composed primarily of blood vessel layer tunica adventitia of blood vessel tunica adventitia vasorum tunica adventitia of vessel The outermost layer of a blood vessel, composed mainly of fibrous tissue. adventitia externa UBERON:0005734 EMAPA:36299 uberon tunica externa vasorum tunica adventitia The outermost layer of a blood vessel, surrounding the tunica media. It is mainly composed of collagen. The collagen serves to anchor the blood vessel to nearby organs, giving it stability[Wikipedia:Tunica_externa_%28vessels%29]. UMLS:C0225342 external coat TODO - note terminological problems. WP says tunica external layer of blood vessel. FMA says tunica externa = adventitia, and is FMA adventitia covers other kinds of vessels. Here we opt for a lengthier name that attempts to disambiguate. There is no precise FMA equivalent, as FMA has tunica externa of arteries, veins, genital ducts etc as siblings adventitia fibrous connective tissue