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Encyclopedia > Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to "tune" the focus, the curvature of the cornea is fixed. Closeup of a blue-green human eye. ... The human iris The iris is the green/grey/brown area. ... The human eye The pupil is the central transparent area (showing as black). ... Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... The focus or image point is the point where light rays, originating from a point in the object, converge [1]. The principal focus or focal point of a lens or parabolic mirror is the point onto which collimated light parallel to the axis is focused. ... Curvature refers to a number of loosely related concepts in different areas of geometry. ...


The cornea has nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 12 mm and a thickness of 0.5 mm - 0.7 mm in the center and 1.0 mm - 1.2 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. Nerves (yellow) Nerves redirects here. ... The mechanism of the reflex arc A reflex action is a stereotyped (involuntary) motor response elicited by a defined stimulus. ... An eyelid is a thin fold of skin and muscle that covers and protects an eye. ... The arterial system The blood vessels are part of the circulatory system and function to transport blood throughout the body. ... Schematic drawing of the effects of diffusion through a semipermeable membrane. ... The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... Neurotrophins are a family of molecules that encourage survival of nervous tissue. ... Trinomial name Homo sapiens sapiens Linnaeus, 1758 Humans, or human beings, are bipedal primates belonging to the mammalian species Homo sapiens (Latin for wise man or knowing man) under the family Hominidae (the great apes). ... A millimetre (American spelling: millimeter, symbol mm) is an SI unit of length that is equal to one thousandth of a metre. ...


In humans, the refractive power of the cornea is approximately 45 dioptres, roughly three-fourths of the eye's total power. Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... A dioptre, or diopter, is a non-SI unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (i. ...


Medical terms related to the cornea often start with "kerat-".

Contents


Layers of the cornea

The cornea consists of five layers. From the outside to the inside they are:

  • Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers
  • Corneal stroma (also substantia propria): a thick, transparent middle layer responsible for most of the focusing that the cornea performs, consisting of regularly-arranged collagen fibers along with (few) fibroblasts (If the stroma is damaged, for example by injury or infection, it can lose its transparency, causing vision problems. The corneal stroma consists of approximately 200 layers of type I collagen fibrils. The ordering of the fibrils is responsible for the transparency of the tissue.)
  • Descemet's membrane (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium

The cornea is composed mostly of dense connective tissue, similar to the surrounding sclera. However, the collagen fibers are arranged in a parallel pattern, allowing light waves to constructively interfere, allowing the light to pass through relatively uninhibited. The cornea is innervated by the long posterior ciliary nerves that branch from the trigeminal nerve's ophthalmic division. The corneal epithelium (epithelium corneæ anterior layer) covers the front of the cornea and consists of several layers of cells. ... In zootomy, epithelium is a tissue composed of a layer of cells. ... Cells in culture, stained for keratin The cell is the structural and functional unit of all living organisms. ... Tears are commonly associated with crying Tears are a liquid produced by the bodys process of lacrimation to clean and lubricate the eyes. ... The Bowmans membrane, or Bowmans layer, is a smooth layer in the eye. ... Tropocollagen triple helix. ... The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent. ... A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue. ... A basement membrane that lies between the proper substance and the endothelial layer of the cornea; posterior limiting layer of the cornea. ... Corneal endothelium is the inner most layer of the cornea, the corneal endothelium is actually a monolayer of squamate epithelial cells lining the anterior chamber of the eye. ... Squamous means covered with or formed of scales; scaly. ... In anatomy, the cuboid bone is a bone in the foot. ... Look up Misnomer in Wiktionary, the free dictionary. ... The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... Human blood smear: a - erythrocytes; b - neutrophil; c - eosinophil; d - lymphocyte. ... In mammals including humans, the lymphatic vessels (or lymphatics) are a network of thin tubes that branch, like blood vessels, into tissues throughout the body. ... The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... The sclera is the white outer coating of the eye. ... The trigeminal nerve is the fifth (V) cranial nerve, and carries sensory information from most of the face, as well as motor supply to the muscles of mastication (the muscles enabling chewing), tensor tympani (in the middle ear), and other muscles in the floor of the mouth, such as the...

Cornea
Vertical section of human cornea from near the margin. (Waldeyer.) Magnified.
1. Epithelium.
2. Anterior elastic lamina.
3. substantia propria.
4. Posterior elastic lamina.
5. Endothelium of the anterior chamber.
a. Oblique fibers in the anterior layer of the substantia propria.
b. Lamellæ the fibers of which are cut across, producing a dotted appearance.
c. Corneal corpuscles appearing fusiform in section.
d. Lamellæ the fibers of which are cut longitudinally.
e. Transition to the sclera, with more distinct fibrillation, and surmounted by a thicker epithelium.
f. Small bloodvessels cut across near the margin of the cornea.
[[Image:|250px|center|]]
Latin l. limitans anterior corneae
Gray's subject #225 1008
System
Precursor
MeSH A09.371.060.217.113
Dorlands/Elsevier l_02/12476272

Schematic diagram of the human eye. Image File history File links Gray871. ... In zootomy, epithelium is a tissue composed of a layer of cells. ... The Bowmans membrane is a smooth layer in the eye. ... The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent. ... A basement membrane that lies between the proper substance and the endothelial layer of the cornea; posterior limiting layer of the cornea. ... The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... The anterior chamber if the fluid-filled space inside the eye between the iris and the corneas innermost surface, the endothelium . ... The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent. ... The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. ... Fusiform is a spindle-like shape that tapers at both ends. ... The sclera is the white outer coating of the eye. ... In zootomy, epithelium is a tissue composed of a layer of cells. ... Latin is an ancient Indo-European language originally spoken in the region around Rome called Latium. ... In biology, an organ is a group of tissues which perform some function. ... Embryology is the branch of developmental biology that studies embryos and their development. ... Medical Subject Headings (MeSH) is a huge controlled vocabulary (or metadata system) for the purpose of indexing journal articles and books in the life sciences. ... Elseviers logo Elsevier, the worlds largest publisher of medical and scientific literature, forms part of the Reed Elsevier group. ... (created with Sodipodi) Schematic diagram of the human eye Aqueous humour Anterior chamber Choroid Cornea Ciliary muscle Iris Lens Optic disc Fovea Optic nerve Posterior chamber Pupil Retina Sclera Suspensory ligament Vitreous humor Zonular fibers File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev...


