192 DIGESTION MASTICATION, INSALIVATION, DEGLUTITION. 



third molars, between the seventeenth and the twenty-first years, and some- 

 times even much later. In some instances the third molars are never de- 

 veloped. 



The upper jaw has ordinarily a somewhat longer and broader arch than 

 the lower; so that when the mouth is closed the teeth are not brought 

 into exact apposition, but the upper teeth overlap the lower teeth both in 

 front and laterally. The lower teeth are all somewhat smaller than the cor- 

 responding teeth in the upper jaw and generally make their appearance a 

 little earlier. 



The physiological anatomy of the maxillary bones and of the temporo- 

 maxillary articulation necessarily precedes the study of the muscles of masti- 

 cation and the mechanism of their action. 



The superior maxillary bones are immovably articulated with the other 

 bones of the head, and do not usually take any active part in mastication. 

 Their inferior borders, with the upper teeth embedded in the alveolar cavi- 

 ties, present fixed surfaces against which the food is pressed by the action of 

 the muscles which move the lower jaw. 



The inferior maxilla is a single bone. Its body is horizontal, of a horse- 

 shoe shape, and in the alveolar cavities in its superior border, are the lower 

 teeth. Below the teeth, both externally and internally, are surfaces for the 



atachments of the muscles concerned in 

 the various movements of the jaw and for 

 one of the muscles of the tongue. 



Temporo- Maxillary A rticii lation. In 

 man the articulation of the lower jaw with 

 the temporal bone is such as to allow an 

 antero-posterior sliding movement and a 

 lateral movement, in addition to the move- 

 ments of elevation and depression. The 

 condyloid process is convex, with an ovoid 

 surface, the general direction of its long 

 aT being transverse, and slightly ob- 



fossa; 5, mental foramen: 6. attachment liq ue f rom without inward and from before 

 of the digastric muscle : 7, depression at 



the site of the facial artery : 8, angle ; 9, backward. This process is received into a 



attachment of the superior constrictor r 



of the pharynx ; 10, coronoid process ; cavity of corresponding shape in the tem- 



11, condyle; 12, sigmoid notch : 13, open- ij . 



ing of the inferior dental canal ; 14, poral bone, the glenoid fossa, Wllicll IS 



groove for the mylo-hyoid muscle ; 15, ;* i i 



alveolar border ; i, incisor teeth : c, ca- bounded anteriorlv by a rounded eminence, 



nine teeth ; b, bicuspid teeth ; m, molars. n -, ,, ",-, i 



called the emmentia articulans. 



Between the condyle of the lower jaw and the glenoid fossa, is an oblong, 

 interarticular disk of fibro-cartilage. This disk is thicker at the edges than 

 in the centre. It is pliable and is so situated that when the lower jaw is pro- 

 jected forward, making the lower teeth project beyond the upper, it is ap- 

 plied to the convex surface of the eminentia articularis and presents a con- 

 cave surface for articulation with the condyle. One of the uses of this 

 cartilage is to constantly present a proper articulating surface upon the 

 articular eminence and thus permit the antero-posterior sliding movement 



