STUDIES OF CALCIUM AND PHOSPHORUS METABOLISM : V. A STUDY OF THE BONE TRABECULÆ AS A READILY AVAILABLE RESERVE SUPPLY OF CALCIUM.

1. Bone trabeculae are easily depleted by the prolonged administration of parathormone, long continued negative calcium balance and growth. 2. A long continued high calcium diet results in a rapid accumulation of the trabeculae. 3. Alizarin red, as has previously been shown in the literature, is deposited in newly formed bone. Its use has made clear that easily mobilizable calcium is not deposited in the shafts of adult animals, but in the trabeculae of bone. 4. The bone trabeculae therefore serve as the storehouse of readily available calcium. 5. The shafts have a slow progressive exchange of inorganic salts and are not influenced except in the case of unusual body demands. 6. It is suggested that the absence of trabeculae in premature infants and their depletion at the end of 4 months in a normal baby might well be an etiological factor in rickets. 7. In our observations, parathormone administration to growing or adult cats has been without effect. 8. Daily injections of parathormone in growing rats results in an increased number of trabeculae and smaller bones.