The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.

An Inside Look at the Eye

The eye has a relatively tough white outer layer (sclera or white of the eye). Near the front of the eye, the sclera is covered by a thin mucous membrane (conjunctiva), which runs to the edge of the cornea and also covers the moist back surface of the eyelids.

Light enters the eye through the cornea, a transparent dome on the front surface of the eye. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the iris), which is actually a hole through the iris. The iris—the circular, colored area of the eye—controls the amount of light that enters the eye so that the pupil dilates (enlarges) and constricts (shrinks) like the aperture of a camera lens. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.

The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors. The high density of photoreceptors in the macula makes the visual image detailed, just as high-resolution film has more tightly packed grains. Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye. The photoreceptors in the retina convert the image into electrical impulses, which are carried to the brain by the optic nerve.

There are two main types of photoreceptor: cones and rods. Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. The rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color. Rods are grouped mainly in the peripheral areas of the retina and do not contribute to detailed central vision as the cones do.

Tracing the Visual Pathways Nerve signals travel from each eye along the corresponding optic nerve and other nerve fibers (called the visual pathway) to the back of the brain, where vision is sensed and interpreted. The two optic nerves meet at the optic chiasm, which is an area behind the eyes immediately in front of the pituitary gland and just below the front portion of the brain (cerebrum). There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side and continue to the back of the brain. Thus, the right side of the brain receives information through both optic nerves for the left field of vision, and the left side of the brain receives information through both optic nerves for the right field of vision. The middle of these fields of vision overlaps. It is seen by both eyes (called binocular vision). An object is seen from slightly different angles by each eye so the information the brain receives from each eye is different, although it overlaps. The brain integrates the information to produce a complete picture.

The eyeball is divided into two sections, each of which is filled with fluid. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor. The pressure generated by these fluids fills out the eyeball and helps maintain its shape.

The anterior segment itself is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.

Last full review/revision August 2006 by James Garrity, MD