The thoracic cavity, also called the chest cavity, is a cavity of vertebrates bounded by the rib cage on the sides and top, and the diaphragm on the bottom. The chest cavity is bound by the thoracic vertebrae, which connect to the ribs that surround the cavity. The thoracic cavity is actually composed of three spaces each lined with mesothelium, a special film-like tissue that separates vital organs. The pleural cavities surround the lungs, while the pericardial cavity surrounds and protects the heart.