The Collaborative International Dictionary of English v.0.48:

Isthmus \Isth"mus\ (?; 277), n.; pl. Isthmuses. [L. isthmus,
   Gr. 'isqmo`s a neck, a neck of land between two seas, an
   isthmus, especially the Isthmus of Corinth; prob. from the
   root of 'ie`nai to go; cf. Icel. ei[eth] isthmus. See
   Issue.] (Geog.)
   A neck or narrow slip of land by which two continents are
   connected, or by which a peninsula is united to the mainland;
   as, the Isthmus of Panama; the Isthmus of Suez, etc.
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   Isthmus of the fauces. (Anat.) See Fauces.
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The Collaborative International Dictionary of English v.0.48:

Fauces \Fau"ces\, n. pl. [L.]
   1. (Anat.) The narrow passage from the mouth to the pharynx,
      situated between the soft palate and the base of the
      tongue; -- called also the isthmus of the fauces. On
      either side of the passage two membranous folds, called
      the pillars of the fauces, inclose the tonsils.
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   2. (Bot.) The throat of a calyx, corolla, etc.
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   3. (Zool.) That portion of the interior of a spiral shell
      which can be seen by looking into the aperture.
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