Respiration. The lungs, the organs of respiration, may be described as fleshy sponges. In human beings there are two; one on each side the heart, fitted within the chest, and reaching from top to bottom of the ribs. These sponges are full of little holes or cells; the number of them is calculated at 174 millions, and each one of these takes in air as it comes down the throat or windpipe. This large number is required for the air to act upon. A man inspires from one to two pints each time he breathes. The air enters into all the cells of the lungs, where one part of it is absorbed or taken up by the blood as it rushes through, and the remainder is breathed out again, mingled with carbonic acid gas and vapour. A man breathes from fifteen to twenty times in a minute. Thus, about 1,000 pints of air enter the lungs every hour, or 3,000 gallons every twentv-four hours.