• WordNet 3.6
    • n adventitia an enveloping or covering membrane or layer of body tissue
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Century Dictionary and Cyclopedia
    • n adventitia In anatomy, any membranous structure covering an organ but not properly belonging to it (in full, membrana adventitia, adventitious membrane); specifically, the outermost of the three coats of a blood-vessel (in full, tunica adventitia, adventitious tunic), consisting of connective tissue.
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In literature:

The veins differ from arteries in possessing thinner walls, less elastic and muscular tissue, and for the most part a stronger tunica adventitia.
"Special Report on Diseases of the Horse" by United States Department of Agriculture
The adventitia or outer coat is exceedingly tough.
"Arteriosclerosis and Hypertension:" by Louis Marshall Warfield