THE MOVEMENTS OF MASTICATION. 299 



masses, a part is bitten off with the incisor teeth, and is afterwards 

 brought under the action of the molar teeth by means of the lips, 

 cheeks, and tongue. 



153. The Movements of Mastication. 



The articulation of the jaw is provided with an interarticular cartilage (Vidius, 

 1567) the meniscus which prevents direct pressure being made upon the 

 articular surface when the jaws are energetically closed, and which also divides 

 the joint into two cavities, one lying over the other. The capsule is so lax that, 

 in addition to the raising and depressing of the lower jaw, it permits of the lower 

 jaw being displaced forwards upon the articular tubercle, whereby the meniscus 

 moves with it, and covers the articular surface. 



The process of mastication consists of the following movements : 

 (a.) The elevation of the ja,w is accomplished by the combined action 

 of the Temporal, Masseter, and Internal Pterygoid Muscles. If the 

 lower jaw was previously so far depressed that its articular surface 

 rested upon the tubercle, it now passes backwards upon the articular 

 surface. 



(b.) The depression of the lower jaw is caused by its own weight, 

 aided by the action of the anterior bellies of the Digastrics, the Mylo- 

 and Genio-hyoid and Platysma (Haller). The muscles act during 

 forcible opening of the mouth. The necessary fixation of the hyoid 

 bone is obtained through the action of the Omo- and Sterno-hyoid, 

 and by the Sterno-thyroid and Thyro-hyoid. 



When the articular surface of the lower jaw passes forwards on to the tubercle, 

 the External Pterygoids actively aid in producing this (Be>ard). 



(c.) Displacement of both or one articular surface forwards or backwards. 

 During rest, when the mouth is closed, the incisor teeth of the lower 

 jaw fall within the arch of the upper incisors. When in this position, 

 the jaw is protruded by the External Pterygoids, whereby the articular 

 surface passes on to the tubercle (and, therefore, downwards), while the 

 lateral teeth are thereby separated from each other. The jaw is 

 retracted by the Internal Pterygoids without any aid from the 

 posterior fibres of the Temporals. When one articular surface is 

 carried forwards, the jaw is protruded and retracted by the External 

 and Internal Pterygoid of the same side. At the same time, there is 

 a transverse movement, whereby the back teeth of the protruded side 

 are separated from each other. 



During mastication, when the individual movements of the lower 

 jaw are variously combined, the food to be masticated is kept from 

 passing outwards by the action of the muscles of the lips (Orbicularis 

 oris) and the Buccinators, while the tongue continually pushes the 

 particles between the molar teeth. The energy of the muscles of 



