The importance of pupil constriction can be demon-
strated by looking at this page through a pinhole in a
piece of paper. You will be able to read with the page
much closer to your eye because the paper blocks out
light from the sides.
The importance of convergence can be demon-
strated by looking at your finger placed on the tip of
your nose. You can feel your eyes move medially
(“cross”) in maximum convergence. If the eyes don’t
converge, the result is double vision; the brain cannot
make the very different images into one, and settles
for two. This is temporary, however, because the brain
does not like seeing double and will eventually sup-
press one image.
You have probably heard of the condition called
“lazy eye” (the formal name is strabismus), in which
a person’s eyes (the visual axis of each) cannot be
directed at precisely the same point. True convergence
is not possible and, if untreated, the brain simply will
not use the lazy eye image. That eye may stop focus-
ing and become functionally blind because the brain is
ignoring the nerve impulses from it. Such loss of
vision is called amblyopia. Correction of a lazy eye
may involve eye exercises (to make the lazy eye
straighten out), a patch over the good eye (to make the
lazy eye straighten out and take over), or surgery to
correct an imbalance of the extrinsic muscles. You can
show yourself the benefits of converging eyes the next
time you are a passenger in a car (notthe driver). As
the car is moving, close one eye. Does the oncoming
landscape seem to flatten out, lose dimension? This is
loss of depth perception and some of the three dimen-
sionality that our brains provide.
THE EAR
The ear consists of three areas: the outer ear, the mid-
dle ear, and the inner ear (Fig. 9–7). The ear contains
the receptors for two senses: hearingand equilib-
rium. These receptors are all found in the inner ear.
OUTER EAR
The outer earconsists of the auricle and the ear
canal. The auricle, or pinna, is made of cartilage cov-
ered with skin. For animals such as dogs, whose ears
are movable, the auricle may act as a funnel for sound
waves. For people, however, the flat and stationary
auricle is not important. Hearing would not be nega-
tively affected without it, although those of us who
wear glasses would have our vision impaired without
our auricles. The ear canalis lined with skin that con-
tains ceruminous glands. It may also be called the
external auditory meatus, and is a tunnel into the
temporal bone, curving slightly forward and down.MIDDLE EAR
The middle earis an air-filled cavity in the temporal
bone. The eardrum, or tympanic membrane, is
stretched across the end of the ear canal and vibrates
when sound waves strike it. These vibrations are
transmitted to the three auditory bones: the malleus,
incus, and stapes (see Fig. 9–7). The stapes then
transmits vibrations to the fluid-filled inner ear at the
oval window.
The eustachian tube(auditory tube) extends from
the middle ear to the nasopharynx and permits air to
enter or leave the middle ear cavity. The air pressure
in the middle ear must be the same as the external
atmospheric pressure in order for the eardrum to
vibrate properly. You may have noticed your ears
“popping” when in an airplane or when driving to a
higher or lower altitude. Swallowing or yawning cre-
ates the “pop” by opening the eustachian tubes and
equalizing the air pressures.
The eustachian tubes of children are short and
nearly horizontal and may permit bacteria to spread
from the pharynx to the middle ear. This is why otitis
mediamay be a complication of a strep throat.INNER EAR
Within the temporal bone, the inner earis a cavity
called the bony labyrinth(a labyrinth is a series of
interconnecting paths or tunnels, somewhat like a
maze but without dead ends; see Fig. 9–7), which is
lined with membrane called the membranous
labyrinth. Perilymphis the fluid found between bone
and membrane, and endolymphis the fluid within the
membranous structures of the inner ear. These struc-
tures are the cochlea, concerned with hearing, and the
utricle, saccule, and semicircular canals, all concerned
with equilibrium (Fig. 9–8).Cochlea
The cochleais shaped like a snail shell with two-and-
a-half structural turns. Internally, the cochlea is parti-
tioned into three fluid-filled canals. The medial canal210 The Senses