The Collaborative International Dictionary of English v.0.48:

Gnathic \Gnath"ic\, a. [Gr. gna`qos the jaw.] (Anat.)
   Of or pertaining to the jaw.
   [1913 Webster]

   Gnathic index, in a skull, the ratio of the distance from
      the middle of the nasofrontal suture to the basion (taken
      equal to 100), to the distance from the basion to the
      middle of the front edge of the upper jaw; -- called also
      alveolar index.
      [1913 Webster]

            Skulls with the gnathic index below 98 are
            orthognathous, from 98 to 103 mesognathous, and
            above 103 are prognathous.            --Flower.
      [1913 Webster]