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Choroid
	Schematic cross section of the human eye; choroid is shown in purple.
	Interior of anterior half of bulb of eye. (Choroid labeled at right, second from the bottom.)
Latin	choroidea
Gray's	subject #225 1009
Artery	short posterior ciliary arteries, long posterior ciliary arteries
MeSH	Choroid

The choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, 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 [1]. Along with the ciliary body and iris, the choroid forms the uveal tract.

[edit] Layers

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

Haller's layer - outermost layer of the choroid consisting of larger diameter blood vessels
Sattler's layer - layer of medium diameter blood vessels
Choriocapillaris - layer of capillaries
Bruch's membrane (synonyms: Lamina basalis, Complexus basalis, Lamina vitra) - innermost layer of the choroid

[edit] Blood supply

There are two circulations of the eye: the retinal and uveal, supplied in humans by posterior ciliary arteries, originating from the ophthalmic artery. The arteries of the uveal circulation, supplying the uvea and outer and middle layers of the retina, are branches of the ophthalmic artery and enter the eyeball without passing with the optic nerve. The retinal circulation, on the other hand, derives its circulation from the central retinal artery, also a branch of the ophthalmic artery, but passing in conjunction with the optic nerve. They are 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 dependent on choroidal blood supply. ^[1]

[edit] Mechanism

Calf's eye dissected to expose the choroid: its tapetum lucidum is iridescent blue

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).