162 ADVENTUKES IN RADIOISOTOPE EESEAECH 



was found in the case of adult rats. The high radioactivities of the bones 

 are due to the fact that in this case an appreciable part of the bones an^ 

 actually grown from blood of high radioactive phosphorus content ; 

 a rapid formation of new cells takes place, in whose building up radio- 

 active phosphorus participates. 



A very conspicuous difference is found between the active phosphorus 

 content of the molars of rapidly growing and of adult rats, the great 

 difference being due primarily to the low^ exchange values in the 

 latter. 



The brain as a whole was found to contain 0.5 per cent of the active 

 phosphorus taken by the animal. 



The ratio between the rad. P content of the muscles and the bones 

 is nearly unity in the case of the young rats, while in adult rats the 

 muscles show a higher rad. P content. 



When we compare the radioactive phosphorus content of the bones 

 of growing rats, we find for example more activity in 100 mgm of the 

 ashes of the bones of animals killed after one week than in those killed 

 after two weeks. This is due chiefly to the fact that the phosphorus atoms 

 present in the bone at a certain time will soon be found in an entireh' 

 new part of the growing skeleton, and will also have a certain chance 

 of leaving the skeleton entirely. If we want to obtain information on the 

 latter point we must compare the "radioactive" phosphorus contents 

 of whole skeletons. We carried out such experiments, comparing the 

 whole of the leg material. Five very young rats having a total weight 

 of only 25 gm were fed on their normal diet plus some radioactive 

 phosphorus (0.50 mgm each). Two were kihed 2 days later and three 

 65 days later. 10 mgm of the ashes of the leg bones of animals killed 

 after 2 days contained 8.4 times as much radioactive phosphorus as 

 that of rats killed after 65 days. The active phosphorus atoms were 

 in fact distributed all through the greatly increased amount of bone 

 tissue ; the leg bones increased in the course of 63 days to about ten times 

 their original weight, as can be seen from Table 9. When we compare 

 the radioactive phosphorus content of the total bone material of the 

 legs, the difference between the rats killed after 2 days and after 65 

 days is much less ; the difference still present is due to the loss of phos- 

 phorus atoms by the bone material. The phosphorus atoms which were 

 present in the bone for a while and left it again will be found partly 

 in the excrements but to some extent also in some of the organic 

 compounds building up the organism. In the course of two months 

 about one third of the phosphorus atoms originally present left the 

 skeleton. 



A comparison of the behaviour of the active phosphorus present 

 in the incisors with that in the bones is difficult in view of the rapid 

 using up and replacement of the incisors. Prof. Holst, Prof. Krogh 



