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Cornea
The cornea is sometimes referred to as the 'window of the eye'. It provides most of the focusing power when light enters your eye. The cornea is composed of 5 layers of tissue. The outer layer (the epithelium), is the eye's protective layer. This layer is made up of cells that have the ability to grow back within 3 days, and therefore, allow for fast healing of superficial injuries. The second layer is Bowman's membrane. This layer provides the smooth surface and anchor for the epithelium. The stromal layer provides the strength and curvature to the cornea, and it accounts for about 90% of the thickness of the eye. Descemet's membrane is the fourth layer, acting as an anchor to the endothelium or fifth layer which helps to pump water out of the cornea maintaining its clarity.
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Iris
This is the colored part you see in people's eyes (i.e. blue/green/brown/hazel). The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris in response to changes in the amount of ambient light.
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Pupil
The pupil is the 'black circle' that you see in people's eyes. It is actually the opening in the surrounding iris. When it is dark, the iris expands to allow more light to reach the back of the eye, and the pupil becomes dilated.
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Lens
The lens is the clear structure behind the iris. Its primary function is to provide fine tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 40 to 50, the lens becomes larger, crowding the muscles that reshape it, and presbyopia sets in. At about the age of 60 or 70, the lens becomes cloudy and hard (cataract formation), preventing light from entering the eye.
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Vitreous Body
This is the clear 'gel like' substance located inside the eye's cavity. Its purpose is to provide a cushion for shock absorbency to the eye. The vitreous may develop small clumps known as 'floaters', which are more common in nearsighted people than in the rest of the population.
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Retina
The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to convert light and image into electrical impulses that are then relayed to the brain by the optic nerve.
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Sclera
This is the 'white part' that we see in people's eyes. The sclera's purpose is to provide structure, strength and protection to the eye. It is very similar to the cornea in structure with the exception that it has no mechanism to pump out water, giving it its characteristic white color.
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Optic Nerve
The optic nerve is the "wiring" that carries images from the retina to the brain. The two nerves (one from each eye) supply greater than one half the total sensory input to the brain. That's more than all the other sensory input from all our other organs combined
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