142 



ROBERT BENNETT BEAN 



The order of eruption is represented by serial numbers and 

 followed by the line with barbs, beginning with the lower first 

 molar and terminating with the upper third molar. The line 

 skips from maxilla to mandible or mandible to maxilla nine 

 times, and five times it goes from one tooth to another in the 

 same jaw. 



There are two waves of growth, one from the median line later- 

 ally, which includes the incisors, canines and premolars, 20 teeth 

 in all, that erupt more rapidly (from 6 to 11 years) than the 

 other wave, which includes the first, second and third molars, 

 only 12 teeth in all, that erupt more slowly (from 6 to 20 years 

 or later). Four molars erupt every 6 years, the first molars at 

 6 years of age, the second molars at 12 years of age, and the 

 third molars at about 18 years of age. We may illustrate the 





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Diagi-ain 5 The eruption of the teeth of the Filipinos and the Eutopeans 

 in two series. A, American: F, Filipino. Each tooth is represented by a dot. 

 Under the dots are symbols for the tooth; i, incisor; c, canines; p, premolars; 

 TO, molars. The order of eruption is represented by the lines. 



two waves of growth by diagram 5, showing the relative time 

 of development of each tooth in the two waves. 



When the permanent teeth begin to erupt the lower teeth erupt 

 before the upper in the first period of acceleration in eruption 

 (the first molars, incisors and canines); the upper teeth erupt 

 before the lower in the second period of acceleration in eruption 

 (the premolars) ; and the lower teeth again erupt before the upper 

 in the final stages of eruption (the second and third molars). 



The alternation in development seems to apply to abnormal 

 as well as normal development, although insufficient data are as 

 yet available to demonstrate this conclusively. If one structure 

 is unusually precocious in the periods of acceleration in develop- 

 ment its complementary structure will be backward in the 

 periods of acceleration, and vice versa. The eruption of the teeth 



