Foods that contain in appreciable quantities all the constituents that sustain life are substantial factors in the diet, though none are so balanced in their constituents as to be a desirable diet alone. Milk is more nearly so for children than other substantial foods are for any one. But even milk when used alone in infancy requires some modification.

The purity of the milk-supply is one of the most important of the food-problems of humanity. Every community is in need of pure milk in abundance. The health and growth of children is largely dependent upon this. Neglect of the milk-supply is negligence toward life itself. Children need care taken for them of the milk they are to drink; they are helpless themselves. (See Milk Commissions, p. 115).

Cleanliness of the environment of milk-cows, of cows themselves, of workers, and of receptacles is an absolute requirement for a clean milk-supply. The health of the animals, their food, the water they drink, the air they breathe, all affect the quality of milk they give. Mixed milk from a number of cows is preferable to milk from one, as such a supply minimizes the probability of poor milk, also of concentration of any unsuspected danger. Milk-cows need constant intelligent inspection and care.

Transportation of milk is to-day almost universal. In such dissemination milk needs protection from dust and contamination, and must be at lowered temperature to prevent not only souring but the development of any bacteria. The delivery of milk, through which it is widely distributed to family consumers, requires no less scientific attention, though it usually receives less. Consumers, too, have a responsibility beyond caring for the milk they use, in the complete cleaning of milk-bottles immediately upon emptying them.