How the Eye Works

It's a simple sphere, about 1" in diameter, yet its power is awesome. It discerns colors and shapes in bright or dim light from near or far. It helps you read books, and situations, and people. It is a vital link to the world around you. And it succeeds through a complex ballet of muscles and nerves.

The eye is the organ of sight, a nearly spherical hollow globe filled with fluids (humors). The outer layer or tunic (sclera, or white, and cornea) is fibrous and protective. The middle tunic layer (choroid, ciliary body and the iris) is vascular. The innermost layer (the retina) is nervous or sensory. The fluids in the eye are divided by the lens into the vitreous humor (behind the lens) and the aqueous humor (in front of the lens). The lens itself is flexible and suspended by ligaments which allow it to change shape to focus light on the retina, which is composed of sensory neurons.

The ability to see starts when light reflects off an object at which we are looking and enters the eye. As it enters the eye, the light is unfocused. Light entering the eye is first bent, or refracted, by the cornea -- the clear window on the outer front surface of the eyeball. The cornea actually provides most of the eye's optical power or light-bending ability.

After the light passes through the cornea, it is bent again, to a more finely adjusted focus, by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved by tiny muscles in the eyeball that change the shape of the lens, bending or flattening it to focus the light rays. This adjustment in the lens, known as accommodation, is necessary for bringing near and far objects into focus. The process of bending light to produce a focused image on the retina is called refraction.

Once the light is focused on the retina, it stimulates cells which send millions of electrochemical impulses along the optic nerve to the brain. The brain interprets the impulses, enabling us to "see" the object.. Because the light rays cross while going through the cornea, the retina reads the image upside down—but the brain readjusts so you stay properly oriented.

The superior rectus muscle is a muscle in the orbit that elevates, adducts, and rotates the eye medially.