THE MEDIASTINAL SEPTUM 1239 



usually encountered, it should be noted that they are subject to variation. The extremes of 

 variation of the anterior lines, as determined by Tanja, are indicated in fig. 1003. 



Blood-vessels. — The vascular networks of the pulmonary pleura are derived from the 

 bronchial artery and probably to some extent from the pulmonary artery which in the dog, is 

 the only source of blood supply. The venous radicles arising from the network enter the lung. 

 (See radicles of the pulmonary vein on page 123.5.) The parietal pleura is supplied by arteries 

 from several sources: internal mammary, mtercostals, phrenics, mediastinal, and bronchial. 

 The veins correspond to the arteries. The lymphatics of the pulmonary pleura form rich 

 networks without definite relations to the lobules of the lung. They accompany the radicles 

 of the pulmonary veins and drain into the bronchial lymph-glands. In the parietal pleura 

 lymph-vessels are present most abundantly over the interspaces; they empty into the sternal 

 and intercostal glands. (See p. 728.) The nerves supplied to the pulmonary pleura are 

 branches from the pulmonary plexus; to the parietal pleura, from the intercostals, vagus, 

 phrenic, and sympathetic. 



• MEDIASTINAL SEPTUM 



The two pleural cavities are separated from each other by the mediastinal sep- 

 tum [septum mediastinale] (fig. 1000). This is a sagittal partition extendirig 

 from the superior aperture of the thorax to the diaphragm between the thoracic 

 vertebrae and the sternum, its free surfaces, right and left, formed by the mediast- 

 inal layers of the pleurae. It is composed of the pericardium and heart and of 

 structures which, for the most part, extend in a longitudinal direction through the 

 thoracic cavity. 



These mclude the oesophagus together with the vagus nerves, the thoracic duct, thoracic 

 aorta and azygos vein ; the trachea, the pulmonary vessels and the arch of the aorta with its 

 great branches, the superior vena cava and its tributaries and the phrenic nerves; the thymus 

 gland, mternal mammary vessels and many lymph glands throughout the septum. These 

 structures are packed together and supported by intervening connective tissue. Moreover, 

 the connection of the sheaths of the great vessels with processes of the cervical fascia and the 

 fixation of the pericardium to the diaphragm, give to the latter a strong support. Owing to 

 the position of the heart, the two sides of the septum are not symmetrical, and it follows from 

 the bulging of the left surface of the mediastinal septum that the left pleural cavity is encroached 

 upon. 



The name mediastinal cavity has been apphed to the two regions of the medi- 

 astinal partition which find themselves located, the one in front, the other behind 

 the plane of the heart. There is in reahty no cavity, the term being used in this 

 connection merely to donate space. Between the two spaces are interposed the 

 pericardium and heart, the great vessels, trachea and bronchi. The anterior 

 mediastinal cavity [cavum mediastinale anterius] is small. Its lateral limits are 

 formed by the mediastinal layers of the pleurae, right and left, which are reflected 

 backward from the costal pleura3 of the anterior thoracic wall. The space is 

 occupied by loose connective tissue, surrounding the thymus gland, the internal 

 mammarj^ vessels and a number of lymph-glands. 



RecaUing the Unes of reflexion of the mediastinal pleurae as above described, the form, 

 position and extent of this space as observed from in front, will be understood; it is widest 

 behind the inferior end of the body of the sternum and fifth and sixth costal cartilages of the 

 left side {area interpleurica inferior); narrowest where the mediastinal layers are approximated 

 behind the body of the sternum, broader again where the laminae deviate posterior to the 

 manubrium sterni {area interpleurica superior). In the latter space lies the thymus gland and 

 the superior portions of the internal mammary vessels. In the area interpleurica inferior 

 the pericardium comes into immediate contact with the anterior thoracic wall, and here the 

 inferior portions of the left internal mammary vessels are found. The lymphatic vessels and 

 glands of the anterior mediastinal space belong to the anterior mediastmal and sternal groups. 



The posterior mediastinal cavity [cavum mediastinale posterius] (fig. 1000), 

 hmited behind by the thoracic vertebrae and laterally by the mediastinal layers of 

 the pleurae where they are reflected forward from the costal pleurae of the pos- 

 terior thoracic walls, is elongated and of more regular form than the anterior space. 

 It includes the thoracic aorta, the oesophagus and vagi, the thoracic duct, azygos 

 vein and lymph glands. 



Within this space are also to be found the origins of the right intercostal arteries, the 

 hemiazygos and, when present, the accessory hemiazygos veins, terminations of some of the 

 left intercostal veins and the greater splanchnic nerves. The lymph glands belong to the 

 posterior mediastinal group. 



(Figs. 993, 994). — A subdivision of the mediastinal septum into anterior, middle, posterior, 

 and superior mediastinal spaces has long been customary, and is useful for descriptive purposes. 



The superior mediastinum is that part of the mediastinum which lies above the level of 



