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42. Composition of the body. Many careful analyses have been made of the composition of the human body, and these analyses have shown that our bodies are made of the same kinds of materials as those found in plants; namely, proteins, fats, carbohydrates, mineral matters, and water.

43. Proteins. The most important substances in the living body are the proteins. As we have already learned,' proteins are essential constituents of the protoplasm of every plant cell, and this is likewise true of the cells in animal and human bodies.

44. Fats and carbohydrates. The amount of fat in the body varies greatly in different individuals, but it is always present in some quantity. Muscle, however lean, contains particles of fat; fat constitutes a small percentage of the blood; it fills the spaces in the interior of bones; and it is often deposited in considerable quantity in the deeper layers of the skin. In the blood and in other animal tissues we find some of the carbohydrate called grape sugar. Another carbohydrate known as animal starch, or glycogen, is found in the liver and in the muscles.

45. Mineral matters. Mineral matters are found in the greatest quantities in the bones and the teeth. When 143, "Plant Biology."

we burn bones, about one third of the weight disappears, the remaining two thirds being bone ash, which is the mineral matter. Every part of the body, however, contains some mineral ingredients; for when muscle, liver, brain, or blood is burned, there remain some traces of ash in each case.

46. Water. The great importance of water in the composition of the human body is evident from the fact that this compound forms about 62 per cent of the weight of an adult. Hence, if all the water were removed from the body of a man weighing one hundred and fifty pounds, the solids that remained would weigh less than sixty pounds. The different organs vary greatly in their percentages of water; bones contain about 22 per cent, muscles have 75 per cent, and the kidneys 82 per cent.


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47. Necessity of foods for growth. During the earlier years of life, as we all know, the human body rapidly increases in weight. A child at birth usually weighs seven to eight pounds, whereas the weight of a fully grown man is often one hundred and fifty pounds or more. Hence during a lifetime there is often a twentyfold increase in weight. To provide for this increase or growth a large amount of new material must of course be taken in by the human being, and this material is supplied by the food.

48. Necessity of foods for repair and for the production of energy. - On the other hand, it is not difficult to prove that throughout life the body tends constantly to decrease in weight. For instance, if one were weighed on accurate scales immediately after eating and then again after several hours had elapsed and before food or drink had been taken, a decrease in weight would be noted. Still more striking is

the loss of weight due to abstaining from food because of illness or other reasons.

It has been found too that when one is engaged in very active exercise, such as playing tennis or football, the loss of weight is greater than when one remains quiet. How, then, can we account for the loss of weight in all the cases that we have been enumerating? We all know that during violent activity considerable quantities of perspiration are given off from the skin, and this has been proved to be true at all other times, though to a less extent. It has also been demonstrated that many waste materials are given off from the lungs, the organs of digestion, and the kidneys.

We have now accounted for the constant loss of weight in our bodies, but we have still to ask ourselves how these waste substances are produced in the body. The two commonest wastes of the body are carbon dioxid and water. These are produced by the oxidation of the carbon (P. B., 80) and the hydrogen in the foods. This has also been proved to be true in animals and in the human body.

49. Definition of a food. The three most important uses of foods have been suggested in the preceding sections. Hence we may say that a food is any substance that yields material for the repair or growth of the body, or that supplies the fuel used by the body for producing heat, or power to do work. It should be understood, however, that no substance should be regarded as a food if it injures the body while supplying materials for growth, repair, or the production of energy.


50. To determine the food substances present in milk. Laboratory demonstration.

1. Shake a bottle containing milk and cream and pour a

small amount into a test tube; add a little strong nitric acid, and boil.

a. Describe what was done.

b. What change in the color of the milk do you observe? c. What food substance do you therefore conclude to be

present in milk?

2. Place a drop of the "mixed milk," used in 1 above, on paper, and allow the paper to dry over a warm radiator. Hold the paper to the light. What kind of food substance is present in considerable quantity in the milk? How do you know?

3. Add a few drops of iodine to some milk. result, and what is your conclusion?

What is the

4. Test another sample of milk with Fehling's solution. State the result and your conclusion from the experiment. The sugar found in milk is known as milk sugar, and when it is heated with Fehling's solution, it is changed to grape sugar.

5. Heat a half spoonful of milk, and hold over it a clean, dry tumbler. What nutrient does this experiment prove to be present? Why?

6. (Optional.) Evaporate to dryness the spoonful of milk, and then burn the solid residue over a very hot flame. Does all the solid disappear, or is something left on the spoon? What is your conclusion as to the presence or absence of mineral matter in milk?

7. As a conclusion from all your experiments, state what food substances or nutrients 1 are present in milk, and what food substances are absent.

51. The composition of other foods. Our study of milk has shown us that this food is composed of the same

1 In our study of plant biology we called the compounds named in this paragraph food substances rather than nutrients, for botanists regard the simpler compounds (carbon dioxid, water, and mineral matters) that plants obtain from the water and air as the nutrients of the plants. By some writers water is not regarded as a nutrient ; since, however, it is an essential constituent of protoplasm, it may well be named among the nutrients.

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FIG. 19.-Composition of common animal foods. (Drawn from Charts of U. S Dept. of Agriculture by Mabelle Baker.)

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