The Special Senses
forms the middle coat of the eyeball. Also
ve-ah; “grape”), this pigmented layer has
three regions: choroid, ciliary body, and iris (Figure 15.4).
is a blood vessel–rich, dark brown
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.
Anteriorly, the choroid becomes the
, a thickened ring of tissue that encircles the lens. Te cili-
ary body consists chieﬂy of interlacing smooth muscle bundles
, which act to control lens shape. Near the
lens, its posterior surface is thrown into radiating folds called
, which secrete the ﬂuid that ﬁlls the cavity of
the anterior segment of the eyeball. Te
) extends from the ciliary processes to the lens.
Tis halo of ﬁne ﬁbers encircles and helps hold the lens in its
, 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-
, 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
. In close vision and bright light,
(circular muscles) contract and the pupil
constricts. In distant vision and dim light, the
(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
, is a slightly irregu-
lar hollow sphere
. Because the eyeball is shaped
roughly like the globe of the earth, it is said to have poles. Its
most anterior point is the
, and its most posterior
point is the
. Its wall is composed of three layers:
the ﬁbrous, vascular, and inner layers. Its internal cavity is ﬁlled
with ﬂuids called
that help to maintain its shape. Te
lens, the adjustable focusing apparatus of the eye, is supported
vertically within the eyeball, dividing it into
Te outermost coat of the eyeball, the
, is com-
posed of dense avascular connective tissue. It has two obviously
diﬀerent regions: the sclera and the cornea.
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 (
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.
Te anterior sixth of the ﬁbrous layer is modiﬁed to
form the transparent
, 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
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.
• Sphincter pupillae
• Dilator pupilla
upil size increases
Pupil constriction and dilation, anterior view.