air sac

from WordNet (r) 3.0 (2006)

air sac
    n 1: a tiny sac for holding air in the lungs; formed by the
         terminal dilation of tiny air passageways [syn: {alveolus},
         {air sac}, {air cell}]
    2: any of the thin-walled extensions of the tracheae of insects
    3: any of the membranous air-filled extensions of the lungs of
       birds

from The Collaborative International Dictionary of English v.0.48

Air sac \Air" sac`\
   1. (Anat.) One of the spaces in different parts of the bodies
      of birds, which are filled with air and connected with the
      air passages of the lungs; an air cell.
      [1913 Webster]

   2. An {alveolus}.
      [PJC]

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