192 



THE TEETH 



is flattened from side to side. As the tooth Ijecomes worn off, 

 the length of the free portion is maintained by a pushing out of 

 the tooth, and a corresponding shortening of the portion that is 

 fixed or imbedded in the jaw. 



The table surface of the unworn incisor tooth is covered with 



enamel, and in the middle por- 

 tion the enamel forms a deep cup. 

 After tlie tooth has become worn 

 the margin of the table portion is 

 then limited by a ring of enamel. 

 This is termed the encircling 

 enamel ring. The central portion 

 of the taljle shows a second ring, 

 the central enamel ring, that 

 limits the cup margin (Fig. 54). 

 As the table surface repre- 

 sents a cross section of the tooth, 

 its appearance and shape will 

 then depend on the portion of the 

 tooth that it represents. From 

 year to year, there is a gradual 

 shortening in the lateral diameter, 

 and an apparent increase in the 

 diameter from before to behind. 

 These changes in shape are from 

 a long, narrow table surface to an 



Fig. 55.— Table surfaces of nippers at OVal, frOUl OVal tO circukr and 

 difforentages. ^ four years; B. six years; f^.^^^j^ circular tO triaUgular (Fig. 

 C , nine years, and /), fifteen years of age. '- \ e^ 



55). As the original free por- 

 tion of the tooth wears off, the cup becomes shallow and smaller 

 until the remnant is represented by a mere dot of enamel that 

 finally disappears from the posterior portion of the table. After 

 the cup has moved from the central portion of the crown 

 and occupies a more posterior position, the dental star, which 



