The respiratory apparatus is truly marvelous in beauty and efficiency. Medical men complain about nature's way of constructing the alimentary canal, saying that it is partly superfluous, but no such complaint is lodged against the lungs and their accessories.

The respiratory system may be likened in form to a well branched tree, with hollow trunk, limbs and leaves: The trachea is the trunk; the two bronchi, one going to the right side and the other to the left side, are the main branches; the bronchioles and their subdivisions are the smaller branches and twigs; the air cells are the leaves.

The trachea and bronchi are tubes, furnished with cartilaginous rings to keep them from collapsing. They are lined with mucous membrane. The bronchi give off branches, which in turn divide and subdivide, until they become very fine. Upon the last subdivisions are clustered many cells or vesicles. These are the air cells and here the exchange takes place, the blood giving up carbonic acid gas and receiving from the inspired air a supply of oxygen. This exchange takes place through a very thin layer of mucous membrane, the air being on one side and the blood capillaries on the other side.

The whole respiratory tract is lined with mucous membrane. This membrane is ciliated, that is, it is studded with tiny hairlike projections, extending into the air passages. These are constantly in motion, much like the grain in a field when the wind is gently blowing. Their function is to prevent the entry of foreign particles into the air cells, for their propulsive motion is away from the lungs, toward the external air passages.

In some of the large cities where the atmospheric conditions are unfavorable and the air is laden with dust and smoke, the cilia are unable to prevent the entrance of all the fine foreign particles in the air. Then these particles irritate the mucous membrane, which secretes enough mucus to imprison the intruders. Consequently there is occasionally expulsion of gray or black mucus, which should alarm no one under the circumstances, if feeling well. Normally the mucous membrane secretes only enough mucus to lubricate itself, and when there is much expulsion of mucus it means that either the respiratory or the digestive system, or both, are being abused. At such times the sufferer should take an inventory of his habits and correct them.

The air cells are made up of very thin membrane. So great is their surface that if they could be flattened out they would form a sheet of about 2,000 square feet. We can not explain satisfactorily why it is that through their walls there is an exchange of gases, nor how the respiratory system can act so effectively both as an exhaust of harmful matter and a supply of necessary elements. The distribution of the blood capillaries, so tiny that the naked eye can not make them out, is wonderful. Under the microscope they look like patterns of delicate, complex, beautiful lace.

The lungs are supplied with more blood than any other, part of the body. A small part of it is for the nourishment of the lung structure, but most of it comes to be purified. After the blood has traveled to various parts of the body to perform its work as a carrier of food, and oxygen and gatherer of waste, it returns to the heart and from the heart it is sent to the lungs. There it gives up its carbonic acid gas and receives a supply of oxygen. Then it returns to the heart again and once more it is sent to all parts of the body to distribute the vital element, oxygen.

The lungs give off watery vapor, a little animal matter and considerable heat, but their chief function is to exchange the carbonic acid gas of the blood for the oxygen of the air. When the fats, sugars and starches, in their modified form, are burned in the body to produce heat and energy, carbonic acid gas and water are formed. The gas is taken up by the blood stream, which is being deprived of its oxygen at the same time. This exchange turns the blood from red into a bluish tinge. The red color is due to the union of oxygen with the iron in the blood corpuscles, forming rust, roughly speaking.

The fine adjustment that exists in nature can be seen by taking into consideration that animals give off carbon dioxide and breathe in oxygen, while vegetation exhales oxygen and inhales carbon dioxide. In other words, animal life makes conditions favorable for plant growth, and vegetation makes possible the existence of animals.

An animal of the higher class can live several days without water, several weeks without food, but only a very few minutes without oxygen. When the blood becomes surcharged with carbonic acid gas, and oxygen is refused admittance to the lungs, life ceases in about five or six minutes. From this it can easily be seen how important it is to have a proper supply of oxygen. Acute deprivation of this element is immediately fatal, and chronic deprivation of a good supply helps to produce early deterioration and premature death. The lungs can easily be kept in good condition, and when we ponder on the beautiful and effective way in which nature has equipped us with a respiratory apparatus and an inexhaustible store of oxygen, surely we must understand the folly of not helping ourselves to what is so vital, yet absolutely free.

Wrong eating and impure air are largely responsible for all kinds of respiratory troubles, from a simple cold to the most aggravated form of pulmonary tuberculosis. Exercise and deep breathing will to a great extent antidote overeating, but there is a limit beyond which the lungs refuse to tolerate this form of abuse.

Experiments have shown that if the carbonic acid gas thrown off daily by an adult male were solidified, it would amount to about seven ounces of solid carbon, which comes from fats, sugars and starches that are burned in the body. It is well to remember that there are various forms of burning or combustion. Rapid combustion is exemplified in stoves and furnaces, where the carbon of coal or wood rapidly and violently unites with oxygen. Slow combustion takes place in the rotting of wood, the rusting of iron and steel and the union of oxygen with organic matter in animal bodies. Both processes are the same, varying only in rapidity and intensity.

People who daily give off seven ounces of carbon are overworking their bodies. They take in too much food and consequently force too great combustion. This forcing has evil effects on the system, for under forced combustion the body is not able to clean itself thoroughly. Some of the soot remains in the flues (the blood-vessels) and is deposited in the various parts of the engine (the body). Result: Hardening, which means loss of elasticity and aging of the body. Aging of the body results in deterioration of the mind. Proper breathing is fine, but unless it is also accompanied by proper eating it does not bring the best results.