AskDefine | Define choroid

Dictionary Definition

choroid n : a highly vascular membrane in the eye between the retina and the sclera [syn: choroid coat]

User Contributed Dictionary

English

Etymology

From sc=polytonic + sc=polytonic.

Pronunciation

  • /ˈkəʊrɔɪd/, a US /ˈkoʊrɔɪd/

Noun

  1. The vascular layer of the eye lying between the retina and the sclera.

Translations

vascular layer of the eye
  • Finnish: suonikalvo
  • Polish: naczyniówka

Extensive Definition

The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina http://www.eyeglossary.net/#C. Along with the ciliary body and iris, the choroid forms the uveal tract.

Layers

The structure of the choroid is generally divided into four layers:

Blood supply

The choroid is supplied in humans by posterior ciliary arteries branching in a segmental distribution to the end arterioles and not anastomoses. This is clinically significant for diseases affecting choroidal blood supply. The macula responsible for central vision and the anterior part of the optic nerve are dependant on choroidal blood supply.

Mechanism

Melanin, a darkly colored pigment, helps the choroid limit uncontrolled reflection within the eye that would potentially result in the perception of confusing images. In humans and most other primates, melanin occurs throughout the choroid. In albino humans, frequently melanin is absent and vision is low. In many animals, however, the partial absence of melanin contributes to superior night vision. In these animals, melanin is absent from a section of the choroid and within that section a layer of highly reflective tissue, the tapetum lucidum, helps to collect light by reflecting it in a controlled manner. The uncontrolled reflection of light from dark choroid produces the photographic red-eye effect on photos, whereas the controlled reflection of light from the tapetum lucidum produces eyeshine (see Tapetum lucidum).

Additional images

References

External links

choroid in Bengali: কোরয়েড
choroid in Catalan: Coroide
choroid in German: Chorioidea
choroid in Modern Greek (1453-): Χοριοειδής χιτώνας
choroid in Spanish: Coroides
choroid in Basque: Koroide
choroid in French: Choroïde
choroid in Hebrew: שכבה דמית
choroid in Lithuanian: Gyslainė
choroid in Dutch: Vaatvlies
choroid in Polish: Naczyniówka
choroid in Slovak: Cievovka
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