286 OP THE PRIMARY TISSUES OF THE HUMAN BODY. 



enamel-prism is originally formed ( 280). In a perfect state, the Enamel con- 

 tains but an extremely minute quantity of animal matter; but if a young tooth 

 be examined, it is found that, after the calcareous matter of the tooth has been 

 dissolved away by an acid, there remains a set of distinct prismatic cells, which 

 formed (as it were) the moulds in which the mineral substance was deposited. 1 

 The Enamel, when once formed, appears to undergo scarcely any further change, 

 and it possesses no power of self-regeneration after loss of substance by injury 

 or disease. 



277. The Cementum or Crusta Petrosa corresponds in all essential particulars 

 with Bone, possessing its characteristic lacunae, and being also traversed by vas- 

 cular medullary canals, which pass into it from its external surface, wherever it 

 occurs in sufficient thickness (as in the exterior of the tooth of the extinct 

 Megatherium, and in the thick plates interposed within the islets of Enamel in 

 the teeth of Ruminants, Rodents, &c.) ; in Man, however, in whose teeth the 

 Cementum is very thin, such vascular canals do not usually exist, though Mr. 

 Tomes states (Op. cit., p. 57) that he has occasionally met with them. The 

 Cementum was formerly supposed to be restricted to the compound teeth of 

 Herbivorous animals \ and its presence in the simple teeth of Man and the Car- 

 nivora can be shown only by the application of the Microscope. In the latter 

 it forms a layer, which invests the fang, and which decreases in thickness as it 

 approaches the crown of the tooth (Fig. 73, 2, 7) ; at the time of the first emer- 

 sion of the tooth, it covers the crown also with a very thin lamina, which is 

 speedily worn away by use ; on the other hand, its thickness around the apex 

 of the fang often undergoes a subsequent increase, especially when chronic 

 inflammation and thickening take place in the membranous contents of the 

 socket (d). 



278. The following are the results of the most recent Chemical analyses of 

 the component structure of Human Teeth : a 



Incisors of Adult Man. 



Dentine. Enamel. Cementum. 



Organic matter . .- . . 28.70 3.59 29.27 



Earthy matter . . ;. . 71.30 96.41 70.73 



100.00 100.00 100.00 



The proportion of these two components varies considerably in different species ; thus 

 the organic basis of the Elephant's tusk forms as much as 43 per cent, of the whole. It 

 would seem even to vary considerably in different individuals of the same species ; thus in 

 the molar teeth of one man, Von Bibra found the organic matter to constitute as little as 

 21 per cent, whilst in another it was 28. The following analyses afford a more particular 

 view of the components of each substance: 



Molars of Adult Man. 



Dentine. Enamel. 



Phosphate of Lime, with trace of fluate of lime 66.72 89.82 



Carbonate of Lime 

 Phosphate of Magnesia 

 Other Salts 

 Chondrin (?) 



3.36 4.37 



1.08 1.34 



0.83 0.88 



27.61 3.39 



Fat ..... 0.40 0.20 



100.00 100.00 



1 The Author has discovered a structure precisely resembling this, in the shells of many 

 Mollusca. See " Reports of the British Association" for 1844 and 1847. 

 3 See Von Bibra's "Chemische Untersuchungen iiber die Knochen und Zahne." 



