THE HEAD AND NECK. 123 



external enamel layer. The flattened, worn surface of 

 an incisor tooth, as seen in 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, shell-like 

 margins surrounding the deep, open cavity, as in 

 Fig. 18 * 



The permanent incisors of the horse (like the 

 molars, as will be explained hereafter) differ from 

 those of most mammals in the great length of their 

 crowns, which do not remain fixed in position when 

 they have once come into place, but continue to 

 push up from below, as they wear away at the ex- 

 posed surface, for a considerable part of the life of 

 the animal. The upper part of the tooth, or that 

 which first appears, is very wide transversely, and 

 narrow from before backwards ; but the form grad- 

 ually alters, becoming narrower from side to side, 

 and finally somewhat triangular in section, flat in 

 front, and with a projecting ridge in the middle, be- 

 hind. Consequently, the shape of the table alters 

 as the wear of the tooth proceeds, and by its form 

 gives indications of great assistance in determining 

 the age of the animal. A considerably worn table 

 shows, in addition to the mark caused by the pit or 



* This appearance has given rise to the term of " shell " 

 teeth, applied to newly-cut, unworn incisors. 



