196 ADVENTURES IN RADIOISOTOPE RESEARCH 



the incisal part ol the enamel is formed by extensive calcification of the outer 

 region of the dentine. In that case, the enamel could not be much less active 

 than the corresponding dentine part. The foundation of the incisal enamel is laid 

 in the apical end and reaches its final position without interchange with the 

 dentine. As both the P and Ca of the teeth have their origin in the plasma, the 

 application of labelled Ca as an indicator can be expected to lead to similar results 

 to those found above. 



EPIPHYSIAL AND DIAPHYSIAL BONE TISSUES 



The epiphysis was found to exchange the P content of its mineral constituents 

 at a higher rate than the diaphysis, as seen in Table 5. 



Table 5. Ratio of the Specific 



Activities of the Epiphysial and 



Diaphysial P of the Tibia of Rabbits 



Tlie level of the activity of the plasma 



inorganic P was kept constant 



all through the experiment 



In experiments with frogs, in which the labelled phosphate was injected into 

 the lymph sac at the start of experiments, taking 1 — 22 days, the ratio 1.3 — 1.6 was 

 found. In these experiments, the tibia and femur were both investigated and the 

 average was taken. In an investigation of the tibia P of rats, to which the labelled 

 phosphate was administered at the start of the experiment, the ratios 3.1, 2.9, 2.5, 

 1.7 and 1.8 were found after ~ hr., 4, 10, 50 and 110 days, respectiveh'. It is 

 of interest to remark that from the finding that the diaphysial P is only about 

 half as active as the epiphysial P 110 days after the start of the experiment, we 

 can conclude that an appreciable part of the skeleton has not been renewed 

 within 110 days. 



In experiments on chickens, Dols et al. [1939] found the above ratio to be 3, 

 22 hr. after administration of labelled phosphate: rachitic chickens gaxc the 

 ratio 2.5. 



The more rapid exchange of the epiphysial P is just what would be expected. 

 The epiphysis is characterized by a poorer mineralization of the matrix than is 

 the diaphysis and contains more organic matter and water than the diaphysis. 

 The circulating lymph, containing the labelled P, will therefore reach the apatite 

 surface more easily in the first-mentioned case. Should the size of the apatite 

 crystals be smaller in the epiphysis than in the diaphysis and theicfore the ratio 

 surface: volume Vjc larger in this type of bone tissue, one would also expect a 

 more rapid exchange of the mineral constituents of the epiphysis. Whethei such 

 a difference in the size of the apatite crystals actuaUy occurs is not known. X-ra> 

 investigations [Bale ct al. 1934] lead to the result that the size of the ultimate 



