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Jai Ganesh

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Registered: 2005-06-28

Posts: 50,857

Cornea

Cornea

Gist

The cornea is the transparent, dome-shaped front part of the eye that acts like a window, allowing light to enter and focus on the retina. It plays a crucial role in vision by refracting light, contributing significantly to the overall focusing power of the eye. The cornea also serves as a protective barrier, shielding the eye from debris, germs, and some UV rays.

Cornea: The transparent part of the eye that covers the iris and the pupil and allows light to enter the inside.

In addition to protecting the eye from outside infiltrates and ultraviolet radiation, the cornea is responsible for approximately 65% to 75% of the refraction of light as it passes through the eye. The cornea performs the initial refraction onto the lens, which further focuses the light onto the retina.

Summary

The cornea is the transparent front part of the eyeball which covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. The cornea can be reshaped by surgical procedures such as LASIK.

While the cornea contributes most of the eye's focusing power, its focus is fixed. Accommodation (the refocusing of light to better view near objects) is accomplished by changing the geometry of the lens.

Structure

The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance, the healthy cornea does not have or need blood vessels within it. Instead, oxygen dissolves in tears and then diffuses throughout the cornea to keep it healthy. Similarly, nutrients are transported via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface. Nutrients also come via neurotrophins supplied by the nerves of the cornea. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue.

The most abundant soluble protein in mammalian cornea is albumin.

The human cornea borders with the sclera at the corneal limbus. In lampreys, the cornea is solely an extension of the sclera, and is separate from the skin above it, but in more advanced vertebrates it is always fused with the skin to form a single structure, albeit one composed of multiple layers. In fish, and aquatic vertebrates in general, the cornea plays no role in focusing light, since it has virtually the same refractive index as water.

Details

Cornea is the dome-shaped transparent membrane about 12 mm (0.5 inch) in diameter that covers the front part of the eye. Except at its margins, the cornea contains no blood vessels, but it does contain many nerves and is very sensitive to pain or touch. It is nourished and provided with oxygen anteriorly by tears and is bathed posteriorly by aqueous humour. It protects the pupil, the iris, and the inside of the eye from penetration by foreign bodies and is the first and most powerful element in the eye’s focusing system. As light passes through the cornea, it is partially refracted before reaching the lens. The curvature of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which images appear elongated or distorted.

The cornea itself is composed of multiple layers, including a surface epithelium, a central, thicker stroma, and an inner endothelium. The epithelium (outer surface covering) of the cornea is an important barrier to infection. A corneal abrasion, or scratch, most often causes a sensation of something being on the eye and is accompanied by intense tearing, pain, and light sensitivity. Fortunately, the corneal epithelium is able to heal quickly in most situations.

The collagen fibres that make up the corneal stroma (middle layer) are arranged in a strictly regular, geometric fashion. This arrangement has been shown to be the essential factor resulting in the cornea’s transparency. When the cornea is damaged by infection or trauma, the collagen laid down in the repair processes is not regularly arranged, with the result that an opaque patch or scar may occur. If the clouded cornea is removed and replaced by a healthy one (i.e., by means of corneal transplant), usually taken from a deceased donor, normal vision can result.

The innermost layer of the cornea, the endothelium, plays a critical role in keeping the cornea from becoming swollen with excess fluid. As endothelial cells are lost, new cells are not produced; rather, existing cells expand to fill in the space left behind. Once loss of a critical number of endothelial cells has occurred, however, the cornea can swell, causing decreased vision and, in severe cases, surface changes and pain. Endothelial cell loss can be accelerated via mechanical trauma or abnormal age-related endothelial cell death (called Fuchs endothelial dystrophy). Treatment may ultimately require corneal transplant.

Additional Information

Your cornea is your eye’s clear, protective outer layer. It acts like a barrier against dirt and germs, and it helps filter out some of the sun's damaging ultraviolet light.

It also plays a key role in vision. As light enters your eye, it gets refracted, or bent, by the cornea’s curved edge. This helps determine how well your eye can focus on objects close up and far away.

If your cornea is damaged by disease, infection, or an injury, the scars can affect your vision. They might block or distort light as it enters your eye.

Layers of the Cornea

Your cornea has six main layers:

Epithelium

The outermost layer protects your eyes from chemicals, water, and microbes and absorbs nutrients from tears and oxygen. It's the most sensitive part of the body.

Bowman's layer

The second layer is made up of a strong protein called collagen. It helps form the shape of your cornea.

Stroma

The third layer is the thickest layer of your cornea. It's made up of water and proteins that strengthen and support your cornea. It's the most important layer for helping your eyes to focus.

Dua's layer

This is the thinnest layer of your cornea. It was only recently discovered, and scientists aren't sure what its function is yet.

Descemet’s membrane

This is a strong layer of tissue that protects your eye against infection and injury.

Endothelium

The innermost layer is a very thin layer of cells on the back of the stroma. It works like a pump to drain excess fluid from the stroma. Without it, fluid would build up in the stroma and your cornea. Your cornea would get opaque and hazy, and so would your vision.

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