Sclera
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] 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.

[edit] 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). 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]

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]

[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-uniform 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. ^[5] 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. ^[6]

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

[edit] Additional images

Limbus is the border between cornea and sclera

Interior of anterior half of bulb of eye.

The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section.

The interior of the posterior half of the left eyeball.

[edit] References

[edit] External links

• Histology at BU 08008loa
• Atlas of anatomy at UMich eye_1 - "Sagittal Section Through the Eyeball"

v • d • e Sensory system – visual system – globe of eye (TA A15.2.1-6, GA 10.1005) Fibrous tunic (outer) Sclera Episcleral layer • Schlemm's canal • Trabecular meshwork Cornea Limbus • layers (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) Uvea/vascular tunic (middle) Choroid Capillary lamina of choroid • Bruch's membrane • Sattler's layer Ciliary body Ciliary processes • Ciliary muscle Iris Stroma • Pupil • Iris dilator muscle • Iris sphincter muscle Retina (inner) Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retinal ganglion cell (Giant retinal ganglion cells, Photosensitive ganglion cell) Muller glia Layers Inner limiting membrane • Nerve fiber layer • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer Outer plexiform layer • Outer nuclear layer External limiting membrane • Layer of rods and cones • Retinal pigment epithelium Other Macula • Foveola • Fovea • Optic disc (Cup) Anterior segment Anterior chamber • Aqueous humour • Posterior chamber • Lens (Capsule of lens, Zonule of Zinn) Posterior segment Vitreous humour Other Ocular immune system • Tapetum lucidum • Keratocytes eye navs: anat/adnexa anat/pathways/physio/dev, noncongen/congen/neoplasia, eponymous signs, proc

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