The choroid itself consists of four layers: an outermost layer of large blood vessels (Haller's layer), a layer of intermediate size blood vessels (Sattler's layer), a layer of capillaries (choriocapillary layer), and an innermost layer (Bruch's membrane).
In primates, including humans, dark-colored melanin in the choroid helps restrict the amount of internal reflection in the eye, which might otherwise result in the perception of multiple, confusing images. Poor vision in albinos stems from a lack of this pigmentation. By contrast, the choroid of many other animals contains reflective materials that help to collect light in dim situations.
The "red eye" seen on photos, especially those taken with a flash, is caused by reflection of light by the choroid. The redness is due to the choroid's numerous blood vessels.
Related category ANATOMY AND PHYSIOLOGY
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