THE HEAD AND NECK 



115 



shell-like margins surrounding the deep, open cavity, 

 as in fig. 18. 1 



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 exposed 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 gradually alters, becoming 

 narrower from side to side, and finally somewhat tri- 

 angular in section, flat in front, and with a project- 

 ing ridge in the middle, behind. 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 infundibulum, another spot, 

 having a cloudy-yellow colour, always situated in front 

 of the pit when the two coexist, and continuing, after 

 the obliteration of the former, quite to the base of the 

 root. This is caused by the pulp-cavity, which has 

 become filled up by an irregular deposit of dentine. 



1 This appearance has given rise to the term of 1 shell ' teeth 

 applied to newly-cut, unworn incisors. 



i 2 



