148 A MANUAL OF DENTAL ANATOMY. 



CALCIFICATION. 



A tissue is said to be " calcified " when the organic struc- 

 tures of which it is composed are hardened and stiffened by 

 impregnation with salts of lime. The impregnation with 

 lime salt may go on so far that the residual organic matrix 

 is reduced to a very small proportion, as is exemplified in 

 the case of adult enamel, in which the organic constituents 

 make up only from one to three per cent, of the whole, so 

 that practically the enamel wholly disappears under the in- 

 fluence of an acid j or the organic matrix may persist in 

 sufficient quantity to retain its structural characteristics 

 after the removal by solution in an acid of its salts, as is 

 the case with dentine, bone, and cementum. There are 

 two ways in which a calcified structure may be built up : 

 the one by the deposition of the salts in the very substance 

 of a formative organ, which thus become actually converted 

 into the calcified structure ; the other by a formative organ 

 shedding out from its surface both the organic and inorganic 

 constituents, and thus, so to speak, excreting the resultant 

 tissue. 



An example of the latter method is to be found in the 

 shells of many mollusks, in which the mantle secretes the 

 shell, and is able to repair fractures in it, without itself 

 undergoing any apparent alteration j while the formation of 

 dentine, bone, and enamel (') are examples of calcification 

 by conversion. 



The insoluble salts of lime are altered in their behaviour 

 by association with organic compounds, a fact which was 

 iirst pointed out by Rainie, and has been more recently 

 worked out by Professor Harting and Dr. Ord. 



If a solution of a soluble salt of lime be slowly mixed 

 with another solution capable of precipitating the lime, the 



( a ) All observers are not, however, agreed as to the formation of the 

 enamel. (Cf. page 157.) 



