THE ALIMENTARY APPARATUS. 



221 



The dentine is formed by the development of odontoblasts 

 in the periphery of the papilla in a similar manner to the develop- 

 ment of osteoblasts in bone. 



The cement is developed from the wall of the dental sac by 

 the intramembranous process of ossification. 



Permanent. The successional permanent teeth, or those re- 

 placing the temporary, are formed in a different manner from 

 the superadded, or three molars in each side of each jaw. 



The former are developed similarly to the temporary teeth, 

 but in a secondary dental groove, from which after their forma- 

 tion they recede behind the germs of the temporary teeth, in- 

 closed in sacs. The molars, or supcradded teeth, however, are 

 formed by extensions backward of a portion of the enamel germ 

 of the tooth immediately in front. 



6-8 

 C.In. 



FIG. 113. TEMPORARY TEETH. 



Eruption, or "cutting of the teeth," takes place by the 

 growth of the fang, the gums being absorbed by the pressure 

 of the advancing crown. The development of the fangs of the 

 permanent causes the absorption of the fangs of the temporary 

 teeth through the. agency of the odontoclasts, multinucleated cells 

 corresponding to the osteoclasts of bone. 



The eruption of the temporary teeth takes place in months 

 in the order shown in Fig. 51, the lower preceding by a short 

 time the upper (according to Dr. C. N. Pierce, in "American 

 System of Dentistry"). The order of the eruptions is : first the 

 lower central incisors, the upper central incisors, lateral incisors, 

 upper and lower, first molars, followed by the canines or cuspids, 

 and ending with the second molars. 



