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The highly vascular tunic of the eye lying between the retina and sclera. Its main function is to nourish the retina (oxygen and nutrients) and remove waste products. It is a thin membrane extending from the optic nerve to the ora serrata. It contains blood vessels, capillaries, nerves, collagen and melanocytes, as well as fibroblasts, macrophages, mast cells and plasma cells. It consists of five main layers from without inward: the suprachoroid (lamina fusca), the layers of vessels (Haller's layer and Sattler's layer), the choriocapillaris and the membrane of Bruch (lamina vitrea). The blood supply is provided mostly by the short posterior ciliary arteries and to a lesser extent by the long posterior ciliary arteries, as well as some branches from anterior ciliary arteries. Venous blood drains into the vortex veins. The posterior choroid is thickest in hyperopia and thinnest in myopia. Note: some authors consider the suprachoroid as belonging to the sclera. However, when choroid and sclera are separated, part of the suprachoroid adheres to the choroid and part to the sclera. See choroiditis; epichoroid; choroidal naevus; choroidal neovascularization; suprachoroidal space.