Foods and the Theory of Digestion : Part 4

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 12 of 27)

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 therefore 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 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., proteins, 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?

5. Name the different kinds of proteins; the different kinds of carbohydrates. Why are proteins called nitrogenous foods and fats and carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteins; without water.

7. What percents of protein, fat, and carbohydrate are found in wheat flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts?

8. State the objection to a meal consisting of beef, eggs, beans, bread, and butter; to one consisting of potatoes, rice, bread, and butter. Which is the more objectionable of these meals and why?

9. State the general plan of digestion.

10. Show that digestion is not a simple process like that of dissolving salt in water.

Practical Work

Elements supplied by the Foods. - The following brief study will enable the pupil to identify most of the elements present in the body and which have, therefore, to be supplied by the foods.

Carbon. - Examine pieces of charred wood, coke, or coal, and also the "lead" in lead pencils. Show that the charred wood and the coal will burn. Recall experiment showing that carbon in burning forms carbon dioxide.

Hydrogen. - Fill a test tube one third full of strong hydrochloric acid and drop into it several small scraps of zinc. The gas which is evolved is hydrogen. When the hydrogen is coming off rapidly, bring a lighted splinter to the mouth of the tube. The gas should burn. Hold a cold piece of glass over the flame and observe the deposit of moisture. Hydrogen in burning forms water. Extinguish the flame by covering the top of the tube with a piece of cardboard. Now let the escaping gas collect in a tumbler inverted over the tube. After holding the tumbler in this position for two or three minutes, remove and, keeping inverted, thrust a lighted splinter into it. (The gas should either burn or explode.) What does this experiment show relative to the weight of hydrogen as compared with that of air?

Nitrogen. - Nitrogen forms about four fifths of the atmosphere, where, like oxygen, it exists in a free state. It may be separated from the oxygen of an enclosed portion of air by causing that gas to unite with phosphorus. Place a piece of phosphorus the size of a pea in a depression in a flat piece of cork. (Handle phosphorus with wet fingers or with forceps.) Place the cork on water and have ready a glass fruit jar holding not more than a quart. Ignite the phosphorus with a hot wire and invert the jar over it, pushing the mouth below the surface of the water. The phosphorus uniting with the oxygen fills the jar with white fumes of phosphoric oxide. These soon dissolve in the water, leaving a clear gas above. This is nitrogen. Place a cardboard under the mouth of the jar and turn it right side up, leaving in the water and keeping the top covered. Light a splinter and, slipping the cover to one side, thrust the flame into the jar of nitrogen, noting the effect. (Flame is extinguished.) Compare nitrogen with oxygen in its relation to combustion. What purpose is served by each in the atmosphere?

Oxygen. - Review experiments showing the properties of oxygen.

D.C. Heath and Co. - Publishers
Original copyright 1909