The Collaborative International Dictionary of English v.0.48:

Mesial \Me"sial\ (?; 277), a. [Gr. me`sos middle.] (Anat.)
   Middle; median; in, or in the region of, the mesial plane;
   internal; -- opposed to lateral.
   [1913 Webster]

   Mesial plane. (Anat.) See Meson.
      [1913 Webster]