AskDefine | Define cornea

Dictionary Definition

cornea n : transparent anterior portion of the outer covering of the eye; it covers the lens and iris and is continuous with the sclera [also: corneae (pl)]

User Contributed Dictionary

English

Etymology

From Latin cornea tela, horny tissue, from cornu, horn

Pronunciation

Noun

  1. The transparent layer forming the front of the eye.

Derived terms

Translations

See also

Italian

Noun

  1. In the context of "anatomy|lang=it": cornea

Derived terms

Adjective

cornea
  1. Feminine of corneo

Extensive Definition

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of the eye's optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance. Medical terms related to the cornea often start with the prefix "kerat-".

Structure

The cornea has unmyelinated 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 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air.
It borders with the sclera by the corneal limbus.

Layers

The human cornea, like that of other primates, has five layers. The corneas of cats, dogs, and other carnivores have only four. From the anterior to posterior they are:
  1. Corneal epithelium: a thin epithelial multicellular tissue layer (stratified squamous epithelium) of fast-growing and easily-regenerated cells, kept moist with tears. Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity.
  2. Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers. This layer is absent in carnivores.

Diseases and disorders

Treatment and management

Surgical procedures

Various refractive eye surgery techniques change the shape of the cornea in order to reduce the need for corrective lenses or otherwise improve the refractive state of the eye. In many of the techniques used today, reshaping of the cornea is performed by photoablation using the excimer laser.
If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted. Because there are no blood vessels in the cornea, there are also few problems with rejection of the new cornea.
There are also synthetic corneas (keratoprostheses) in development. Most are merely plastic inserts, but there are also composed of biocompatible synthetic materials that encourage tissue ingrowth into the synthetic cornea, thereby promoting biointegration.

Non-surgical procedures

Orthokeratology is a method using specialized hard or rigid gas-permeable contact lenses to transiently reshape the cornea in order to improve the refractive state of the eye or reduce the need for eyeglasses and contact lenses.

References

External links

cornea in Arabic: قرنية
cornea in Bengali: কর্নিয়া
cornea in Catalan: Còrnia
cornea in German: Cornea
cornea in Spanish: Córnea
cornea in Esperanto: Korneo
cornea in Basque: Kornea
cornea in French: Cornée
cornea in Galician: Córnea
cornea in Korean: 각막
cornea in Hindi: स्वच्छमण्डल
cornea in Indonesian: Kornea
cornea in Italian: Cornea
cornea in Hebrew: קרנית
cornea in Lithuanian: Ragena
cornea in Hungarian: Szaruhártya
cornea in Dutch: Hoornvlies
cornea in Japanese: 角膜
cornea in Norwegian: Hornhinne
cornea in Polish: Rogówka
cornea in Portuguese: Córnea
cornea in Russian: Роговица
cornea in Simple English: Cornea
cornea in Slovak: Rohovka
cornea in Slovenian: Roženica
cornea in Finnish: Sarveiskalvo
cornea in Swedish: Hornhinna
cornea in Turkish: Kornea
cornea in Yiddish: קארניע
cornea in Chinese: 角膜
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