Blood Vessel Wall

external elastic membrane (also called the external elastic lamina)

Separating the tunica media from the outer tunica externa in larger arteries

tunica externa

The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.

tunica intima

The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium that is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation.This is the layer on the inner most part of a vessel

internal elastic membrane (also called the internal elastic lamina)

the boundary between the tunic intima and the the tunica media. The internal elastic membrane is not apparent in veins

nervi vasorum

nerves of the vessel, that run within the walls of blood vessels- they regulate vasodilation and vasoconstriction- These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction

tunica media

the substantial middle layer of the vessel wall. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers.