Primarily, three groups of cases were selected for blood studies: cases (1) of milk rickets; (2) of typical severe rickets, and (3) of active tetany showing rachitic changes. The cases covered prematurely born, full-term, breast-fed and artificially fed infants. The majority of the infants were between 4 and 12 months of age. The calcium and inorganic phosphorus content of the blood plasma was determined before treatment, and in some cases after treatment. The studies were started with a possibility in mind that rickets might be classified into low phosphorus and low calcium groups, and a third intermediate group; and that certain clinical manifestations might be anticipated by a study of the blood in infants.
Howland and Kramer1 determined the inorganic phosphate of the serum in a series of normal children. They found the concentration to average 5.4 mg. per hundred cubic centimeters, with a minimum of 4 mg. and