Milk is first of all the food for the young,—until a certain age the only food, and a perfect food. It contains but little refuse or waste and is under favorable conditions almost wholly digestible. NUTRIENTS All foods contain certain groups of nutrients which may be classified according to various functions in nutrition and their chemical composition. =Protein.=—The protein group of nutrients contains nitrogen and is necessary for building up the tissues of the body, the muscles and the tendons which also contain nitrogen. Only by this group can tissues wasted by constant wear and tear be rebuilt. Proteins are the flesh-forming group. To some degree the proteins or albuminoids are also active in producing fat in the body, but the other groups of nutrients, especially the fats, also contribute. =Fats and Carbohydrates.=—Another important function of food is to produce and maintain the animal heat. The main sources of this necessity are the fats and the carbohydrates, so called because they consist of the element carbon combined with oxygen and hydrogen, the last two in the exact proportion in which they are combined in water. All of these three groups furnish the fuel, so to speak, for the body, but they are not equal in this respect. Pound for pound, when burned in the body, the fats yield 2¼ times as much heat as protein or the carbohydrates. =Mineral Matters.=—Finally there are in all foods the mineral matters, a group containing a number of salts which are indispensable because they are constituents of every part of the body. Phosphate of lime, for instance, makes up one-half of the substance of the bones, and the sulphates and chlorides of potash and soda, iron, etc., are everywhere present in smaller quantities. No food in which any of them is lacking is complete. =Relation.=—The value of a food depends largely upon the relation of one group of nutrients to another. Proper nutrition can only be obtained when a sufficient amount of flesh-forming as well as of heat-producing nutrients are present, when the “nutritive ratio” is properly balanced for the particular purpose in view, be it the growth of the child, the maintenance of the body, the restoration of matter consumed by labor of body or brain, or the supply of heat to make up for cold surroundings. =Milk contains all= of these groups of nutrients. The protein is represented in milk by the casein and albumin, the fats by the butter-fat, the carbohydrates by milk-sugar, and the mineral matters by the milk-ash. Human milk contains them in a perfect proportion for infants, and for all purposes of nutrition cow’s milk may be used to make up a “balanced ration,” if not alone, then in connection with other food.