(_a_) _Muscles of Mastication._
=M. digastricus= (Fig. 65, _b_).--The digastric is a thick prismatic
muscle lying mediad of the angle of the lower jaw, connecting it with
the base of the skull.
_Origin_ by fleshy fibres from the outer surface of the jugular process
of the occipital bone, and by a thin tendon from the tip of the mastoid
process and from the ridge between the mastoid and the jugular
processes. The muscle passes craniad, becoming at the same time broader
and more voluminous.
_Insertion._--The ventral border of the mandible craniad of the caudal
border of the molar tooth. The insertion area extends slightly onto both
surfaces of the bone and is triangular, with its apex craniad.
_Relations._--Outer (ventral) surface with the following, beginning at
the caudal end: the rectus capitis lateralis, the cleidomastoid (_h_),
the submaxillary gland (2), the stylohyoid (_d_), a large lymph-gland
(3), and the integument. Inner (dorsal) surface with the jugulohyoid,
the tympanic bulla, the masseter (_a_), the hyoglossus, and the
mylohyoid (_c_).
_Action._--Depressor of the lower jaw.
=M. masseter= (Figs. 65 and 66, _a_).--The masseter forms the projecting
mass so prominent in the cat behind and below the eye. Its outer surface
is covered by a strong aponeurosis. The muscle is divided into three
layers which are distinct as to origin, insertion, and direction of
fibres, but which are otherwise not distinct.