The human eye is a marvelous structure designed to provide us with a visual view of the world in which we live.
The cornea is the thin, dome-shaped structure that serves as the outermost layer of the eye. The cornea protects the eye and, along with the lens, helps focus light rays onto the back of the eyeball. The lens of the eye is a nearly transparent, biconvex structure suspended behind the iris. The iris is responsible for regulating the amount of light entering the eye.
The function of the lens is to focus light rays onto the retina. The retina is the light sensitive membrane in the back of the eye that functions like the film in a camera and registers the images we see. The retina covers an area of approximately one square inch and contains about 137,000,000 light-sensitive cells.
The lens is composed of the lens capsule, which forms the outermost layer of the lens, and the lens fibers, which form the bulk of the interior of the lens. The cells of the lens epithelium are located between the lens capsule and the outermost layer of lens fibers. This layer constantly lays down fibers in the developing embryo and fetus as well as in the infant and adult stages of life.
The lens capsule is a smooth membrane that completely surrounds the lens. The capsule is made up of unusual elongated cells that contain numerous and very durable crystallin proteins synthesized by the lens epithelium that gives the lens elasticity. This allows the lens to change shape in order for the eye to focus on objects at various distances.
The shape of the lens can be altered by the relaxation and contraction of the ciliary muscles that surround it. The lens itself lacks nerves, blood vessels, or connective tissue. The cells of the lens epithelium regulate most of the homeostatic functions of the lens. Ions, nutrients, and liquids enter and leave the lens by way of the aqueous humor which bathes the anterior surface of the lens.
Larrie Stone is a retired Dana College science professor.