FOOD AND DIGESTION 75 



importance in constituting an essential part of hemo- 

 globin, the oxygen-carrying pigment of the blood. 



Even if the diet contains sufficient amounts of carbo- 

 hydrate, fat, and protein, with an adequate supply of 

 mineral salts and water, it is found that nutritional 

 disturbances will occur eventually, unless certain or- 

 ganic substances in addition be included. These sub- 

 stances may be nitrogenous in nature or may be free 

 from nitrogen. They are not energy-yielding foods, 

 nor are they used for constructive purposes; but, ap- 

 parently, they exert a very marked influence on the 

 processes of metabolism. They are known as "vita- 

 mines " or as " growth-promoting factors. ' ' The first one 

 to be studied is that occurring in the pericarp (the part 

 which surrounds the kernel) of rice, the absence of 

 which gives rise to the peculiar disease known as "beri- 

 beri." Scurvy, also, was formerly held to be a disease 

 due to the absence of vitamines from the diet, though 

 this has been denied recently. Certain fats contain the 

 requisite "growth-promoting" factors, while others do 

 not; butter and cod-liver oil being instances of the 

 former, while lard is an example of the latter class. 



