eyelids and drain into the nasal cavity, thereby producing
a runny nose.
The eye has external and internal muscles (
Figure 8.7
).
Contraction of different combinations of the six external
muscles enables the eye to move rapidly in many directions
(also see Figure 6.15). Internal muscles of the
iris
control
the size of the
pupil
, regulating the amount of light that
enters the eye. Other internal muscles change the shape of
the lens to focus light refracted by the lens onto the retina.
The Eyeball Is Composed
of Three Structural Layers
The eyeball is a layered structure that consists of the outer
coat or
fibrous tunic
, a middle
vascular tunic
(vascu-
lar layer) that supplies blood to the eye’s tissues and se-
cretes fluids, and an inner
neural
tunic (retina)
, which
transduces
incoming light to nerve impulses
(
Figure 8.8
). Inside the eye are
two chambers, the
anterior cavity
in front of the
lens
and the
posterior cavity
behind it. The anterior cavity is
filled with a liquid called
aqueous
humor
, which helps maintain the
shape of the eyeball and nourish-
es the lens and
cornea
. Aqueous
humor has a composition similar
to that of cerebrospinal fluid. The
posterior cavity is filled with a
jelly-like substance called the
vit-
reous body
, which prevents the
eyeball from collapsing and holds
the retina in place.
The retina has layers of con-
necting
ganglionic
cells
,
inter-
neurons,
supporting
cells,
and
photoreceptors
. The photorecep-
tors consist of two types,
rods
and
cones
, which allow us to see color
and to see in different light levels.
transduce
To
change from one form
to another.
aqueous humor
(AK-we-us HO-mer) The
watery fluid that fills
the anterior cavity of
the eye between the
cornea and the lens.
vitreous body
(VIT-
re-us) A soft, jellylike
substance that fills the
posterior cavity of the
eyeball between the
lens and the retina.
photoreceptor
A
receptor that detects
light shining on the
retina of the eye.
M uscles o f th e ey e • Figure 8.7
The eye has exterior and interior muscles to allow it to move and
to control the entry of light.
Exterior Eye Muscles
Levator
palpebrae
superiors muscle
moves the upper eyelid up and down.
Superior rectus muscle
moves the eye up.
Optic -
nerve
Inferior rectus muscle
moves the eye down.
Circular muscles constrict the pupil, and radial muscles dilate it.
Pupil
Pupil constricts as
circular muscles
of iris contract
Pupil dilates as
radial muscles
of iris contract
Bright light
Normal light
Anterior views
Interior eye muscles
Dim light
Inferior oblique muscle
moves the eye down, while
superior oblique muscle
(not shown) moves the eye up.
236 CHAPTER 8
Somatic Senses and Special Senses
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