Diseases and disorders

This is a partial list of human eye diseases and disorders. ...

Treatment and management of corneal diseases and disorders

Surgical procedures involving the cornea

Various refractive eye surgery techniques change the shape of the cornea in order to reduce the need for glasses or otherwise improve the refractive state of the eye. In the techniques used today, parts of the cornea are removed with lasers. Refractive eye surgery is any eye surgery used to improve the refractive state of the eye and decrease dependency on glasses or contact lenses. ... Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments. ...


If the corneal stroma has developed opaque patches known as leukomas, a cornea of a deceased donor can be transplanted. Because there are few blood vessels in the cornea, there are also few problems with rejection of the new cornea. Cornea Transplant A cornea transplant, also known as a corneal graft or penetrating keratoplasty, is a surgical procedure where a damaged or diseased cornea is replaced by a donor. ...


There are also synthetic corneas in development. Most are merely plastic inserts, but there are also some made of plastics that encourage the eye tissue to grow into the synthetic cornea, making it a full replacement.


Non-surgical procedures involving the cornea

Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses. Please wikify (format) this article as suggested in the Guide to layout and the Manual of Style. ... A contact lens (also known simply as a contact) is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye. ...


References

  • Daxer A et al. Collagen fibrils in the human corneal stroma: structure and ageing. Invest Ophthalmol Vis Sci 1998;39:644-648.
  • Daxer A and Fratzl P. Collagen fibril orientation in the human corneal stroma and its implication in keratoconus. Invest Ophthalmol Vis Sci 1997;38:121-129.
  • Fratzl P and Daxer A. Structural transformation of collagen fibrils in corneal stroma during drying: An X-ray scattering study. Biophys J 1993;64:1210-1214.

See also:

Sensory system - Visual system - Eye - edit
Anterior segment | Posterior segment 

Sclera | Schlemm's canal | Trabecular meshwork | Cornea | Conjunctiva  In ophthalmology, astigmatism is a refractive error of the eye in which there is a difference in degree of refraction in different meridians. ... This article or section may be confusing for some readers, and should be edited to be clearer or more simplified. ... The visual system is the part of the nervous system which allows organisms to see. ... Closeup of a blue-green human eye. ... anterior segment ... The posterior segment is the back two-thirds of the eye that includes the anterior hyaloid membrane and all structures behind it: the vitreous humor, retina, choroid, and optic nerve. ... The sclera is the white outer coating of the eye. ... Schlemms canal, also known as canal of Schlemm, is a circular channel in the eye that collects aqueous humor from the anterior chamber and delivers it into the bloodstream . ... This is an area of tissue located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humour from the eye via the anterior chamber (the chamber on the front of the eye covered by the cornea). ... The conjunctiva is a membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids. ...


Pupil | Choroid | Tapetum lucidum | Ciliary body | Iris | Anterior chamber | Posterior chamber  The human eye The pupil is the central transparent area (showing as black). ... Schematic cross section of the human eye; choroid is shown in yellow. ... tapetum lucidum in a calf eye, with the retina hanging down The tapetum lucidum (Latin: bright carpet) is a reflecting layer immediately behind, and sometimes within, the retina of the eye of many vertebrates (though not humans); it serves to reflect light back to the retina, increasing the quantity of... Schematic diagram of the human eye The ciliary body is the part of the eye containing the ciliary muscle and ciliary processes. ... The human iris The iris is the green/grey/brown area. ... The anterior chamber if the fluid-filled space inside the eye between the iris and the corneas innermost surface, the endothelium . ... The posterior chamber is a narrow chink behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes. ...


Retina | Macula | Optic fovea | Optic disc  Human eye cross-sectional view. ... Human eye cross-sectional view. ... The fovea, a part of the eye, is a spot located in the center of the macula. ... The optic disc or optic nerve head is the point in the eye where the optic nerve fibres leave the retina; it is not sensitive to light and thus also known as the blind spot or anatomical blind spot. Inspection of the optic disc by ophthalmoscopy can give an indication...


Aqueous humour | Vitreous humour | Lens  The aqueous humour is the clear, watery fluid that fills the complex space in the front of the eye which is bounded at the front by the cornea and at the rear by the front surface or face of the vitreous humour. ... Vitreous humour is the clear aqueous solution that fills the space between the lens and the retina of the vertebrate eyeball. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ...

Sensory system - Visual system - edit
Eye | Optic nerve | Optic chiasm | Optic tract | Lateral geniculate nucleus | Optic radiation | Visual cortex

  Results from FactBites:
 
Cornea transplant - Wikipedia, the free encyclopedia (1739 words)
A cornea transplant, also known as a corneal graft or penetrating keratoplasty, is a surgical procedure where a damaged or diseased cornea is replaced by donated corneal tissue which has been removed from a recently deceased individual having no known diseases which might affect the viability of the donated tissue.
The cornea is the clear part of eye in front of the iris and pupil.
Patients with synthetic corneas have been reported to be able to see into the near-ultraviolet spectrum, as the synthetic material is transparent to near-UV while a natural cornea is not.
  More results at FactBites »

 
 

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