thorax

Part of the body in four-limbed vertebrates containing the heart and lungs, and protected by the ribcage. It is separated from the abdomen by the diaphragm. During breathing (ventilation) the volume inside the thorax is changed. This then causes air to move in or out of the air passages that lead to the lungs. The volume of the thorax is altered by the contraction of muscles in the diaphragm and the contraction of muscles between the ribs - the intercostal muscles.

In arthropods, the thorax is the middle part of the body, between the head and abdomen. In insects the thorax bears the legs and wings. The thorax of spiders and crustaceans, such as lobsters, is fused with the head, to form the cephalothorax.

Structure of the thorax In humans the thoracic inlet contains the oesophagus (gullet) and trachea (windpipe), and those arteries and veins leading from and to the heart through the neck, together with certain nerves. The thoracic outlet is filled by the diaphragm. The intercostal spaces between adjacent ribs are sealed by three layers of intercostal muscles. During inspiration these muscles lift the ribs and the diaphragm flattens, causing an increase in volume of the thorax. The lungs expand to fill this extra space, drawing in air. During expiration the relaxation of the intercostal muscles and diaphragm, combined with the elastic recoil of the lungs, forces air out of the lungs.