combined. Blood-formers, heat-producers, and nutritive salts, are not separately foods, but only factors of food, each as indispensable for the vital processes as air and water, but each incapable by itself of supporting life. One cannot live on albumen alone or on fat alone. Without lime-phosphate no bone would be formed, no matter how much pure albumen and fat we ate; and without albumen no muscular tissue would be formed, though we were to gorge ourselves with sugar and salt; finally, without fat, no brain. But we properly enough give the name of foods to meat, milk, and bread, for in them all the three conditions are present.
Fortunately, these nutritive principles are found by no means sparingly distributed throughout Nature; under the most varied forms they occur in almost every food-stuff used by man. We not only find the blood-formers in the shape of fibrine in the blood and muscles of animals, of albumen in eggs, of casein in milk, of lime and areolar tissue in cartilage, sinew, and skin, but also in the vegetable kingdom; we discover them in the gluten of grains, in the legumine of pulse, in the vegetable albumen of sundry roots, leaves, and fruits. Heat-producers are furnished, not only by the animal kingdom in its fats, but by plants also. Sundry seeds give a small quantity of oil, but the principal supply of heat-producing elements derived from the vegetable world appears in the shape of starch, gum, and sugar—substances which, during the process of digestion, are transformed into fats, and therefore may replace the latter. Finally, we have salts in the water we drink, and more abundantly in nearly every animal and vegetable substance we consume. Hence, it might seem to be an easy thing to find wholesome food, as though one had only to seize blindly the store of food offered by Nature, in order to get all that our organism requires to keep it vigorous. But it is not to be forgotten that, while the nutritive elements make up the losses of the organism, and renew the body, as it were, still they must be taken in certain definite proportions. Now, in Nature they are not distributed in any such proportion. There the greatest diversity is found; one food-stuff consists principally of blood-formers, another of heat-producers; this one contains only one of the nutritive salts, that one, another. If, then, we let chance decide in this matter, it might easily happen that we would take one element in excess, while we took none at all of another. If this were the case again and again, or permanently, the organism must suffer or perish utterly; for, suppose only a single organ to be improperly nourished, i. e., to receive food insufficient to make up its losses, the check given to this one will affect all the rest. Science can now pretty accurately determine the composition of every article of food, and hence its contents of the various nutritive principles. This is done by chemical analysis. Chemical analysis, however, is of little assistance to us in determining accurately the constituents of
