ADAih<!, I'lrYLOGEXY OF THE JAW MUt<VLEi< 117 



riatc X, Fi.^s. ">. 



The relations of the jaw iniuscles to tlie dentition and form of jaw and 

 skull are nowhere more apparent than in the rodents. 



The rodents have a great development and difl'erentiation of the masse- 

 ters which overshadows the temporals. This is characteristic of the group 

 and the jaw action is very complex. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (.INNERVATED BY V^) 



Masseter Pterygoideus intei-nus 



Temporalis Digastricus (pars anterior) 



Pterygoideus externus 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Digastricus (pars posterior) 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V^) 



Masseter. — This muscle is very large and much subdivided. It arises 

 along the zygomatic arch, extending some distance in front of the eyes 

 where the z3'gomatic process of the mandil)le is much expanded to give it 

 a foundation. 



The posterior part of the expanded mandible forms the base of the 

 insertion. This insertion is very large in comparison with that of most 

 mammals. 



Temporalis. — This muscle arises in the temporal fossa and is inserted 

 on the inner side of the mandible. It is of medium size in Mus, although 

 in some of the rodents it is much reduced. 



Pterygoideus externus. — This muscle arises on the pterygoid fossa and 

 is inserted on the mandible, where it has a good hold on the expanded 

 angle. It holds against the pull of the masseters and p\dls the mandible 

 forward. 



rterygoideus internus. — This muscle is much larger than the external 

 ]>terygoid, arising in the pterygoid fossa and inserted with a large inser- 

 tion on the large expanded angle of the mandible. It holds against the 

 pull of the other temporal muscles and prevents the tearing apart of the 

 jaws where the sym])hysis is weak, as it is in many rodents. 



