_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. The =superficial layer= takes _Origin_ by the superficial aponeurosis from the ridge which runs lengthwise of the outer surface of the malar bone, and by fleshy fibres from the surface of the malar bone ventrad of this ridge. The fibres pass obliquely caudad to their insertion. _Insertion._--The caudal half of the lateral margin of the ventral border of the mandible, by a sheet of fascia which lies on the inner surface of the muscle. The superficial fibres curve ventrad of the mandible and are inserted into a tendinous intersection between this muscle and the internal pterygoid (Fig. 66, _c_). The =middle layer= takes _Origin_ from the ventral border of the malar bone, from opposite the molar tooth to within three millimeters of the mandibular fossa. The fibres pass ventrad and have their _Insertion_ into the mandible at the ventral border of the external coronoid fossa by means of the aponeurosis which covers the inner surface of the muscle. [Illustration: FIG. 65.--MUSCLES ON THE VENTRAL SURFACE OF THE THORAX, NECK AND HEAD.