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(1) the providing of man with sensation, by means of which he gets in touch with the world about him; (2) the connection of organs in different parts of the body so that they act as a united and harmonious whole; (3) the giving to the human being a will, a provision for thought. Coöperation in word and deed is the end attained. We are all familiar with examples of the coöperation of organs. You see food; the thought comes that it is good to eat; you reach out, take it, raise it to the mouth; the jaws move in response to your will; the food is chewed and swallowed; while digestion and absorption of the food are taking place, the nervous system is still in control. The nervous system also regulates pumping of blood over the body, respiration, secretion of glands, and, indeed, every bodily function. Man is the highest of all animals because of the extreme development of the nervous system. Man is the thinking animal, and as such is master of the earth.

REFERENCE READING FOR THIS AND SUCCEEDING CHAPTERS ON HUMAN BIOLOGY

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

Davison, The Human Body and Health. American Book Company.

Eddy, General Physiology. American Book Company.

Hall, Elementary Physiology. American Book Company.

Clodd, Primer of Evolution. Longmans, Green, and Company.

Clodd, The Story of Primitive Man. Longmans, Green, and Company.
Ritchie, Human Physiology. World Book Company.

ADVANCED

Halliburton, Kirk's Handbook of Physiology.

P. Blakiston's Son and Company.

Hough and Sedgwick, The Human Mechanism. Ginn and Company.
Howell, Physiology, 3d edition. W. B. Saunders Company.

Schafer, Textbook of Physiology.

Stewart, Manual of Physiology.

The Macmillan Company.

W. B. Saunders Company.

Verworn, General Physiology. The Macmillan Company.

XXIV. FOODS AND DIETARIES

Problem XLII. A study of food values and diets. (Laboratory Manual, Prob. XLII.)

(a) Food values and cost.

(b) Nutritive values as compared with cost.

(c) The family dietary.

(d) Food values.

Why we need Food. We have already defined food as anything that forms material for the growth or repair of the body of a plant or animal, or that furnishes energy for it. The millions of cells of which the body is composed must be given material which will form more living matter or material which can be oxidized to release energy when muscle cells move, or gland cells secrete, or brain cells think. Food, then, not only furnishes our body with material to grow, but also gives us the energy we expend in the acts of walking, running, breathing, and even in thinking.

Nutrients. Certain nutrient materials form the basis of food of both plants and animals. These have been stated to be proteids (such as lean meat, eggs, the gluten of bread), carbohydrates (starches, sugars, gums, etc.), fats and oils (both animal and vegetable), and mineral matter and water. The parts of the human body, be they muscle, blood, nerve, bone, or gristle, are built up from the nutrients in our food.

Proteids. Proteids, in some manner unknown to us, are manufactured in the bodies of green plants. Proteid substances contain the element nitrogen. Hence such foods are called nitrogenous foods. Man must form the protoplasm of his body (that is, the muscles, tendons, nervous system, blood corpuscles, the living parts of the bone and the skin, etc.) from nitrogenous food. Some of this he obtains by eating the flesh of animals, and some he obtains directly from plants (for example, peas and beans). Because of their chemical composition, proteids are considered to

be flesh-forming foods. They are, however, oxidized to release energy if occasion requires it.

Fats and Oils. Fats and oils, both animal and vegetable, are the materials from which the body derives part of its energy. The chemical formula of a fat shows that, compared with other food substances, there is very little oxygen present; hence the greater capacity of this substance for uniting with oxygen. The rapid burning of fat compared with the slower combustion of a piece of meat or a piece of bread illustrates this. A pound of butter releases over twice as much energy to the body as does a pound of sugar or a pound of steak. Human fatty tissue is formed in part from fat eaten, but carbohydrate or even proteid food may be changed and stored in the body as fat.

Carbohydrates. We see that the carbohydrates, like the fats, contain carbon, hydrogen, and oxygen. Here, however, the oxygen and hydrogen are united in the molecule in the same proportion as are hydrogen and oxygen in water. Carbohydrates are essentially energy-producing foods. They are, however, of use in building up or repairing tissue. It is certainly true that in both plants and animals, such foods pass directly, together with foods containing nitrogen, to repair waste in tissues, thus giving the needed proportion of carbon, oxygen, and hydrogen to unite with the nitrogen in forming the protoplasm of the body.

