The Collaborative International Dictionary of English v.0.48:

Pons \Pons\, n.; pl. Pontes. [L., a bridge.] (Anat.)
   A bridge; -- applied to several parts which connect others,
   but especially to the pons Varolii, a prominent band of
   nervous tissue situated on the ventral side of the medulla
   oblongata and connected at each side with the hemispheres of
   the cerebellum; the mesocephalon. See Brain.
   [1913 Webster]

   Pons asinorum. [L., literally, bridge of asses.] See
      Asses' bridge, under Ass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pons asinorum
    n 1: a problem that severely tests the ability of an
         inexperienced person