adventitia

ad·ven·ti·tia   (ād'věn-tĭsh'ə, -vən-)    n.  The membranous outer covering of an organ or a blood vessel. [New Latin, from Latin adventīcius, foreign; see adventitious.] ad'ven·ti'tial adj.

Main Entry: ad·ven·ti·tia
Pronunciation: "ad-v&n-'tish-(E-)&, -(")ven-
Function: noun
: the outer layer that makes up a tubularorgan or structure and especially a blood vessel, is composed of collagenous and elastic fibers, and is not covered with peritoneum called also tunica adventitia, tunica externa

adventitia ad·ven·ti·tia (ād'věn-tĭsh'ə, -vən-)
n.
The outermost membranous covering of an organ or structure, especially the outer coat of an artery.