The tongue and other structures forming the floor of the mouth have been removed, showing the roof of the mouth. On the right side the masseter and pterygoideus internus have been removed, _a_, _a′_, M. masseter; _b_, M. pterygoideus externus; _c_, M. pterygoideus internus; _d_, _d′_, M. tensor veli palatini; _e_, _e′_, M. levator veli palatini; _f_, M. constrictor pharyngis superior. 1, aponeurosis of the velum palatinum; 2, ridges formed by the palatines and pterygoid portions of the sphenoid; 3, hamular processes; 4, free caudal border of the velum palatinum; 5, bulla tympani.]
_Relations._--Ventral surface with the styloglossus and the soft palate. Dorsal surface with the bulla tympani (5) and base of the skull. Cranial surface with the pterygoideus externus (_b_), with which it is partly united.
_Action._--Assists the pterygoideus externus.
=M. stylohyoideus= (Fig. 65, _d_).--The stylohyoid is a ribbon-like muscle about three millimeters wide. It lies on the superficial surface of the digastric, caudad and ventrad of the angle of the jaw.
_Origin_ by fleshy fibres from the outer surface of the stylohyal bone near its middle. The muscle curves over the outer surface of the digastric (_b_), and ends about one centimeter from the middle line in a slender flat tendon.
_Insertion_ into the middle of the ventral surface of the body of the hyoid bone. Some of the fibres of the mylohyoid (_c_) are inserted into its tendon.
_Relations._--Outer surface with the submaxillary gland (2) and the submentalis muscle. Inner surface with the digastric (_b_), the hyoglossus (Fig. 67, _h_), and the geniohyoid (Fig. 67, _g_).
=M. geniohyoideus= (Fig. 67, _g_).--The geniohyoid is a long slender muscle which lies next to the median line between the symphysis menti and the hyoid bone, closely applied to its fellow of the opposite side and within (dorsad of) the mylohyoid (Fig. 65, _c_).
_Origin_ from the ventral half of the inner surface of the mandibula for about one millimeter from the symphysis.
_Insertion_ by muscle-fibres into the lateral half of the ventral surface of the body of the hyoid bone.
_Relations._--Outer (ventral) surface with the stylohyoid (Fig. 65, _d_) and the mylohyoid (Fig. 65, _c_); medial border with the muscle of the opposite side. Lateral border with the hyoglossus (Fig. 67, _h_) and genioglossus (Fig. 67, _f_). Inner (dorsal) surface with the genioglossus.