Chapter 15
The Special Senses
549
15
Vascular Layer
Te
vascular
layer
forms the middle coat of the eyeball. Also
called the
uvea
(u
9
ve-ah; “grape”), this pigmented layer has
three regions: choroid, ciliary body, and iris (Figure 15.4).
Choroid
Te
choroid
is a blood vessel–rich, dark brown
membrane (
choroid
5
membrane) that forms the posterior five-
sixths of the vascular layer. Its blood vessels nourish all eye lay-
ers. Its brown pigment, produced by melanocytes, helps absorb
light, preventing it from scattering and reflecting within the eye
(which would cause visual confusion). Te choroid is incom-
plete posteriorly where the optic nerve leaves the eye.
Ciliary Body
Anteriorly, the choroid becomes the
ciliary
body
, a thickened ring of tissue that encircles the lens. Te cili-
ary body consists chiefly of interlacing smooth muscle bundles
called
ciliary muscles
, which act to control lens shape. Near the
lens, its posterior surface is thrown into radiating folds called
ciliary processes
, which secrete the fluid that fills the cavity of
the anterior segment of the eyeball. Te
ciliary zonule
(
suspen-
sory ligament
) extends from the ciliary processes to the lens.
Tis halo of fine fibers encircles and helps hold the lens in its
upright position.
Iris
Te
iris
, the visible colored part of the eye, is the most
anterior portion of the vascular layer. Shaped like a flattened
doughnut, it lies between the cornea and the lens and is contin-
uous with the ciliary body posteriorly. Its round central open-
ing, the
pupil
, allows light to enter the eye. Te iris is made
up of two smooth muscle layers with bunches of sticky elastic
fibers that congeal into a random pattern before birth. Its mus-
cle fibers allow it to act as a reflexively activated diaphragm to
vary pupil size
(Figure 15.5)
. In close vision and bright light,
the
sphincter pupillae
(circular muscles) contract and the pupil
constricts. In distant vision and dim light, the
dilator pupillae
(radial muscles) contract and the pupil dilates, allowing more
light to enter. Sympathetic fibers control pupillary dilation and
parasympathetic fibers control constriction.
Changes in pupil size may also reflect our interests and emo-
tional reactions. Our pupils o±en dilate when the subject matter
the deviant eye, which then becomes functionally blind. Strabis-
mus is treated either with eye exercises to strengthen the weak
muscles or by temporarily placing a patch on the stronger eye,
which forces the child to use the weaker eye. Surgery is needed
for unyielding conditions.
Structure of the Eyeball
Te eye itself, commonly called the
eyeball
, is a slightly irregu-
lar hollow sphere
(Figure 15.4)
. Because the eyeball is shaped
roughly like the globe of the earth, it is said to have poles. Its
most anterior point is the
anterior pole
, and its most posterior
point is the
posterior pole
. Its wall is composed of three layers:
the fibrous, vascular, and inner layers. Its internal cavity is filled
with fluids called
humors
that help to maintain its shape. Te
lens, the adjustable focusing apparatus of the eye, is supported
vertically within the eyeball, dividing it into
anterior
and
poste-
rior segments
.
Fibrous Layer
Te outermost coat of the eyeball, the
fibrous layer
, is com-
posed of dense avascular connective tissue. It has two obviously
different regions: the sclera and the cornea.
Sclera
Te
sclera
(skle
9
rah), forming the posterior portion
and the bulk of the fibrous layer, is glistening white and opaque.
Seen anteriorly as the “white of the eye,” the tough, tendon-
like sclera (
sclera
5
hard) protects and shapes the eyeball and
provides a sturdy anchoring site for the extrinsic eye muscles.
Posteriorly, where the sclera is pierced by the optic nerve, it is
continuous with the dura mater of the brain.
Cornea
Te anterior sixth of the fibrous layer is modified to
form the transparent
cornea
, which bulges anteriorly from its
junction with the sclera. Te crystal-clear cornea forms a win-
dow that lets light enter the eye, and is a major part of the light-
bending apparatus of the eye.
Epithelial sheets cover both faces of the cornea. Te external
sheet, a stratified squamous epithelium that protects the cornea
from abrasion, merges with the bulbar conjunctiva at the cor-
neoscleral junction. Epithelial cells that continually renew the
cornea are located here. Te deep
corneal endothelium
, com-
posed of simple squamous epithelium, lines the inner face of
the cornea. Its cells have active sodium pumps that maintain the
clarity of the cornea by keeping its water content low.
Te cornea is well supplied with nerve endings, most of
which are pain receptors. (For this reason, some people can
never adjust to wearing contact lenses.) When the cornea is
touched, blinking and increased tearing occur reflexively. Even
so, the cornea is the most exposed part of the eye and is vulner-
able to damage from dust, slivers, and the like. Luckily, its capac-
ity for regeneration and repair is extraordinary.
Te cornea has no blood vessels and so it is beyond the reach
of the immune system. As a result, the cornea is the only tissue
in the body that can be transplanted from one person to another
with little risk of rejection.
Iris
(two muscles)
• Sphincter pupillae
• Dilator pupilla
e
Sphincter pupilla
e
muscle contracts:
P
upil siz
e decr
eases
.
Dilator pupilla
e
muscle contracts:
P
upil size increases
.
Sympathetic +
Parasympathetic +
Figure 15.5
Pupil constriction and dilation, anterior view.
(
1
means activation.)
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