STRUCTURE OF THE EXAMEL, 



789 



Another kind of irregularity in the structure of the dentine gives rise to 

 the granular layer of Purkinje ; the peculiarity of which consists in the 

 presence of a number of minute cell-like cavities, which break up the 

 uniformity of the matrix, and by branches anastomose one with another 

 and receive terminations of dental tubuli. They are found principally in 

 a layer beneath the cement, and also beneath the enamel. The circum- 

 stance of their forming connections with the tubules points to a difference 

 in nature between these cavities and the much larger iuterglobular spaces. 



The surface of the dentine where it is in con- 

 tact with the enamel is marked by undulating 

 grooves and ridges, and also by numerous minute 

 hexagonal depressions, to which the microscopic 

 fibres of the enamel are accurately adapted. 



B. The enamel is that hard white covering 

 which encrusts and protects the exposed portion 

 or crown of a tooth. It is the hardest of all the 

 dental tissues, but it is gradually worn down by 

 protracted use. It is thickest on the grinding 

 surface and cutting edges of the teeth, and be- 

 comes gradually thinner towards the neck, where 

 it ceases. Its extent and thickness are readily 



Fig. 548. THIN SECTION OF THE ENAMEL AND A PART 

 OF THE DENTINE (from Kb'lliker). r f 



a, cuticular pellicle of the enamel ; 6, 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). 



seen on charring the tooth, by which the dentine 



becomes blackened, whilst the enamel, owing to 



the very small quantity of animal matter in its 



composition, remains white. According to Bibra 



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 



of magnesia and other salts 1'3 ; and has only 



3-5 per cent, of animal matter. 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, composed almost wholly of earthy matter, arranged closely to- 

 gether, side by side, and set by one extremity upon the subjacent surface of 

 the dentine. On the summit of the coronal portion of the tooth these 

 enamel fibres are directed vertically, but on the sides they are nearly hori- 

 zontal. As seen in a section they are disposed in gently waving lines, 

 parallel with each other, but not so regular as the curvatures of the tubuli 

 of the dentine, with which they have no agreement. The concurrence of 

 these parallel curvatures produces, as in the case of the dentine, an appear- 

 ance of concentric undulations in the enamel, which may be seen with a 

 lens of low power. A series of concentric lines is likewise to be seen 

 crossing the enamel fibres, as the contour lines cross the dentine : these are 

 termed coloured lines from their brown appearance, but they seem rather to 

 depend on lamination than on pigmentary deposit. Minute fissures not 

 unfrequently exist in the deep part of the enamel, which run between 



