THE MEDIASTINUM. 



Ill 



Not less important is the fact that a comparatively small area of the pericardium is in im- 

 mediate contact anteriorly with the inner surface of the thoracic wall. This place is situated 

 where the two anterior pleural limits diverge close to the left sternal border from the fifth to the 

 seventh costal cartilage. In withdrawing fluid from the pericardial cavity it is best to make 

 the puncture in the fifth intercostal space close to the sternal margin. If the puncture is made 

 further to the left, there is danger of injuring the left pleural sac. 



A knowledge of the relation of the pericardium to the thoracic wall is also necessary when 

 it is desired to open retrosternal abscesses or to remove tumors from behind the sternum by a 

 partial resection of this bone, since the pleural and pericardial cavities should not be clumsily 

 nor unnecessarily opened. 



Erector spinae muscle Trapezius muscle 



Scapula" 



Spinal cord--' 

 Pleural cavity - 



Subscapular 



vessels. 



Bifurcation of 



trachea 

 Azygos major vein - - 



Right lung 



Superior vena cava 



Sinus transversus 



pericardii 

 Ascending aorta 



Deltoid muscle 



Subscapularis 

 '-Descending aorta 

 -Seiratus magnus 

 - -Esophagus 



Left pulmonary 

 - artery 



_JLeft lung 

 __Lymphatic glands 



_ Pectoralis minor 

 Pectoralis major 



Internal mammary artery with vein 



FIG. 53. A cross-section of the thorax at the level of the tracheal bifurcation (frozen section). The left lung 

 was diseased at its apex and diminished in size as a whole; the right lung was correspondingly enlarged. 



Since the mediastinum is not a rigid partition between the two pleural cavities, but rather 

 a movable and elastic septum, as is necessary for costal breathing, it is clear that when a pleural 

 cavity is distended by abnormal contents (pyothorax, pneumothorax) or when one lung requires 

 more space than normally belongs to it, the mediastinum together with the heart will be pushed 

 toward the opposite side.' This may be recognized by the physician by the displacement of the 

 cardiac dulness. A left-sided pleural exudate, for example, may push the right border of the 

 normal cardiac dulness far beyond the right margin of the sternum. If the lung becomes con- 

 tracted, as in cirrhosis, and adherent to the mediastinal pleura, the mediastinum will be drawn 

 toward the same side, and the opposite lung, when healthy, may undergo a compensatory enlarge- 