Inorganic Foods. Water forms a large part of almost every food substance. The human body, by weight, is composed of about 60 per cent water. It is used to make the blood, and a sufficient quantity is most essential to health. When we drink water, we take with it some of the inorganic salts used by the body in the making of bone and in the formation of protoplasm. Sodium chloride (table salt), an important part of the blood, is taken in as a flavoring upon our meats and vegetables. Phosphate of lime and potash are important factors in the formation of bone.

Phosphorus is a necessary substance for the making of living matter, milk, eggs, meat, whole wheat, and dried peas and beans containing small amounts of it. Iron also is an extremely important mineral, for it is used in the building of red blood cells. Meats, eggs, peas and beans, spinach and prunes, are foods containing some iron.

Some other salts, compounds of calcium, magnesium, potassium, and

phosphorus, have been recently found to aid the body in many of its most important functions. The beating of the heart, the contraction of muscles, and the ability of the nerves to do their work appear to be due to the presence of minute quantities of these salts in the body.

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How the Exact Nutritive Value of Food has been Discovered. - For a number of years, experiments have been in progress in different parts of the civilized world which have led to the beliefs regarding food just quoted. One of the most accurate and important series of experiments was made a few years ago by the late Professor W. O. Atwater of Wesleyan University, in coöperation with the United States Department of Agriculture. By means of a machine called the respiration calorimeter (Latin, calor heat + metrum measure), which measures both the products of respiration and the heat given off by the body, it has been possible to determine accurately the value of different kinds of food, both as fuel and as tissue builders. This respiration calorimeter is described by Professor Atwater as follows:

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"Its main feature is a copper-walled chamber 7 feet long, 4 feet wide, and 6 feet 4 inches high. This is fitted with devices for maintaining and measuring a ventilating current of air, for sampling and analyzing this air, for removing and measuring the heat given off within the chamber, and for passing food and other articles in and out. It is furnished with a folding bed, chair, and table, with scales and appliances for muscular work, and has telephone connection with the outside. Here the subject stays for a period of from three to twelve days, during which time, careful analyses and measurements are made of all material which enters the body in the food, and of that which leaves it in the breath and excreta.

1 W. O. Atwater, Principles of Nutrition and Nutritive Value of Food, U.S. Department of Agriculture, 1902.

Record is also kept of the energy given off from the body as heat and muscular work. The difference between the material taken into and that given off from the body is called the balance of matter, and shows whether the body is gaining or losing material. The difference between the energy of the food taken and that of the excreta and the energy given off by the body as heat and muscular work, is the balance of energy, and, if correctly measured, should equal the energy of the body material gained or lost. With such apparatus it is possible to learn what effect different conditions of nourishment will have on the human body. In one experiment, for instance, the subject might be kept quite at rest, and in the next do a certain amount of muscular or mental work with the same diet as before, then by comparing the results of the two, the use which the body makes of its food under the different conditions could be determined; or the diet may be slightly changed in the one experiment, and the effect of this on the balance of matter or energy, observed. Such methods and apparatus are very costly in time and money, but the results are proportionately more valuable than those from simpler experiments.”

Fuel Values of Nutrients. In experiments performed by Professor Atwater and others, and in the appended tables, the value of food as a source of energy is stated in heat units called calories. A calorie is the amount of heat required to raise the temperature of one kilogram of water from zero to one degree Centigrade. This is about equivalent to raising one pound four degrees Fahrenheit. The fuel value of different foods may be computed in a definite manner. This is done by burning a given portion of a food (say one pound) in the apparatus known as a calorimeter. By this means may be determined the number of degrees the temperature of a given amount of water is raised during the process of burning.

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The Best Dietary. Inasmuch as all living substance contains nitrogen, it is evident that proteid food must form a part of the dietary; but proteid alone is not usable. If more proteid is eaten than the body requires, then immediately the liver and kidneys have to work overtime to get rid of the excess of proteid which forms a poisonous waste harmful to the body. We must take foods that will give us, as nearly as possible, the proportion of the different chemical elements as they are contained in protoplasm. It has been found, as a result of studies of Atwater and others, that a man who does muscular work requires a little less than one quarter of a pound of proteid, the same amount of fat, and about one pound

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