It is the posterior part of the tunica media of the eyeball. The choroid is attached to the inner surface of the sclera, from the optic papilla to the serrated edge of the ciliary body, accounting for 5/6 of the posterior middle layer of the ocular wall. It is dark brown, rich in blood vessels and pigments, and has the functions of supplying eyeball nutrition and isolating light. Its tissue structure is divided into four layers: choroidal upper layer, vascular layer, choroidal capillary layer and vitreous membrane.

From the outside to the inside, it is divided into five layers: 1, suprachoroidal space: connected by connective tissue bundles and sclera, containing elastic fibers, pigment cells and smooth muscle fibers. Long posterior ciliary artery, short posterior ciliary artery and ciliary nerve all pass through it.

2. Great vascular layer: composed of arteries and anastomotic veins, with pigment cells and a small amount of smooth muscle fibers between the blood vessels. The arteries in this layer mainly branch from the posterior ciliary artery.

3. Middle vascular layer: There is no obvious boundary with the large vascular layer, only the blood vessels gradually taper. There is no large vascular layer in the macula, only a closely arranged middle vascular layer. This layer of pigment is less.

4, capillary layer: a layer of capillaries, no pigment.

5. Glass film: 2 ~ 4 microns thick near the optic papilla, thinner towards the periphery, only 1 ~ 2 microns thick. Located between retina and choroid. Under light microscope, it can be divided into two layers: epidermis, pigment epithelium basement membrane and elastic layer.

Electron microscope observation can be divided into five layers: pigment epithelial basement membrane, inner collagen band, elastic layer, outer collagen band and choroidal capillary basement membrane from inside to outside.