TEETH. 



53 



FIG. 22. 



Examined under the microscope, enamel is found composed 

 of fine hexagonal fibres (Figs. 22 and 

 23), which are set on end on the sur- 

 face of the dentine, and fit into cor- 

 responding depressions in the same. 

 They radiate in such a manner from 

 the dentine that at the top of the 

 tooth they are more or less vertical, 

 while towards the sides they tend to 

 the horizontal direction. Like the 

 dentine-tubules, they are not straight, 

 but disposed in wavy and parallel 

 curves. The fibres are marked by 

 transverse lines, and are mostly solid, 

 but some of them contain a very 

 minute canal. 



The enamel itself is coated on the 

 outside by a very thin calcified mem- 

 brane, sometimes termed the cuticle of 

 the enamel. 



The crusta petrosa, or cement, is 

 composed of true bone, and in it are 

 lacunae and canaliculi which some- 

 times communicate with the outer 

 finely-branched ends of the dentine- 

 tubules. 



Development of Teeth. The teeth 

 are developed after the following 

 manner : Along the free edge of the 

 toothless gum in the foetus, there ex- 

 tends a groove, or small trench, the 

 primitive dental groove (Goodsir), and 

 from the bottom of this project ten 

 small processes of mucous membrane, 

 or papillae, containing bloodvessels and 

 nerves. As these papillce grow up from 

 below, the edges of the small trench 

 begin to grow in towards each other, 



and overshadow them, at the same time that each papilla is 

 cut off from its neighbor by the extension of a partition wall 

 from the gum, which grows in from each side to separate the 

 one from the other. Thus closed in above and all around, 

 each dental papilla is at length contained in a separate sac, 

 and gradually assumes the character of a tooth by deposition 

 on its surface of the various hard matters which have been 

 just enumerated as composing the greater part of a tooth's 



7) 



Thin section of the enamel 

 and a part of the dentine 

 (from Kolliker) &%&. a, cu- 

 ticular pellicle of the enamel ; 

 b, enamel fibres, or columns 

 with fissures between them 

 and cross striae ; c, larger cavi- 

 ties in the enamel, communi- 

 cating with the extremities 

 of some of the tubuli (d). 



