38 ANATOMY AND PHYSIOLOGY 



The permanent teeth are thirty-two in number. At the age 

 of six years the first permanent molar ("six-year molar") should 

 appear; the others follow in order somewhat like the following: 



Four first molars, i right, i left, in each jaw. ... at 6 years. 



Eight incisors, 2 central, 2 lateral, in each jaw . . at 7 to 8 years. 



Eight bicuspids, 2 right, 2 left, in each jaw at 8 to 10 years. 



Four canines, i right, i left, in each jaw at 12 to 14 years. 



Four second molars, i right, i left, in each jaw. . at 12 to 15 years. 



Four third molars, i right, i left, in each jaw... . at 17 to 25 years. 

 (The third molars are called "wisdom teeth.") 



Thirty-two teeth in the permanent set at twenty-five years of age. 



Clinical notes. Caries, or decay of teeth, is due to bacterial 

 action. This is favored by the accumulation of particles of food, 

 the warmth and moisture of the mouth furnishing perfect conditions 

 for the development of bacteria. Careful cleansing with brush or 

 dental floss, or both, will prevent this and thus aid in preserving 

 the teeth. Care is important in the use of brush or floss or 

 toothpick, not only that the removal of injurious particles may be 

 well done but in order to avoid wounding the mucous membrane 

 which covers the gums, thus exposing them to bacterial invasion. 



Recession of the Gums. Any irritation (as by bacteria) of the 

 gums may be followed by their recession, which exposes the dentine 

 where it is not protected by enamel. 



Sudden changes of temperature, as from hot to cold liquids, is 

 injurious to the enamel. Acids, as ordinarily taken in food, have 

 no special action upon the teeth, but sweets may do harm by 

 their fermentation in a mouth where teeth are not kept clean. 



The sockets of the teeth are lined wi'th periosteum (dental 

 periosteum). It is reflected at the bottom of the socket to the root 

 of the tooth and covers the cement; this portion is called the 

 peri-cemental membrane (or periosteum). 



Bacterial invasion of the gums may extend beneath the periosteum, caus- 

 ing a chronic inflammation with suppuration, called pyorrhea alveolaris. 

 Often the condition is painful, mastication is difficult and the teeth loosen and 

 almost fall out of themselves. 



