1084 



THE EESPIEATOEY SYSTEM. 



marked projection of the heart to the left side, and to the position of the thoracic 

 aorta on the left side of the median plane, the left pleural chamber, although it 

 is deepei than the right, is greatly reduced in width. The two pleural cavities, 

 therefore, are very far from being symmetrical in form, and consequently the 

 mediastinal septum tends to extend ^o the left of the median plane of the body. 



Each pleural cavity is completely lined by a separate serous membrane termed 

 the pleura. The portion of this membrane which clothes the mediastinum or 

 intervening partition forms the lateral boundary of an area termed the mediastinal 

 or interpleural space, within which the parts which build up the mediastinal 

 septum are placed. 



PLEUK^. 



The pleura of each side not only lines the corresponding pleural cavity, but at 

 the pulmonary root, it is prolonged on to the lung so as to give it a complete 

 investment. It is customary, therefore, to recognise a pulmonary or investing part 

 (pleura pulmonalis) and a parietal or lining part (pleura parietalis). The inner 

 surface of the pleura (i.e. that surface which is turned towards the interior of the 

 cavity) is coated with squamous endothelium, and presents a smooth, glistening, and 

 polished appearance ; further, it is moistened by a small amount of serous fluid. 



In consequence of this the surface 

 of the lung covered by pulmonary 

 visceral pleura pleura can glide on the wall of the 

 cavity, lined as it is by parietal 

 pleura, with the least possible 

 degree of friction. In the patho- 

 logical condition known as pleurisy 

 the inner surface of the pleura 

 becomes roughened by inflam- 

 matory exudation, and the so-called 



Costal part of parietal pleura 



Pleural cavity v 

 Visceral pleura 



Costal 



friction sounds" are heard when 

 the ear is applied to the chest. 



Pleura Pulmonalis. The pul- 

 monary pleura is very thin, and is 

 so firmly bound down to the surface 

 of the lung that it cannot be de- 

 tached without laceration of the 



FIG. 862.-DIAGRAM SHOWING ARRANGEMENT OP PLEURAL Pulmonary substance, and then 

 SACS AS SEEN IN TRANSVERSE SECTION. only m small pieces. It dips into 



the fissures of the lungs, lines them 



down to the very bottom, and thus completely separates the different lobes of the 

 lungs from each other. The pulmonary pleura becomes continuous with the 

 mediastinal pleura at the root of the lung, and also through the ligamentum 

 pulmonale. 



Pleura Parietalis. Different names are applied to the parietal pleura as it 

 lines the different parts" of the wall of the cavity in which the lung lies. Thus 

 there are the costal pleura, the diaphragmatic pleura, the mediastinal pleura, 

 and the cupula pleura? ; but it must be borne in mind that these terms are 

 merely used for convenience in description, and that the portions of the pleura so 

 designated are all directly continuous with one another. 



The cupula pleurae or the cervical pleura rises into the root of the neck, through 

 the superior aperture of the thorax, and forms a dome-shaped roof for the pleura] 

 cavity. Its highest point or summit reaches the level of the inferior border of the 

 neck of the first rib ; but owing to the great obliquity of the first costal arch, thit 

 point is placed from one to two inches above to the ventral or anterior extremit) 

 of the first rib, and from a half to one and a half inches above the clavicle. Tht 

 cupula pleurae is supported on the lateral side by the scalenus anterior anc 

 scalenus medius muscles, whilst the subclavian artery, arching laterally, lie; 

 in a groove on its medial and ventral aspects a short distance below its summit 



