ACTION OP THE MUSCLES OF MASTICATION. 297 



backwards and outwards, to be inserted into the internal 

 and anterior part of the neck of the lower jaw and the in- 

 terarticular cartilage. 



Function. When one muscle acts, it turns the jaw to the 

 opposite side; when both act alternately, they give the ro- 

 tatory or grinding motion. If they act together, the jaw is 

 thrown forward. 



4. Pterygoideus internus, (Fig. 84.) 



This muscle is thick and short, and situated behind the 

 ramus and angle of the inferior maxilla. It is inferior to 

 the external pterygoid, and parallel to the superficial layer 

 of the masseter. 



It arises fleshy and tendinous from the inner surface of 

 the external pterygoid plate, and pterygoid process of the 

 palate bone, occupies the greater portion of the pterygoid 

 fossa, and passes downwards and outwards to be inserted 

 tendinous and fleshy on the inner surface of the angle of 

 the jaw. 



Function. It is a rotator of the jaws and thus co-operates 

 with the external pterygoid in triturating the food. 



The two pterygoid muscles arising so near each other 

 and passing in different directions, the one downwards, 

 and the other upwards and outwards, a triangular space is 

 left between them, containing a quantity of fat, the inter- 

 nal maxillary artery and vein, the inferior dental and gus- 

 tatory nerves, and a portion of the parotid gland. The in- 

 ternal maxillary artery occasionally passes between the 

 origins of the external pterygoid muscles, as it is about en- 

 tering the spheno-maxillary fossa. 



COMBINED ACTION OF THE MUSCLES OF MASTICATION. 



The conjoint action of these muscles is the effective 

 agent in mastication. The masseter and temporal are 

 principally employed in raising the lower jaw to the 

 upper,, and thus dividing the food, while the pterygoid 

 in rotating the lower jaw upon the upper, produces the 

 grinding motion, and thus reduces the food to the smallest 

 portions, suitable for deglutition. The anterior layer of the 



