THE PLEURAE. 



715 



THE PLEURAE. 



Each lung is invested, upon its external surface, by an exceed-jjigly delicate 

 serous membrane, the pleura, which incloses the organ as far as its root, and is 

 then reflected upon the inner surface of the thorax. The portion of the serous 



Fig. 364. A Transverse Section of the Thorax, showing the Relative Position of the Viscera, 

 and the Rellections of the Pleurae. 



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Meant ftuimanaUt 

 Pleura CoiLaLis 



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Pomterier 



membrane investing the surface of the lung is called the pleura pulmonalis 

 (visceral layer of pleura), whilst that which lines the inner surface of the chest 

 is called the pleura costalis (parietal layer of pleura). The interspace or cavity 

 between these two layers is called the cavity of the pleura. Each pleura is 

 therefore a shut sac, one occupying the right, the other the left half of the 

 thorax; and they are perfectly s'eparate, not communicating with each other. The 

 two pleurae do not meet in the middle line of the chest, excepting at one point in 

 front ; an interspace being left between them, which contains all the viscera of the 

 thorax, excepting the lungs : this is the mediastinum. 



Reflections of the pleura (fig. 364). Commencing at the sternum, the pleura 

 passes outwards, covers the costal cartilages, the inner surface of the ribs and 

 Intercostal muscles, and at the back of the thorax passes over the thoracic 

 ganglia and their branches, and is reflected upon the sides of the bodies of the 

 vertebrae, where it is separated by a narrow interspace from the opposite pleura, 

 the posterior mediastinum. From the vertebral column, the pleura passes to the 

 side of the pericardium, which it covers to a slight extent ; it then covers the 

 back part of the root of the lung, from the lower border of which a triangular 

 fold descends vertically by the side of the posterior mediastinum to the Diaphragm. 

 This fold is the broad ligament of the lung, the ligamentuni latum pulmonis, and 

 serves to retain the lower part of the organ in position. From the root, the 



