The amount of air entering and leaving the lungs varies greatly in ordinary and forced respiration, being often three times as much in the latter. The volume of air is determined by the spirometer. Mr. Hutchinson, who invented the spirometer, has defined the vital capacity of the lungs as that amount of air which can be expelled by the most forcible expiration, and so the measure of the individual’s respiratory power. The vital capacity varies according to a number of conditions, as age, sex, weight, but, most important of all, is the height. It has been found that between five and six feet the vital capacity increases eight cubic inches for each inch in height. The vital importance of the rôle which oxygen plays in the health and life of the individual may be better understood from the facts that about 10,000 liters of air are breathed daily, which makes the amount of food and drink consumed daily seem almost infinitesimal, and, important as the quality of the food is, the quality of the air is much more so, and, finally, that one can live for some days without either food or drink, but dies in a few minutes if the supply of air is cut off. _Secretion of the Lungs._—Like the lips and mouth, the lungs are invested on their free inner surface by a delicate mucous membrane, which constantly secretes a clear viscous fluid, the mucus. The lungs, therefore, like the nose, are always moist, and just as the nose is cleared by blowing it, so the lungs are cleared by hawking or coughing. A sense of discomfort or a feeling of irritation of the windpipe induces a deep inspiration, followed by an explosive expiration, which quickly brings up the mucus, so that it can be expectorated. The only difference between the mucus of the lungs and that of other organs is that the former is mixed with air and has, therefore, a frothy appearance. The secretion of the lungs naturally flows down and accumulates, until it is voluntarily brought up and expelled. Any one with a cold on the chest, or who is subject to catarrh, will notice that, on moving about in the morning in making the toilet, especially on raising the arms to dress the hair, expectoration is greatly facilitated, and that this is followed by a feeling of clearing out of the throat and lungs. Because of the great aid given to the lungs in clearing them of mucus, moderate exercise in the open air is a much better treatment of an ordinary cold than a prolonged stay in bed. And for the same reason, in the treatment of lung troubles, so soon as the temperature of the patient is down to normal, and her strength makes it safe to allow her to move about, the recovery of the patient is hastened by getting up and moving about the house.