Choroid

see also: Etymology of Choroid

The choroid is one of the layers of the wall of the Eye-ball, located between the sclerotic outside and the Rétine inside. It is a richly vascularized layer which ensures the nutrition of the iris and of the retinal Photorécepteur S. With the Body ciliaire and the iris, the choroid forms the Uvée. Its structure provides a double function to him: on the one hand, it forms a screen which maintains the interior of the eye in darkroom, on the other hand, it protects it from the thermal point of view, preventing the passage of most of the external radiations.

Anatomical structure

One usually distinguishes four or five layers in the choroid:

the layer To haul - external layer of the consistent choroid in conjunctive Fabric elastic, pigmented conjunctive cells, and containing blood-vessels of large diameter. (posterior arteries ciliaires)
the layer of Sattler - layer of the consistent choroid in blood-vessels of average diameter traversing a very also pigmented conjunctive fabric.
Ici fits in certain animals (majority of the mammals, but not the human one, neither the pig, nor the rabbit) the Tapetum lucidum, layer little pigmented which diffracts and reflects the rays and acts like an amplifier of light by leading this one to return to strike the photoreceivers of the retina. This is why the tapetum lucidum is particularly important at the active species in the twilight. It is also responsible for the illumination characteristic of the eyes of these animals when one lights them in the darkness. The tapetum lucidum can be made up, either of cells applaties (as in the carnivores), or of crystalline substance as in the dog, or of especially ordered conjunctive fibers, as in the horse or the ruminants.
the layer of the choriocapillaires - fine network of capillaries which ensure the constant food of the external layer of the Rétine.
the Membrane of Bruch - layer interns choroid directly coupled with the pigmentary layer of the retina.

The Optical nerve allows a contact between the pia mater and the choroid while crossing the latter.

Variations according to the species

The choroid of the Primates (of which the human one) is papered cells rich in Mélanine, and thus very dark, which absorbs the visible Radiation S and thus avoids inappropriate reflections inside the eye. The primates Albinos frequently suffer from a poor vision caused by the lack of pigments in the choroid.

Many other animal species have a choroid papered contrary to reflective materials which enable them to better collect the light in the darkness.

Choroid and photography

The effect red eyes in Photographie is in fact a diffusion of light by the blood-vessels (red) of the choroid.

See too

Chorioretinitis
Uvéite (ignition of the uvea)

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