The Work Of The Body

How does eating help to keep us alive? Life involves activity; work, play, activity of any sort, makes us hungry; food gives us energy to go on working and playing. Any activity that uses up energy is, in the scientific sense, work. Your muscles work as hard in playing a game as in going on an errand; and just as truly does the heart work in pumping blood, the stomach in digesting food, the brain and nerves in giving rise to thought and feeling.

1 Sometimes called ptyalin.

2 A mixture of copper sulphate, caustic potash or soda, and Rochelle salts.

What else does food do for us ? Breathe on your fingers; your breath is warm. Evidently heat is produced in the body.


Blow into lime-water through a glass tube for a minute or two. What is the effect on the lime-water? In what other way have you produced the same effect? (p. 5.) What gas must there be in your breath?

Air from the lungs and air in which something has been burned both turn lime-water cloudy, because both contain carbon dioxide. A slow burning goes on in the body all the time. The oxygen in the air we breathe in unites with substances in the body. Carbon dioxide is formed and goes back through the lungs into the air.

This process of slow combustion is necessary to the life of both plants1 and animals. Without oxygen, life would go out as a flame does.

The Body Compared To A Steam-Engine

Just as heat and mechanical power are produced by burning fuel under the boiler of a steam-engine, so energy and heat are produced by the oxidation of food in the tissues of the body. Starch slowly oxidized in the body gives off just as much heat and energy as if burnt {i.e., rapidly oxidized) in the air. Thus food serves as fuel to warm the body and to keep its

1 Plants breathe through their leaves, taking in oxygen and giving off carbon dioxide, night and day. The making of starch, during which the plant takes in carbon dioxide, is a distinct process, which goes on only in the light (p. 68).

The Body, Unlike An Engine, Repairs Itself

A steam-engine differs from the body, however, in one important respect, - it cannot repair itself. No fuel we can feed it with will stop a leak in the boiler or restore a missing rivet; but food renews the tissues of the body as fast as they wear out, making bone, nerve, muscle, and skin for us continually. Then, too, in a steam-engine, fuel and air meet and unite in one place, whereas in the body combustion goes on in all its parts.


Starch is one of a class of foodstuffs called carbohydrates. As the name indicates, carbohydrates are composed of carbon and of hydrogen and oxygen in the right proportions to form water. They are good fuel foods. They cannot, however, build tissue, except fatty tissue, which is stored-up fuel rather than living tissue, such as muscle is.


Throw a bit of butter or lard and a bit of starch on the fire and see which burns best.

Fat contains the same three elements that carbohydrates do, but the proportion of oxygen is much smaller. Hence it unites with more oxygen and so burns better.

Brief Reference List

For further development of topics treated in this section see: -

Bevier and Van Meter : Selection and preparation of food. Pp. 48-52.

Thorpe : Dictionary of applied chemistry. V. 4, p. 149.

Sherman : Food products. Pp. 7-10, 259-263.

Bigelow: Applied biology. Pp. 191-196.

Ritchie : Primer of physiology. Ch. 15, Foods and why we need them.

Forster and Weigley : Foods and sanitation.