GENERAL INTRODUCTION TO VITAMIN THEORY 17 



grains, such as would ensure protection against beriberi, were fed. 

 From this they proceeded to a thorough study of experimental scurvy 

 in the guinea pig, the results of which (to be described in Chapter 

 IV) indicated clearly that scurvy is due to the deficiency in the diet 

 of a substance soluble in water and rather easily destroyed by heating 

 under ordinary conditions, but more stable in acid than in neutral or 

 alkaline medium. 



Shortly after this, Osborne and Mendel (1913a) found that their 

 experimental animals became subject to a characteristic eye disease 

 when kept upon diets devoid of fat, while the simple addition of butter 

 fat to the diet sufficed either to prevent or to cure the disease. Further 

 experiments showed that cod-liver oil shared this property with butter 

 fat, and that beef fat showed it in lesser degree, while it was not shown 

 under like conditions by certain other fats such as lard or cottonseed 

 oil. As this eye disease involves an infection it is not so purely a "de- 

 ficiency disease" as scurvy and beriberi now appear to be, yet this dis- 

 tinction does not seem to us to be particularly significant since the 

 susceptibility to the eye trouble induced by a deficiency in the diet is the 

 chief determining cause of the disease. (It may also be noted that many 

 of the physicians who have had to do with beriberi and scurvy have 

 emphasized the view that the clinical forms of these diseases usually 

 show infection superimposed upon the nutritional deficiency.) 



Thus observations upon disease led to the conception of the exist- 

 ence of several substances of the vitamin type, needed for the pre- 

 vention of the "deficiency" diseases and for the maintenance of that 

 condition of healthy resistance which safeguards against susceptibility 

 to infections in other ways than through the ordinary immunological 

 mechanism. 



While in this way the conception of the vitamins has arisen largely 

 in connection, and is intimately associated, with that of the deficiency 

 diseases, yet from our present standpoint an even greater interest at- 

 taches to vitamins as chemical substances occurring in natural food 

 materials and having important functions in the normal processes of 

 nutrition. 



Early Evidence from Experiments upon Normal Nutrition 



In 1881, Lunin, in the course of a report upon investigations of 

 the significance of certain inorganic substances in animal nutrition, 

 remarked that since mice can be successfully nourished under labora- 

 tory conditions upon milk but not upon purified proteins, fats, carbo- 

 hydrates, salts and water, it follows that milk must contain other 



