STRUCTURE OF THE INCISORS. 



23 



Three substances enter 

 into the structure of the 

 teeth, — one fundamental, 

 the dentine or the ivory, 

 and two covering sub- 

 stances, which differ from 

 each other very much, and 

 are known as the cement 

 and the enamel. 



1. The Cement (Fig. 

 12, G O'— The cement 

 forms the most superficial 

 layer; it is deposited di- 

 rectly upon the enamel over 

 the whole surface of the 

 tooth, and dips into the 

 cup, which it fills more or 

 less completely in different 

 subjects. Sometimes it is 

 excessively thin, 2 to 3 

 mm. ; at otlier times it is 

 from 10 to 15 or even 20 

 mm. in thickness. It is 

 generally tliicker in the 

 lower teeth than in the 

 superior. MM. Chauveau 

 and Arloing, in France, 

 and Mr. Mayhew, in Eng- 

 land, were the first to call 

 attention to the importance 

 of this in judging age. 



It will be readily seen 

 that the size of the cavity 

 in the dental cup depends 

 greatly upon the amount 

 of the cement which is 

 deposited in it. It is 



Fia. 12. 

 Longitudinal and antero-posterior section of i»er- 

 manent incisor (enlarged). F A, anterior, F P. pos- 

 terior face: C, cement: E, enamel ; I, ivory, or den- 

 tine : T c, external dental cavityj or cup of enamel 

 lined with cement ; P, internal dental or pulp cavity. 



