CHANGES IN THE TEETH FEOM AGE. 753 



enamel forms a depression that is more or less filled with cement, 

 which soon becomes discoloured by the food the animal eats. The 

 hole thus made in the tooth is called the " mark.'' As this layer 

 of cement varies from one-tenth to one-half of an inch in thickness, 

 the " mark ' wears out in the teeth of some horses, much quicker 

 than it does in those of others. The outer enamel which surrounds 

 the crowns of the teeth is, in the first instance, covered with a 

 very thin layer of cement, which is soon rubbed off. After an 

 incisor has been a short time in use, its table presents two more 

 or less irregular rings of enamel (Fig. 204, p. 757) :■ the outer or 

 external enamel, and the inner or central enamel. In the upper 

 back teeth, the central enamel forms two " marks,'' circumscribed 

 by an irregulrir ring of external enamel (Fig. 206, p. 759). Al- 

 though the enamel of the back teeth of the lower jaw does not form 

 hollows on the tables of these teeth ; " marks " are, however, made 

 by the doubling in of the interior face of the enamel (Fig. 205, 

 p. 759). 



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

 bottom of the " mark," and between this depression and the ex- 

 ternal enamel of the tooth (Fig. 203, p. 756). 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 

 is 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 by the Teeth with 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. Our own teeth remain stationary in length, after 

 they have attained their full size. 



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

 placed by permanent ones. The permanent incisors push out, from 

 behind, the milk ones, the roots of which, being squeezed between 

 the jaw and the new teeth, waste away ; so that the milk-teeth, 

 usually, readily drop out. They may, however, remain as a second 

 row in front, and should, in this case, be removed by some suitable 

 instrument. 



3. The teeth show wear. The tables of the permanent incisors, 

 as they become rubbed down, change their form in the manner 

 alluded to on p. 752, and as illustrated by Fig. 202 (p. 755) and 

 by the plates giving the different ages. As a rule, the tushes 



48 



