7 RADIOSTRONTIUM IN HUMAN BONES A list of human bone specimensto the end of 1956 analyzed at Woolwich is given in Ap- pendix 3, and the radiostrontium results* on 34 femora and 3 tibiae among them are shownin Fig. 7. The specimens come mostly from South East England and the Midlands, but there were a few from the West and Northwest, and these included the two showing the highest radiostrontium activity, namely, 1.55 S.U. in a two-year-old Plymouth child and 1.3 S.U. in a three-monthold child from Carlisle.t There is a considerable scatter in the results on bones from infants under 1 year, but thereafter there is a fall off in the Sr*’/Ca ratio with age. a -— a— 1.2 Pre —— gx 4 1.0 -— — T 5 - bx _ S osx- x x ¢ x xx * O6 — | * CX — _ 0.4 x mi ; x a *x x x * x —4 x! eel ttt tiittiitiuit ih it x { 2 3 4 5 6 y, 8B 9 10 4 #12 AGE, YEARS #13 4 «15 «#16 26 40 60 Fig. 7—Sr™ in human bone in 1956. The average Sr°? activity of femora in various age groups is given in Table 8 at the end of this paper. As the minimum weight of bone for a reasonably accurate Sr°? estimation is 50 g, it was not possible to make a detailed study of distribution within the bone, but two femora from older children were each divided into four portions which were analyzed separately as shown in Table 6. The pattern is the same in both specimens, with a maximum in the new sub-epiphyseal bone and a minimum in the old bone in the centre of the shaft. Stable Sr measurements have been made on a number of human bones, and the results are given in Appendix 3. The average at all ages in the present series is 290 ug of Sr per gram of Ca, which is not significantly different from the average of 270 ug of Sr per gram of Ca found in milk. Since, however, the stable Sr content of foods not derived from milk is about 10 times greater than this, it is by no means certain that milk is the main source of stable Sr in human bone. *As stated in the paragraph on methods, the practice with human bones has been to measurethetotal radiostrontium activity and attribute it to sr®®, On specimens for which y*" has been measured independ- ently, the Sr*® activity deduced has been within 15 per cent of that estimated by the total strontium method. + Provisional results of 2.3 S.U. in a one-year-old Cumberland child and 2.4 S.U. in a six-month-old Liverpool child have been obtained in 1957. 220