32 



A COURSE ON ZOOLOGY. 



FIG. 17. 



germs of the teeth of both dentitions already formed, 

 that is, with fifty-two dental embryos enclosed in the 

 jaws. When the second teeth fall they are not replaced. 

 There are then three kinds of teeth : incisors, canines, 

 and molars, and they are symmetrically arranged. An 

 adult man has on each side of the 

 face, and in each jaw, two incisors, 

 one canine, five molars, which are 

 classified as three true molars and 

 two premolars or bicuspids. 



The incisors occupy the front of 

 the mouth ; next to them are the 

 canines, and next to these the mo- 

 lars. The last molars do not often 

 appear before adolescence; some- 

 times they are much later, and more 

 rarely they are entirely wanting. 

 They are commonly called wisdom 

 teeth, 



The teeth are formed of a pecu- 

 liar substance called dentine or ivory, 

 and that portion of the dentine 

 which projects beyond the gums, and 

 which is called the crown, is covered 

 with a thin layer of a hard sub- 

 stance, called enamel. The root of 

 the tooth that is, the portion which 

 is embedded in the cavities or alveoli of the jaws is cov- 

 ered with a thin coating of osseous tissue, called cement. 

 The roots are single in the incisors, canines, and pre- 

 molars ; double and often triple in the true molars. 



The teeth, like all other organs, are provided with 

 nerves and nourishing vessels. These nerves and vessels 



VERTICAL SECTION OF A 

 MOLAR TOOTH, moder- 

 ately magnified. 1 , 

 enamel, the lines of 

 which indicate the ar- 

 rangement ; 2, dentine 

 or ivory ; 3, pulp, con- 

 taining nerves and 

 nourishing vessels ; 4, 

 root, covered with ce- 

 ment. 



