EXAMINATION OF THE MOUTH FOE AGE. 79 



Fig. 8) taper gradually down to the end of the fang 

 (the part of the tooth below the gum), when viewed 

 from the front, or from behind ; looking at them in 

 profile, they are, if anything, thicker away from the 

 croivn (the portion of the tooth above the gum), than on 

 it. Hence, the cutting surface, or table, which at first 

 is broad from side to side, and narrow from front to rear, 

 becomes in time, as it gets worn down, narrower and 

 narrower from side to side, and broader and broader from 

 front to rear. This is well shown by Fig. 10, and by the 

 plates of the teeth at different ages. 



Structure of the teeth. — The body of a tooth (see 

 Fig. 11) is composed of an ivory-like substance called 

 dentine, and has a hollow (the iml^-cavity) extending 

 from its base up its centre, in which cavity the blood 

 vessels, nerves, the tooth imlp, consisting of secreting 

 cells, etc., that are concerned in the nourishment of the 

 tooth, are lodged. The dentine is more or less covered by 

 a layer of white and very hard material termed enamel, 

 which furnishes the cutting or grinding surface with 

 which the animal masticates his forage. In the milk 

 incisors (see Fig. 7), the enamel does not extend below 

 the crown. In all the permanent teeth, the enamel 

 covers the greater part of the fang. Over the whole 

 surface of each unused tooth, there is an envelope of 

 cement, which is nearly similar in structure to bone. On 

 the upper surface of the incisors the enamel forms a 



