ANIMAL DENTISTRY. 73 



first triangular, then round and finally in extreme age flat- 

 tened from side to side. The triangular shape makes its ap- 

 pearance almost as soon as the cups disappear in the central 

 and lateral incisors, while the corners maintain their ob- 

 long shape until after the age of twelve, and even never 

 become as distinctly triangular as the former. (See Fig. 47.) 



2. Receding of the alveolar margin is more prominent 

 in the herbivora than in the other species, although it con- 

 stitutes a feature of dental retrogression in all animals. 

 Through its influence the teeth lose their implantation by 

 gradual stages until in extreme old age they are held in 

 position only by the gums and small pits in the jaw, which 

 retain the remaining roots. 



In the incisor arcades the alveolar margin recedes much 

 faster than the teeth wear at the table, which circumstance 

 results in the elongation of the crowns as the animal be- 

 comes older. In rare cases the reverse occurs and the 

 crowns become shorter A\'ith age, but this incident results 

 only when the teeth are defective in cpiality. The long 

 crowns of the incisor teeth of old horses due to the re- 

 ceding jaw, is responsible for the general impression that 

 the teeth grow in length and that they are longer in old 

 than in young horses. 



The receding process is also responsible for the gradual 

 change in the inclination (angle) of the incisor teeth. The 

 angle formed by the incisor teeth at the contact point is 

 obtuse in the young, but changes gradually into a very acute 

 one in the aged animal. This change is the result of the 

 great curvature in the incisor tooth, which in the young 

 animal is implanted into a deep curved cavity. As the 

 tooth wears the part projecting forwards becomes exposed 

 and the increased inclination results. The incisor teeth, like 

 all teeth of herbivora, are longest in the young animal, but 



