26 AGE OF THE DOMESTIC ANLMALS. 



Histology. — The enamel is of epithelial origin, formed from 

 the superior papilla of the primitive follicle. On microscopical 

 examination it is found to be composed of an infinity of little 

 hexag-onal prisms, intimately joined and directed obliquely to 

 the subjacent surface. The deepest layers lie immediately over 

 the periplieral lacunae of the dentine. 



3. The Fundamental Substance, Dentine, Eburnated 

 Substance, or the Ivory (Fig-. 12, /). — This constitutes the 

 major part of the tooth. It is produced by tlie inferior papilla 

 or pulp, has a deep depression in its free extremity for the dental 

 cup, and is covered by the enamel. It forms the walls of the 

 pulp-cavity. In the forming tooth, the dentine consists of a thin 

 layer juxtaposited to the layer of enamel formed by the superior 

 papilla (Fig. 11, d); but as the tooth develops, successive layers 

 are deposited on the interior of the first, and the ivory gradually 

 encroaches upon and diminishes the size of the papilla lodged 

 in the pulp-cavity until the latter entirely disappears. 



The deeper layers have a darker color than the first, as in 

 their continual infringement on the vascular papilla they im- 

 prison a portion of its organic matter and blood-supply until 

 they have produced a complete atrophy of their progenitor. 



This discoloration of the dentine we shall find later, on the 

 table of tlie tooth, under the name of the " dental star," which 

 becomes an important factor in judging of age after eight years. 

 Strictly speaking, there are two stars, but the posterior is very 

 rudimentary and of no practical importance. It is occasionally, 

 however, seen very distinctly. 



In Fig. 14 we have a series of longitudinal antero-posterior 

 sections of incisor teeth from horses of different ages, which 

 show the gradual diminution of the pulp-cavity from the en- 

 croachment of the dentine. The dentine toward the root of the 

 tooth is always of a darker color than that toward the crown. 

 The ivory is less hard than the cement, but much more resisting- 

 than either the cement or the bone in wliich the tooth is imbedded. 



Histology. — The dentine is channeled by a multitude of 

 canalicuU, which radiate from the pnlp and after frequent anas- 



