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The outermost connective tissue covering of any organ, vessel, or other structure (or part thereof) not covered by a serous coat (visceral peritoneum); instead, the covering is properly derived from without (that is, from the surrounding connective tissue) and does not form an integral part of such organ or structure. Terminologia Anatomica lists adventitia (tunica adventitia) of the following organs: blood vessels, ductus deferens, esophagus, renal pelvis, seminal glands, and ureters.
[L. adventicius, coming from abroad, foreign, fr. ad, to + venio, to come]
membrana adventitia(1) Tunica externa (see there); tunica adventitia [NA6].
(2) Capsular decidua; decidua capsularis [[NE3]].