of the eye, is the eye's most prominent structure. The iris is composed of smooth muscles that contract and expand to alter the size of the pupil and control the amount of light that enters the eye. Also visible from the front of the eye is the pupil, the opening in the center of the iris that resembles a large black dot. The sclera is the tough, white connective tissue that provides a protective outer layer for the eye.

Also at the front of the eye are the cornea, lens, and conjunctiva. The cornea is a transparent, dome-shaped disk that covers the iris and pupil. Beneath the cornea is a transparent, elastic structure called the lens. The cornea does about three quarters of the work of focusing light on the retina; the lens does the rest. The conjunctiva is a thin, lubricating mucous membrane that covers the sclera and lines the inside of the eyelids.

Inside the sclera lie two more layers. The middle layer, called the choroid, contains a dark pigment that minimizes scattering of light inside the eye. It is rich in blood vessels that supply nutrients to the retina, the innermost layer of the eye that consists of light-sensitive nerve tissue. The retina functions like film in a camera, receiving an imprint of an image and sending it via the optic nerve to the brain to be "developed," or interpreted. The macula, at the center of the retina, is a small area responsible for central vision, color perception, and the kind of detailed vision required for reading.

The vitreous humor is a thick, gel-like substance that fills the back of the eyeball behind the lens. The aqueous humor is a watery fluid located in front of the lens. Along with the vitreous humor, the aqueous humor maintains intraocular pressure (the internal pressure of the eye), which is needed to prevent the eyeball from collapsing. Intraocular pressure is controlled by specialized cells that produce aqueous humor and by ducts and canals that drain this fluid from the eye.

Publication Review By: Susan B. Bressler, M.D., Harry A. Quigley, M.D., Oliver D. Schein, M.D., M.P.H.

Published: 24 Feb 2011

Last Modified: 15 Dec 2014