THE MOVEMENTS OF MASTICATION. 271 



vating action, because the other muscles would tend also to draw it 'forward at 

 the same time. (3) When the lower jaw is displaced laterally, the elevating action 

 of the temporal muscles is lost. 



(b) The downward movement of the lower jaw is partly due to its 

 weight and partly to moderate contraction of the anterior bellies of the 

 digastric and by the mylohyoid and geniohyoid muscles. These mus- 

 cles act more powerfully when the mouth is opened widely and forcibly. 

 The fixation of the hyoid bone necessary for this purpose is effected by 

 the omohyoid and sternohyoid, as well as the combined action of 

 the sterno thyroid and thyrohyoid muscles. 



As the articular heads of the bones move forward upon the articular tubercles 

 when the inferior maxilla is greatly depressed, it has been assumed that, in this 

 case, the external pterygoid muscles actively favor this displacement. When the 

 mouth is opened to an especially marked degree, the head is bent backward, 

 and, with the hyoid bone fixed, the posterior bellies of the digastric muscles, as 

 well as the stylohyoid muscles, enter into action. Some animals possess upper 

 jaws capable of movement upward and downward, as, for instance, parrots, croco- 

 diles, snakes, and fish. 



(c) Displacement of one or of both articular heads of the inferior 

 maxillary bone forward or backward, (i) Projection forward of the 

 lower jaw is caused by the action of the external pterygoid muscles. 

 As under such circumstances the articular head of the bone slips upon 

 the articular tubercle, and therefore also moves downward, the surfaces 

 of the lateral teeth must separate from each other in this position. 

 (2) Backward displacement is caused by the action of the internal 

 pterygoid muscles. (3) The articular head on one side is drawn for- 

 ward and then backward again by the external and internal pterygoid 

 muscles of the same side, a transverse movement of the inferior maxilla 

 taking place at the same time. The more the lower jaw is depressed, 

 the more ineffective are these movements. 



In the movements of mastication, with which both elevation and de- 

 pression of the lower jaw, as well as with a transverse grinding movement 

 are often combined, the food to be masticated is kept between the 

 opposing surfaces of the teeth by the muscles of the lips (orbicularis 

 oris) and the buccinators from without and by the action of the tongue 

 from within. The sensibility of the masticatory muscles, together with 

 the sensibility of the teeth and the mucous membrane of the mouth and 

 lips, determines the amount of force to be expended by the muscles of 

 the lower jaw for the purpose of mastication. By reason of simultane- 

 ous insalivation, the divided particles cohere, so that they can be readily 

 formed into an oval bolus on the dorsum of the tongue. 



The muscles of mastication, together with the mylohyoid and the anterior 

 belly of the digastric, receive their motor nerves from the motor portion of the 

 third division of the trigeminal nerve. The hypoglossal nerve innervates the 

 geniohyoid, thyrohyoid, omohyoid, and sternohyoid muscles, as well as the sterno- 

 thyroid. The buccinator, the posterior belly of the digastric, the stylohyoid and 

 the muscles of the face that take part in opening and closing the mouth are sup- 

 plied by the facial nerve. The common nervous center for the movements o 

 mastication lies in the medulla oblongata. 



When the mouth is shut, the permanent position of the jaws in contact with 

 each other is due to atmospheric pressure, as the buccal cavity is made completely 

 free of air, while the entrance of air is prevented anteriorly by the lips and pos- 

 teriorly by the veil of the palate. The pressure of the atmospheric air corresponds 

 to a column of mercury of from 2 to 4 mm. high. 



