34 A MANUAL OF DENTAL ANATOMY. 



the temporals, attached to the outer sides of the jaw ; and 

 the external and internal pterygoids, attached to its inner 

 sides. 



The masseter, temporal, and internal pterygoid muscles 

 close the jaws and press the teeth against one another, and 



FIG. 15 

 XT" 



! i!f'"////'/ 



this is their principal action. They are antagonised by the 

 digastric, the mylohyoid, and the geniohyoid muscles, which, 

 aided perhaps by the platysma, depress the lower jaw when 

 the hyoid bone is fixed by its own depressor muscles. 



The external pterygoid draws the jaw forward, and so in 

 some measure tends to open it ; as the two muscles do not 

 always, or indeed generally, act together, they give a lateral 

 movement to the jaw. The superficial portions of the 

 masseter and the internal pterygoid are ordinarily supposed, 

 as their direction is slightly backwards, to assist in drawing 

 the jaw forwards, but Langer, one of the most recent inves- 

 tigators of their action, attaches very little importance to 

 this, and indeed considers that, when the jaw has been 

 pulled forwards by the external pterygoid, the combined 

 action of the internal pterygoid, the temporal, and the mas- 

 seter, may bring it back again. 



( l ) Pterygoid muscles. 1. Upper, and 2. Lower heads of external 

 pterygoid muscle. 3. Internal pterygoid muscle. 



