CHAPTER XII 



THE TEETH 

 AND THEIR CARE 



Continuous growth of teeth. 



Fangs of snake inject venom. 



What are some causes of bad teeth ? What is meant by the hygiene 

 of the teeth ? Why are the teeth of the average American probably 

 inferior to those of his great-grandparent? 



Detailed structure of teeth. Not only are teeth important in 

 helping to prepare food for use in the body, but they are invaluable 

 in maintaining the health of the body. The teeth, embedded in 

 sockets in the upper and lower jawbones, are especially fitted for 

 the work that they have to do. Each tooth has a root which con- 

 sists of one or more divisions contained in the socket; a crown 

 which projects above the gums ; and a neck which is the narrow 

 portion at the edge of the gum. 



Each tooth is composed principally of dentine. In the center 

 of the dentine is the pulp cavity which tapers into the root canal 

 and ends in a small opening at the extremity of the root. The 

 pulp cavity is filled with loose connective tissue containing blood 

 vessels and nerves which enter through the root canal. The 

 dentine is crowned with a hard layer of enamel composed mostly 

 of calcium. The dentine of the root is covered with cement which 

 has practically the same composition and structure as bone, but 

 is much harder. Every normal person has two sets of teeth dur- 

 ing his life, the first or temporary, and the permanent. There are 

 twenty temporary teeth, ten in each jaw : four sharp-edged in- 



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