THE DENTAL TISSUES. 57 



of dentine which differs most markedly from bone ; in other 

 words, which has the most typical " dentinal " structure ; 

 and for that purpose the tissue met with in the teeth of man 

 and the majority of mammalia, (though it is by no means 

 confined to that class,) and known under the name "hard" 

 or " un vascular" dentine may be selected. 



Dentine is a hard, highly elastic substance, in colour 

 white with a slight tinge of yellow, and to some extent 

 translucent, its transparency being often made more striking 

 by contrast with the opacity which marks the first advent of 

 dental caries. When broken a silky lustre is seen upon the 

 fractured surfaces, which being in the main due to the presence 

 of air in its tubes, is more apparent in dry than in fresh 

 dentine ; its fracture is sometimes described as finely fibrous. 



The mass of the dentine consists of an organic matrix 

 richly impregnated with calcareous salts ; this matrix is 

 everywhere permeated by parallel tubes, which run, with 

 some deviations, in a direction at right angles to the surface 

 of the tooth. 



The Matrix. The exact chemical composition of the 

 matrix is not known ; in man the proportion borne by the 

 organic to the inorganic constituents varies in different 

 individuals, and very probably in the same individual at 

 different ages, so that analyses can only give approximate 

 results. In a fresh human tooth 62 per cent, of its weight 

 was found to be inorganic salts, the tooth cartilage being 

 28 per cent., leaving a residue of 10 per cent, of water. 



Von Bibra gives the following analysis of perfectly dried 

 dentine : 



Organic matter (tooth cartilage) . 27*61 



Fat 0-40 



Calcium phosphate, and fluoride . 66*72 



Calcium carbonate . . . . 3 '36 



Magnesium phosphate . . . . 1*18 



Other salts 0'83 



