It has long since been realized that rickets runs an irregular course, characterized by remissions and exacerbations. Physicians came to this conclusion as the result of direct clinical observation, which was confirmed more recently by a study of roentgenographic films; it is a point emphasized by Schmorl and other pathologists, as well as by those who investigated the mineral metabolism of this disorder. This striking irregularity in the progress of rickets is due to the fact that there are exceptionally many factors which tend to bring about fluctuations in its course; among this number may be mentioned starvation, exposure to a greater or less degree of sunlight, and variations in the diet. No doubt, there are still other factors which tend to alter the course of a disorder which at present appears so complex in its etiology. Although the frequent occurrence of relapses or remissions is well recognized, the incidence