Milk is one of the most important articles of food for the sick, and enters largely into the composition of various diets. It is constituted essentially of four elements—albuminoid, fatty, saccharine, and saline —and therefore contains all the materials necessary for the growth and nutrition of tissues. The nitrogenous constituent is casein, an albuminoid substance, but which differs from ordinary albumen in that it is combined with a larger proportion of alkali, and is not coagulable by heat. The fatty element is butter, which contains several neutral fats. The composition of butter is not exactly the same in all kinds of milk, the difference being due chiefly to a volatile principle upon which the special taste of each variety depends. The saccharine element is a crystallizable sugar, known as lactin or lactose, a substance which easily decomposes into lactic acid by a process of fermentation in which the casein plays the part of a ferment. The mineral constituents of milk are, chlorides of sodium and potassium, phosphates of lime, soda, magnesia, and iron. The most important of these is the phosphate of lime. The amount of these salts varies from ·5 to •8, and rarely exceeds one per cent (Parkes). The French commission, appointed by the Prefect of Police of Paris, reported upon the analysis of milk made in various countries, and concluded that the following figures represent the composition of this fluid when of good quality (Tardieu) :

Water........................................................ 87

Total solids.................................................. 13

Casein, extractive matters, and salts.                      Butter.                               Lactin.

The commission fixed the minimum standard of good milk at—

Water............. 88·50

Casein, extractives, and salts.......  4·00

Solids.............11·50 Butter..........................  2·70 to 3·00

Lactin..........................  450

When perfectly fresh, milk is usually neutral in reaction, or it may be a little alkaline. After a short time—especially in summer—it becomes acid by a process of fermentation in which the lactin is converted into lactic acid, and the casein coagulates. The fluid portion is called whey, and the semi-solid casein curds. By the fermentation of mare's-milk an alcoholic liquor, named koumiss, is prepared in Tartary, and has been introduced into medical practice as a remedy for phthisis.

The proportion of cream in good milk ranges from 10 to 15 per cent by volume. By churning, the fat of the cream is collected and is then known as butter. This important article of food has the following composition (Fonssagrives) :

Margarin...................................................... 68

Butyrolin.....................................................30

Butyrin, caprin, and caproin..................................... 2

Butter readily undergoes decomposition becomes rancid—capric and butyric acids separating from the base glycerin. This process is one of fermentation {butyric), and is due to the action of a special organism, the growth of which is favored by air, light, and imperfect separation of milk in the process of churning.

After the process of churning, which separates the butter, the resultant liquid, known as buttermilk, contains the casein, lactin, and the salts, and is therefore a nutritious article of food.

As the milk of other animals than the cow is sometimes prescribed in medical practice, the comparative chemical constitution of this fluid should be studied. The following table (Pereira) shows at a glance the difference in composition of the milk from several animals :

Whenever fresh and pure milk can be procured, this only should be prescribed for the sick, but in large cities it is not always practicable to obtain it. Under these circumstances "condensed milk" must be used. This preparation is made by evaporation of the water of the milk and the addition of some sugar. It is found in two forms, dependent on the extent to which the abstraction of water is carried : as a granular solid and as a soft semi-solid. The addition of warm water to the condensed milk furnishes a palatable fluid, of the appearance and composition of fresh warm milk.

Fresh milk, boiled and corked up in bottles to exclude the air, will keep for a considerable length of time. To prevent fermentation, some sulphite of lime may be added to it. For temporary preservation of milk in the summer-time, especially when intended for food for infants, a little bicarbonate of soda and sugar may be used.

Cheese contains all the constituents of milk, except the water and some salts and lactin removed by expression. In the preparation of cheese the casein of the milk is coagulated by rennet, the butter and a portion of the lactin and salts are entangled in the meshes of the casein, and the mass is subjected to powerful compression. The peculiar flavor and quality of the cheese depend upon the nature and richness of the milk, and upon certain fermentative changes which take place, developing volatile, odorous, and sapid constituents. The following table of the composition of cheese illustrates its nutritive qualities:

Water......................................................  36·8

Albuminates.................................................  33·5

Fats.......................................................  24·8

Salts........................................................    5'4

It is evidently a concentrated food. The digestibility of cheese depends in part on its freshness, in part on its composition. When fresh and of good quality, it does not ordinarily disagree with the stomach. A small quantity of cheese taken after dessert in some cases assists digestion; but many dyspeptics and persons of weak digestion can not make use of it under any circumstances.