12 



AGE OF THE DOMESTIC ANIMALS. 



have a smooth and poUshed surface as the animal becomes older. 

 The posterior face is concave from above to below, and slightly 

 convex from side to side. The internal border of each tooth is 

 thicker than the external. In a virgin tooth (Fig. 3), or one 

 which is not worn by use, the free portion of crown is divided 

 from in front to behind and limited by two borders, one anterior, 

 a, and the other posterior, h, which are separated by a cavity 

 known as the cup, c. The anterior border is the highest and 

 longest ; it is shaped convex transversely, and is the first portion 

 of the tooth to come through the gum. The posterior border is. 



"be a 

 Fig. 3. 

 Longitudinal section of a virgin milk-tooth in its alveolar cavity. 



shorter and appears later, but soon reaches, however, the level 

 of the anterior border from the wearing down of the latter. 

 From the same cause the cavity. or cup, which first existed, 

 gradually disappears. This is known as the leveling of the 

 teeth. The imbedded portion or root also contains a cavity, 

 known as the internal dental cavity, or pulp-cavity {d), which 

 protects the papilla or pulp of the tooth ; but as the animal be- 

 comes older the tooth elongates by the growth of its imbedded 

 portion, and the internal cavity diminishes in calibre and is 

 nearly obliterated by a deposit of bony substance ; at the same 



