198 A MANUAL OF DENTAL ANATOMY. 



The eruption of the permanent teeth is a process closely 

 analogous to that of the temporary set. Rapid absorption 

 of the bone, especially on the exterior surface of the crypts, 

 takes place, and an orifice very much larger than the crown 

 of the tooth is quickly opened out. 



Hence it is that the slightest force will suffice to determine 

 the direction assumed by the rising crown : a fragment of a 

 root of a temporary tooth, the action of the lips and tongue, 

 &c., are all potent agencies in modifying the arrangement of 

 the teeth. 



The temporary teeth stood vertically, the permanent 

 teeth in front of the mouth stand obliquely, thus opening 

 a space between the lateral incisors and the first bicuspid 

 for the canine, which during development was out of the 

 line altogether. And, inasmuch as the crowns of the teeth 

 are on the whole much larger than their necks, it would 

 be manifestly impossible for them all to come down simul- 

 taneously. 



^ The permanent teeth usually make their appearance in 

 the following order : First permanent molars, about the 

 seventh year ; a little later, the lower central incisors, upper 

 centrals and laterals, the first bicuspids, the canines, the 

 second bicuspids, the second permanent molars, the third 

 permanent molars. 



The period of eruption is variable. From a comparison 

 of several tables, I find the principal discrepancies to relate 

 to the date of the appearance of the canines and the second 

 bicuspids. The canine would certainly appear to belong to 

 the eleventh and twelfth years ; but some authors consider 

 that the second bicuspid is usually cut earlier, others later 

 than this date. 



We may now revert to the phenomena observed in the 

 alveolar processes. They were first built up as crypts with 

 overhanging edges enclosing the temporary teeth : then they 

 were swept away, in great part, to allow of the eruption of 



