290 Chapter 10
The eyes transduce energy in the electromagnetic spectrum
( fig. 10.26 ) into nerve impulses. Only a limited part of this spec-
trum can excite the photoreceptors—electromagnetic energy with
wavelengths between 400 and 700 nanometers (1 nm 5 10 2 9 m,
or one-billionth of a meter) constitutes visible light. Light of lon-
ger wavelengths in the infrared regions of the spectrum is felt
as heat but does not have sufficient energy to excite the photo-
receptors. Ultraviolet light, which has shorter wavelengths and
more energy than visible light, is filtered out by the yellow color
of the eye’s lens. Honeybees—and people who have had their
lenses removed—can see light in the ultraviolet range.
The structures of the eyeball are summarized in table 10.4.
The outermost layer of the eye is a tough coat of connective tis-
sue called the sclera, which can be seen externally as the white
of the eyes. The tissue of the sclera is continuous with the trans-
parent cornea. A clear epithelium covers the cornea and is con-
tinuous with the conjunctiva, a mucous membrane that covers
the sclera and the internal surface of the eyelids. At the juncture
of the corneal epithelium and conjunctiva is a small region of
membrane containing stem cells that can renew and repair the
cornea. In an exciting recent report, scientists cultured these
stem cells obtained from the contralateral eyes of patients with
burned corneas and used these to successfully restore transpar-
ent, self-renewing corneas in the majority of the patients.
Light passes through the cornea to enter the anterior chamber
of the eye. Light then passes through an opening called the pupil,
which is surrounded by a pigmented muscle known as the iris.
After passing through the pupil, light enters the lens ( fig. 10.27 ).Clinical Investigation CLUES
Susan was told she would need to use a hearing aid.- What type of hearing loss does Susan have?
- How would a hearing aid help her?
spiral ganglion can be electrically stimulated to produce action
potentials and convey information of low, medium, and high
sound frequencies to the brain. Bilateral cochlear implants
allow varying abilities to hear speech and permit sound to be
localized, but they do not restore normal hearing.
| CHECKPOINT
- Use a flowchart to describe how sound waves in air
within the external auditory meatus are transduced
into movements of the basilar membrane. - Explain how movements of the basilar membrane
affect hair cells, and how hair cells can stimulate
associated sensory neurons. - Explain how sounds of different intensities affect the
function of the cochlea. How are different pitches of
sounds distinguished by the cochlea?
10.6 The Eyes and Vision
Light from an observed object is focused by the cornea
and lens onto the photoreceptive retina at the back of
the eye. The focus is maintained on the retina at different
distances between the object and the eyes by muscular
contractions that change the thickness and degree of cur-
vature of the lens.
Figure 10.26 The
electromagnetic spectrum. Different
parts of the electromagnetic spectrum
( top ) are shown in Angstrom units
(1Å 5 10 2 10 meter). The visible
spectrum ( bottom ) constitutes only a
small range of this spectrum, shown in
nanometer units (1 nm 5 10 2 9 meter).
See the Test Your Quantitative Ability
section of the Review Activities at the
end of this chapter.
10 -4 10 -3 10 -2 10 -1 110102103 104105 106107 108 10910101011101210131014Gamma rays X−rays UltravioletVisible lightInfrared Microwaves Radio wavesWavelength (A) ̊
Visible light400 500 600 700
Wavelength (nm)LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the structures of the eye, and how these
focus light onto the retina. - Explain how accommodation at different distances is
accomplished. - Explain common disorders of refraction.