Gnathic adj. (Anat.) Of or pertaining to the jaw. 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. "Skulls with the gnathic index below 98 are orthognathous, from 98 to 103 mesognathous, and above 103 are prognathous."
