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formed by the twelve thoracic vertebrae of the spine, the sternum in front and the twelve pairs of ribs, which encircle the heart and lungs from the vertebral column at the back to the sternum in front. The thoracic cage protects these vital organs from damage. The spaces between the ribs contain the intercostal muscles, which connect them and give a degree of movement to the whole ribcage. The ribs are articulated at each end, so they can move upwards and outwards like the handle of a bucket.
There are two lungs, one on each side of the chest, separated by the heart and its major blood vessels. Lung tissue is pink in colour in its unpolluted state, because of its many blood capillaries. It is elastic, porous and spongy and, because of the air in its alveoli, it floats on water. The right lung is made up of three lobes or compartments, while the left has only two because the heart takes up room in the left side of the ribcage. The lungs are cone-shaped and consist of an apex, a base, costal surface and medial surface. The apex is rounded and rises into the root of the neck, about 25 mm (1 inch) above the level of the middle third of the clavicle. The structures associated with it are the first rib and the blood vessels and nerves in the root of the neck.
The base is concave and semilunar in shape, and is closely associated with the thoracic surface of the diaphragm. The costal surface is convex and is closely associated with the costal cartilages, the ribs and the intercostal muscles. The medial surface is concave and has a roughly triangularshaped area, called the hilum, at the level of the 5th, 6th and 7th thoracic vertebrae. Structures which form the root of the lung enter and leave at the hilum. These include the primary bronchus, the pulmonary artery supplying the lung and the two pulmonary veins draining it, the bronchial artery and veins, and the lymphatic and nerve supply.
The area between the lungs is called mediastinum. It is occupied by the heart, great vessels, trachea, right and left bronchi, oesophagus, lymph nodes, lymph vessels and nerves.
