Sclera


Sclera

Infobox Anatomy
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Caption = Schematic diagram of the human eye.

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Artery = anterior ciliary arteries, long posterior ciliary arteries, short posterior ciliary arteries
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MeshName = Sclera
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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 eye containing collagen and 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.

Histology

From outer to innermost, the four layers of the sclera are:
* Episclera
* Stroma
* Lamina fusca
* Endothelium

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.


=Additional

References

External links

*
* - "Sagittal Section Through the Eyeball"


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Look at other dictionaries:

  • scleră — scléră s. f., pl. sclére Trimis de siveco, 10.08.2004. Sursa: Dicţionar ortografic  SCLÉRĂ s.f. (anat.) Tunica solidă a globului ocular. [< germ. Sklera]. Trimis de LauraGellner, 13.09.2007. Sursa: DN  SCLÉRĂ s. f …   Dicționar Român

  • Sclera — Sclera, die weiße Augenhaut (s. Tafel »Auge II«, mit Text) …   Meyers Großes Konversations-Lexikon

  • Sclera — Sclera, die harte Haut (s. Auge) …   Kleines Konversations-Lexikon

  • Sclera — Sclera, Lederhaut, harte Augenhaut, äußere Hüllschicht des ⇒ Einzellinsenauges der Wirbeltiere, die sich in seinem lichtseitigen vorderen Bereich in die Cornea fortsetzt …   Deutsch wörterbuch der biologie

  • sclera — / sklɛra/ s.f. [dal gr. sklērós duro ]. (anat.) [parte non trasparente della tunica esterna dell occhio] ▶◀ sclerotica …   Enciclopedia Italiana

  • Sclera — vgl. Sklera …   Das Wörterbuch medizinischer Fachausdrücke

  • sclera — Medical L., from Gk. sklera (menix) the hard (membrane), fem. of skleros hard (see SCLEROSIS (Cf. sclerosis)) …   Etymology dictionary

  • sclera — [sklir′ə] n. pl. scleras or sclerae [ModL < Gr sklēros, hard < IE base * (s)kel , to dry out > SHALLOW] the outer, tough, white, fibrous membrane covering all of the eyeball except the area covered by the cornea: see EYE scleral adj …   English World dictionary

  • Sclera — Anatomischer Aufbau des Auges. Sclera (Lederhaut) hellgelb. Sclera (von griech. skleros „hart“) ist der medizinische Fachausdruck für die Lederhaut des Auges. Aufgrund ihrer weißlichen Farbe wird sie auch „weiße Augenhaut“ genannt. Sie umschließt …   Deutsch Wikipedia

  • Sclera — The tough white outer coat over the eyeball that covers approximately the posterior five sixths of its surface. The sclera is continuous in the front of the eye with the cornea and in the back of the eye with the external sheath of the optic… …   Medical dictionary


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