THE SKIN 



Enamel worn 

 away 



Crown 



Neck 



are so far back in the mouth that they are not much 

 used ; therefore they do not usually remain so long as the 

 others, 



21. Milk Teeth. The teeth just described are those 

 of the permanent set. The first or temporary set of 

 teeth (Fig. 19) consists of teeth the same in name and 

 number as those of the permanent set except that the 

 three large molars in each half jaw, 



twelve in all, are lacking; so there 

 are twenty teeth in the temporary 

 set (Fig. 19). The milk teeth come 

 during the first two years of life, and 

 begin during the sixth or seventh 

 year to be pushed out and replaced 

 by the permanent teeth. They are 

 all gone by the twelfth year. The 

 milk teeth have much smaller roots 

 than the permanent teeth. Since 

 a tooth does not enlarge after it 

 develops, the two sets of teeth are 

 a beautiful provision for preserving 

 the regularity of the teeth as the 

 jaws increase in size. If a milk 

 tooth remains so long as to cause 

 the tooth of the second set to grow 



r i .-i r- ,1 i_ 11 FIG. 20. Vertical Section of a 



OUt Of place, the first tOOth Should Bicuspid Tooth, magnified. 



be removed. 



22. The main part of the body of a tooth is dentine (Fig. 

 20). The dentine of the teeth of elephants and other large 

 animals is the ivory of commerce. It is commonly used 

 on piano keys, fans, etc. In the central part of the den- 

 tine is a space called the pulp cavity. The soft substance 

 called the pulp contains the nerves and blood vessels which 

 enter at the tip of the root. In the socket the dentine 

 is covered by cement; above the socket it is covered by 



c 



Fang 



