38^2 F. C. MCLEAN AND R. E. ROWLAND 



The resorption caxities are filled in hv formation of new bone in 

 eoncentric layers or lamellae, insuring a constant snpplv of new or 

 metabolic bone; this new bone takes up radioisotopes, such as Ca^' 

 and Ra"", preferentially, leading to concentration in hotspots, de- 

 monstrable bv autoradiography. As metabolic bone matures it loses 

 its reactivity, and adds to the mass of structural bone. It is suggested 

 that maturation of the bone, with a decrease in its reactivity, repre- 

 sents a change in the chemical nature of the mineral. 



The reactive bone in the newly forming osteons participates in 

 the exchange of Ca++ between bone and blood; this is in part under 

 control of the parathyroid glands, and in part independent of them; 

 the parathyroids, however, serve to control the level of Ca++ in 

 the blood plasma. It is suggested also that vitamin D mav plav a 

 part in the regulation of exchange of calcium between blood and 

 bone. 



Acknowledgment. This study was aided by Research Grant A-4225 

 from the National Institute of Arthritis and Metabolic Diseases, National 

 Institutes of Health, United States Public Health Service. 



Kefkhences 



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