THE TEETH 



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the second or ^' permanent^' set there are thirty-two teeth. These 

 teeth are divided, according to their structure, into four groups. 

 In the center of each jaw in front are four teeth with chisel-hke 

 edges (eight in all) ; these are the inci'sors, or cutting teeth. Next 

 to them on each side is a single tooth (four in all) ; these have 

 rather sharp points and are called the canines. Then come two 

 teeth on each side, eight in all, called premolars. Lastly, at the 

 back are the flat-top molars, or grinding teeth, of which there are 

 six in each jaw. Food is 

 caught between irregular 

 projections on the surface 

 of the molars and crushed 

 to a pulpy mass. 



Each tooth, as the figure 

 shows, is composed chiefly 

 of hard bone or dentine. 

 The crown of the tooth is 

 covered with enamel, the 

 hardest substance in the 

 body. In the interior is 

 a pulp cavity, which dur- 

 ing the life of the tooth 

 contains blood vessels and 

 nerves, which give the 

 tooth its nourishment. 

 The tooth is held in its 

 bony socket in the jaw by 

 cement. 



When a tooth dies, bacteria often set up an irritation at its base 

 and form a center of focal injection from which a stream of poison 

 gets into the blood. As a result of this infection, very serious 

 diseases may occur, of which the most common are rheumatism 

 of the joints and neuritis or inflammation of the nerves. Infected 

 teeth should be extracted, as this removes the cause of the trouble. 



Care of the Teeth. — Too much emphasis cannot be placed on 

 the proper care of the teeth, for many ills beside those already 

 mentioned may be laid to neglect. The teeth should be care- 

 fully brushed each morning and before you go to bed. Use a 



Teeth of the upper jaw, from below. 

 1, 2, incisors; 3, canine; 4. 5, premolars; 

 6, 7, S, molars. II. longitudinal section of a 

 tooth. 



