THE METABOLIC MACHINERY OF ANIMALS 279 



Vitamin D, better known as the antirachitic vitamin, is chiefly con- 

 cerned with maintaining an adequate supply of phospliorus and 

 calcium in the blood, bones, and teeth. The discovery of this vitamin 

 is associated with a study of rickets. Early workers noted that cod- 

 Uver oil had a beneficial effect. The cure was attributed to Vitamin 

 A until, in 1923, McCollum of the Johns Hopkins University and his 

 co-workers showed that the efficacy of cod-liver oil remained even 

 after treatment which destroyed Vitamin A, an observation which 

 led to the identification of Vitamin D. The best sources of this 

 vitamin are cod-liver oil, butter, and egg yolk. 



More recently it has been shown that the precursor or "pro- 

 vitamin" of Vitamin D, a substance known as ergosterol, will yield the 

 vitamin after irradiation with ultraviolet light. Ultraviolet rays of 

 the sun, or X-rays, are likewise a great help in overcoming rickets. 

 At the present time four methods are used to increase the amount of 

 Vitamin D in the bodj' : (1) irradiation of the skin by exposure to 

 sunlight or other sources of ultraviolet light ; (2) the addition of 

 cod-liver oil to the diet ; (3) the introduction of irradiated ergosterol 

 (viosterol) ; and (4) the use of Vitamin D concentrates in foods. This 

 latter method has been most successfully introduced by Zucker, by 

 the addition of this concentrate to milk, thus facilitating its adminis- 

 tration to young children. 



A survey of the prevalence of rickets shows that this disease is 

 much more common than has been supposed, especially in young 

 children, a fact strikingly brought out when 83 per cent of a group 

 of over 200 children from New Haven, Connecticut, who were exam- 

 ined by X-ray, showed mild evidence of rickets. 



Vitamin E is commonly known as the antisterility vitamin. This 

 important substance has been shown to be present in greatest quantity 

 in lettuce, whole wheat, and, to a somewhat lesser degree, in egg yolk 

 and milk. It is fat-soluble and quite resistant to heat. There is 

 evidence suggesting that the animal body has the ability to store this 

 vitamin. 



The Activators — Enzymes 



It will be recalled that the metabolic processes of plants and animals 

 include about every type of reaction known to the chemist. It has 

 been demonstrated that enzymes not only are essential for diges- 

 tion, but also that all chemical changes in the body are mediated by 

 enzymes. Glucose may be taken as an example. The decomposition 



H. W. H. — 19 



