From Wikipedia, the free encyclopedia
The sclera, also known as the 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 fiber.[1] It is derived from the neural crest.[2] 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, the hole that is formed by the optic nerve. At the optic disk the outer two-thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve. The inner third with, on the other hand, along 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 sclera's blood vessels are mainly on the surface, and together with the conjunctiva (which lies on top) This is a thin layer covering the sclera. Along with the vessels of the conjunctiva, those of the sclera renders the inflamed eye bright red. [3]
Limbus is the border between cornea and sclera
[edit] Histology
The collagen of the sclera is continuous with the cornea. 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, as opposed to near-uniformn thickness and parallel arrangement of the corneal collagen. 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. [4] 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. [5]
Sometimes, very small gray-blue spots can appear on the sclera, a condition called scleral melanocytosis. They are not harmful.
[edit] Additional images
Interior of anterior half of bulb of eye.
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The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.
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The interior of the posterior half of the left eyeball.
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[edit] References
- ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.
- ^ Hermann D. Schubert. Anatomy of the Orbit http://www.nyee.edu/pdf/schubert.pdf
- ^ "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
- ^ "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
- ^ "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
[edit] External links
