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1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   Mesencephalon \Mes`en*ceph"a*lon\, n. [NL. See Meso- and
      Encephalon.] (Anat.)
      The middle segment of the brain; the midbrain. Sometimes
      abbreviated to mesen. See Brain.
      [1913 Webster]