Choroid

The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye lying between the retina and the sclera, with a thickness about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina.

Along with the ciliary body and iris, the choroid forms the uveal tract. In humans and other primates, darkly colored melanin pigment in the choroid helps limit reflections within the eye that would potentially result in the perception of confusing images. Poor vision frequently results from lack of this pigmentation in human albinos. By contrast, the choroid of many other animals contains reflective materials that help to collect light in dim situations; this is one type of tapetum lucidum.

The red eye effect on photos is caused by the reflection of light from choroid. It appears red because of the choroid's blood vessels.

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 - innermost layer of the choroid