l^(^ ADVEXTURES IX RADIOISOTOPE RESEARCH 



are to be found after the lapse of 5 clays in a single tooth. Making use 

 of the figures quoted in Table 20 we find that a canine takes up 0.005 

 mgm and a molar 0.003 mgm. 



THE BEHAVIOUR OF THE ENAMEL 



The difference in the mechanical properties of dentine and enamel 

 is very pronounced. The hardness of anterior enamel is nearly half as 

 (Treat as that of hardened toolsteel, while dentine compares closely 

 with brassi. The hardness is taken as the pressure in kilograms necessary 

 to push a steel ball into the test piece. 



The above mentioned difference is not due to a pronounced difference 

 in the relative abundance of the mineral constituents of dentine and 

 enamel, as discussed on p. 5, but to the following conditions. The amount 

 of organic constituents +w^ater found in dentine is about six times as 

 large as the amount present in enamel, the calcification of the enamel 

 tissue being thus carried through much more effectively than that of 

 the dentine tissue. Bowes and Murray^ found organic matter in human 

 enamel to an extent of only 1%. As there is more organic matter^ in 

 enamel near the junction with the underlying tissue, the dentine, than 

 in the part equidistant from the dentine and the surface of the teeth, 

 the outer part of enamel must contain even less than 1% organic matter. 

 The latter appears to be* a protein containing tyrosin and resembling 

 reticulin. 



Another outstanding difference between dentine and enamel seems 

 to be the size and degree of orientation of the crystahites present in 

 these. As to the orientation it has been stated^ that enamel of high 

 quality gives X-ray diagrams of a high degree of orientation, while 

 enamel of poor quality does not. On igniting dentine an X-ray diagram 

 characteristic of ^-Ca3(P04)2 is often but not always observed'; this is 

 never shown by ignited enamel. As it was found' that i3-Csi^{F0^)^ is 

 formed when an excess of PO^-ion is present, it was concluded that the 

 dentine apatite often adsorbs an excess of phosphate ion which promotes 

 the formation of j3-(\{V0^)^ on ignition. In the case of enamel forming 

 larger crystallites, no excess of PO^-ions being present, no j3-(\{FO^).2^ 



1 H. C. Hodge, J. Dent. Res. 15, 251 (193(J). 



2 J. H. Bowes and M. M. Murray, Biochem, J. 29, 721 (1935). 



3 C. F. BoDECKER, J. Dent. Res. 6, 2, 117 (1923). 

 * P. PiNCUS, Nature 138, 970 (1936). 



5 J. Thewlis, Naturw. 25, 42 (1937). 



6 W. F. Baxe, M. L. Lefevre and H. C- Hodge, Naturw. 24, 976 (1936). 

 " G. Trommel and H. Moller, Z. anorg. Chetn. 206, 227 (1932). 



