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Cornea is also a commune in Caraş-Severin County, Romania, see Cornea, Caraş-Severin. The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to "tune" the focus, the curvature of the cornea is fixed. 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 cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5 mm - 0.6 mm in the center and 0.6 mm - 0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. In humans, the refractive power of the cornea is approximately 43 dioptres, roughly three-fourths of the eye's total refractive power.* Medical terms related to the cornea often start with "kerat-".
Layers The human cornea, like that of other primates, has five layers. The corneas of cats, dogs, and other carnivores have only four. Innervation The cornea is one of the most sensitive tissues of the body, it is densely innervated with sensory nerve fibres via the ophthalmic division of the trigeminal nerve by way of 70 - 80 long ciliary nerves and short ciliary nerves. The nerves enter the cornea via three levels, scleral, episcleral and conjunctival. Most of the bundles give rise by subdivision to a network in the stroma, from which fibres supply the different regions. The three networks are midstromal, subepithelial/Bowman's layer, and epithelium. The receptive fields of each nerve ending are very large, and may overlap. Diseases and disorders Surgical procedures involving the cornea Various refractive eye surgery techniques change the shape of the cornea in order to reduce the need for corrective lenses or otherwise improve the refractive state of the eye. In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer laser. If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea. There are also synthetic corneas (keratoprostheses) in development. Most are merely plastic inserts, but there are also composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration. Non-surgical procedures involving the cornea Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses. See also | |||||||||||||||||||||||||||
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