The effects of deprivation of carotene and anterior pituitary hormone on the partition of vitamin A in the blood and milk of the cow

Abstract

CALCIUM EXCHANGE IN BONE 391 phase certainly occupies a longer period in the calcium experiments than it did in the phosphate ones. If Neuman & Weikel's explanation, postulated for P04 exchange, is accepted it can be concluded that the exchange of calcium in this particular bone is mainly limited to the surface of the crystal, and the exchange of surface and sub- surface ions plays a very small part. The exchangeable calcium fraction of nearly 33 % is higher than that found by Falkenheim et al. (1951) in ashed rabbit bone (25 %). A pointer towards an explanation is that Robinson (1951), from electron microscope studies, found that the average bone crystal size was greater in cattle than in man, dog or cat (deceasing in this order). Whether there is sufficient difference in the crystal size, and, therefore, of the surface area available for ex- change, of cattle and rabbits to explain the differ- ences in calcium exchange is not known. If this is so, the amount of exchangeable calcium will depend on the species, age and type of bone. It may also depend on the technique used in measuring it; as Neuman & Neuman (1953) pointed out, any mani- pulative procedure, even washing with water, can introduce marked alterations in the state of an apatite crystal. SUMMARY 1. The percentage of exchangeable bone Ca in unashed compact ox bone has been determined as 33 % by the loss of 45Ca ions both from a radioactive solution, and from bone previously equilibrated with 46Ca. The results were similar.