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.
Unique to the veins are venous valves, which permit unidirectional flow toward the heart. The deep vein wall consists of three layers. The outside layer, called the adventitia, is made of collagen, vasa vasorum and nerve cells. The middle layer, or media, is comprised of smooth muscle.