THORACIC CAVITY 



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posteriorly ; and the inferior mediastinum is subdivided into 

 anterior, middle, and posterior portions. The anterior 

 mediastinum is the part anterior to the pericardium, the 

 posterior mediastinum the part posterior to the pericardium, 

 whilst the pericardium and the heart with the great vessels 

 and the phrenic nerves with their accompanying vessels form 

 the middle mediastinum. It is customary, however, to speak of 

 the mediastinum as if it were a space, and to say that the various 

 viscera, vessels, etc., lie in the mediastina (Fig. 19, p. 44). 

 The lateral portions of the thoracic cavity are known as 



Costal part of parietal pleura 



Pleural cavity x 

 Visceral pleura 



Costal part of parietal pleura 

 Pleural cavity 

 Visceral pleura 





FIG. 4. Diagrammatic representation of a cross section through 

 the Thorax. 



the pleural spaces ; each contains the corresponding lung 

 surrounded by an invaginated serous membrane called the 

 pleural sac. There are therefore two pleural sacs, and each is 

 so disposed that it not only lines the chamber in which the lung 

 lies, but is also reflected over the surface of the lung, so as to 

 give it an external covering which is intimately connected with 

 the pulmonary substance. Consequently, the wall of each 

 pleural sac is separable into two portions, an investing or 

 visceral part which covers the surface of the lung, and a lining or 

 parietal part which clothes the inner surfaces of the boundary 

 of each lateral part of the thoracic cavity. It must be clearly 

 understood, however, that the two terms are merely applied 

 to indicate different portions of a continuous membrane. 



