242 



THE ENAMEL. 



ment and constitute the structure which I have described 

 and demonstrated by the name of baccated fibre." See 

 Fig. 65, A. 



FIG. 67. 



ABC 



Mr. Tomes makes three stages in the formation of den- 

 tine or ivory. 1st. The Areolar, consisting of a very fine 

 tissue, which he calls the preparatory stage. 2d. Cellular, 

 (Fig. 6 I 7, A,) in which the cells are scattered irregularly and 

 have no definite arrangement. 3d. The Linear, where the 

 cells (Fig. 67, B) are disposed in regular rows, vertical to the 

 coronal surface. From this third stage, Mr. Tomes thinks, 

 the regular, continuous, permanent tubes of dentine re- 

 sult these vertical cells becoming united, end to end, 

 opening into each other and forming a communication : so 

 that if the baccated appearance of Mr. Nasmyth is seen, it 

 is regarded as an arrest of development, and exhibiting 

 dentine in an imperfect state. 



The Enamel (Fig. 61, A) covers the crown of the tooth ; is 

 thickest upon the cutting and grinding surfaces, and is 

 much the hardest portion. Its hardness is so great as to 

 strike fire with steel, and resist the file or saw ; hence it is 

 well suited to oppose the pressure and friction to which it 

 is constantly liable. Its color is a pearly white, and it is 

 very brittle. It is found to consist of fibres, or minute 



FIG. 67, A represents the Dentine in its second stage. 

 FIG. 67, B represents the Dentine in the early part of the third stage. 

 FIG. 67, C represents the Dentine at the completion of the third and most 

 perfect stage. 



