the disk in the fundus of the eye marking the point at which the optic nerve enters; it is accompanied by blood vessels, is oval, light in color and the blind spot of the retina.
ARTERIOLES. In arterioles, the tunica intima consists of a continuous endothelium and a very thin subendothelial layer. In addition, a thin, fenestrated internal elastic lamina is present in larger arterioles but absent in terminal arterioles. Generally, the tunica media consists of 2 layers of smooth muscle cells.
Veins of small and medium size are characterized by a thin media containing only a few layers of smooth muscle cells. These vessels have a much thicker adventitia composed of collagen and occasionally some longitudinal smooth muscle fibers. In general, veins are larger in diameter and have thinner walls than arteries.