465
CHAPTER TWELVE
Nervous System III
The Outer Tunic
The anterior sixth of the outer tunic bulges forward as the
transparent
cornea
(kor
ne-ah), the window of the eye that
helps focus entering light rays. It is largely composed of con-
nective tissue with a thin surface layer of epithelium. The
cornea is transparent because it contains no blood vessels
and the collagenous F
bers form unusually regular patterns.
The cornea is well supplied with nerve F
bers that enter
its margin and radiate toward its center. These fibers are
associated with many pain receptors that have very low
thresholds. Cold receptors are also abundant in the cornea,
but heat and touch receptors are not.
In 1905, doctors transplanted the cornea of an eleven-year-old boy
who lost his eye in an accident into a man whose cornea had been
destroyed by a splash of a caustic chemical, marking one of the F
rst
successful human organ transplants. Today, corneal transplants are
commonly used to treat corneal disease, the most common cause of
blindness worldwide. In this procedure, called a
penetrating kerato-
plasty,
a piece of donor cornea replaces the central two-thirds of the
defective cornea. These transplants are highly successful because the
cornea lacks blood vessels, and therefore, the immune system does
not have direct access to the new, “foreign” tissue.
Along its circumference, the cornea is continuous with
the
sclera
(skle
rah), the white portion of the eye. The sclera
makes up the posterior F
ve-sixths of the outer tunic and is
opaque due to many large, seemingly disorganized collag-
enous and elastic F
bers. The sclera protects the eye and is an
attachment for the extrinsic muscles.
antagonists. ±or example, when the eyes move to the right,
the lateral rectus of the right eye and the medial rectus of the
left eye must contract. At the same time, the medial rectus
of the right eye and the lateral rectus of the left eye must
relax. A person whose eyes are not coordinated well enough
to align has
strabismus.
Table 12.5
summarizes the muscles
associated with the eyelids and eye.
When one eye deviates from the line of vision, the person has double
vision (diplopia). If this condition persists, the brain may eventually
suppress the image from the deviated eye. As a result, the turning eye
may become blind (suppression amblyopia). Treating the eye devia-
tion early in life with exercises, eyeglasses, and surgery can prevent
such monocular blindness. ±or this reason, vision screening programs
for preschool children are important.
PRACTICE
29
Explain how the eyelid is moved.
30
Describe the conjunctiva.
31
What is the function of the lacrimal apparatus?
32
Describe the function of each extrinsic eye muscle.
Structure of the Eye
The eye is a hollow, spherical structure about 2.5 centime-
ters in diameter. Its wall has three distinct layers—an outer
f
brous tunic,
a middle
vascular tunic,
and an inner
nervous
tunic.
The spaces in the eye are F
lled with fl
uids that support
its wall and internal structures and help maintain its shape.
Figure 12.25
shows the major parts of the eye.
TABLE
12.5
|
Muscles Associated with the Eyelids and Eyes
Skeletal Muscles
Name
Innervation
Function
Muscles of the eyelids
Orbicularis oculi
±acial nerve (VII)
Closes eye
Levator palpebrae superioris
Oculomotor nerve (III)
Opens eye
Extrinsic muscles of the eyes
Superior rectus
Oculomotor nerve (III)
Rotates eye upward
and toward midline
Inferior rectus
Oculomotor nerve (III)
Rotates eye downward
and toward midline
Medial rectus
Oculomotor nerve (III)
Rotates eye toward
midline
Lateral rectus
Abducens nerve (VI)
Rotates eye away from
midline
Superior oblique
Trochlear nerve (IV)
Rotates eye downward
and away from midline
Inferior oblique
Oculomotor nerve (III)
Rotates eye upward
and away from midline
Smooth Muscles
Name
Innervation
Function
Ciliary muscles
Oculomotor nerve (III)
parasympathetic F
bers
Relax suspensory ligaments
Iris, circular muscles
Oculomotor nerve (III)
parasympathetic F
bers
Constrict pupil
Iris, radial muscles
Sympathetic F
bers
Dilate pupil
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