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.
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.
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.
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.
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.
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.
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.
The choroid lies between the sclera and the retina. It supplies the blood vessels that nourish the outer 2/3 of the 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.
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.
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).
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.
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).
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).
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.
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.