We learned in our last lesson about rapid combustion, by which we obtain heat to cook our food. In this lesson we are to learn what food is, and about animal heat, a result of slow combustion.

Food is anything that nourishes the body, or helps to support life.

To live, to grow, and to be in health, the human body must have the power to move, and must be kept warm. When the voluntary muscles of the body lose their power of motion, they are said to be paralyzed; and when the heart ceases to move the whole body soon after dies.

Motion, whether of the whole body or of its smallest part, results in waste. New material, equal to the waste, must be supplied, or the body will be entirely worn out; and until maturity there should be material for growth as well as for repair. The particles of worn-out tissue must be removed from the body to keep it in a healthful state.

The living human body always has internal warmth. No matter how the temperature of the external air may change, the internal temperature keeps almost exactly the same. A variation of a few degrees always causes death.

Food, therefore, in order to support and nourish life, must supply the body not only with building material, but also with fuel to develop animal heat.

How is this supplied ?

All living bodies, whether vegetable or animal, are capable of growth from within, and of repairing their own waste. But lifeless substances, such as manufactured articles, are constantly wearing out and cannot restore themselves, but must be repaired by some power from without.

The living plant, if surrounded by air and a suitable soil, light, and the proper temperature, has the power, through its leaves and roots, to take from these sources all that it needs for growth and nourishment.

The living animal also has similar power, but animals wander about from place to place, and are not, like plants, always in contact with their food. Sometimes they have work to do, and cannot be seeking food. For these reasons animals are provided with a storehouse in which to lay by, at intervals, a supply of material for growth and repair. They are also provided with a monitor, in the shape of nervous structure, to tell them when the storehouse is empty, and are surrounded by an almost unlimited supply of material from which to select, according to their need.

All that is required of them, therefore, is to heed the voice of this nervous monitor, replenish the storehouse at the proper time, with a suitable amount of the right kind of food; and the remaining work - the making of the food into a part of themselves - will be done by the energy within.

How does food build up the body ?

Food, as we see it on our tables, bears little resemblance to our bodies ; but if we study and compare them we shall find that they are composed of similar sub stances. The solid part of the flesh and blood is largely fibrin and albumen, substances similar to the fibres and juices of meat and fish. Eggs, milk, peas, beans, and grains also contain other varieties of fibrin and albu-men. When we analyze these substances still farther, we find nitrogen is the element common to all; and it is from these nitrogenous foods that the bodily substance is chiefly built up.

A small part of the body is fat, a substance similar to the fat which we eat in the form of butter, oil, fat meat, etc. Fats, from whatever source obtained, when analyzed, are found to consist of carbon, oxygen, and hydrogen.

The bones, teeth, skin, hair, and nails contain, in addition to these elements, a larger proportion of mineral matter. Grains, vegetables, fruit, meat, milk - in fact, all things we eat - contain mineral matter, oftentimes in solution.

Although dissimilar in appearance, the body and our food are both made up of substances which contain the same elements, the principal of which are oxygen, hydrogen, carbon, and nitrogen. There are small quantities of phosphorus, sulphur, iron, potassium, silicon, calcium, etc.

But food, in the form in which it is eaten, cannot nourish the body and sustain life. It must first be changed, and converted into a fluid that can pass through very small channels into the blood. The blood, laden with food, and enriched with oxygen in the lungs, is carried by the arteries to the capillaries, which penetrate every part of the body. There it is taken up by the living cells, and changed by them into their own tissues.

Each little cell or particle of tissue, whether of bone, flesh, brain, nerve, hair, nail, or gland, has the power to select from this common supply such material as it especially requires, and convert it into its own substance.

By this power our food is assimilated, or becomes a part of ourselves ; but only for a time, for the cells are as constantly breaking down as building up. The wearing-out process goes on incessantly, creating a demand for new material as long as life lasts.

The particles of worn-out tissue and the surplus of new material are taken out of the way, partly by the capillaries and partly by the lymphatic vessels, and carried as the venous blood to the lungs, where they are got rid of or changed, as we shall learn in the next section.

How does food keep the body warm ?

To obtain heat in the stove we need carbon and hydrogen to be burned and oxygen to burn them. To obtain animal heat, the same elements are essential. All our food contains compounds of carbon and hydrogen, the same elements that are found in wood and coal. Many of our foods contain compounds of nitrogen. These carbonaceous and nitrogenous foods are eaten in a natural or in a prepared state, and, after undergoing certain processes of digestion, are absorbed into the general circulation. Through the lungs, oxygen from the air enters the blood. So we have in the arterial blood all the elements we had in the coal fire, - carbon and hydrogen in the form of new material and worn-out tissue, and oxygen taken in at every breath. This blood is carried all over the body; and then in the cells, chiefly in the muscles, the oxygen combines with the carbon and hydrogen, producing carbonic acid gas and watery vapor.

This chemical action develops heat just as truly, though at a lower temperature, as in the coal fire. But instead of combining so rapidly as to produce fire and light, this animal combustion goes on so slowly and continuously as scarcely to be noticed except when vigorous exercise increases the amount of fire, or lack of fuel diminishes it.

Carbonic acid gas and watery vapor - products of combustion - are given off from the lungs in the exhalations. The mineral salts and the nitrogenous residue, together with the larger part of the water, escape through the kidneys and skin.

Some of the food, being indigestible, never enters the blood, but leaves the system as excrement.

In the coal fire, we have been advised to " keep the grate free from ashes and clinkers when a bright fire is needed." Equal care is essential in respect to the fire within our bodies. The pores of the skin must be kept open by frequent bathing, and a suitable amount of water be taken daily, to aid in digestion and assimilation of food, and in the removal of the waste products.

Coal burned in a stove gives out heat to warm our rooms and to cook our food, and if burned in an engine, converting water into steam, gives force with which to do mechanical work. So, in animal combustion, the burning of the foods by the oxygen liberates their stored-up energy. This energy is given out partly as heat - keeping the body temperature at 98° - and partly in the form of mechanical work.

Thus our food renders a threefold service to that most wonderful machine, the human body : furnishing heat to keep it warm, material to build it up, make it grow, and keep it in repair, and energy with which it may do its work, whether that work be the voluntary activity of our hands, feet, and brains, or the involuntary motions by which are performed the great functions of respiration and circulation.