A study of the literature on the etiology of rickets shows that practically no progress was made in the elucidation of the cause of this condition from the time of Glisson1 until the investigations of the past ten years. The theories advanced were almost without number2 and were apparently limited only by the imagination of the different investigators. Credit belongs to Mellanby,3 in 1918, for directing attention to the presence and importance of an antirachitic factor, or vitamin, in the prevention of this disease. Whether this factor was identical with or just allied to vitamin A was not clear until McCollum, Simmonds, Becker and Shipley,4 in 1922, produced conclusive evidence of the presence of a separate and distinct antirachitic vitamin which has since been designated as vitamin D. The most concentrated source of this vitamin is cod liver oil.
The curative action of ultraviolet rays from