The Lungs lie within the pleural cavities of the thorax. A perfectly healthy lung lies quite free within its own pleural sac, attached only by its root; but as a rule adhesions between the lung and pleura are found. The right lung is larger than the left, the proportion being about eleven to ten. It is also shorter and wider.

The lungs take the shape of the pleural cavities, and are conical structures having an apex which projects above the level of the first rib for about half an inch, and a concave base which is adapted to the cupola of the diaphragm, on which it rests. The outer surface is smooth and convex, except for shallow grooves made by the ribs, and the inner surface has the impressions of the organs against which it lies. The lower border of the outer surface of the lung is thin and reaches down between the diaphragm and the ribs to the level of the lower border of the sixth rib in front; the eighth rib in the mid-axillary line; then passes horizontally inwards, reaching the vertebral column at the level of the tenth rib. The bases of the lungs are in relation with some of the abdominal organs, the diaphragm intervening. Thus, the base of the right lung rests upon the right lobe of the liver, whilst the base of the left lung is in relation with the left lobe of the liver, the fundus of the stomach, and the spleen.

On the anterior and lower part of the inner surface of each lung is a deep recess made by the pericardium (this is deeper in the left than the right), and above the pericardial concavity is the hilum of the lung, where the bronchial vessels and nerves enter, constituting the root of the lung. On the left lung, above and behind the hilum is a broad shallow groove made by the descending thoracic aorta.

The left lung is divided into two lobes by a groove reaching into the hilum; it starts above on the outer surface just below the apex and passes obliquely forwards and downwards to the base near the middle line. The right lobe is divided by a similar line into two, which, however, cuts the base nearer the outer side. The inner and upper lobe is then subdivided by a horizontal line passing inwards and forwards from about the middle of the first line.

The lungs entirely fill the thoracic cavity except for the space in the middle occupied by the trachea, oesophagus, large vessels, heart, and roots of the lungs.

The Roots of the Lungs are formed by a number of structures which pass into each lung at the hilum; they consist of the pulmonary veins, the pulmonary artery and the bronchus, nerves, lymphatic vessels and glands.