ANIMAL DENTISTRY. 



131 



The pterygoid internus muscle occupies the same relative 

 position on the internal aspect of the ramus as the masseter 

 does externally. Its fixed attachment is the palatine crest 

 and the movable insertion the internal face of the lower 

 jaw. Its function is that of producing lateral motion and as- 

 sisting the masseter to close the jaws, as follows: The left 

 muscle acting alone draws the jaw to the right and the right 

 one to the left, and when acting together they assist the 

 masseter in closing the mouth. 



The temporalis muscle occupies the space over the parie- 

 tal bones, and is inserted to the coronoid process and anterior 

 border of the ramus of the jaw. Its function is that of clos- 

 ing the jaw, and when acting singly it produces lateral mo- 

 tion. 



The pterygoid extemus is a short, thick muscle situated 

 internally and in front of the temporo-maxillary articulation. 

 It originates on the sphenoid bone and is inserted at the neck 

 of the jaw just below the condyle. Acting together they 

 pull the jaw forward and singly they produce a lateral mo- 

 tion. 



The stylo-maxillaris originates at the styloid process of 

 the occipital and is inserted to the posterior border of the 

 ramus of the jaw. Its function is that of drawing the jaw 

 backward and depressing it. 



The digastricus muscle is located in the maxillary space, 

 originating on the styloid process with the stylo-maxillaris 

 and is inserted along the inner border of the ramus toward 

 the symphysis. Its function is that of retracting and de- 

 pressing the jaw. 



The stemo-maxillaris muscle originates on the cariniform 

 cartilage of the sternum and is inserted at the angle of the 

 jaw. Its function is that of depressing the jaw. 



The lower jaw^ is moved in every direction by the muscles 

 of mastication; it is advanced, retracted, elevated, depressed 



