MASTICATION. 



247 



incisors of a horse, the free surface, with the exception of the crown, is 

 covered with enamel alone (Figs. 95 and 96). On the biting surface of 

 the incisor tooth, when freshly erupted, is always found a central spot 

 composed of cement, the enamel dipping in to form a cavity or depres- 

 sion on the free biting surface of these teeth. By the change in shape 



FIG. 95 DIAGRAM OF FRESHLY-ERUPTED 

 INCISOR OF LOWER JAW OF HORSE. 

 (Nuhn.) 



c, depression in table of tooth : *, cement, which rapidly 

 disappears except from infundibuluin ; z, enamel ; c, 

 dentine. 



X 



FIG. 96. LOWER INCISOR TOOTH OP 

 HORSE. (Nuhn.) 



c, worn-down surface of table of tooth, showing the 

 alternate layers of enamel, s : z, dentine ; and x, dis- 

 colored cement filling infundibulum. 



Cl 



of this central depression in the incisor teeth of the horse, through the 

 gradual wearing down of the surface, an index is furnished of the age 

 of the horse, a matter which will subsequently be alluded to. 



The molar teeth of herbivorous animals are chiefly compound teeth, 

 that is, the enamel dips down below the surface of the crown, and in 

 some animals, as in the elephant, the com- 

 pound teeth" may be regarded as a series 

 of flattened teeth arranged side by side in 

 the jaw, and connected only by the 

 cement, or crusta petrosa (Figs. 97 and 

 98). This substance is like that invari- 

 ably found covering the fangs of teeth, 

 but which only in compound teeth appears 

 upon the crown. The pointed fang or 

 fangs of teeth are pierced by an opening 

 which communicates with a cavity in the 

 centre of the body of the tooth, called 

 the pulp-cavity, which contains blood- 

 vessels and nerves which enter through 

 the opening in the fang, and in the pulp- 

 cavity ramify over a delicate fibro-cellular 

 structure constituting the pulp (Fig. 99). 



The pulp is continuous over its surface with an infinite number of small 

 projections which extend into the tubes of dentine in the inner structure 

 of the tooth. 



These three different substances, which constitute the substances of 



FIG. 97. SECOND UPPER MOLAR OF 

 HORSE, SHOWING WEAR OF 

 TABLE. (Nuhn.) 



<', depression on table; pi, depression on 

 side; s, enamel ; 2, dentine; Cae, cement; a, ex- 

 ternal or buccal surface; t, internal or oral 



