172 ADVENTURES IN RADIOISOTOPE RESEARCH 



calcium by, for example, magnesium takes place, governed chiefly by 

 solubility (chemical affinity) conditions, and it is quite conceivable 

 that in the course of time more and more calcium is replaced by magne- 

 sium if the magnesium: calcium ratio is in favour of such an exchange. 

 The enamel being characterised by a decidedly poorer lymph circulation 

 than the dentine, the much lower magnesium content of the former can 

 easily be accounted for by a reference to the above considerations. 

 The 0.42% magnesium found in the human enamel possibly got into 

 the latter wholly or to a large extent during the formation of the enamel 

 tissue. The presence of as much as -i^j^ of magnesium in elephant den- 

 tine is possibly due to a high magnesium content of its food or to a 

 high magnesium retention in its blood, ft is also of interest to remark 

 that the magnesium content of the teeth found in prehistoric skelet- 

 ons is only one third of that found in leeth of recent generations, fur- 

 thermore that carious teeth^ show a greatly increased magnesium con- 

 tent. Besides the elements discussed above, spectroscopic investiga- 

 tion^ revealed the presence of traces of Na, Ag, Sr, Ba, Cr, Sn, Zn, Mn, 

 Ti, Ni, V, Al, Si, B and Cu in dental tissue. 



That the concentration of the minor constituents of the teeth does 

 not fluctuate between still wider limits is due to the narrow limits 

 within which the concentration of most elements in the blood plasma 

 is restricted. This is caused partly by a prevention of the resorption 

 of excessive quantities of the elements, conspicuously show^n in the case 

 of calcium, and partly by prompt removal chiefly through the kid- 

 neys of excessive amounts of the mineral constituents present in the 

 plasma. But even in spite of this levelling mechanism of the blood 

 plasma some of the mineral constituents are deposited to a remark- 

 able extent in the tooth tissue, as is seen above in the case of flu- 

 orine, and it is quite possible that exon an excessive replacement of, 

 for example, the calcium by magnesium, sodium, or potassium might 

 lower the resistance of teeth to disease. 



While the conclusions given above are based partly on hypothetical 

 assumptions, in the case of lead, which also replaces calcium in the 

 crystal lattice, the accumulation in the teeth with time can clearly be 

 shown. While small children have only negligible amounts of lead in 

 their teeth, the lead content increases with age^, the increase being 

 markedly greater in the case of carnivorous than herbivorous animals, 

 presumably on account of a greater lead intake in their normal nourish- 

 ment. In the case of lead poisoning the lead content of teeth is greatly 



(i) T. Francia, Ann. Clin. Odoniat. 8, (iil.'j, (1931); M. M. Murray uirI 



J. H. Bowes, Brit. Dent. J. 61, 473, (1936). 

 ^2) E. LowATER and M. M. Murray, Biochem. J. 31, S37, (1937). 

 ^3) F. Pfrieme, (1934) Arch, f. Hyg. Ill, 232. 



