THE THORAX 939 



health no such cavity exists, the parietal and visceral pleurae being 

 at all times in intimate contact with each other. The attached sur- 

 faces of the parietal and visceral pleurae are roughened by fibrous 

 processes, by means of which they are connected to the parts which 

 they cover. 



The costal pleura is the thickest, and can readily be stripped fiom 

 the inner surfaces of the ribs and internal intercostal muscles. 

 There is a fairly thick laj-er of subserous areolar tissue on its 

 attached surface. The diaphragmatic and pericardial pleurae are 

 thinner than the costal pleura, and are more adherent to the sub- 

 jacent structures. The pulmonarj^ pleura is the thinnest and most 

 adherent. Beneath it there is a layer of subserous areolar tissue, 

 containing much elastic tissue, and this is in continuity with the 

 areolar tissue which pervades the lung. 



Differences between the Two Pleural Sacs. — The right pleural sac rises 

 higher into the root of the neck, and is shorter and w-ider, than the left. These 

 differences are due partly to the projection formed by the liver on the right 

 side, and partly to the greater inclination of the heart to the left of the sternum 

 than to the right. 



Blood-supply. — ^The parietal -pleura receives its arteries from 

 (i) the aortic intercostals, and (2) the superior phrenic, anterior 

 intercostal, anterior mediastinal, pericardial, and musculo-phrenic 

 branches of the internal mammary. 



The veins pursue courses corresponding to the arteries. 



Nerve-supply. — ^The nerves are derived from the sympathetic 

 and phrenic ner\'es. 



Lymphatics— Visceral or Pulmonary Pleura. — The lymphatic 

 vessels of this part of the pleura open into the superficial lymphatics 

 of the lungs. Parietal Pleura. — The lymphatics of the costal pleura 

 open into the lymphatics of the internal intercostal muscles, which 

 terminate in (i) the sternal or internal mammary' glands, and (2) the 

 intercostal glands. The l\Tnphatics of the diaphragmatic pleura 

 open into the lymphatics of the diaphragm. The Ijmiphatics of the 

 mediastinal pleura open into (i) the anterior mediastinal glands, 

 and (2) the posterior mediastinal glands. 



Structure. — The pleura is a typical serous membrane like the serous portion 

 of the pericardium, the peritoneum, and the tunica vaginalis. Such mem- 

 branes are called serous because their free surfaces are moistened by a small 

 quantity of serous fluid. Briefly stated, the pleura consists of a homogeneous, 

 connective-tissue basement membrane, containing elastic tissue, and lined 

 with endothelium. It presents many stotnata, which communicate with the 

 pleural cavity and with the pleural lymphatic vessels. 



Development. — The pleura is developed from the walls of the coelom, or 

 body-cavity, which is the cleft in the mesoblast separating the splanchno- 

 pleure and somatopleure. 



Mediastinum Thoracis. — ^The mediastinum thoracis is formed by 

 the approximation of the two pleural sacs in the region of the 

 median antero-posterior line of the thorax. The inter\-al between 

 the two sacs is called the mediastinal space, and its boundaries are 



