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.