The _Lungs_ are irregular conical organs rounded at the apex, situated within the chest, and filling the greater part of it, since the heart is the only other organ which occupies much space in the thoracic cavity. The lungs are convex externally, and conform to the cavity of the chest, while the internal surface is concave for the accommodation of the heart. The size of the lungs depends upon the capacity of the chest. Their color varies, being of a pinkish hue in childhood but of a gray, mottled appearance in the adult. They are termed the _right_ and _left_ lung. Each lung resembles a cone with its base resting upon the diaphragm, and its apex behind the collar-bone. The right lung is larger though shorter, than the left, not extending so low, and has three _lobes_, formed by deep fissures, or longitudinal divisions, while the left has but two lobes. Each lobe is also made up of numerous _lobules_, or small lobes, connected by cellular tissue, and these contain great numbers of cells. The lungs are abundantly supplied with blood-vessels, lymphatics, and nerves. The density of a lung depends upon the amount of air which it contains. Thus, experiment has shown that in a _foetus_ which has never breathed, the lungs are compact and will sink in water; but as soon as they become inflated with air, they spread over a larger surface, and are therefore more buoyant. Each lung is invested, as far as its root, with a membrane, called the _pleura_, which is then continuously extended to the cavity of the chest, thus performing the double office of lining it, and constituting a partition between the lungs. The part of the membrane which forms this partition is termed the _mediastinum_. Inflammation of this membrane is called _pleurisy_. The lungs are held in position by the root, which is formed by the pulmonary arteries, veins, nerves, and the bronchial tubes. Respiration is the function by which the venous blood, conveyed to the lungs by the pulmonary artery, is converted into arterial blood. This is effected by the elimination of carbonic acid, which is expired or exhaled from the lungs, and by the absorption of oxygen from the air which is taken into the lungs, by the act of inspiration or inhalation. The act of expiration is performed chiefly by the elevation of the diaphragm and the descent of the ribs, and inspiration is principally effected by the descent of the diaphragm and the elevation of the ribs. [Illustration: Fig. 44. A representation of the heart and lungs. 4. The heart. 5. The pulmonary artery. 8. Aorta. 9, 11. Upper lobes of the lungs. 10, 13. Lower lobes. 12. Middle lobe of the right lung. 2. Superior vena cava. 3. Inferior vena cava.]