Either of two small elevations on the inner surface of each side of the symphysis (abnormal attachment) of the lower jaw of which the upper one on each side provides attachment for the genioglossus (tongue muscle) and the lower for the geniohyoid muscle which consists of the chin and hyoid (U-shaped bone at the root of the tongue): The medical instructor, Dr. Reynolds, explained to his students the anatomical structures of the mental spines in the mouths of humans.