390
UNIT 3
Organ Systems
[CN] VII) and functions to close the eyelids; (2) the
levator palpe-
brae superioris muscle
, a thin fl at striated muscle that originates
in the orbit, passes forward, and inserts into the upper eyelid. It
is innervated by the oculomotor nerve (CN III) and is responsible
for opening the eyelid and holding it open; and (3) the
superior
tarsal muscle
(
Müller muscle
), a bundle of smooth muscle that
arises from the interstitia of the levator muscle and inserts on
the upper end of the tarsal plate and superior conjunctiva of
the eyelid. The superior tarsal muscle is innervated by sympa-
thetic nerve F
bers from the superior cervical ganglion and helps
to raise the upper eyelid.
Tunica Fibrosa (Tunica Externa)
The outermost structures of the eye are the
cornea
and
sclera
(±ig. 20-1).
THE CORNEA
is a transparent tissue that covers the anterior
sixth of the eye (±igs. 20-5 and 20-6). The cornea contains no
blood vessels and aids in focusing the visual image onto the
retina. It consists of F
ve layers. The thickest layer, the
stroma
,
comprises 90% of the thickness of the cornea and consists
of collagen F
bers and F broblasts embedded in an extracel-
lular matrix (±ig. 20-6B). The anterior surface of the cornea
is covered by a thin layer of
pavement epithelium
(stratiF ed
squamous epithelium) resting on the
Bowman membrane
. The
posterior surface of the cornea is covered by a layer of corneal
endothelium
(simple squamous epithelium) that is only one cell
thick and rests on the
Descemet membrane
(±igs. 20-5A and
20-6B).
THE SCLERA
is a tough, thin structure consisting of dense,
irregular, opaque connective tissue that comprises the posterior
F
ve sixths of the outer surface of the eyeball (±igs. 20-1, 20-11A,
and 20-12A). The cornea and sclera are continuous with each
other at the
limbus
(±ig. 20-11A). The extraocular muscles,
which move the eyes in their orbits, insert in the sclera. The con-
junctival tissue, which covers the inner surfaces of the eyelids,
also attaches to the sclera.
Refractive Media of the Eye
THE LENS
is a transparent, fl
exible, biconvex structure that
is suspended from the ciliary processes by
zonular ±
bers
. The
curvature of the lens can be changed by contraction or relax-
ation of the
ciliary muscles
(under control of
parasympathetic
nerve F
bers of the oculomotor nerve) so that the image of
nearby or distant objects can be focused on the retina. The
lens has three components: the
lens capsule
, the
subcapsular
epithelium
, and the
lens ±
bers
(±igs. 20-7 and 20-8). The
lens
capsule
is a transparent basement membrane that surrounds
the entire lens. Immediately beneath it, on only the anterior
surface of the lens, is a single layer of squamous cells, the
sub-
capsular epithelium
(±ig. 20-10A). In the region of the equator
of the lens, proliferating epithelial cells become elongated, are
displaced toward the center of the lens, and lose their nuclei.
They are then called
lens f
bers
and comprise the major bulk of
the lens (±ig. 20-10A).
THE AQUEOUS HUMOR
is a thin, watery, transparent fl
uid
that is produced continuously by the
ciliary
body
and F
lls the
anterior chamber
. It exits the anterior chamber in the region
of the
angle of the anterior chamber
. It is produced by the
ciliary processes
, which are rich in capillaries (±igs. 20-11C and
20-12C).
THE VITREOUS BODY
is a transparent gelatinous substance
that F
lls the eye between the
posterior surface of the lens
and the
retina
(±ig. 20-1). Its composition is predominantly
water
with
small amounts of
collagen
and
hyaluronic acid
. The surface of
the vitreous body is covered by a layer of condensed vitreous
F bers called
hyaloid membrane
. It is in contact with the poste-
rior lens capsule, the zonular F
bers, the posterior portion of the
ciliary epithelium (pars plana), the retina, and the optic nerve
head (±ig. 20-1). The vitreous body is important in maintaining
the transparency and shape of the eye.
Tunica Vasculosa (Tunica Media)
The
tunica vasculosa
(sometimes called the
uveal tract
) lies just
internal to the tunica externa and consists of the
iris
(anteriorly),
the
ciliary body
, and the
choroid
(posteriorly).
THE IRIS
is a thin diaphragm of tissue in the anterior cham-
ber, composed of a highly vascularized, loose connective tissue
stroma
, two groups of
contractile elements
, the
anterior iridal
border
, and the
posterior iridal border
. The
posterior iridal bor-
der
contains two layers of
pigmented epithelium
, the
anterior
iridal epithelium
(anterior pigmented epithelium) and the
pos-
terior iridial epithelium
(posterior pigmented epithelium). Two
groups of muscle F
bers regulate the diameter of the
pupil
, the
circular hole in the center of the iris, and adjust the amount of
light entering the eye (±ig. 20-9). The circular
constrictor pupil-
lae muscle
(smooth muscle F
bers) reduces the size of the pupil
under the infl uence of parasympathetic nerve F
bers; the radial
F bers of the
dilator pupillae muscle
(myoepithelial cells) act to
increase the size of the pupil under the infl
uence of
sympathetic
nerve ±
bers
.
THE CILIARY BODY
lies interior to the anterior margin of
the sclera, between the choroid and the iris. It is composed of
two concentric rings of tissue, the
pars plicata
and the
pars
plana
, and includes epithelial tissue, a stroma of connective tis-
sue, and smooth muscle F
bers (±igs. 20-9A and 20-11). The
muscles of the ciliary body control the curvature of the lens
and, therefore, function to focus the visual image on the retina.
The epithelium of the ciliary body has two layers,
a pigmented
layer
and a
nonpigmented layer
. The latter secretes the
aqueous
humor
, which F
lls the anterior chamber of the eye and leaves
the anterior chamber in the region of the
anterior chamber
angle
(±ig. 20-12C).
THE CHOROID
is a highly vascularized tissue containing
some collagen F
bers that is loosely attached to the overlying
sclera (±ig. 20-13A). The inner surface of the choroid adheres
tightly to the pigment epithelium layer of the retina. The inner-
most layer of the choroid is the
choriocapillaris
, which supplies
oxygen and nutrients to the outer layers of the retina.
Bruch
membrane
delineates the junction between the choriocapillaris
and the
retinal pigment epithelium
(±ig. 20-13B).
Retina (Tunica Interna)
The
retina
consists of a thin sheet of neurons that covers the
inner surface of the posterior two thirds of the eye and a layer
of cuboidal epithelial cells that sit on the choroid and contain
previous page 405 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 407 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off