70 ANIMAL DENTISTRY. 



at work to a remarkable degree, while in the carnivora and 

 omnivora only the second influence is concerned in the de- 

 structive process. In the herbivora the length of the teeth 

 varies from the long, three or four-inch tooth of the young 

 subject, to the short, half-inch tooth of extreme old age 

 (from the wear of mastication), while the alveolar cavity 

 varies from the exceedingly deep excavation of youth to 

 the shallow, flattened cavity of old age. The diminution in 

 the depth of the alveolar cavity occurs in all the animals, and 

 is solely responsible for senile self-extraction in the animals 

 having simple teeth. In the animals having simple teeth the 

 table retains its enamel covering through life, and the tooth 

 does not diminish in length from wear. 



I. Wear from mastication. — When the teeth of the ru- 

 minant and soliped first enter into the process of mastica- 

 tion they are covered with enamel over the grinding sur- 

 face. The enamel in addition dips deeply into the body of 

 the tooth around the infundibula. As soon as wear begins 

 the surface enamel of the table at once wears off, exposing 

 the dentine and leaving only the edges of the enamel organ 

 on the grinding surface. The dentine, ordinarily a sensitive 

 substance, becomes hard and insensitive along the table 

 surface, by the deposit of dentinal cells within its tubules. 

 The area of insensitiveness extends throughout the entire 

 table and to the depth of one to two millimeters during the 

 first year of wear, but in the subsequent three or four years 

 it descends to the depth of one centimeter or even more. At 

 the age of ten to twelve years it has descended to more than 

 one-third of the entire tooth in the molars, and to the depth 

 of about two centimeters in the incisors. When the animal 

 has passed the age of eighteen the tooth is practically a dead 

 organ so far as the sensibility of the dentine is concerned. 

 That the stimulus to the proliferation of dentinal cells (os- 

 teo-dentine) is wear is demonstrated in the fact that teeth 



