HUMAN BIOLOGY

280 Chapter 14

Disorders of the eye

problem. The eyeball is “taller” than it is wide (or the lens

is “lazy”), so close images are focused behind the retina

(Figure 14.21B).

the eyes also are vulnerable to infections

and cancer

The eyes are vulnerable to pathogens including viruses,

bacteria, and fungi. Health authorities estimate that in the

United States, about one in every fifty visits to a doctor’s

office is for conjunctivitis, inflammation of the transparent

membrane (the conjunctiva) that lines the inside of the eye-

lids and covers the sclera (the white of the eye). Symptoms

include redness, discomfort, and a discharge. In children,

conjunctivitis usually is caused by bacteria; in adults it more

often is triggered by allergy (Figure 14.22). Most cases of

bacterial conjunctivitis are easily treated with antibiotics.

Herpes simplex, a virus that causes cold sores and

genital herpes, can infect the cornea. Because blindness

can result from a herpes infection in the eyes, a pregnant

woman who has a history of genital herpes likely will

deliver by cesarean section to avoid any chance of exposing

her newborn to the virus.

Malignant melanoma is the most common eye cancer.

It typically develops in the choroid (the eye’s middle layer)

and may not trigger noticeable vision problems until it has

spread to other parts of the body. About 1 in 20,000 babies is

born with retinoblastoma, a cancer of the retina. Because

Problems that disrupt normal eye functions range from

injuries and diseases to inherited abnormalities and natural

changes associated with aging.

Some eye disorders are inherited

Red–green color blindness is a common inherited abnor-

mality. It shows up most often in males, for reasons you can

read about in Chapter 20. The retina lacks some or all of

the cone cells with pigments that normally respond to light

of red or green wavelengths. Most of the time, color-blind

people have trouble distinguishing red from green only in

dim light. However, some cannot distinguish between the

two even in bright light.

Occasionally, some or all of the cone cells that selectively

respond to light of red, green, or blue wavelengths are

missing. The rare people who have only one of the three

kinds of cones are totally color-blind. They see the world

only in shades of gray.

Some inherited vision problems are due to misshapen

eye parts that affect the eye’s ability to focus light. In

astigmatism, one or both corneas curve unevenly, so they

can’t bend incoming light rays to the same focal point.

In myopia, or nearsightedness, the eyeball is wider than

it is high, or the ciliary muscle responsible for adjusting

the lens contracts too strongly. Then, images of distant

objects are focused in front of the retina instead of on it

(Figure 14.21A). Hyperopia, farsightedness, is the opposite

close

object

(focal

point)

B

distant

object

(focal

point)

A

Figure 14.21 A Nearsighted and b farsighted vision are the most common vision problems. (© Cengage Learning)

© Elena Dijour/Shutterstock.com

14.10

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).