thorax, body division found in certain animals. In humans and other mammals it lies between the neck and abdomen and is also called the chest. The skeletal frame of the thorax is formed by the sternum (breastbone) and ribs in front and the dorsal vertebrae in back. Within the thoracic cavity are the heart, lungs, and esophagus. The chest and abdominal cavities are separated by a muscular structure, the diaphragm. In insects and other arthropods the thorax is a body division consisting of several segments lying between the head and abdomen, to which are attached legs or other appendages.