MASTICATION. 223 



the incisor teeth, and is afterwards brought under the action of the molar teeth by 

 means of the lips, cheeks, and tongue. 



153. MASTICATION. The articulation of the jaw is provided with an interarticular cartilage 

 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, whereby the meniscus moves with it, 

 and covers the articular surface. 



The process of mastication embraces : (a) The elevation of the jaw, accom- 

 plished 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. The muscles act especially 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 {B6rard). 



(c) Displacement of the Articular Surfaces. During rest, when the mouth is 

 closed, the incisor teeth of the lower jaw are 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 Exter- 

 nal 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, the individual movements of the lower jaw are variously com- 

 bined, and especially with lateral grinding movements, while 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 mastication is regulated by the 

 sensibility of the teeth, and the muscular sensibility of the muscles of mastication, 

 as well as by the general sensibility of the mucous membrane of the mouth and 

 lips. At the same time, the mass is mixed with saliva, the divided particles cohere, 

 and are formed into a mass or "bolus, of a long, oval shape, by the muscles of the 

 tongue. The bolus then rests on the back of the tongue, ready to be swallowed. 



Nerves of Mastication. The muscles of mastication receive their motor nerves from the third 

 branch of the trigeminus, the mylo-hyoid and the anterior belly of the digastric being supplied 

 from the same source. The genio-, omo-, and sterno-hyoid, sterno-thyroid, and thyro-hyoid 

 are supplied by the hypoglossal, while the facial supplies the posterior belly of the digastric, the 

 stylo-hyoid, the platysma, the buccinator, and the muscles of the lips. The general centre for 

 the muscles of mastication lies in the. medulla oblongata ( 367). 



When the mouth is closed, the jaws are kept in contact by the pressure of the air, as the 

 cavity of the mouth is rendered free from air, and the entrance of air is prevented anteriorly 

 by the lips, and posteriorly by the soft palate. The pressure exerted by the air is from 2 to 4 

 mm. Hg. (Metzger and Bonders). 



[Effect on the Circulation. Marey found that mastication trebled the velocity of the blood- 

 current in the carotid (horse), while Francois Frank observed that the circulation of the brain 

 (in man) is increased ; hence it is evident that mastication implies an increased supply of blood 

 to the nerve-centres.] 



154. STRUCTURE AND DEVELOPMENT OF THE TEETH. A tooth is just a papilla of the 

 mucous membrane of the gum, which has undergone a characteristic development. In its simplest 

 form, as in the teeth of the lamprey, the connective-tissue basis of the papilla is covered with 

 many layers of corneous epithelium. In human teeth, part of the papilla is transformed into a 

 layer of calcified dentine, while the epithelium of the papilla produces the enamel, the fang of 

 the tooth being covered by a thin accessory layer of bone, the crusta petrosa or cement. 



