TEETH. 



289 



endure neither without becoming diseased. Teeth vary with the 

 class of animal in number, size, form, structure, position, and 

 attachment, but in all cases they are in correlation with the food 

 and generic habits of the animal. Thus, in herbivora, wheie 

 grinding the food is necessary, the contacting surfaces of the 

 molars are rough and flat. In carnivora, where tearing and 

 crushing are requisite, the molars are sharp, pointed, and serrate ; 

 in omnivora, where the food is general, the teeth are mixed in 

 their character. The form of the teeth thus depends upon the 

 natural food of the animal, and there is always a certain harmony 

 between their disposition and the conformation of correlated 

 organs. They are not found in all animals. Birds have none, 

 while the typical number of mammalian teeth as viewed by Pro- 

 fessor Owen is forty-four,. 



Three hard structures enter into the formation of the teeth 

 Dentine, or Ivory; Enamel ; and Cementum, or Crusta petrosa. 



DENTINE. 



Dentine constitutes the major part of the tooth, and is a 

 hard, yellowish substance, consisting of very minute tubuli, em- 

 bedded in a dense, granular, intertubular 

 matrix, which contains the bulk of the 

 earthy matter, the latter being about 

 thirty per cent, of the whole. The 

 tubuli commence at the pulp cavity, and 

 radiate to the superficies of the dentine, 

 where they anastomose andtheir branches 

 terminate in minute cavities, the dentinal 

 cells, which perhaps are analogous to the 

 lacunaB of bone. The tubuli are about 

 the ToW of an inch in diameter, and, 

 in the fresh state, they contain processes 

 from the pulp. 



During the development of a tooth 



the calcification of the dentine proceeds from centres, around 

 which the calcific material becomes deposited. Thus nodules are 

 formed, which, by expanding, finally coalesce and form the solid 

 matrix. 



FIG. 93. 



Oblique section of deutioe, show- 

 ing the tubuli and coalescence of 

 the calcified nodules. 



