44 



STRUCTURE OF TEETH 



of the nature of teeth is founded. 



I ^ ir^ M . 



ITi 



It has been pointed out 

 that the scales of the Elas- 

 mobranch fishes consist of 

 a cap of enamel upon a 

 Ijase of dentine, the former 

 Ijeing derived from the epi- 

 dermis and modelled upon 

 a papilla of the dermis 

 whose cells secrete the 

 dentine. TJie fact that 

 similar structures arise 

 within the mouth {i.e. the 

 teeth) is explicable when 

 it is remembered 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 

 dentine, and 

 cement (or crusta petrosa 



Fig. 33. — Diagrammatic sections of various forms 

 of teeth. /, Incisor or tusk of Elephant, 



ivitli pulp cavity persistently open at base ; the enamel 

 //, Hnman incisor during development, with 

 root imperfectly formed, and pulp cavity 



widely open at ba.se ; III, completely formed aS it is Sometimes Called) 



Human incisor, with pulp cavity opening by . j. • i- . i mi i j.j. 



a contracted aperture at base of root; IV, IS not indicated. 1 he latter 



Human molar with broad crown and two J^aS the closest resemblance 

 roots ; V, molar of the Ox, with the enamel . . 



covering the crown deeply folded, and the tO bonC. The dentine IS 



depressions filled up with cement ; the sur- traversed by fine canals 



lace IS worn by use, otherwise the enamel . ■' 



coating would be continuous at the top of the wllich run parallel to Cach 



ridges. In all the figures the enamel is black, „4-V,p,, and amstni-nncp liovo 



the pulp white ; the dentine represented by °^^^^ ^^^ auastomose lieie 



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



(After Flower and Lydekker.) o i p i 



lormed of long prismatic 

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



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



