The Lungs, roughly conical in shape, occupy the thoracic cavity, being separated from one another by the mediastinum. The right lung is broader, shorter, and slightly larger than the left. The apex projects into the root of the neck, corresponding posteriorly to the spine of the seventh cervical vertebra, and anteriorly to a point nearly 1 inch above the inner end of the clavicle. The lines of the anterior borders correspond to those of the pleural reflections already given (q.v.). The anterior border of the left lung presents a notch or incisura corresponding to the cardiac apex, which is thereby left uncovered by lung. The bases, however, do not reach as low as the pleurae. Thus, on the right side the base of the lung extends from the sixth intercostal space in front to the lower border of the eighth rib in the axillary line, and that of the tenth rib in the scapular line, and then runs horizontally inwards to the vertebral extremity of the eleventh rib, the lowest level reached being the ninth intercostal space in front of the scapular line. The lower border of the left lung is similar, but rather lower. In consequence of the pleurae descending below the lower limit of the lung, it is possible to get wounds which involve the pleura, but do not damage the lung. Both lungs present a deep fissure, which commencing posteriorly about 3 inches below the apex, opposite the third dorsal spine, extends first horizontally on a level with the vertebral extremity of the spine of the scapula, and then is directed downwards and forwards, dividing the lung into two lobes, and terminating in front at the level of the lower border of the sixth rib just in front of the mammary line. The right lung generally presents a second fissure, which divides the upper lobe into two. It thus presents three lobes. The mediastinal surface of each lung is concave, and particularly on the left is deeply indented by the pericardium. The left lung is also grooved above the hilum by the arch of the aorta and left subclavian artery, while behind the hilum the right ig grooved by the vena azygos major and oesophagus, and the left is in relationship to the oesophagus and thoracic duct. The hilum is situated on the inner surface, rather above and posterior to the centre of the lung, opposite the fifth, sixth, and seventh dorsal spines. It is oval in section, and is continuous below with the ligamentum latum pulmonis. The structures forming the root of the lung are the bronchus, pulmonary artery, and three pulmonary veins, the artery lying below the bronchus on the right side, and above it on the left, while the veins on both sides lie below the other structures. The branches of the pulmonary vessels follow the bronchi, break up into fine capillaries, which project into the lumen of the alveoli, thus producing the oxygenation of the blood, which then returns by the pulmonary veins to the left auricle. In addition to the structures mentioned, there are small bronchial arteries and veins and lymphatic glands, pulmonary lymphatic vessels and nerves, and a small amount of connective tissue. The bronchial arteries supply the lung tissue, and vary from one to three for each lung, being derived from the aorta or an intercostal vessel. They lie on the posterior surface of the bronchus. The blood is returned partly by the pulmonary and partly by the bronchial veins, which latter open into the azygos veins. The pulmonary nerves are derived from the vagus and sympathetic.

Wounds of the lung are occasionally met with, and cases are recorded where rupture of the lung has occurred without either external wound or fracture of the ribs. In bleeding from the lung the blood may be effused into the tissue of the organ, giving rise to pulmonary apoplexy, or into the alveolar spaces, and so to the bronchi, causing hemoptysis, or, where the visceral pleura is wounded, into the pleural cavity, causing hemothorax. In cases of desperate bleeding from the lung, it may be necessary to open the chest-wall, and collapse the lung. The lung is frequently affected by emboli brought to it by the systemic veins, or from the right side of the heart. These get arrested in the pulmonary capillaries, and, if septic (as would be the case when conveyed from septic sigmoid sinus thrombosis), give rise to lung abscesses, or, where they have been detached from some sarcomatous tumour, give rise to secondary tumours in the lung. Thus, the lung should be carefully examined, where extension either of a venous septic process or of sarcoma is suspected. Fat emboli from injury to the medulla of bone, and air emboli from wound of one of the large veins in the neck, also occur in the lung, and the latter frequently give rise to asphyxia.

Where from any cause-as, for instance, in mitral or aortic disease-the return flow of blood from the lungs is obstructed, the lung becomes greatly engorged. The bronchial vessels anastomose with the pulmonary, but they are of small size, and only serve to supply the lung tissue.

Cavities in the lung arise perhaps most frequently from tubercular disease, but may also be caused by abscesses, or gangrene, following pneumonia, the entrance of foreign bodies, or by extension from the liver. Hydatid cysts are also sometimes met with. In such cases a pneumotomy may be performed in order to reach and drain the cavity. Such an operation is generally best performed in two stages-the first opening the pleural cavity, and the second, after the pleurae have become soldered round the edges, opening into the lung substance, the cautery being sometimes used for this purpose, where serious bleeding is feared. On the other hand, wThere the lung is affected-for example, by a primary sarcomatous tumour-it may be necessary to excise a portion, or, where it is riddled with tubercle, it is occasionally advisable to remove the whole lung. This operation is known as pneumonectomy, and consists, first, in the removal of several ribs, as in Estlander s operation, and, at a later stage, of ligature of the affected portion and its removal, or, where complete extirpation is required, ligature of all the structures at the root of the lung, and removal of the lung in toto (Macewen). Such an operation on a healthy lung would probably cause immediate death of the patient, but, in disease, the lung is almost, if not entirely, functionless at the time of its removal, and good results are hence obtained, the more normal lung frequently benefiting by the removal, as cross infection by the bronchi and trachea is arrested.