STllUCTURE OF THE ENAMEL. 



311 



Fiff. 212, 





■ikSm 



jfHi; 



The enamel is that hard uhite covering which encrusts and pro- 

 tects the exposed portion or crown of a tooth. It is the hardest of all 

 the dental tissues, but is gradually worn down by protracted use. It 

 is thickest on the grinding surface and cutting edge of the teeth, and 

 becomes gradually thinner towards the neck, where it ceases. Its 

 extent and thickness are readily seen on charring the tooth, by which 

 the deniine becomes blackened, whilst the enamel, owing to the very 

 small quantity of animal matter in its composition, remains white. 

 According to Bibru it contains of earthy constituents 96-5 per cent., 

 viz., phosphate of lime with traces of fluoride of calcium 89'8, carbonate 

 of lime 4-4, phosphate ot magnesia and other 

 salts I'o ; and of animal matter only 3'5 per 

 cent. Berzelius, however, gives the proportion 

 of carbonate of lime as 8, and of animal matter 

 as only 2 per cent. 



The enamel is made up entirely of very hard 

 and dense microscopic fibres or prisms, ar- 

 ranged closely together, side by side, and set 

 by one extremity upon the subjacent surface 

 of the dentine (fig. 212). The fibres are 

 disposed in ranges which are set vertically 

 on the grinding surface, but on the sides of 

 the crown get more and more horizontal. Near 

 the dentine the fibres cross one another in the 

 alternate ranges, but become more parallel as 

 they approach the surface of the tooth ; from 

 this intercrossing the ranges appear on a 

 section as alternate light and dark stripes 

 passing tli rough the enamel from its inner to 

 its outer surface (as in figs. 201 and 210). 

 This is no doubt owing to the manner in Avhich 

 the different strata affect the light. A series 

 of concentric lines is likewise to be seen 

 crossing the enamel fibres : these are termed 

 coloured linos irom their brown appearance, but 

 whether caused by pigmentary deposit or 

 otherwise is unascertained. Minute fissures not 

 unfrequcntly exist in the deep part of the 

 enamel, Avhicli run between clusters of the 

 fibres down to the suriace of the dentine (fig. 

 212, c) ; and other much larger and more 

 evident fissures are orten observed leading 

 down from tlie depressions or crevices between 

 the cusps of the molar and premolar teeth. 

 The unworn surface of the enamel is marked 

 by concentric ridges, which may be distin- 

 guished with a common magnifying glass. 



The eufimcl-nbres (fig. 21.j) have the form 

 of solid six-sided prisms. Their diameter is 



ordinarily about ^-^^-^-^jth of an inch. They are marked at small intervals 

 by dark transverse lines. The inner ends of the prisms are implanted 

 in minute hexagonal depressions found on tlie surface of the dentine ; 

 whilst the outer ends, somewhat larger in diameter, are free, and 



Fig. 212. — Thin .Section 

 OF THE Enamel and a 

 Part op the Dentine 

 (from Kolliker). 350 



Diameters. 



a, cxiticular pellicle of 

 tlie enamel ; h, enamel- 

 fibres or columns with 

 fissures between tliem and 

 cross striffi; c, clefts in the 

 enamel communicating with 

 the extremities of some of 

 the tubuli [d). 



