The collagen serves to anchor the blood vessel to nearby organs, giving it stability.

The tunica externa is made from collagen and elastic fibers in a loose connective tissue.

[1] This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.

[1] A common pathological disorder concerning the tunica externa is scurvy, also known as vitamin C deficiency.

[citation needed] This article incorporates text in the public domain from page 499 of the 20th edition of Gray's Anatomy (1918)