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.