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definitions

tunica externa

[ĭk-stûr′nə]
n.
  1. The outer of two or more enveloping layers of any structure, especially the fibroelastic coat of a blood or lymph vessel.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.