io8 THE MUSCLES. 



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 (/;), the submaxillary gland (2), the stylohyoid 

 (^), 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 ^6, 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. 



Itisertion. — 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 inter- 

 section 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. 



The deep layer takes 



Origin by a strong tendon from the ventral border of the 

 zygoma just craniad of the mandibular fossa. The fibres pass 

 cranioventrad, diverge and have their 



