Chapter 15
The Special Senses
585
15
also at risk for certain conditions that cause blindness, such as
macular degeneration, glaucoma, cataracts, atherosclerosis, and
diabetes mellitus.
Hearing and Balance
Te ear begins to develop in the three-week embryo. Te inter-
nal ears develop first, from thickenings of the surface ectoderm
called the
otic placodes
(o
9
tik plak
9
ōds), which lie lateral to the
hindbrain on each side. Te otic placode invaginates, forming the
otic pit
and then the
otic vesicle
, which detaches from the surface
epithelium. Te otic vesicle develops into the membranous laby-
rinth. Te surrounding mesenchyme forms the bony labyrinth.
Te middle ear cavity and pharyngotympanic tube of the
middle ear develop from
pharyngeal pouches
, lateral outpock-
etings of the endoderm lining the pharynx. Te auditory ossi-
cles develop from neural crest cells.
Te external acoustic meatus and external face of the tym-
panic membrane of the external ear differentiate from the
pha-
ryngeal clef
(
branchial groove
), an indentation of the surface
ectoderm, and the auricle develops from swellings of the sur-
rounding tissue.
Newborn infants can hear, but early responses to sound are
mostly reflexive—for example, crying and clenching the eyelids
in response to a startling noise. By the fourth month, infants will
turn to the voices of family members. Critical listening begins as
toddlers increase their vocabulary, and good language skills are
closely tied to the ability to hear well.
Homeostatic Imbalance
15.15
Congenital abnormalities of the ears are fairly common. Exam-
ples include partly or completely missing pinnae and closed or
absent external acoustic meatuses. Maternal rubella during the
first trimester commonly results in sensorineural deafness.
Except for ear inflammations, mostly due to infections, few
problems affect the ears during childhood and adult life. By
the 60s, however, deterioration of the spiral organ becomes no-
ticeable. We are born with approximately 30,000 hair cells, but
their number declines as they are damaged or destroyed by loud
noises, disease, or drugs (for example, the antibiotic streptomy-
cin). Te hair cells
are
replaced, but at such a slow rate that there
really is no functional regeneration. It is estimated that if we were
to live 140 years, we would lose all of our hearing receptors.
Te ability to hear high-pitched sounds leaves us first. Tis
condition, called
presbycusis
(pres
0
bĭ-ku
9
sis), is a type of sen-
sorineural deafness. Although presbycusis is considered a dis-
ability of old age, it is becoming much more common in younger
people as our world grows noisier.
Check Your Understanding
17.
What age-related changes make it more difficult for the
elderly to see at night?
For answers, see Appendix H.
Our abilities to see, hear, taste, and smell—and some of our
responses to the effects of gravity—are largely the work of our
disagreeable odors. Additionally, their sense of taste is poor.
Tis along with the decline in the sense of smell makes food
taste bland and contributes to loss of appetite.
Vision
By the fourth week of development, the eyes begin forming as
the
optic vesicles
that protrude from the diencephalon (see
Figure 12.1c on p. 429). Soon these hollow vesicles indent to
form double-layered
optic cups
, and their stalks form the optic
nerves and provide a pathway for blood vessels to reach the eye
interior.
Once an optic vesicle reaches the overlying surface ecto-
derm, it induces the ectoderm to thicken and then form a
lens
vesicle
that pinches off into the cavity of the optic cup, where
it becomes the lens. Te lining (internal layer) of the optic cup
becomes the neural layer of the retina, and the outer layer forms
the pigmented layer. Te rest of the eye tissues and the vitre-
ous humor are formed by mesenchymal cells derived from the
mesoderm that surrounds the optic cup.
In the darkness of the uterus, the fetus cannot see. Nonethe-
less, even before the light-sensitive portions of the photorecep-
tors develop, the central nervous system connections have been
made and are functional. During infancy, synaptic connections
are fine-tuned, and the typical cortical fields that allow binocu-
lar vision are established.
Homeostatic Imbalance
15.14
Congenital problems of the eyes are relatively uncommon, but
their incidence is dramatically increased by certain maternal in-
fections, particularly rubella (German measles) during the criti-
cal first three months of pregnancy. Common rubella sequels
include blindness and cataracts.
As a rule, vision is the only special sense not fully functional
at birth. Because the eyeballs are foreshortened, most babies
are hyperopic. Te newborn infant sees only in gray tones, eye
movements are uncoordinated, and o±en only one eye at a time
is used. Te lacrimal glands are not completely developed until
about two weeks a±er birth, so babies are tearless for this period,
even though they may cry lustily. By 5 months, infants can fol-
low moving objects with their eyes, but visual acuity is still poor
(20/200).
By the age of 3 years, depth perception is present and color
vision is well developed. Because the eyeball has grown, visual
acuity has improved to about 20/30, providing a readiness to be-
gin reading. By first grade, emmetropia has usually replaced the
initial hyperopia, and the eye reaches its adult size at 8–9 years
of age. Emmetropia usually continues until presbyopia begins to
set in around age 40 owing to decreasing lens elasticity.
With age, the lens loses its crystal clarity and discolors. As a
result, it begins to scatter light, causing a glare that is distress-
ing when driving at night. Te dilator pupillae muscles become
less efficient, so the pupils stay partly constricted. Tese two
changes together decrease the amount of light reaching the ret-
ina, and visual acuity is dramatically lower in people over 70. In
addition, the lacrimal glands are less active and the eyes tend to
be dry and more susceptible to infection. Elderly persons are
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