VITAMINS — HOPKINS 269 



experiments of E. Mellanby on the production of rickets were already 

 in progress. A few years later, interest in the subject penetrated 

 into every European country, and research became everywhere very 

 active. Recently, publications dealing with vitamins have reached 

 a total of a thousand in a single year. 



Today we have knowledge of some eight or nine vitamins, each 

 proved to have its own specific influence in maintaining the normal 

 course of events in the living body, and each exercising its func- 

 tions when in exceedingly small concentrations. Happily the actual 

 chemical constitution of some of them is now known. 



It is, of course, impossible in a brief review to recount all the 

 stages of discovery in the case of each of these substances. The 

 existence of individual vitamins, each with its special influence in 

 the body, has in the majority of cases been revealed by the experi- 

 mental feeding of animals on the following general lines. Natural 

 products or preparations — crude when experiments began — from nat- 

 ural sources, animal or vegetable, when simultaneously added in 

 characteristically small amounts to a vitamin-free dietary, were 

 found to render it capable of supporting normal nutrition. The 

 tendency at first was to assume that each effective addendum con- 

 tained one active ingredient. The next step in progress, however, 

 involved the fractionation of each crude preparation, and this in 

 many cases revealed the presence of more than one vitamin, with 

 obviously distinct functions, each calling therefore for separate en- 

 deavors toward its isolation and purification. It may be mentioned 

 in illustration that yeast, which, because it represents a concentrated 

 mass of living cells capable of active growth, and at the same time 

 is available in large amounts, was early and justifiably looked to as a 

 probable source of vitamins, has yielded some of them in a complex 

 which even today has perhaps not been fully analyzed. 



The position of knowledge at the present moment will be made 

 sufficiently clear if the most salient characteristics of each recognized 

 vitamin are very briefly reviewed. Unfortunately, it is impossible 

 at the same time to give credit to the many who have shared in the 

 heavy labors involved in the remarkable recent advances in the 

 subject. 



Vitamin A. — This vitamin is found in association with animal fats 

 and exists in specially high concentration in the livers of fishes. It 

 was discovered and studied in cod-liver oil and at first was not 

 distinguished from vitamin D, but by 1922 it had become clear 

 that there were two " fat soluble " vitamins with functions entirely 

 distinct. 



Vitamin A exerts an important influence in the body. In its ab- 

 sence young animals fail to grow. Lack of a proper supply leads to 



