f EEM. 



85 



Enamel is an exceeding hard substance, containing only 

 3|- per cent, of animal matter, while the rest of it consists 

 of phosphate of lime and other 

 earthy salts. It is composed 

 of solid vertical columns or 

 prisms, the sides of which 

 closely fit to one another, like 

 pillars of basalt, but are by 

 no means strictly parallel, for 

 they interlace to a certain 

 extent. They are about -g-^Vfr 

 inch diameter, and have a 

 transverse striatioii which is 

 particularly distinct in the 

 young state. The enamel 

 prisms are crossed by bands 

 of a brownish tinge called 

 coloured lines, which have 

 probably a similar origin to 

 the contour lines of the den- 

 tine, both being most fre- 

 quent in old teeth. 



Fig. 46. ENAMEL, a, a, 

 tudinal section; &, free 



longi- 

 outer 



The crusta petrosa or cement extremities of the 'prisms, seen 

 (fig. 45, d,) is softer than in perspective; c, dentine, 

 dentine, and consists of a deposit of layers of solid bone- 

 matrix, in which are sparsely scattered lacunae, with very 

 irregular canaliculi coming off from them. In the human 

 teeth it is confined to the root, but in the complex crowned 

 teeth of some animals, for example, the molars of the ox, it 

 fills up depressions which dip deeply down into the crown, 

 and is there situated superficial to the enamel. When such 

 a tooth is worn, as it soon is after coming into use, the dark 

 coloured crusta petrosa, filling up the complicated depres- 

 sions, is seen surrounded by lines of pure white enamel, on 

 the other side of which is the yellow dentine, somewhat 

 hollowed out, as is also the crusta petrosa, from being softer 

 than enamel. 



55. Teeth, in their first development, have a consider- 

 able resemblance to hairs ; for both make their first ap- 

 pearance, in the embryo, from tegumentary depressions 



