\Air" blad`der\
1. (Anat.) An air sac, sometimes double or variously lobed,
   in the visceral cavity of many fishes. It originates in
   the same way as the lungs of air-breathing vertebrates,
   and in the adult may retain a tubular connection with the
   pharynx or esophagus.
2. A sac or bladder full of air in an animal or plant; also
   an air hole in a casting.