The Digestive Fluids.—Several fluids—saliva, gastric juice, pancreatic juice, bile, and intestinal juice—are employed in the digestion of the food. The composition of these fluids is in keeping with the nature of the digestive process. While all of them have water for their most abundant constituent, there are dissolved in the water small amounts of active chemical agents. It is the work of these agents to convert the insoluble nutrients into substances that are soluble in water. The digestive fluids are thus able to act in a double manner on the nutrients—to change them chemically and to dissolve them. The chemical agents which bring about the changes in the nutrients are called enzymes, or digestive ferments.
Foods Classed with Reference to Digestive Changes.—With reference to the changes which they undergo during digestion, foods may be divided into three classes as follows:
1. Substances already in the liquid state and requiring no digestive action. Water and solutions of simple foods in water belong to this class. Milk and liquid fats, or oils, do not belong to this class.
2. Solid foods soluble in water. This class includes[pg 133] common salt and sugar. These require no digestive action other than dissolving in water.
3. Foods that are insoluble in water. These have first to be changed into soluble substances, after which they are dissolved.
Summary.—Materials called foods are introduced into the body for rebuilding the tissues, supplying energy, and aiding in its general work. Only a few classes of substances, viz., proteids, carbohydrates, fats, water, and some mineral compounds have all the qualities of foods and are suitable for introduction into the body. Substances known as drugs, which may be used as medicines in disease, should be avoided in health. Before foods can be passed into the body proper, they must be converted into the liquid form, or dissolved. In this process, known as digestion, water is the solvent; and certain chemical agents, called enzymes, convert the insoluble nutrients into substances that are soluble in water.
Exercises.—1. How does oxidation at the cells make necessary the introduction of new materials into the body?
2. What different purposes are served by the foods?
3. What is a nutrient? Name the important classes.
4. What are food materials? From what sources are they obtained?