Tobacco and Digestion.-Tobacco frequently plays an important role in influencing the digestion of food. It is a wellknown fact that the chewing of tobacco increases the salivary secretion, frequently reduces the appetite, and increases the movements of the bowels. As a rule, it is better to smoke after meals than before, the irritating effect of tobacco being thus lessened. In acute gastric disturbances tobacco should be interdicted entirely, and in chronic forms the smoking should be limited to a very few cigars a day.

Tobacco acts as an excitant to the nervous system, and should be prohibited in all nervous diseases.

CLASSES OF FOODS.

ANIMAL FOODS.

ANIMAL foods contain much digestible matter, chiefly proteins, a considerable quantity of fat, in some foods carbohydrates, and, in addition, water and mineral salts. Being thoroughly digested, they leave but little residue in the intestine. The various forms of animal foods-milk, eggs, meat, fish, and gelatin-will now be described under these headings.

MILK AND MILK PRODUCTS.

Milk, the most important of animal foods, contains all the elements necessary for the maintenance of life, and constitutes a complete food.

Composition. Milk contains varying proportions of each of the four classes of food principles, protein, fats, carbohydrates, and mineral salts, and from 84 to 90 per cent. of water; this latter varying with the quality of the milk. In a general way this is true of all milks, which are more or less alike, but which contain different percentages of the constituents.

Milk forms the exclusive diet for the young, growing mammals, but owing to the fact that the proportions of proteins and fat are in excess of the carbohydrates, it is unsuited as an exclusive diet for adults. Unless otherwise stated cows' milk is meant by milk in this volume. Cows' milk is most extensively used for food, but the milk of goats and asses and some other animals is used to some extent.

Fresh cows' milk has a sweetish taste, a characteristic odor, and is yellowish-white in color; on standing it separates into two distinct layers, the upper being more yellow in color, of lighter specific gravity, and containing more fat. For dietetic purposes it is well to think of cream as a milk containing varying percentages of fat. The lower part, called "skim milk" after the removal of the cream, is of a bluish-white color, and may be considered relatively free from fat. The specific gravity of milk varies from 1.027 to 1.035 and it freezes at a slightly lower temperature than water.

There are numerous statements and theories concerning the reaction of cows' and human milk. Freshly drawn, the milk of most carnivora is acid to the litmus reaction; human milk is alkaline, sometimes acid or amphoteric, and cows' milk amphoteric-turning red litmus blue and vice versa. On exposure to the air all milks will turn blue litmus red, owing to the conversion of the milk-sugar into lactic and other acids.

The microscopic examination with a low power shows the fat globules and some leukocytes and foreign matter if present; with the immersion lens the bacterial contents may be studied. Kastle and Roberts give the scheme on p. 67 compiled by Van Slyke and Babcock.

The principal nitrogenous compound of milk is casein, which differs from the other protein compounds in that it contains both phosphorus and sulphur. Casein is not coagulated by heat, but this change may be effected by adding acid or rennet. The casein clot formed by adding acids may be dissolved by neutralizing the acid, while that formed by rennet is not affected by the addition of an alkali.

Milk also contains other proteins, as lactalbumin, which is similar to the serum-albumin of the blood, lactoglobulin, and lactomucin. The total proteins average about 3.3 per cent. of the bulk of the milk, or about 25 per cent. of the total solids.

The fats of milk consist of the glycerids of palmitic, stearic, and oleic acids. In addition to these, milk contains several other fats in smaller proportions, to which the flavor of butter is in part due. The fat is suspended in the milk in the form of minute globules, which give the milk its white color and opacity. The fat globules in some milks are larger than in others. They are smallest from a herd of mixed common cows and largest in the milk of Jerseys and Guernseys. Fat averages about 4 per cent. of the milk, or about 31 per cent. of the total solids.

The chief carbohydrate of milk is lactose, or milk-sugar, which is not nearly so sweet as ordinary sugar, and is less soluble in water. It reacts to Fehling's solution like glucose, and in the presence of the lactic acid bacillus it is converted into lactic acid, which causes the milk to turn sour. Lactose forms about 38 per cent. of the total solids.

Milk contains about 0.7 per cent. of salts, which exist chiefly in the form of phosphates, chlorids, and sulphates. Potassium salts occur in larger quantities than do sodium salts.

Calcium salts are very essential to young, growing animals,

inasmuch as they play a very important part in the formation of bone. The relative percentages of salts in the ash of human milk are shown by the following table:

When the cow is diseased, various substances not present in normal milk may be discovered, as urobilin and bile. Milk may also contain odoriferous substances from things which the cow has eaten, as wild garlic. Milk also absorbs odors from the air.

Variations in Milk.-There are wide variations in the composition of the milk of different animals. While human milk contains more sugar and less protein than cows' milk, the fuel-value is about the same. Dogs' milk seems to be the richest, whereas that which comes from the horse is exceedingly poor, as may be seen from the following table:

Comparative Composition of Various Kinds of Milk.1

Not only is there a wide variation in the milk of different animals, but cows' milk itself is subject to great changes in the percentage composition of its ingredients. These may be attrib

1 König, Chemie der menschlichen Nährungs- und Genussmittel, 3d ed., vol. i., pp. 267-362.