Eustachian \Eu*sta"chi*an\, a. [From Eustachi, a learned Italian physician who died in Rome, 1574.]

(Anat.)
   (a) Discovered by Eustachius.
   (b) Pertaining to the Eustachian tube; as, Eustachian catheter.

Eustachian catheter, a tubular instrument to be introduced into the Eustachian tube so as to allow of inflation of the middle ear through the nose or mouth.

Eustrachian tube (Anat.), a passage from the tympanum of the ear to the pharynx. See Ear.

Eustachian valve (Anat.), a crescent-shaped fold of the lining membrane of the heart at the entrance of the vena cava inferior. It directs the blood towards the left auricle in the fetus, but is rudimentary and functionless in the adult.

Source: Webster's Revised Unabridged Dictionary (1913)