THE TEETH. 561 



On the first appearance of the bony jaws, the teeth lie in 

 continuous grooves extending all round their free borders. 

 Partitions are soon formed, dividing these grooves into separate 

 compartments or alveoli, which grow round the teeth so as to 

 closely embrace them. 



The order of appearance of the milk teeth takes place in 

 regular sequence ; but the actual dates at which the several 

 teeth emerge, or are ' cut,' vary within certain limits. The 

 cutting of the milk teeth usually commences about seven months 

 after birth, and is completed by the end of the second year. 

 The central lower incisors appear first, about the seventh month ; 

 the upper incisors two or three months later ; a few months later 

 still the lower lateral incisors, and the first premolars ; four or 

 five months later the canines ; and about the end of the second 

 year, the second premolars. 



The permanent teeth are developed in the same manner as 

 the milk teeth. From the stalk or neck of the enamel organ of 

 each milk tooth, a small outgrowth arises at a very early stage, 

 about the sixteenth week, which becomes the enamel organ of 

 the corresponding permanent tooth. A dental papilla is formed 

 opposite each enamel organ, and the permanent teeth are formed 

 in the jaw, a little way behind and below the corresponding milk 

 teeth, and in precisely similar fashion. 



The three hinder grinding, or molar teeth, which have no 

 milk predecessors, are formed by extension backwards of the 

 original common enamel germ, from which the milk teeth are 

 developed. The enamel organ for the first permanent molar 

 appears about the fifteenth week of embryonic life ; that for 

 the second permanent molar about seven months after birth ; 

 and that for the third permanent molar, or ' wisdom tooth,' not 

 until the third year. 



The eruption, or cutting, of the permanent teeth of the lower 

 jaw takes place at the following dates, the teeth of the upper 

 jaw usually appearing a little later : 



Molars, first 6 years. 



Incisors, central 7 



lateral 8 



Bicuspids, anterior 9 



posterior 10 



Canines 11 to 12 



Molars, second 12 13 



third (or wisdom) 17 25 



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