TEETH. 



53 



Fig. 22. : 



The enamel, which, is by far the hardest portion of a 

 tooth, is composed, chemically, of the same elements that 

 enter into the composition of dentine and bone. Its 

 animal matter, however, amounts only to about 2 or 3 per 

 cent. 



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 

 corresponding 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-tu- 

 bules, 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 

 membrane, sometimes termed the 

 cuticle of the enamel. 



The crusta petrosa, or cement, is composed of true bone, 

 and in it are lacunse and canaliculi which sometimes 

 communicate with the outer finely-branched ends of the 

 dentine- tubules. 



* Fig. 22. Tliin section of the enamel and a part of the dentine 

 <from Kolliker) ^ , cuticular pellicle of the enamel ; b, enamel 

 fibres, or columns with fissures between them and cross striae ; c, larger 

 cavities in the enamel, communicating with the extremities of some of 

 the tubuli (d). 



