FOOD DIGESTION AND RESPIRATION 61 



was found to require at least sodium, potassium, calcium, and 

 magnesium. 



Accessory Factors 



We have seen that we must provide for a supply of carbon and 

 nitrogen in certain forms, and also salts. But there is something 

 else to be considered. 



Suppose that we give to a growing animal a diet of pure protein, 

 pure fat and pure carbohydrate, together with salts, that is, all the 

 actual chemical compounds required and in sufficient amount, we 

 find that it does not grow. But if we add a very small quantity of 

 milk or of turnip juice it grows as well as on its normal food. 

 There is evidently something wanting in the pure materials, of 

 which only a small amount is needed, but which is, nevertheless, 

 indispensable. 



We do not know yet what this "accessory factor" is. It has 

 been called " vitamine," owing to a mistaken view of its chemical 

 nature. There is ground for believing that there are several kinds, 

 because different diseases develop in the absence of particular 

 constituents present in some foods, not in others. For example, 

 beri-beri, after having made its appearance, can be cured by the 

 addition of a small quantity of the outer layer of rice, whereas 

 scurvy cannot be cured by this, but needs the juice of oranges or 

 other fresh vegetable. This latter fact was known to Captain Cook, 

 who discovered, in his second voyage round the world, that scurvy, 

 then so serious a difficulty in long voyages, could be prevented by 

 adding fresh vegetables to the preserved diet whenever a chance 

 presented itself. Some other diseases are turning out to be, in all 

 probability, " deficiency diseases." 



Although there seems to be some variety in these factors, they 

 fall into two main groups, one soluble in fat (" fat-soluble A-factor "), 

 the other soluble in water (" water-soluble B-factor"). That con- 

 tained in butter is typical of the former, that in wheat-germ, of the 

 latter. Both are necessary for normal growth. 



These factors are somewhat easily destroyed by cooking, es- 

 pecially if heated in alkaline solution, and by preservation with 

 salt. Hence the importance of fresh food, especially fruit and 

 vegetables. Fruit contains an unusually large amount of the 

 anti-scorbutic factor, which seems to belong to a special class. 



Notwithstanding their great importance, very little is known 

 about the nature of these substances or the way in which they act. 

 They seem to behave like those agents called "catalysts," about 

 which we shall learn more presently. They are not subject to 

 chemical change in the course of their activity (P., pp. 258, etc.). 



