Respiration under Nervous Control.--The muscular movements which cause an inspiration are partly under the control of the will, but in part the movement is beyond our control. The nerve centers which govern inspiration are part of the sympathetic nervous system. Anything of an irritating nature in the trachea or larynx will cause a sudden expiration or cough. When a boy runs, the quickened respiration is due to the fact that oxygen is used up rapidly and a larger quantity of carbon dioxide is formed. The carbon dioxide in the blood stimulates the nervous center which has control of respiration to greater activity, and quickened inspiration follows. Need of Ventilation.--During the course of a day the lungs lose to the surrounding air nearly two pounds of carbon dioxide. This means that about three fifths of a cubic foot is given off by each person during an hour. When we are confined for some time in a room, it becomes necessary to get rid of this carbon dioxide. This can be done only by means of proper ventilation. A considerable amount of moisture is given off from the body, and this moisture in a crowded room is responsible for much of the discomfort. The air becomes humid and uncomfortable. It has been found that by keeping the air in motion in such a room (as through the use of electric fans) much of this discomfort is obviated. The presence of impurities in the air of a room may easily be determined by its odor. The odor of a poorly ventilated room is due to organic impurities given off with the carbon dioxide. This, fortunately, gives us an index of the amount of waste material in the air. Among the factors which take oxygen from the air in a closed room and produce carbon dioxide are burning gas or oil lamps and stoves, and the presence of a number of people. [Illustration: Three ways of ventilating a room. _i_, inlet for air; _o_, outlet for air. Which is the best method of ventilation? Explain.] Proper Ventilation.--Ventilation consists in the removal of air that has been used, and the introduction of a fresh supply to take its place. Heated air rises, carrying with it much of the carbon dioxide and other impurities. A good method of ventilation for the home is to place a board two or three inches high between the lower sash and the frame of a window or to have the window open an inch or so at the top and the bottom. An open fireplace in a room aids in ventilation because of the constant draft up the flue. Sweeping and Dusting.--It is very easy to demonstrate the amount of dust in the air by following the course of a beam of light in a darkened room. We have already proved that spores of mold and yeast exist in the air. That bacteria are also present can be proved by exposing a sterilized gelatin plate to the air in a schoolroom for a few moments.[47]