COMPOUNDS OP NUTRITION 27 



The same classification is used for the compounds of 

 both the vegetable and animal , kingdoms, which are 

 divided into the following groups : 



Water, 



Ash (mineral compounds). 



Protein (nitrogenous compounds), 



Carbohydrates (and related bodies). 



Fats (or oils). 



Variously named compoimds partially theoretical 

 and unclassified: Enzyms, antigens hormones, 

 vitamines accessories, activators. (These terms 

 are in part synonyms.) 



Accuracy is here sacrificed to convenience. These 

 class names have come to be regarded, more or less, as 

 representing entities having fixed properties and func- 

 tions, whereas each class contains numerous compounds 

 differing widely in their characteristics and in their nutri- 

 tive value and office. Moreover, these terms have a 

 variable significance as used under different conditions. 

 No one of them, except water, imiformly represents just 

 the same mixture of compoimds when applied to unlike 

 substances. 



33. Distribution of elements. — ^It is well to gain a 

 clear imderstanding of the relation which the fifteen 

 elements previously mentioned sustain to these classes 

 of substances. This relaltion can be shown for the five 

 main classes of nutrients, but not for the imclassified or 

 theoretical bodies. Just what elements enter into these 

 is not known. So far as known they do not have a con- 

 structive function. The distribution of the elements can 

 be seen most readily by a tabular display: 



