REFLECTIONS OF PLEURA. 307 



THE PLEURAE. 



The pleurae are two serous membranes, or closed sacs, which are re- 

 flected around the lungs in the cavity of the thorax. One occupies the 

 right, and the other the left half of the cavity ; they approach each other 

 along the middle line of the body, forming a thoracic partition or medias- 

 tinum. 



Each pleura is conical in shape ; its apex projects into the neck above 

 the first rib (fig. 97), and its base is in contact with the diaphragm. The 

 outer surface is rough, and is connected to the lung and the wall of the 

 thorax by areolar tissue, but the inner surface is smooth and secerning. 

 Surrounding the lung, and lining the interior of one-half of the chest, the 

 serous membrane consists of a parietal part pleura costalis, and of a vis- 

 ceral part pleura pulmonalis. 



There are some differences in the shape and extent of the two pleural 

 bags. On the right side the bag is wider and shorter than on the left ; 

 and on the latter it is narrowed by the projection of the heart to that side. 



The continuity of the bag of the pleura over the lung and the wall may 

 be traced circularly from a given point to the same, in the following man- 

 ner: Supposing the membrane to be followed outwards from the sternum, 

 it may be traced on the wall of the chest as far as the spinal column ; 

 here it is directed forwards to the root of the lung, and is reflected over 

 the viscus, covering its surface, and connecting together its different 

 lobules. From the front of the root the pleura maybe followed over the 

 side of the pericardium to the sternum. Below the root the pleura gives 

 rise to a thin fold, the ligamentum latum pulmonis, which intervenes be- 

 tween the inner surface of the lung and the side of the pericardium. 



If the serous sac be traced above the root of the lung, it describes a 

 circle without deflection over a viscus. 



The mediastinum. The median thoracic partition, or the mediastinum, 

 is formed by the approximation of the pleural bags along the middle line, 

 and is constructed of two layers one being derived from each sac. About 

 midway between the sternum and the spine the contiguous strata of the 

 mediastinum are widely separated by the heart ; but in front of, and be- 

 hind the heart they approach near each other. To the parts before and 

 behind that viscus the terms " anterior and posterior mediastina" are 

 sometimes applied. 



The part in front of the heart (anterior mediastinum) extends from the 

 back of the sternum to the pericardium. Behind the second piece of the 

 sternum the pleural bags touch each other, but above and below that spot 

 they are separated by an interval ; so that the space between them (inter- 

 pleural) is narrowed at the centre, and is inclined below to the left of the 

 middle line. In the upper part of the space are the remains of the thymus 

 gland, and the origin of some of the hyoid and thyroid muscles ; and in 

 the lower part is some areolar tissue, together with the triangularis sterni 

 muscle of the left side. 



The part behind the heart (posterior mediastinum) intervenes between 

 the back of the pericardium with the roots of the lungs, and the spinal 

 column. Its lateral boundaries are the opposite pleural sacs, which are 

 separated here by a larger interpleural interval than in front of the heart. 

 If the pleura be divided behind the lung on the right side, the extent of 

 the space and its contents will appear : in it are contained the different 

 bodies on the front of the spine, viz., the aorta, the vena azygos, the thoracic 



