LIDS(palpebra)
The eyelids are moveable folds of tissue which serve to protect
the eye. The skin of the lids is loose and elastic, permitting
extreme swelling and subsequent return to normal shape and size.
CONJUNCTIVA
The conjunctiva is a thin, transparent mucous membrane which lines
the anterior scerla up to the edge of the cornea and the posterior
surface of the lids. There are two divisions of the conjunctiva:
palpebral--lining the inner surface of the eyelids, and bulbar--
covering th surface of the sclera. Underneath the bulbar conjunctiva lies
a thick white connective tissue (Tenon's capsule) which covers the
extraocular muscles.
CORNEA
The cornea is transparent, avascular tissue comparable to the the
crystal of a small wristwatch. The junction between the sclera and the
cornea serves as a refracting lens and a protective "window" through
which light rays pass en route to the retina. The average adult cornea
is 1.0 mm thick and 11.5 mm in diameter. There are five distinct layers
of the cornea: epithelium, Bowman's membrane, stroma, Descemet's membrane,
and the endothelium. Ninety percent of the corneal thickness is the stroma;
it consists of lamellae which are arranged in a very intricate pattern that
allows light to pass through giving transparency to the cornea. The lamellae
are spongelike and normally would saturate with fluid, changing the delicate
orientation of the lamellae and causing the cornea to swell and become cloudy.
The single layer of endothelium cells act as tiny powerful pumps to keep fluid
away from the stroma.
LENS
The lens is a biconvex, avascular, colorless, and almost completely transparent
structure about 4 mm thick and 9 mm in diameter. It is held just behind the
pupil by suspensory ligaments known as zonules which connect the lens to the
focusing (ciliary) muscles. The interaction between the lens and the ciliary muscles
results in the ability to focus near and distance known as accommodation.
ORBIT
The bony orbits are the sockets containing the eyeballs and associated structures.
The other structures include the extraocular muscles, cushioning orbital fat, blood
vessels, and nerves. The eyeball occupies only about 20% of the volume of the orbit.
MUSCLES
Six extraocular muscles control the movement of each eye: four rectus muscles and
two oblique muscles. The rectus muscles are the medial, lateral, superior, and
inferior. They move the eyeball horizontally and vertically. The superior and inferior
obliques help in torsional movements like tilting the head to one side of looking
up or down at an angle.
SCLERA
The sclera is the fibrous outer protective coating of the eye. It is dense, white, and
continuous with the cornea anteriorly. Its only opening is posterior, where
the optic nerve and retinal vessels enter the eye. Its outer surface is covered by a thin
vascular covering called episclera. Like the cornea, the sclera has many lamellae,
but they are relatively hydrated (full of fluid) and less uniform in structure; this makes
the sclera nontransparent.
CHOROID
The choroid lies between the sclera and the retina. It supplies the blood vessels that nourish
the outer 2/3 of the retina.
RETINA
The retina is delicate, highly specialized brain tissue that forms the vision, much like the film
in a camera. Normally, it is transparent, but appears gray after death.
IRIS
The iris is the anterior extension of the ciliary body (muscles). It is a thin, vascular tissue
that has a central aperture called the pupil. A series of specialized pupil muscles allows
the pupil to rapidly enlarge or constrict to regulate the amount of light entering the eye.
OPTIC NERVE
The optic nerve is a trunk of nerve tissue that is actually an extension of the brain, covered with
the same meningeal skin as the brain. Fibers from the retina converge to form the nerve which
exits the eye and travels through the orbit, into the skull bone and along the underside of the
brain. Fibers then separate into both halves of the brain and travel to the vision area of the
brain posteriorly (occipital lobe).
AQUEOUS HUMOR
The aqueous humor is the fluid that fills the space between the lens and the inside surface of the
cornea. It carries the nutritional elements needed for the avascular cornea and lens.
VITREOUS HUMOR
The vitreous humor is a clear, transparent gel of semisolid consistency which fills the chamber
between the posterior surface of the lens and the inner surface of the retina. It is 99% water,
with the remainder being mostly a viscous substance called hyaluronic acid. It occupies two-thirds
of the volume of the globe (eyeball).
CORNEAL SCARS
Opaque or cloudy spots as well as irregularities that distort the corneal refracting surface can
interfere with vision. Some inconsistencies are due to injury. Infections can also result in
visually significant scarring: Contact lens-related bacterial infection, Herpes simplex(fever blister),
or Herpes zoster(shingles).
CORNEAL SWELLING
Injury to normal endothelial cells can result in loss of their pump function, allowing fluid to enter
the corneal stroma and cause swelling with resultant loss of transparency. Folds in Descemet's membrane
can sometimes be seen. Cataract/implant surgery is the most common surgical trauma leading to corneal
swelling (edema). Other intraocular surgeries can also result in an opaque cornea. Fuch's corneal
dystrophy (hereditary) results in weak endothelial pumps cells that become fragile and begin to fail
under normal aging conditions; routine cataract surgery will cause extreme injury to the abnormal
endothelial cells and permanent loss of transparency is noted following such surgery.
CORNEAL SHAPE PROBLEMS
Rare conditions such as keratoconus or keratoglobus can cause refractive distortions of
the cornea that cannot be corrected with glasses or contact lenses. Replacing the distorted portion of
the cornea with a normal cornea can return visual functioning.
OTHER OPACITIES
Rare inherited corneal clouding conditions can interfere with corneal transparency and require corneal
transpants to replace the central clouded cornea with a clear corneal button.