EXAMIXATIOX OF THE MOUTH FOR AGE. 83 



outer, let us call the outer enamel; the inner, the 

 central enamel. In the upper back teeth, the central 

 enamel forms two " marks," which are circumscribed 

 by an irregular ring of outer enamel (see Fig. 13). 

 Although the enamel of the back teeth of the lower jaw 

 does not form hollow^s on the cutting, or, rather, grinding 

 surfaces ; " marks " are, however, made by the doubling 

 in of the interior face of the enamel (see Fig. 14). 



The pulp-cavity in tlie incisors extends, at first, above 

 the bottom of the " mark," and betAveen this depression 

 and the outer enamel of the tooth (see Fig. 11), After 

 the tooth has made its appearance, the tooth-pulp 

 commences and continues to secrete a new supply of 

 dentine, which is of a yellow colour and is darker in 

 hue than the original dentine. As soon as the tooth 

 becomes somewhat worn down, the new dentine becomes 

 exposed; the stain thus made on the cutting surface 

 of the incisor, being called the dental star. 



Changes undergone hj the teeth ivith age. — The chief 

 changes are as follows : 



1. Owing to the pulp-cavity being continually filled 

 from behind by new dentine, the teeth are gradually, 

 though slowly, forced out of their sockets. I may 

 remark, that our own teeth remain stationarv in leno-th, 

 after they have attained their full size. 



2. The milk-teeth become gradually worn down, and 

 are replaced by permanent ones. The permanent in- 



G 2 



