The tunica externa, also known as the tunica adventitia (or adventitia), is the outermost layer of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and is supported by external elastic lamina. The collagen serves to anchor the blood vessel to nearby organs, giving it stability.
A common pathological disorder concerning the tunica externa is scurvy, also known as vitamin C deficiency. Scurvy occurs because vitamin C is essential for the synthesis of collagen, and without it, the faulty collagen cannot maintain the vein walls and rupture, leading to a multitude of problems.
Additional images 
Anatomy of the arterial wall
Microphotography of arterial wall with calcified (violet colour) atherosclerotic plaque (haematoxillin & eosin stain)
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This article incorporates text from a public domain edition of Gray's Anatomy.