DENTIN. 



53 



Enamel. This covers the crown and extends to the root. It is 

 the hardest part of a tooth indeed, it is the hardest tissue in the 

 human body and protects the softer and more sensitive portion 

 beneath in the process of mastication or chewing. It is made up 

 of elongated hexagonal prisms, enamel-prisms, which are placed 

 at right angles to the dentin (Fig. 46). 



Chemical analyses of enamel vary to a considerable extent. 

 Hoppe-Seyler gives the following : Calcium carbonate and phos- 

 phate, 96 per cent. ; magnesium phosphate, 1 per cent. ; and 

 organic substances, 3 per cent. Other chemists state the amount 

 of organic matter to be from 2 to 10 per cent. ; but the most recent 



- Enamel. 



- Branching of the 

 dentinal tubules. 



Dentinal tubules. 



Interglobular J 

 space. 



FIG. 46. A portion of a ground tooth from man, showing enamel and dentin ; 

 X 170 (Bohin and Davidoff). 



analyses seem to show that the organic matter present in the 

 enamel of a fully formed tooth is too minute to be weighed. 



Cement or Crusta Petrosa. At the point where the enamel 

 ends the cement begins, and forms a covering of the dentin as 

 far as the tip of the root. It is both structurally and chemically 

 identical with bone, possessing both lacunae and canaliculi. The 

 presence of Haversian canals is claimed by some histologists, es- 

 pecially in the thicker portions ; while others deny it in normal 

 teeth. Like bone, the cement is covered with periosteum, which 



