CHAPTEE II 

 THE TEETH IN GENEEAL 



Definition. The term teeth is applied to all 

 hard, usually calcined substances placed at the 

 orifice of the alimentary canal. They are gen- 

 erally confined to the cavity of the mouth, but are 

 sometimes found in the pharynx and rarely in 

 the oesophagus. The name is, however, confined 

 to those structures located in the oral cavity which 

 contain a calcified tissue known as dentin. 



Origin. The teeth of animals are derived from 

 the layers of the skin and hence are but special- 

 ized dermal structures. In the Invertebrates, 

 they are evolved from the superficial layers of the 

 derm, and are therefore called ecderonic, while 

 the teeth of Vertebrates are derived from the 

 deeper portions, the corium of the integument, 

 and are hence called enderonic. The dentin is 

 derived from the mesoderm, but the enamel is a 

 calcified substance derived from epithelium, which 

 is readily demonstrated by its histological ele- 

 ments. 



Tissues. The dental tissues are three in num- 

 ber, — enamel, dentin and cementum. The cemen- 



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