EXCHANCiK <)1' I'JtOSI'HOins IX TKKTH 1^5 



in fully grown ones and also thai while in the i'ormer case the canines 

 lake up 3 1() 4 times as much labelled P (per mgm ash) as the molars, 

 in the latter case no such difference is found. As has already been men- 

 tioned above the figures for the two sets of experiments are not entirely 

 comparable, but no objection can be raised against a comparison of the 

 ratio of the canine and molar uptake, which differs very markedly in 

 the case of growing rats from the ratio for fully grown animals. The 

 following is a possible explanation of this difference: the lal)elled P 

 uptake in the teeth of young rats is due partly to a growth of the teeth 

 and not to an exchange process; since in the cat the canines grow faster 

 than the molars the uptake is greater in the former case. One would 

 be inclined to object to this explanation in view of the short dura- 

 tion of the experiment, as the growth in the course of few hours may 

 l)e entirely negligible. This objection is however unwarranted. The 

 molars of the growing cat weighed 116 mgm and those of the fully 

 grown animal 691 mgm. It does not take longer than a few years for the 

 growing cat to become fully grown so the yearly growth of a molar will 

 be above 100 mgm. 



Let us now calculate the amount of tooth ash formed on the 

 assumption that the labelled phosphorus found in the tooth is 

 due to growth. A molar of the growing cat took up 0.016% labelled 

 P during 3.5 hours. The labelled P which we injected into growing cats 

 had in most cases a negligible weight originally, but very soon after 

 the injection it mixed with the inorganic phosphate of the plasma 

 (corresponding to about 5 mgm P) and from that moment we must 

 consider the labelled P as having a weight of about 5 mgm 0.016% 

 of the labelled P will therefore correspond to 0.0008 mgm P. The next 

 step is that a large part of the labelled phosphorus leaves the plasma 

 and is replaced by other phosphorus atoms coming from different bodily 

 organs and also from the blood corpuscles. The result is that 0.016% 

 of the activity given no longer represents 0.0008 mgm P but a greater 

 weight, our scale of indication becoming less and less sensitive. From the 

 experiences of Prof. Lundsgaard and one of us on the exchange of 

 phosphorus present in the plasma we can estimate roughly that the amount 

 of P which corresponds after the lapse of 3.5 hours to 0.016% of activity 

 is about 0.008 mgm in the case discussed. To transform from phosphorus 

 weight to ash weight we have to multiply by six. The weight of the 

 tooth thus increases by 0.04 mgm in 3.5 hours and about 100 mgm 

 in a year. The order of magnitude of the growth observed and that 

 calculated on the assumption that the uptake of labelled P is due to 

 growth is thus the same. 



A very simple but instructive calculation can be carried out in tiie 

 case of a fully grown cat into which as much as 120 mgm labelled P 

 was injected. We can calculate how many milligrams of these 120 mgm 



