The outer connective tissue covering of an organ, vessel or other structure
(a.k.a. tunica adventitia) is the strong outer covering of arteries and veins. It is composed of connective tissue as well as collagen and elastic fibers. These fibers allow the arteries and veins to stretch to prevent overexpansion due to the pressure that is exerted on the walls by blood flow.
The outer most layer of an arterial wall
The superficial layer of connective tissue surrounding an internal organ; fibers are continuous with those of surrounding tissues, providing support and stabilization.