THE HEAD AND NECK. 93 



THE MUSCLES OF MASTICATION. 



TJic muscles of mastication are the temporal, buccinator, the internal and 

 external pterygoids, and the masseter. (Figs. 56 and 57.) These are the power- 

 ful muscles that act on the mandible, by whose action the food is cut by the 

 incisors, torn by the canines, and triturated by the molars. It would seem that 

 the muscles that depress the jaw belong to the same category; hence we will 

 consider them incidentally here, in detail in another section. 



The temporal muscle (Fig. 52) arises from the complete temporal ridge, 

 from the temporal fossa, and from the under surface of the temporal fascia. It 

 is inserted into the coronoid process of the mandible, as far forward as the last 

 molar tooth. The muscle is triangular ; hence, according to rule, it will be found 

 taking its nerve-supply near the apex. It derives its nerve-supply from the 

 third division of the trigeminus. (Fig. 53.) In your dissection you find this 

 muscle related superficially to the arcus zygomaticus, the temporal fascia, the 

 temporal branches of the seventh nerve, the superficial temporal arteries and 

 veins, and the auriculo-temporal branch of the fifth nerve. (Figs. 15-17.) 

 Deeply the muscle is related to the temporal fossa and the external pterygoid 

 muscle. 



The buccinator muscle (Fig. 62) arises from the alveolar processes above 

 the upper and below the lower molar teeth of the upper and lower jaws respec- 

 tively ; and from the pterygo-maxillary ligament. It is inserted into the outer 

 part of the orbicularis oris. Its function is (i) to draw the corners of the mouth 

 outward and backward against the teeth ; (2) to antagonize the tongue by per- 

 mitting no food to be pushed by this organ beyond the dental arches, into the 

 vestibule during mastication. 



The internal pterygoid muscle (Fig. '57) arises from the inner surface of the 

 external pterygoid plate, from the tuberosity of the palate bone. It is inserted 

 into the inner surface of the ramus of the mandible, as high as the inferior dental 

 foramen and mylo-hyoid groove. The muscle is quadrangular ; hence, according 

 to rule, it takes its nerve-supply in the centre. The action of the muscle is to 

 close the mouth and draw the jaw forward and to the mid-line. 



Name the agents thai depress the lower jatv. 



The digastric muscle, the platysma myoides, and gravity. 



/ ? 'liicJi muscles act on the mandible, to protrude the lower ja^iv ? 



The external pterygoid muscle and the superficial part of the masseter 

 muscle. 



Hoiv is the mandible returned to its original position in the glcnoid ? 



It is drawn back by the posterior fibres of the temporal muscle, and by the 

 deep fibres of the masseter muscle. 



\ \ liat are the ifiuscles of trituration ? 



The external pterygoids. They act alternately, drawing the mandible 

 forward and to the opposite side : this movement produces trituration. 



WJiat is the function of the temporal, masseter, and internal pterygoid muscles ? 



They raise the mandible against the upper jaw with great force. 



Has the temporal muscle an independent, specific action ? 



Yes, its specific action is to accentuate incision. The quick snapping move- 

 ment of the jaw is done by this muscle. 



What is meant by superficial and deep part of the masseter muscle / 



Your knowledge of the action of the masseter muscle on the mandible is 

 imperfect without an understanding of these two parts : In reality each masseter 

 is two muscles. Each part of the masseter muscle has a separate origin, separate 

 function, separate insertion. One the superficial is strong, tendinous, and 



