"Food may be said to be any substance which when introduced into the body supplies the material which renews some structure or maintains some vital process." Alcohol cannot be considered as a food, except to the extent that it reduces waste of tissue. As a heat producer it is inferior to fat. Hunger and thirst are the demands of our bodies for food. Thirst is far less endurable than hunger; liquids enter into every part of the body.

The ordinary diet should include all the factors which are found in the various tissues together with enough water to amply supply the system. Foods are divided into the nitrogenous or gluten-bearing class, such as meats, and the non-nitrogenous such as fats, starch, sugar, etc. A brain worker requires more fats, and a muscle worker more nitrogenous foods. Over brain exercise sometimes produces insensibility to hunger, and students, after light suppers and long night study, find themselves unable to sleep, although not conscious of lack of food. A light lunch is often a cure for this condition, and is to be advised after prolonged mental effort. The average man requires the food elements in about the proportion of 4 oz. nitrogenous, 3 oz. fats, and 13 oz. sugar, starch, etc. Lack of any one of the elements is sure to produce a diseased state of the body. Scurvy results from lack of fresh vegetables or fresh metals.

Saliva, the first modifying agent which affects the food, is secreted by three sets of glands in the mouth, in variously estimated amounts of from seven to seventy ounces. This liquid is thoroughly mixed with the food by the process of mastication. The secretion of saliva is increased by tobacco, diminished by alcohol, and by violent excise and fear. The saying, "his tongue clove to the roof of his mouth from fear," is a physiological fact.

The next lecture will consider the functions of the stomach and processes of digestion.