mesen

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]