THE DEVELOPMENT OF THE TEETH. 



943 



Previous to the i rmnn> 'at teeth penetrating the gum, the bony partitions 

 which separate their sacs from the deciduous teeth are absorbed, the roots of 

 the temporary teeth disappear by absorption through the agency of particular 

 multinucleated cells, called odontoclasts, which are developed at the time in the 

 neighborhood of the root, and the permanent teeth become placed under the 

 loose crown of the deciduous teeth ; the latter finally become detached, and the 

 permanent teeth take their place in the mouth (Fig. 568). 



FIG. 568. The milk-teeth in a child of about four years. The permanent teeth are seen in their alveoli. 



L*f*r "\ 



(Cryer.) 



Calcification of the permanent teeth proceeds in the following order : First 

 molar, soon after birth ; the central incisor, lateral incisor, and cuspid, about six 

 months after birth ; the bicuspids, .at the second year or later ; second molar, end 

 of second year ; third molar, about the twelfth year. 



The eruption of the temporary teeth commences at the seventh month, and is 

 complete about the end of the second year. 



The periods for the eruption of the temporary set are (C. S. Tomes) 



Lower central incisors 



Upper incisors 



Lower lateral incisors and first molars 



Canines 



Second molars 



6 to 9 months. 

 8 to 10 " 



15 to 21 " 



16 to 20 " 

 20 to 24 " 



The eruption of tin' permanent teeth takes place at the following periods, the 

 teeth of the lower jaw preceding those of the upper by a short interval : 



6i years, first molars. 

 7th year, two middle incisors. 

 8th year, two lateral incisors. 

 9th year, first bicuspid. 



10th year, second bicuspid, 

 llth to 12th year, canine. 

 12th to loth year, second molars. 

 17th to 21st year, third molars. 



