VEIN, ILLUSTRATION. Anatomy of a medium caliber vein 4 layers Representation of a vein in cutawayview showing its different structures. From the outside toward the inside, the tunica externa in blue or adventitia made of collagen fibers, the tunica media or media made of muscular fibers in pink and the tunica intima or intima constituted of a subendothelial layer or basal lamina in orange and the vascular endothelium purple. The venous valves prevent the blood to reflux backwards.