The eye is a complex and sensor organ that is gathering
of visual information.
The eye is made up of
three main parts: eyeball (globe), orbit (eye socket), accessory (adnexal)
The eye’s accessory
structures contain the eyelids, conjunctiva, caruncle and lacrimal glands.
The eyelids are folds of
skin that protect and cover the eye. It contains glands, which produce an oily
secretion which in turns covers the tear layer and prevents tears from
evaporating and the eyelids from sticking.
The conjunctiva is a
clear mucous membrane that lines the inner surface of the eyelids and the outer
surface of the eye. The conjunctiva secretes mucus to lubricate the eyeball and
keep it moist.
The caruncle is the
small, pinkish portion of the innermost
corner of the eye that contains sweat glands, oil, and conjunctival tissue.
The lacrimal gland (tear
gland) is located at the upper and outer corner of each eye. The lacrimal gland
secretes tears to keep the surface of the eye and lining of the eyelids moist
and lubricated. Tears help to reduce friction and prevent infection by removing
dust and debris from the eye.
The orbit (eye socket) is made up of bone formed
from the skull that contains the eyeball and the connective tissues. Both bone
and connective tissues protect the eye. The muscles that attached to the
eyeball make it easly move in different directions.
The eyeball (globe) is rich in blood
vessels. Human eyes are roughly spherical, ?lled with a transparent gel-like
substance called the vitreous humor. It
supports the internal structures and maintains the shape of the eye.
The eyeball consists of
three concentric layers (tunics), whose names re?ect their basic functions: a
?brous tunic, consisting of the sclera and the cornea; a vascular pigmented
tunic, comprising the choroid, ciliary body, and iris; and a nervous tunic, the
retina, see Figure(3).
ü The Fibrous Tunic
The ?brous tunic is the outermost layer
of eye tissue. It is a tough and inelastic corneoscleral envelope. The cornea
is dense fibrous connective tissue. It is perfectly
transparent to allow the light. The sclera is
tough and white connective tissue. Function: protection and support.
ü Vascular tunic
The vascular tunic is the middle layer, also known as
uvea. It consists of the choroid, the ciliary body and the iris, which is
perforated by the pupil. The iris is thin, colored part of the eye. It is
located in front of the eye between the lens and the cornea.
secreted by the ciliary body, ?ows into the posterior chamber, through the
pupil and out of globe through drainage apparatus at the angle of the anterior
chamber. The choroid contains blood vessels that supply necessary oxygen and
remove the waste products of the retinal cells. Function: nutritive.
ü The nervous tunic
tunic is the inner sensory which includes the retina and lens. The retina
consists of receptors and neurons and concerned with the initial processing of visual information.
Within the concavity of the retina, the vitreous body is ?lling the space
behind the lens and ciliary body, helping to maintain the shape of the eye. Function:
The cornea and sclera together form the outer
fibrous tunic of the eye and withstand both internal and external forces to
maintain the shape of the eyeball. Although both of these structures consist
mainly of collagen fibrils, their optical properties are different.
The cornea must be transparent, refract light,
contain the IOP and provide a protective interface with the environment. Each
of these functions is provided by a highly specialized substructural
organization. The cornea does not contain blood vessels,
it receives nutrients from tears and the aqueous humor in the anterior chamber.
The cornea acts as the eye’s outermost lens. It
acts as a window that focuses and controls the amount of light to the eye. The
cornea contributes between 65-75 % of the eye’s total focusing power.
When light strikes the cornea, it refracts the
incoming light onto the lens. The lens further refocuses that light onto the
retina. The retina is a layer of light-sensing
cells lining in the back of the eye that starts the translation of light into
The cornea also acts as a filter, screening out some
of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this
protection, the lens and the retina would be highly sensitive to injury from UV radiation.
The corneal tissue is arranged in ?ve layers. They
are: epithelium, Bowman’s membrane, the stroma, Descemet’s membrane and the
endothelium, see Figure (4).
The epithelium is the cornea’s outermost
region. It is about 5–6 cell layers thick and ?lled with tiny nerve endings.
The epithelium functions primarily to: (1) block the passage of foreign
material, such as water, dust, and bacteria, into the eye and other layers of
the cornea; and (2) provide a smooth surface that absorbs oxygen and cell
nutrients from tears then distribute these nutrients to the rest of the
cornea’s layer. The epithelium is filled with thousands of tiny nerve endings that
make the cornea highly sensitive to pain when rubbed or scratched. The basement
membrane (foundation) is the part of the epithelium where the epithelial cells
anchor and organize.
