The mediastinum is the space and the mediastinal pleurae that enclose this space between the right and left pleural cavities. It is divided by the heart into three transverse divisions. The middle division is further divided by the heart into two more divisions dorsal and ventral to it.

The cranial mediastinum (mediastinum craniale) () is the portion of the mediastinum lying cranial to the heart that contains the trachea, esophagus, thymus, sternal and cranial mediastinal lymph nodes, and many vessels and nerves. Dorsally it attaches to the longus colli muscles and ventrally it attaches to the sternum. Ventrally it refects over the internal thoracic vessels as they course toward the sternum to pass deep to the transverse thoracis muscles.

The ventral mediastinum (mediastinum ventrale) is that portion ventral to the heart that contains the thymus cranially and the phrenicopericardial ligament (lig. phrenicopericardiacum), a band of connective tissue between the fibrous pericardium and the diaphragm.

The middle mediastinum (mediastinum medium) () is the portion containing the heart. Here the mediastinal pleura are fused with the fibrous pericardium. A single layer of tissue separates the pleural cavity from the pericardial cavity. From the pleural cavity to the pericardial cavity, this layer consists of the mediastinal pleura, the fibrous pericardium and the parietal layer of the serous pericardium. On each side the phrenic nerves course across the heart through the fibrous pericardium where they are covered by middle mediastinal pleura. At the base of the heart the vagal nerves cross the heart deep to the middle mediastinal pleura.

The dorsal mediastinum (mediastinum dorsale) is that part dorsal to the heart containing the major pulmonary vessels, the aorta and its initial branches, thoracic duct, tracheal bifurcation, esophagus, and lymph nodes. At the tracheal bifurcation the dorsal mediastinal pleura refects on to the lungs as the pulmonary pleura.

The caudal mediastinum (mediastinum caudale) is that part of the mediastinum lying caudal to the heart. It is refected to the left by the accessory lung lobe of the right lung and contains the aorta and thoracic duct dorsally and the esophagus with vagal nerves more centrally. A separate fold of the right caudal mediastinal pleura projects dorsally from the level of the sternum to surround the caudal vena cava and right phrenic nerve approximately one third of the distance between the sternum and the vertebral column. This fold is the plica vena cava (plica venae cavae). The pleural cavity space created between this plica and the caudal mediastinum is the mediastinal recess (recessus mediastini) that contains the accessory lobe of the right lung. In the dog the tissue in the mediastinum is extremely scanty, but the pleura that covers it is not fenestrated. Therefore one lung can be collapsed independently of the other.