252 STRUCTURE AND ADAPTATION OF TEETH 



extremities of the lower incisors, which are 

 straight, and, as in the hog, project directly for- 

 wards. The powerful upper canines are compa- 

 ratively short, and are worn obliquely in front, in 

 consequence of their abrasion against their 

 antagonists in the lower jaw. The lower canines 

 are still more massive, and form the segment of 

 a circle, the extremity being worn away on the 

 inner or concave side ; and as the convex outer 

 sides are furnished with thick enamel, which is 

 not present on the inner, a cutting-edge of 

 irresistible power is constantly maintained. 



Unlike the teeth of the ox or horse, there is in 

 these no distinction of root or crown, but the 

 tooth is of equal diameter throughout. The 

 hollow implanted extremity is occupied by a per- 

 sistent formative pulp, which undergoes progres- 

 sive calcification, and the tooth glides forward in 

 its curvilinear alveolus, to compensate for the 

 wearing down of the free extremity. 



This mode of growth, however advantageous 

 in some respects it may appear, is occasionally 

 productive of unfortunate results. 



In the specimens on the table we have examples 

 of the wrong direction taken by the tooth in 

 describing an arc of too small a circle. 



In one case the apex has returned upon its base, 

 pressing on the outside of the thin wall of the 

 pulp cavity ; and in another it has entered the 

 formative pulp itself, forming a complete ring; 



