Thoracic cavity The thorax is the most cranial of the three body cavities. The dorsal boundary of the thorax is the thoracic vertebrae, of which the cat has thirteen. The thorax extends ventrally and terminates at the sternum. Laterally, the thorax consists of the ribs and costal cartilage. The thorax is separated from the abdomen by the diaphragm. When the diaphragm increases the size of the abdominal cavity, the thoracic cavity is subsequently reduced. The layer of connective tissue lining the thoracic cavity is called the endothoracic fascia. Within this lining there are two pleural cavities, each lined with pleura. The region in between the two pleural cavities is called the mediastinum and most of the organs and other structures within the thoracic cavity are located here, with exception of the lungs and their closely related tissues. The thoracic cavity contains the lungs, heart, oesophagus, trachea and thymus. The major blood vessels in the thorax are the aorta, pulmonary artery and the vena cava. The phrenic nerve and vagus nerve are also found in the thorax.