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Caption = Schematic diagram of the human eye.
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anterior ciliary arteries, long posterior ciliary arteries, short posterior ciliary arteries
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The sclera, called the white or white of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer of the
eyecontaining collagenand elastic fibers.Cassin, B. and Solomon, S. "Dictionary of Eye Terminology". Gainesville, Florida: Triad Publishing Company, 1990.] It is derived from the neural crest[Hermann D. Schubert. "Anatomy of the Orbit" http://www.nyee.edu/pdf/schubert.pdf] . In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, however, fatty deposits on the sclera can make it appear slightly yellow.
The sclera forms the posterior five sixths of the connective tissue coat of the globe. It is continuous with the dura mater and the cornea, and maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions. The sclera is perforated by plenty of nerves and vessels passing through the posterior scleral foramen. At the optic disk, two-thirds of the sclera continues with the dural sheath, the other third with some choroidal tissue forms a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi). The thickness of the sclera varies from 1mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. The main parts of the eye also include the
conjunctiva. This is a thin layer covering the sclera.
From outer to innermost, the four layers of the sclera are:
* Lamina fusca
The sclera is opaque due to the irregularity of the collagen fibers. To avoid confusion, the cornea, which is an outer continuation from the sclera, has 5 layers. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils. The middle, thickest layer is also called the stroma.
LASIK(laser-assisted in situ keratomileusis) surgery to correct vision is usually done in this region of the cornea. The outside layer contains all the blood vessels of the stroma and along with the conjunctival vessels, are the cause of the bright redness of the inflamed eye. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells.
Sometimes, gray-blue spots can appear on the sclera, a condition called
scleral melanocytosis. They are not harmful.
* - "Sagittal Section Through the Eyeball"
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