88 



HUMAN BIOLOGY 



placed by the growth of the permanent set. -There are but twenty 

 teeth in the milk set and their arrangement is as follows : 



namel 



entine 



u , 



Bicuspids are, therefore, wanting, and the milk molars occupy the 

 position in each half jaw that later is filled by the two bicuspids 



of the permanent set. The teeth 

 appear gradually, the lower inci- 

 sors usually being the first to push 

 through the gums at about the sixth 

 month. The third permanent mo- 

 lars of each half jaw often appear 

 "cavity as } a te as the twentieth year ; they 

 are called the wisdom teeth. 



gum 



118. Structure of teeth. The 

 exposed portion of a tooth is 

 called the crown (Fig. 30). It is 

 covered with a layer of enamel, 

 which is the hardest tissue in the 

 body. The root of the tooth is 

 imbedded in a socket in the bone 

 of the jaw. It has no enamel, 

 but, instead, its outer layer is a 

 modified bone tissue called ce- 

 ment. The incisors and canines 

 usually have but a single root, 

 the bicuspids may have two, and the molars are often held in 

 the jawbone by three, four, or five roots. In the region be- 



neck 



root 



FIG. 30. Longitudinal section of 

 a canine tooth. 



