THE HEAD AND NECK 



113 



long- continued use, they presented an external ring of 

 enamel, surrounding a core of the dentine, or ivory, of 

 which the bulk of the tooth is composed. This is the 

 condition of the incisor teeth in the great majority of 

 mammals. The first modification from this simple 

 form consisted in the development of a ridge along 

 the hinder border of the base of the crown, as seen 

 in fig. 17. There was then a groove between this ridge 

 and the rest of the tooth. By the continuous increase 



Fig. 17. — Incisor tooth Fig. 18. — Unworn crown of 



of Anchitherium an- temporary incisor tooth of 



n Uanense. young horse. 



of the ridge, and its union with the edges of the main 

 part of the crown on each side, the groove became 

 converted into a deep pit (infundibulum), the orifice of 

 which is transversely elongated, and placed behind and 

 rather below the cutting-edge of the tooth. This is the 

 condition seen in a colt's incisor which has just cut the 

 gum (fig. 18). As wear takes place, the surface, besides 

 the external enamel layer, as in the ordinary simple 

 tooth, shows, in addition (see fig. 19), a second, inner 



I 



