62 



HAND-BOOK OF PHYSIOLOGY. 



From this formula it will be seen that the two bicuspid teeth in the 

 adult are the successors of the two molars in the child. They differ from 

 them, however, in some respects, the temporary molars having a stronger 

 likeness to the permanent than to their immediate descendants, the so- 

 called bicuspids. 



The temporary incisors and canines differ from their successors but 

 little except in their smaller size. 



The following tables show the average times of eruption of the Tem- 

 porary arfd Permanent teeth. In both cases, the eruption of any given 

 tooth of the lower jaw precedes, as a rule, that of the corresponding tooth 

 of the upper. 



Temporary or Milk Teeth. 

 The figures indicate in months the age at which eacti tooth appears. 



Molars. Canines. Incisors. Canines. Molars. 



18 12 24 



Permanent Teeth. 



The age at which each tooth is cut is indicated in this table in years. 

 Molars. Bicuspid. Canines. Incisors. Canines. Bicuspid. Molars. 



The times of eruption put down in the above tables are only approxi- 

 mate: the limits of variation being tolerably wide. Some children may 

 cut their first teeth before the age of six months, and others not till nearly 

 the twelfth month. In nearly all cases the two central incisors of the 

 lower jaw are cut first; these being succeeded after a short interval by the 

 four incisors of the upper jaw, next follow the lateral incisors of the lower 

 jaw, and so on as indicated in the table till the completion of the milk 

 dentition at about the age of two years. 



The milk-teeth usually come through in batches, each period of erup- 

 tion being succeeded by one of quiescence lasting sometimes several 

 months. The milk-teeth are in use from the age of two up to five and a 

 half years: at about this age the first permanent molars (four in number) 

 make their appearance behind the milk-molars, and for a short time the 

 child has four permanent and twenty temporary teeth in position at once. 



It is worthy of note that from the age of five years to the shedding of 

 the first milk-tooth the child has no fewer than forty-eight teeth, twenty 

 milk-teeth and twenty-eight calcified germs of permanent teeth (all in 

 fact except the four wisdom teeth). 



