STRUCTURE OF THE TEETH. 



113 



Mastication. In man, the introduction of food into the mouth 

 is generally accomplished by artificial means, so that the biting 

 teeth (incisors) and the tearing teeth (canines) are comparatively 

 little used for obtaining a suitable morsel of food (Fig. 46). In 

 the mouth the all-important act of chewing or mastication is 

 accomplished by means of the motion of the lower jaw, the 

 tongue and the cheeks. This process of breaking up the solid 

 parts of the food ought to be continued until all hard substances 

 are ground into a soft pulp. 



Structure of the Teeth. The exposed part of the teeth is cov- 

 ered by a dense substance of flinty hardness called enamel, which 



FIG. 46. 



FIG. 47. 



Fio. 46. Vertical section of the Canine Tooth of a man. (o) Enamel ; (6) Dentine ; (c) 

 Pulp eavity ; (d) Crusta petrosa. (Cadiat.) 



FIG. 47. Structural elements of the Enamel of Tooth. A. Prisms cut across showing 

 the hexagonal section. B. Isolated prisms. (KOlliker ) 



is developed from the epithelium, and consists of hexagonal 

 prisms set on end, which are really modified epithelial cells but 

 only contain about two per cent, of animal matter (Fig. 47). The 

 bulk of the tooth is made up of dentine, a substance like bone in 

 composition, pierced by numerous fine canals dentine tubes 

 which radiate toward the surface, from the pulp cavity, in the 

 centre of the tooth. Processes of protoplasm run in the den- 

 tine tubes from the tooth cells, which line the pulp cavity and 

 preside over the nutrition of the tooth. The cavity contains 

 vessels, nerves, etc., which enter at the root of the tooth, which 

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