The Full Wiki

Sclera: Wikis

  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

Sclera
Schematic diagram of the human eye en.svg
Schematic diagram of the human eye.
Gray's subject #225 1006
Artery anterior ciliary arteries, long posterior ciliary arteries, short posterior ciliary arteries
MeSH Sclera

The sclera, also known as the white part 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] In the development of the embryo, the sclera 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, fatty deposits on the sclera can make it appear slightly yellow.

Contents

Structure

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 many nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve. At the optic disc 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 joins with some choroidal tissue to form a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi) pass. 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. Along with the vessels of the conjunctiva (which is a thin layer covering the sclera), those of the sclera render the inflamed eye bright red.[3]

In many vertebrates, the sclera is reinforced with plates of cartilage or bone, together forming a circular structure called the scleral ring. In primitive fish, this ring consists of four plates, but the number is lower in many living ray-finned fishes, and much higher in lobe-finned fishes, various reptiles, and birds. The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals.[4]

The eyes of all non-human primates are dark with small, barely visible sclera. See Cooperative eye hypothesis

Histology

The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are:

The sclera is opaque due to the irregularity of the collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils.

The cornea, unlike the sclera, has 5 layers. The middle, thickest layer is also called the stroma. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells. [3]

Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.

Additional images

Limbus is the border between cornea and sclera

References

  1. ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainesville, Florida: Triad Publishing Company, 1990.
  2. ^ Hermann D. Schubert. Anatomy of the Orbit http://www.nyee.edu/pdf/schubert.pdf
  3. ^ a b "eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009
  4. ^ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 461. ISBN 0-03-910284-X. 

External links








Got something to say? Make a comment.
Your name
Your email address
Message