VIII. REQUIREMENTS 253 



(ion in milligrams per cent is less than 30, rickets exists; if it is above 40, 

 rickets is not present or is healing. Studies on in vitro calcification indicate 

 that the blood of the normal child is slightly undersaturated with respect 

 to secondar}^ calcium phosphate, and that equilibration experiments with 

 bone and artificial sera, as well as similar experiments using either tertiary 

 calcium phosphate or hydroxyapatite as the substrate, all point to the fact 

 that the determining factor in precipitation is the solublity product of this 

 compound (secondary calcium phosphate). Since both components enter- 

 ing into this product are represented by the first power, it follows that there 

 must be some relationship between the actual ion product and the product 

 expressed in this simple non-chemical way in terms of milligrams of each 

 component in 100 ml. of serum. 



It must be emphasized, however, that the product is not the only factor 

 determining calcification. In rapidly growing premature infants osteoid 

 formation may proceed so rapidly that calcium salts deposition fails to 

 keep pace although the Ca X P product is normal. 



Obviously the Ca X P product is only one basic essential for calcifica- 

 tion. Only certain tissues will calcify, although all are bathed presumably 

 with the same tissue fluid having its origin in the same plasma. Local pro- 

 cesses probably bring about a further increase of the product, presumably 

 by increasing the anion level. The many factors which may play a role in 

 this complicated process have already been discussed. Alkaline phosphatase 

 is the only one of the enzymes involved which can be readily measured. 

 The number of units reported in a particular case represents the number of 

 milligrams of inorganic phosphorus liberated by the amount of enzyme in 

 100 ml. of serum. The normal plasma values for children vary between 5 

 and 15 units. In active rickets this value is increased, and it is diminished 

 during healing. As the excess tissue either becomes calcified or resorbed, the 

 phosphatase concentration in the plasma tends to return to normal. 



VIII. Requirements 



A. OF ANIMALS 



JAMES H. JONES 



So many factors influence the re(iuireni.ents for vitamin D that it is diffi- 

 cult to express (luantitatively the need of animals for this \itamin unless 

 the environment and diet are known. The most important of these factors 

 is sunlight. It is well recognized that exposure to sufficient summer sun- 

 shine makes the giving of vitamin D uiuiecessary. Consequently, the de- 



