30 ANATOMY OF THE WOOD RAT 



M. pterygoideus externus (figs. 23, 24) has origin 

 from the ectal surface of the parapterygoid plate, with 

 insertion upon the medial face of the mandibular condyle. 



M. pterygoideus internus (figs. 8, 23, 24) originates 

 from the pterygoid fossa, and insertion is onto practically 

 the whole of the medial face of the angular process of the 

 mandible. 



The innervation of these four muscles of mastication is 

 by the N. trigeminus. 



7/7. Interramal musculature 

 Of these there are four, as follows : 



Mm. digastricus transmandibularis 



stylohyoideus mylohyoideus 



M. digastricus (figs. 7, 23 ^14, 25) has origin from the 

 paroccipital process. Passing craniad it becomes largely 

 but not entirely tendinous at a point laterad to the hyoid. 

 There is here tendinous attachment to the raphe of the 

 mylohyoid. Craniad to this point the muscle once more 

 broadens, becoming entirely fleshy, and is inserted upon 

 the ventral border of the mandible caudad to the symphysis 

 menti. There is little difference in size between the pos- 

 terior and anterior bellies. 



In Neotoma the anterior belly is considerably broader 

 than the posterior and the tendinous constriction between 

 them is more sharply defined and more tendinous. In 

 Teonoma the anterior belly is much larger than the posterior, 

 and larger than in Homodontomys, completely hiding the 

 transversus mandibularis. The tendinous constriction be- 

 tween the bellies is larger and more muscular. 



As previously mentioned, there would be some grounds 

 for placing this muscle with the masticatory group, but 

 its innervation is different. The anterior belly is served 

 by a branch of the inferior alveolar nerve, and the posterior 

 by the N. faciahs. 



