[Illustration:  Fig. 63.—­Lymphatics and Lymphatic Glands of the Axilla.]

The intense cold makes such a drain upon the heat-producing power of the body that only food containing the largest proportion of carbon is capable of making up for the loss.  In tropical countries, on the other hand, the natives crave and subsist mainly upon fruits and vegetables.

165.  The Kinds of Food Required.  An appetite for plain, well-cooked food is a safe guide to follow.  Every person in good health, taking a moderate amount of daily exercise, should have a keen appetite for three meals a day and enjoy them.  Food should be both nutritious and digestible.  It is nutritious in proportion to the amount of material it furnishes for the nourishment of the tissues.  It is digestible in a greater or less degree in respect to the readiness with which it yields to the action of the digestive fluids, and is prepared to be taken up by the blood.  This digestibility depends partly upon the nature of the food in its raw state, partly upon the effect produced upon it by cooking, and to some extent upon its admixture with other foods.  Certain foods, as the vegetable albumens, are both nutritious and digestible.  A hard-working man may grow strong and maintain vigorous health on most of them, even if deprived of animal food.

While it is true that the vegetable albumens furnish all that is really needed for the bodily health, animal food of some kind is an economical and useful addition to the diet.  Races of men who endure prolonged physical exertion have discovered for themselves, without the teaching of science, the great value of meat.  Hence the common custom of eating meat with bread and vegetables is a sound one.  It is undoubtedly true that the people of this country, as a rule, eat meat too often and too much at a time.  The judicious admixture of different classes of foods greatly aids their digestibility.

The great abundance and variety of food in this country, permit this principle to be put into practice.  A variety of mixed foods, as milk, eggs, bread, and meat, are almost invariably associated to a greater or less extent at every meal.

Oftentimes where there is of necessity a sameness of diet, there arises a craving for special articles of food.  Thus on long voyages, and during long campaigns in war, there is an almost universal craving for onions, raw potatoes, and other vegetables.

166.  Hints about Meals.  On an average, three meals each day, from five to six hours apart, is the proper number for adults.  Five hours is by no means too long a time to intervene between consecutive meals, for it is not desirable to introduce new food into the stomach, until the gastric digestion of the preceding meal has been completed, and until the stomach has had time to rest, and is in condition to receive fresh material.  The stomach, like other organs, does its work best at regular periods.[25]