Topic Overview

The eye is shaped like a round ball, with a slight bulge at the front.

See a picture of the eye.

The eye has three main layers. These layers lie flat against each other and form the eyeball.

• The outer layer of the eyeball is a tough, white, opaque membrane called the sclera (the white of the eye). The slight bulge in the sclera at the front of the eye is a clear, thin, dome-shaped tissue called the cornea. See a picture of the cornea.
• The middle layer is the choroid. The front of the choroid is the colored part of the eye called the iris. In the center of the iris is a circular hole or opening called the pupil.
• The inner layer is the retina, which lines the back two-thirds of the eyeball. The retina consists of two layers: the sensory retina, which contains nerve cells that process visual information and send it to the brain; and the retinal pigment epithelium (RPE), which lies between the sensory retina and the wall of the eye.

The inside of the eye is divided into three sections called chambers.

• Anterior chamber: The anterior chamber is the front part of the eye between the cornea and the iris.
  • The iris controls the amount of light that enters the eye by opening and closing the pupil.
  • The iris uses special muscles to change the size of the pupil. These muscles can control the amount of light entering the eye by making the pupil larger (dilated) or smaller (constricted).
• Posterior chamber: The posterior chamber is between the iris and the lens. See a picture of the iris and lens.
  • The lens is behind the iris and is normally clear. Light passes through the pupil to the lens.
  • The lens is held in place by small tissue strands or fibers (zonules) extending from the inner wall of the eye.
  • The lens is very elastic. Small muscles attached to the lens can change its shape, allowing the eye to focus on objects at varying distances.
  • Tightening (contraction) or relaxing these muscles causes the lens to change shape, allowing the eyes to focus on near or far objects (accommodation).
• Vitreous chamber: The vitreous chamber is between the lens and the back of the eye.
  • The back two-thirds of the inner wall of the vitreous chamber is lined with a special layer of cells (the retina): millions of highly sensitive nerve cells that convert light into nerve impulses.
  • Nerve fibers in the retina merge to form the optic nerve, which leads to the brain. Nerve impulses are carried through the optic nerve to the brain.
  • The macula, near the center of the retina at the back of the eyeball, provides the sharp, detailed, central vision for focusing on what is in front of you. The rest of the retina provides side (peripheral) vision, which allows you to see shapes but not fine details.
  • Blood vessels (retinal artery and vein) travel along with the optic nerve and enter and exit through the back of the eye.

Fluid fills most of the inside of the eye. The chambers in front of the lens (both the anterior and posterior chambers) are filled with a clear, watery fluid called aqueous humor. The large space behind the lens (the vitreous chamber) contains a thick, gel-like fluid called vitreous humor or vitreous gel. These two fluids press against the inside of the eyeball and help the eyeball keep its shape.

The eye is like a camera. Light passes through the cornea and the pupil at the front of the eye and is focused by the lens onto the retina at the back of the eye. The cornea and lens bend light so it passes through the clear substance (vitreous gel) in the back chamber of the eye and is projected onto the retina. The retina converts light to electrical impulses. The optic nerve carries these electrical impulses to the brain, which converts them into the visual images that you see. See a picture of the optic nerve.