Bowman’s membrane lies directly below
the basement membrane of the epithelium. It is a transparent sheet of tissue,
is composed of strong layered of collagen ?brils. The di?cult access to
Bowman’s membrane protects the cornea from injury. Once injured, it resiliently
regenerates, leaving a scar when the injury is deeper. If these scars are large
and located in the center, some vision loss can occur.
stroma lies under Bowman’s layer. It is the thickest layer (about 500µm),
dominating the mechanical response of the cornea to injury and accounting about
90 % of the cornea’s thickness. It consists primarily of water 78%, collagen
16%, and non-collagenous proteins 7%. It
does not contain any blood vessels. Collagen gives the cornea its elasticity,
strength, and form. The unique shape and arrangement
of collagen are essential in producing the cornea’s light-conducting
The Microscopic Organization of Collagen
The corneal stroma depends mostly on the
degree of its collagen ?brils (spatial order) which are narrow in diameter and
closely packed in a regular array. Di?erent types of the collagen present in
the human cornea (I, III, VI, XII). By scanning electron microscopy, it seems that the average diameter of collagen ?brils is
highly uniform (about 31nm), remaining constant across the cornea before rising
at the limbus. There is a significant increase in
spacing from the central cornea (about 57nm) to the peripheral cornea (about
62nm), followed larger increase at the limbus
itself. The proteoglycan matrix (also known as ground substance) is a gel-like
substance, consisting mainly of water 14.
The collagen in stroma also plays an
important role on the macroscopic level, where it confers shape and strength.
The stromal ?brils are organized into three hundred to ?ve hundred ?at bundles,
or lamellae, which run uninterrupted from limbus to limbus like thin belts up
to 0.2mm broad and about 1 ? 2cm thick. Fibrils within a given lamella run
approximately parallel but tend to make
large angles with those in adjacent lamellae, Figure (5).
Descemet’s membrane is located
beneath the stroma, it is a thin but strong sheet of tissue. It is composed of collagen fibers (different
from those of the stroma) and is made by the endothelial cells that lie below
it. Descemet’s membrane plays an important role in
corneal hydration and in the maintenance of the endothelium after wounding and
surgery, regenerating readily after injury. Considering its thickness (? 10µm) and unique composition, it maybe speculated whether
Descemet’s layer has a specialized function,
besides the function as a basement membrane that could be in mechanical
support, ?ltration or liquid barrier 15.
endothelium is the thin and the innermost layer of the cornea. Endothelial
cells are essential in keeping the cornea clear. Normally, fluid leaks slowly
from inside the eye into the stroma. The excess fluid pumps out of the stroma
by the endothelium. Without this pumping,
the stroma would swell with water. A perfect balance in a healthy eye is
maintained between the fluid moving into the cornea and fluid being pumped out
of the cornea. Once endothelium cells are destroyed by disease or trauma, they
do not recover. Too much damage to endothelial cells can lead to corneal edema
(swelling caused by excess ?uid) and blindness, with corneal transplantation
the only available therapy.
The limbus is the area
that the cornea meets the sclera and conjunctiva. Functions
of limbus are nourishing
the peripheral cornea, providing an outflow
for the aqueous humor, assisting in corneal epithelial regeneration.
There is histological evidence
supporting structural di?erentiation of sclera from the clear cornea, but anatomically it is di?cult to determine exactly
where the cornea ends and the sclera begins. Transitional zone is approximately
1.5?2mm with its internal edge being called the corneal limbus and its external
edge the scleral limbus. Within the limbal zone, the orderly packing corneal
collagen gives way to the coarse interweaving of scleral ?bres, and the ?bril
diameter increases markedly from the narrow range of ?ne ?bril diameters in the
cornea to the broad range found in the sclera.
Using synchrotron X-ray di?raction demonstrated the presence of circumferential annulus of collagen ?brils located in
the limbus of the human eye 16. Since the preferred orientation of the
collagen ?brils is circular at the limbus, it is the weakest region in the
corneoscleral envelope by IOP pressure. From a consideration of the mechanics
of the system, it seems probable that the
purpose of this annulus is to help maintain the correct curvature of the cornea; studies on bovine tissue, proposed
microstructural models of possible integration arrangements between the central
cornea and the limbus 17 support this assumption.
The sclera gives the eye most of its white color. It is
relatively avascular and consists almost entirely of collagen. The sclera
protects the intraocular contents from injury and the function of the collagen
in the sclera is obviously structural. The strength and resilience of the
sclera are imparted by the close interlacing of the collagen fibers which
account for 80 % of the dry weight. Its mechanical
strength serves to contain the IOP and at the same time prevents deformations
of the globe by resisting the stresses induced by contractions of the
extraocular muscles. The thickness of sclera is not uniform, being thinner in females
than in males. There is also increase in scleral thickness, together with
opacity in relation to age 18.
The sclera is pierced by the optic nerve, forming a thin netlike lamina.
This structure provides support and anchorage for the optic nerve ?bres passing
through it and also reinforces the globe
at its weakest point. The bands of collagen bundles of the sclera are mostly
parallel to the surface, but they cross each other in all directions and may
divide and reunite, see Figure (6). Within each bundle,
the collagen ?brils are parallel and show wide variation in diameter and
spacing (ranging from 25–230 nm), which is distinctly di?erent from that of the
cornea and account for the opacity of the sclera 19.