Section 20. Eye Disorders

Chapter 224. Biology of the Eyes

Topics: Introduction | Structure and Function | Muscles, Nerves, and Blood Vessels | Protective Features | Effects of Aging

Structure and Function

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.

See the figure 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 membrane (conjunctiva), which runs to the edge of the cornea and also covers the insides 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 area in the middle of 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 and constricts 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 pupillary sphincter muscle.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. For the eye to focus on nearby objects, small muscles (called the ciliary muscles) contract, allowing the lens to become thicker. For the eye to focus on distant objects, the same muscles relax, allowing the lens to become thinner.

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 sharp, 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 vision and color vision and are clustered mainly in the macula. The rods are responsible for night and peripheral vision; they 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 visual clarity as the cones do.

The optic nerve connects the retina to the brain in a split pathway. Half the fibers of this nerve cross over to the other side at the optic chiasm, an area immediately in front of the pituitary gland just below the front portion of the brain. The bundles of nerve fibers then come together again just before they reach the back portion of the brain, where vision is sensed and interpreted. In this arrangement, impulses from each side of the eye's visual field are sent to the other side of the brain. Because of this structure, damage to the optic chiasm can lead to specific patterns of vision loss (see Section 20, Chapter 235).

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 that 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. These fluids fill out the eyeball and help 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 at the edge of the iris.