Structure and Function of the Eye

The eye of an adult measures about one inch from the front to the back of the eye; a child's eye measures about three-quarters of one inch. The eye has three layers:

Sclera ­ the outer protective white coating of the eye

Choroid ­ the middle layer which contains blood vessels to nourish the eye

Retina ­ the inner layer which contains the nerves that breing information to the brain for seeing

 

The cornea is the clear protion of the front of the eye which bends light rays. The conjunctiva is a thin tissue which lines the eyelids and the eyeball up to the edge of the cornea. The iris is the colored portion of the eye which is made up of a spongy tissue and is an extension of the choroid. The pupil is the opening in the iris (black) which allows light into the eye. The lens helps focus light rays onto the retina the way a camera lens focuses light onto film; the lens can change shape, or accommodate, to focus on near or distant objects.

The eye is filled with fluids which help nourish and maintain the pressure within the eye. The anterior chamber, the front portion of the eye between the iris and the cornea, is filled with aqueous humor, a watery fluid which nourishes the lens and mantains the pressure within the eye. The back portion of the eye is filled with vitreous humor, a transparent gel. The retina is made up of ten layers and contains over one million cells. The optic nerve has nerve fibers which transmit information to the brain for interpretation of objects seen.

The macula is the area of the retina that is responsible for central vision; its central portion is referred to as the fovea and is responsible for the sharpest vision. The macula houses the highest concentration of the cones which area responsible for color and sharp vision. The rods, which compose the rest of the retina, are more sensitive to light and are responsible for night vision and peripheral vision. Attached to the globe of the eye are six muscles which aid in the movement of the eye. Movement of the eye may be caused by one, a few, or all of the muscles working together.

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Structure of the Cornea Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and proteins. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas. The corneal tissue is arranged in five basic layers, each having an important function. These five layers are: Epithelium The epithelium is the cornea's outermost region, comprising about 10 percent of the tissue's thickness. The epithelium functions primarily to: (1) Block the passage of foreign material, such as dust, water, 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 distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane. Bowman's Layer Lying directly below the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman's layer. It is composed of strong layered protein fibers called collagen. Once injured, Bowman's layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur. Stroma Beneath Bowman's layer is the stroma, which comprises about 90 percent of the cornea's thickness. It consists primarily of water (78 percent) and collagen (16 percent), and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency. Descemet's Membrane Under the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. Descemet's membrane 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 is regenerated readily after injury. Endothelium The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque. In a healthy eye, a perfect balance 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 are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy.

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stroma -- The middle tissue layer of the eye that makes up about 90 percent of the cornea. The stroma is composed of densely packed layers similar to pages of a book. KeraVision's Intacs is designed to be surgically placed within these dense layers. Here it is designed to remain permanently without maintenance, but it can also be removed by the surgeon. KeraVision, ICR, ICRS, the KeraVision Logo and Shaping the Way the World Sees are registered trademarks or trademarks of KeraVision, Inc., in the U.S. and foreign countries. ©1997, KeraVision, Inc. All rights reserved. Source Site

Photograph of the stroma showing its lamellar (layered) structure. Source Site