24 



AGE OF THE DOMESTIC ANIMALS. 



exceedingly rare to find excess of depth due to increased depth 

 of the enamel, which condition will, however, be considered in 

 the study of the abnormalities of the teeth. 



The cement is a protecting layer which offers only a mod- 

 erate resistance to the friction of food and other substances, and 

 disappears at an early date from the periphery of the tooth, while 



Fig. 13. 



('^mentation of thi' roots of the incisors. A, dental tables; B, roots in their 

 alveolar cavities. 



it persists in the cup of the tooth so long as the latter remains 

 on the table, where it forms a wliitish spot, surrounded by a 

 band of enamel. (Fig. 10, A, B, and C) The cement is shown 

 by microscopical examination to be a bony formation, secreted 

 by the alveolar periosteum (germ sack), and is not a transformed 

 ivory or dentine as given by Simonds. 



It is found in greatest quantity at the crown or free 



