Findlay (18) has continued as has also Paton (19), to emphasize that the opportunity which is afforded for pups to exercise is the determining factor as to whether or not they will be likely to develop rickets. Recently these investigators have been inclined to accept infection as playing the major role in the production of rickets.

It will be evident to one who gives careful scrutiny to the recorded experimental studies of this subject, that as yet no thoroughgoing research has been carried out for the purpose of making clear the etiology of rickets. Shipley, Park, Simmonds and McCollum (20) have approached the problem from an essentially new viewpoint. They observed in the course of extensive studies of nutrition problems with a large rat colony, that certain animals whose diets were faulty in one or more respects, developed a condition which presented the gross appearance of rickets both as respects external appearance and conditions which could be observed at autopsy. They were convinced that only by a most elaborate study of the subject, that is, by systematically varying the composition of the diet with respect to one or more factors, and by means of careful histological studies of the bones of their animals, could one expect to reveal the relation of the diet to the disease. Since their animals were all kept in a north-east room, into which sunlight enters only in small amount, and all of the colony were under essentially uniform conditions as respected illumination, temperature, ventilation and opportunity for exercise, they were convinced that any differences which could be demonstrated in the histological structure of the bones must be due to the character of their experimental diets, which formed the only variable. Radiographs cannot be of much value in studies in this kind since they do not reveal the nature of the anatomic changes which take place under experimental conditions. These studies have progressed far enough to establish certain important facts concerning the etiology of rickets.