But for rickets vitamin D would not have been discovered. Its discovery was the secret to rickets; its use is essentially the therapy of that disease. The most direct way to furnish the foundation for intelligent use of vitamin D is to review briefly the sources and requirements of this factor from the point of view of practical therapeutics.
At the present time no less than ten forms of vitamin D are known,1 but only two have practical importance. One is activated ergosterol of plant origin, in its pure form known as calciferol and given the name of viosterol by the Council on Pharmacy and Chemistry; the other is activated 7-dehydrocholesterol, of animal origin, the form of vitamin D discovered by Waddell2 and the one which develops in the skin under the action of ultraviolet rays.3 The short ultraviolet rays of the sun or those derived from