The long-continued action of such impurities on the olfactory nerves may ultimately induce, through the central nervous system, alterations in the respiration, circulation, and nutrition. When moderately vitiated air is breathed more or less continuously, the individual becomes pale and loses her appetite; after a time there is a decline in the muscular strength and animal spirits. The aëration and nutrition of the blood is interfered with, and the general tone of the system falls below par. It has further been maintained that metabolism is hindered by much-breathed atmosphere. In addition to the ordinary symptoms of discomfort, the long occupancy of so-called stuffy rooms so lowers the resistance as to be conducive to the contraction of colds and even to more serious infections. People in this lowered condition of health, which is very common among those who spend the greater part of the day indoors, in offices, houses, schools, factories, and workrooms, offer much less resistance to attacks of acute diseases than do people who lead an outdoor life. In considering the ventilation of a house, the purity of the air, the temperature, and the dryness of the air must all be considered. The test now generally accepted as the standard of purity of the air is not the chemical one of the estimation of the amount of carbonic acid contained in the air of a closed space, but that, on entering a room or closed space from the outside fresh air, no sense of impurity or closeness should be noticeable. The so-called natural ventilation of houses, which takes place through the porosity of the walls, the cracks around the doors and windows, is generally too inconsiderable to be taken into account. Where houses are heated by furnaces, a certain amount of ventilation is furnished by this means, but the air is by no means so pure as the air of a house heated by hot air or steam pipes. In the latter case, the greatest drawback is the dryness of the air. As the air contained in an inhabited room cannot be kept as pure as the outside air, the object of ventilation is, by the admission of the pure external air, so to minimize the impurities that the air respired may not be detrimental to health. The most effective means for the ventilation of houses and apartments is the throwing wide open all doors and windows; the windows must be opened at both top and bottom, as the hot impure air rises and the cold air falls to the floor. The length of time which the house should be left open will depend on the outside temperature and the velocity of the winds. This ventilation of the entire house should be carried out three times a day—in the early morning, at noon, and again in the evening.