The known compounds that belong to life in all its forms are of great number and variety, and doubtless many are yet to be discovered. These sustain important relations to human needs, some serving as food, some as medicine, and some in the arts. It is fortunate that comparatively few must be considered in discussing the science of human nutrition. Moreover, it is convenient that the compounds which play a leading part in human nutrition are designated, especially for practical purposes, in classes rather than singly, even though this custom.tends to more or less looseness of expression and definition.

The same classification is used for the compounds of both the vegetable and animal kingdoms, and they are now divided into the following general groups:

Water

Ash (mineral compounds)

Proteins (nitrogenous compounds)

Carbohydrates

Fats (or oils)

In this instance, accuracy is sacrificed to convenience. The class names have come to be regarded, more or less, as representing entities having fixed properties and functions, whereas each class contains numerous compounds differing widely in their characteristics and in their nutritive value and office. Moreover, these terms have a variable significance as used under different conditions. No one of them except water uniformly represents just the same mixture of compounds when applied to foods of unlike source.

22. Distribution Of Elements In The Classes Of Compounds

Before passing to a detailed description of these compounds, singly or in groups, it will be well to gain a clear understanding of the relation which the fifteen elements previously mentioned sustain to these classes of substances. This can be seen most readily by a tabular display.

The ash, which, on the average, constitutes about one-twentieth of the plant, and never more than one-tenth of the animal, may contain thirteen of the fifteen elements, while the larger proportion of living matter consists mostly of the compounds of three or four elements, in no case of more than six or seven. From this point of view, it becomes strikingly evident that the dominant elements of life,. quantity alone considered, are those derived from the air and water.