44 



STRUCTURE OF TEETH 



of the nature of teeth is founded. 



I ^ Mm m: 



ITi 



It has been pointed out 

 that the scales of the Elas- 

 niobranch fishes consist of 

 a cap of enamel upon a 

 ])ase of dentine, the former 

 l)eing derived from the epi- 

 dermis and modelled upon 

 a papilla of the dermis 

 whose cells secrete the 

 dentine. The fact that 

 similar structures arise 

 within the mouth {i.e. the 

 teeth) is explicable wdien 

 it is rememl)ered that the 

 mouth itself is a late in- 

 vagination from the out- 

 side of the body, and that 

 therefore the retention by 

 its tissues of the capacity 

 to produce such structures 

 is not remarkable. 



The relations of the 

 three constituents of the 

 tooth in its simplest form 

 is shown in the accom- 

 panying diagram, where 

 the intimate structure of 

 the enamel, dentine, and 

 cement (or crusta petrosa 



Fig. 33. — Diagrammatic .sections of various forms 

 of teetli. /, Incisor or tusk of Elepli.ant, 

 with pulp cavity persistently open at liase ; 

 //, Human incisor during development, with 

 root imperfectly formed, and pulp cavity 

 widely open at base ; ///, completely formed aS it is Sometimes called) 

 Human incisor, with pulp cavity opening by 

 a contracted aperture at base of root ; /I', 

 Human molar with broad crown and two 

 roots ; T', molar of the Ox, with the enamel 

 covering the crown deeply folded, and the 

 depressions filled up with cement ; the sur- 

 face is worn by use, otherwise the enamel 

 coating would be continuous at the top of the wlllch run parallel to eacll 

 ridges. In all the figures the enamel is black, ^^i^^^. .^^^^^^ aiiastomose here 

 the pulp white ; the dentine represented by 



horizontal lines, and the cement by dots, and there. The enamel is 

 (After Flower and Lydekker.) formed of long prismatic 



fibres, and is excessively hard in structure, containing less animal 

 matter than the other tooth tissues. To this fact is frequently 



is not indicated. The latter 

 has the closest resemblance 

 to bone. The dentine is 

 traversed by fine canals 



