100 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 45. A diagrammatic representation of the projections of the heart, of the pleural limits, and of the lungs 

 upon the anterior thoracic wall. The lung is indicated by yellow, and the pleura by red lines. The lungs are represented 

 in a state of moderate distention, so that their borders correspond to a position between deepest expiration and deepest 

 inspiration. 



FIG. 46. A diagrammatic representation of the projections of the pleural limits, of the lungs, and of the bifurcation 

 of the trachea upon the posterior thoracic wall. The lung is indicated by yellow and the pleura by red lines. 



costal spaces is designated as the costal pleura, while that situated upon the upper surface of 

 the diaphragm is known as the diaphragmatic pleura. The costal pleura forms the outer wall 

 of the pleural cavity, while the diaphragmatic pleura forms its floor. The inner wall is known 

 as the mediastinal pleura; it extends in a sagittal plane from the vertebral column toward the 

 sternum, and in these situations becomes continuous with the costal pleura. The costo-medi- 

 astlnal sinus is situated anteriorly at the junction of the costal and the mediastinal pleurae. The 

 costo-phrenic sinus (Fig. 49) is formed by the reflection of the costal pleura upon the diaphragm. 

 The sinuses are complementary spaces for the accommodation of the margins of the lungs dur- 

 ing inspiration. During respiration the lungs glide inaudibly upon the smooth inner surface 

 of the pleura. If inflammatory exudates are deposited upon the pleura, however, the physician 

 may hear friction sounds through the thoracic wall. 



Owing to the thinness of the thoracic wall, pathologic changes in the breathing space or 

 in the lung-tissue or abnormal collections in the pleural sacs may be recognized by typical varia- 

 tions from the normal pulmonary sounds. The pleural cavity may contain air, entering through 

 an external wound, but more frequently through a bronchus after injury to the lung (pneumo- 

 thorax), a serous transudate (hydrothorax), pus (empyema), or blood (hematothorax). 



The costal, diaphragmatic, and mediastinal pleurae together form the parietal pleura, in 

 contradistinction to the visceral pleura, which is intimately adherent to the surface of the lung. 

 The two layers are continuous with each other at the root of the lung. 



The mediastinal pleura is best studied by removing the lateral w T alls of the thorax, together 

 with the costal pleura, and taking out the lungs by cutting through their roots (and also through 

 the small and insignificant ligamentum latum pulmonis, which runs downward from the root 

 of the lung to the diaphragm). It will then be seen that the two mediastinal pleurae form a par- 

 tition between the two pleural cavities which is not exactly in the median line and which extends 

 from the vertebral column to the sternum. This is the mediastinum.* It consists of the two 

 layers of the mediastinal pleura and of the space enclosed between them. Superiorly, where 

 the mediastinal pleura becomes continuous with the costal pleura above the superior aperture 

 of the thorax, the mediastinal contents become continuous with the structures of the neck; in- 

 feriorly, the mediastinum is fixed to the diaphragm. 



The dome of the pleura extends upward posteriorly to the level of the upper margin of the 

 first thoracic vertebra, and slopes down anteriorly to the upper margin of the first costal carti- 



* Among the old anatomists the word "mediastinum" was equivalent to "partition"; for example, mediastinum 

 auris-membrana tympani. As the diaphragm is a horizontal partition between the thorax and abdomen, so is the medias- 

 tinum a vertical partition in the thorax. It is formed by the two layers of mediastinal pleura, which barely touch at 

 any point, but which enclose a space completely filled by many organs. A " mediastinal cavity " consequently has no 

 existence. 



