PLEURAE. 



781 



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

 thorax ; and they are perfectly separate, 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. 431). 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 Dia- 

 phragm. This fold is the broad ligament of the lung, the ligamentum latum 



Fig. 431. A Transverse Section of the Thorax, showing the relative Position of the Yiscera, 



and the Reflections of the Pleura. 



TRIANOULABU STCRNI 



Internal Mammary Vetaelt. \ 



ttftfhrtnif tfm'e 



Ptfum flutmonalt* 

 Pleura CosLdLit 



j Sympntllriio ATcrvf 

 ' Thoraoia Duct 



I PoMttrio 

 'ntumagaftnc Xtrvei ) 



pulmonis, and serves to retain the lower part of that organ in position. From 

 the root, the pleura may be traced over the convex surface of the lung, the 

 summit and base, and also over the sides of the fissures between the lobes. It 

 covers its anterior surface, and the front part of its root, and is reflected upon 

 the side of the pericardium to the inner surface of the sternum. Below, it 

 covers the upper surface of the Diaphragm. Above, its apex projects, in the 

 form of a cul-de-sac, through the superior opening of the thorax into the neck, 

 extending about an inch above the margin of the first rib, and receives the 

 summit of the corresponding lung ; this sac is strengthened, according to Dr. 

 Sibson, by a dome-like expansion of fascia, derived from the lower part of the 

 Scaleni muscles. 



A little above the middle of the sternum, the contiguous surfaces of the two 

 pleurae are sometimes in contact for a slight extent; but above and below this 

 point, the interval left between them forms the anterior mediastinum. 



