104 TOPOGRAPHIC AND APPLIED ANATOMY. 



PLATE 8. 



a, A dissection of the mediastinum seen from the left; almost the entire left half of the thorax has been removed 

 and the left lung has been cut away at its root, b, The mediastinum seen from the right. Formalin preparations from 

 a child. 



THE MEDIASTINUM. 



After the removal of the lateral thoracic walls and of the lungs, the mediastinum is exposed 

 from both sides (see Plate 8). Upon the cut surfaces of the roots of the lungs may be seen the 

 divided bronchi, pulmonary arteries, and pulmonary veins, which pass from the mediastinum 

 into the lungs. 



The contents of the mediastinum are most numerous where the two pleural sacs are most 

 widely separated in the median line. This is the case behind the sternum, where the heart is 

 situated within the pericardium between the two layers of mediastinal pleura. If the anterior 

 thoracic wall is also removed by dividing the sternal attachments of the mediastinum, and if 

 the two layers of mediastinal pleura are slightly reflected backward, the pericardium may be 

 easily exposed. If the pericardium be opened and the heart excised it will at once be appreciated 

 that the two pleural cavities and the lungs, together with the pericardium and heart, occupy 

 almost the entire thoracic space. In addition to the heart and the pericardium the contents 

 of the mediastinum include: the remains of the thymus gland, a large portion of the trachea with 

 its bifurcation, many large and small vessels and nerves, the esophagus, lymphatic vessels includ- 

 ing the thoracic duct, and many important lymphatic glands. These structures are bound together 

 by a loose connective tissue which becomes directly continuous with the loose connective tissue 

 of the neck and furnishes a path along which inflammations (suppurations, abscesses) may 

 easily extend from the neck into the mediastinum through the superior aperture of the thorax. 



The chief contents of the mediastinum may be seen through the mediastinal pleura after 

 the removal of the lungs, if the membrane retains its normal transparency and has not become 

 opaque and thickened by inflammation. The contents become still more distinct if the two 

 mediastinal layers are removed. 



Upon the right side may be seen: portions of the heart through the pericardium (during 

 childhood, at least), the superior vena cava, the phrenic nerve (upon the superior vena cava and 

 running upon the pericardium to the diaphragm), the comes nervi phrenici, the vena azygos 

 major and its termination in the superior vena cava, the right intercostal vessels, the esophagus, 

 the vagus nerve, the right sympathetic and splanchnic nerves, and the lymphatic glands at the 

 root of the lung. 



Upon the left side the visible structures are: the heart with the left phrenic nerve, the comes 

 nervi phrenici, the descending aorta, the left subclavian artery, the left internal mammary artery, 

 the left innominate vein* receiving the termination of the vena azygos minor,f the vagus nerve, 

 the left sympathetic nerve, the left intercostal vessels, and numerous lymphatic glands. 



In children the thymus gland may be seen from either side; it is situated in front of and above 

 the heart. 



* Erroneously designated as V. Cava sup. in the plate. 



f In Plate 8, a, the vena azygos minor (vena hemiazygos) joins a very large superior intercostal vein (see page 117). 



