This section is from the book "Practical Cooking And Serving", by Janet McKenzie Hill. Also available from Amazon: Practical Cooking and Serving: A Complete Manual of How to Select, Prepare, and Serve Food .
Fundamentally, in structure, milk is an emulsion consisting of fine oil globules swimming in a colorless fluid. These oil globules are called cream, milk fat, or butter, and are the most easily digested of all fats. Cream skimmed by hand from milk contains almost all the fat present in the milk - for, being lighter than the other constituents, it rises to the surface - and also a small proportion of the casein. The skimmed milk contains most of the casein, albumen, and milk-sugar, with the water, and though it contains a large proportion of water, it is still nutritious. In large institutions, hospitals, schools, etc., whole milk rather than cream should be purchased, since the skimmed milk can be used to increase the nutritive value of many a dish. In buying cream, the cost of the milk from which the cream was taken is included.