Thorax, a division of certain animal bodies. It is the middle section of the body, situated between the head or neck and the abdomen. In most crustaceans (including lobsters and shrimps) and in spiders and other arachnids, the thorax is fused with the head into a cephalothorax. The thorax usually contains parts of the digestive and respiratory systems. In insects and some crustaceans, the thorax is divided into segments to which legs and some other appendages are attached.
Most vertebrates have a thorax. Its bony parts are the ribs, sternum (breastbone), and certain vertebrae of the backbone. It contains the heart, lungs, and portions of the trachea and esophagus. In mammals the thorax is separated from the abdomen by a thin muscular partition called the diaphragm. In humans the thorax is called the chest.