For a certain period of their lifetime the secretion of the mammary gland forms the only food of all mammals, and it is the one natural product which when taken alone affords adequate nutriment.

It consists of a slightly alkaline watery fluid, containing - 1. Proteids, casein and albumin in solution.

2. Fats, finely divided to form perfect emulsion.

3. Carbohydrate, sugar in solution.

4. Salts, in solution.

5. Water.

Owing to the action of certain organisms which readily propagate in milk, if exposed to the air at a warm temperature for some time, it loses its alkaline reaction, and becomes sour from the formation of lactic acid from the milk sugar, by a kind of fermentation, the probable equation for which may be written thus: -

C6H1206 == 2C3H603.

Milk Sugar. Lactic Acid

If fresh good milk be allowed to stand, the fatty particles tend to float to the surface, thus forming a layer of cream.

The milk of different animals is similar in all essential points, but differs slightly in. the relative proportion of the ingredients, as may be seen in the following table: -

Milk varies both in the amount of solids in solution and fat, according to the age and general condition of the animal, period of lactation, time of day, etc.

Since human milk is much poorer in proteid, fat and salts (see Table), and richer in sugar, than that of the cow and other domestic animals, it is necessary to dilute the latter with water, and add sugar, when it is substituted for human milk in feeding infants.

The great value of milk as nutriment depends upon the fact that it contains every class of food stuff, viz., proteids, fat, carbohydrates, salts and water, in the proportion demanded by the economy; the salts in milk being those required for building up the bones of the infant, viz., phosphates and carbonates of lime, etc.

The normal variations in these proportions are not very great, but as adulteration with water is common, a knowledge of the method of testing the purity of milk is necessary.