114 THE STRUCTURE OF THE HOUSE 



ring of the same substance surrounding the pit, 

 which, of course, adds greatly to the efficiency of the 

 tooth as an organ for biting tough, fibrous substances. 



The bottom and sides of the 

 pit are partially lined with 

 cement, but a considerable 

 cavity remains, generally filled, 

 in the living animal, with par- 

 ticles of food, and, being con- 

 spicuous from its dark colour, 

 it constitutes the 4 mark ' by 

 which the age of the horse is 

 judged. In consequence of 

 its only extending to a certain 

 depth in the crown, it becomes 

 obliterated as the tooth wears 

 away, the section of which then 

 assumes the character of that 



^z^JZ^ of - - di ^y incisor > 



ally worn showing the pit in g Q f only a CQre of dentine, 

 surrounded by its enamel ° J 



layer, outside of which is surrounded by an external 



the dentine with its exter- 

 nal enamel covering. The enamel layer. The flattened, 

 enamel is represented # 

 white, the dentine grey, worn surface of an incisor 

 and the pit black. 1 0 1 n . 



tooth, as seen m fig. 19, is 



called, in works of descriptive veterinary anatomy, ' the 



table.' It is totally different in appearance from the 



summit of an unworn tooth, with its thin, rounded, 



