144 



CHORDATE ANATOMY 



Membrane bone is formed around the root of the teeth to form the alveoli 

 of the jaw-bone and to hold the teeth firmly in place. 



The mechanics of the eruption of teeth is a problem which needs 

 further elucidation. Among the factors which operate is the elongation 

 of the root, although teeth erupt before the root has completed its growth. 

 The eruption of the deciduous teeth begins during the seventh month 

 after birth, and is usually completed by the end of the second year. Of 



PERMANENT- 

 INCISORS 



DEC IDUOUS — c — r^3 

 INCISORS. \ \V 



SECOND 

 PERMANENT- 

 MOLAR. 



PERMANENT- 

 PREMOLARS 



PERMANENT- 

 CANINE 



PERNMNENT 

 MOLAR. 



'^^^f^^y^ PERMANENT 

 INCISORS- 



FiG. 132. — The teeth of a five-year-old child. Portions of the jaws have been 

 removed so as to expose the roots of the milk teeth and the anlagen of the permanent 

 teeth. The latter are stippled in the figure. (Redrawn after Sobotta.) 



the permanent set, the first to erupt are the first molars which appear 

 during the sixth year. The last to erupt are the third molars, which 

 frequently become impacted in the jaw-bone so that eruption is impossible. 

 The shape of a tooth is determined by that of the tooth-germ. If 

 the layer of ameloblasts is folded, the enamel is correspondingly modified, 

 and teeth such as those of ruminants and elephants, which become 

 ridged by wear as the result of the difference in hardness of enamel and 

 dentine, owe this adaptive characteristic to the folding of the ameloblast 

 and odontoblast layers. The multipHcation of roots as in molar teeth is 

 produced by the budding of the odontoblast layer of the dental papilla. 



