372 THE TISSUES. 



the thickness of the enamel; and the dentinal tubes often extend to 

 some distance between them. 



Chemical Composition. Enamel contains but two parts of water 

 in IjOOO, 1 and is the hardest substance in the human body. It can 

 hardly be touched by the knife, and yields sparks with steel. Ena- 

 mel does not, like dentine, contain cartilage (osteine) ; but its organic 

 matter only 2 to 6.6 per cent, of the dried mass appears like a 

 membranous tissue, after being treated with acids. According to 

 Lehmann, dry enamel contains 81 to 88 per cent, phosphate of lime, 

 with 7 or 8 per cent, of carbonate of lime; and Berzelius found 

 3.2 per cent, of fluoride of calcium in the enamel of a human tooth. 

 The following is Yon Bibra's analysis: 



Organic matter . . . 3.59 

 Inorganic matter . . . 96.41 



Cartilage (?) 3.39 



Fat 0.20 



Phosphate of Lime with some Fluoride of Calcium 89.82 



Carbonate of Lime 4.37 



Phosphate of Magnesia 1.34 



Salts 0.88 



In young or developing teeth, the enamel is soft, and may be cut 

 with a knife; and here the fibres may be easily isolated. They 

 also show transverse striaB, somewhat like striated muscular fibre, 

 (Fig. 235,) especially after the addition of hydrochloric acid ; but a 

 further addition converts the fibres into transparent tubes, and 

 finally entirely dissolves the latter. It follows that neither this acid, 

 nor any other agent that acts upon the enamel, should form a part of 

 a dentifrice, or a wash for the teeth. 



III. THE CEMENTUM. 



The cementum is a layer of true osseous tissue covering the fangs 

 and the necks of the teeth. It commences where the enamel ter- 

 minates, as a very thin layer, and increases in thickness to the end 

 of the fang. Internally it is very intimately united with the den- 

 tine, but without any intermediate substance. Externally it is very 

 closely surrounded by the periosteum of the alveoli; but it is less 

 firmly united with the gum. It is the softest of the three dental 



1 Robin and Verdeil, vol. ii. p. 115. 



