VI.

In determining the nutritive value of aliments by the study of their chemical composition, we cannot adhere strictly to the results furnished by analysis. For, says Baron Liebig, we cannot reckon upon results in the human stomach with the same regularity as we would in the alembics of our laboratories.

Physiologists divide alimentary substances into two classes: the nitrogenous, which, according to Dumas, supply the demands of assimilation, and the non-nitrogenous, which are called by Liebig respiratories, from furnishing the products consumed by respiration. Neither the one nor the other will alone support life indefinitely, and when one or the other decreases below well-defined limits, health declines, and finally life becomes extinct from inanition.

Milne Edwards gives, as the mean amount of these two classes, required for all climates, not less than three hundred and fifteen grains of nitrogen and thirty-three hundred and fifty grains of carbon in the twenty-four hours. These views are adopted by most physiologists; yet the analyses of Schlossberger and Kemp indicate that the idea of estimating the value of food by the quantity of nitrogen it contains is a fallacious one.

The beautiful experiments of Bernard and the modern physiologists have unfolded many of the laws that regulate digestion and assimilation. Yet the human researches in the great arcana of nature are extremely limited, in comparison with the vast range of physical phenomena, and every day we are reminded of the remarks of Boerhaave to his students: "Let all these heroes of science meet together; let them take bread and wine, the food which forms the blood of man, and by assimilation contributes to the growth of the body; let them try by all their art, and assuredly they will not be able from these materials to produce a single drop of blood,--so much is the most common act of nature beyond the utmost efforts of the most extended science."

The composition of the typical food of nature is revealed to us in the analysis of human milk.