296 THE VITAMINS 



evidence of the existence of vitamin D, as an entity distinct from 

 vitamin A. 



They reported experiments comparing : ( 1 ) the potency of different 

 oils to cure xerophthalmia; (2) the values of these same oils in pro- 

 moting growth of young rats on diets very deficient in calcium ; and 

 (3) the values of these oils for inducing deposition of calcium salts 

 in rachitic bones by means of the "line test" (described further on 

 in this chapter). In discussing and summarizing their results, they 

 stated that "cod-liver oil oxidized for 12 to 20 hours does not cure 

 xerophthalmia in rats." It does, however, cause the deposition of calcium 

 in the bones of young rats which are suffering from rickets. This shows 

 that oxidation destroys fat-soluble A without destroying another sub- 

 stance which plays an important role in bone growth. They wrote : 



"Cocoanut oil is shown to be lacking in fat-soluble A since it will 

 neither prevent nor cure xerophthalmia. This oil. on the other hand, 

 contains a substance which stimulates the deposition of calcium salts 

 in a manner similar to cod-liver oil. It is, like butterfat, far less effective 

 from a quantitative standpoint. 



"The evidence set forth in this paper demonstrates that the power 

 of certain fats to initiate the healing of rickets depends on the pres- 

 ence in them of a substance distinct from vitamin A. These experi- 

 ments clearly demonstrate the existence of a fourth vitamin whose 

 specific property, as far as we can tell at present, is to regulate the 

 metabolism of the bones. 



"Animals can undergo apparently normal development when fur- 

 nished a very small amount of the calcium-depositing vitamin, pro- 

 vided the diet contains somewhere near the optimal content of calcium 

 and phosphorus. The adjustment of the calcium content of these diets 

 at a very low level has enabled us to demonstrate the difference in 

 the quality of fats in respect to their content of calcium-depositing 

 vitamin in a manner that would have been impossible if the diets had 

 contained a more favorable concentration of calcium and phosphorus." 



Steenbock and Nelson (1923) confirmed McCollum's finding that 

 the antirachitic and antiophthalmic vitamins are distinct entities in 

 a very extensive and carefully planned series of experiments. 



Ultra-violet Light and Vitamin D 



In 1913 Raczynski found sunlight favorable to calcium assimila- 

 tion in puppies, and definitely suggested a connection between the ab- 

 sence of sunlight and rickets. 



Huldschinsky (1919, 1920) reported that ultra-violet light cured 



