APPLICATIONS OF ISOTOPIC INDICATORS 



943 



the specific activity of the bone apatite P woukl be only 1/100 of that 

 of the free plasma P. 



The determination of the degree of renewal based on the said adminis- 

 tration is made difficult by the fact that the specific activity of the 

 free plasma phosphorus varies throughout the experiment. After ad- 



+ 2 



+ 1 



- 



■I - 



50 



100 



150 



200 



Hours. 



Fig. 4. — Change with time in the logarithm of the labelled P 

 content of the plasma after intravenal injection of labelled phosphate 



into a rabbit. 



ministration by the mouth or by subcutaneous injection, the specific 

 activity first increases and subsequently decreases with time, while after 

 administration by intravenal injection it first decreases very rapidly, 

 and later at a moderate rate, as seen in Figs. 3 and 4.^*^^ The deter- 

 mination of the extent of renewal of the mineral P in the skeleton is 

 much facilitated by keeping the free plasma P activity at a constant 

 or almost constant level. This can be attained by repeated injections 

 of varying amounts of labelled phosphate throughout the experiment. 

 Making use of this technique, the data given in Table 5 were obtained 

 for the extent of renewal of the different parts of the skeleton of the 

 adult rabbit in the course of 50 days^*°\ As seen in Table 5, 70 per 

 cent of the epiphysis and 93 per cent of the diaphysis remained unchan- 



