742 EXAMINATION FOR AGE. 



the incisors, the 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. 199): the outer or external enamel, and the inner or central 

 enamel. In the upper back teeth, the central enamel forms two 

 " marks," which are circumscribed by an irregular ring of ex- 

 ternal enamel (Fig. 201). Although 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. 200). 



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

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

 external enamel of the tooth (Fig. 198). 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 



thick layer over the crown, as in Ruminants, a cartilaginous cement organ is 

 formed, and we have a calcification analogous to formation of bone in cartilage. 

 Thus the cement organ is found in those animals only which have coronal cement, 

 such as the Herbivora " (C S, Tomes). 



