1090 SPLANCHNOLOGY 



exists behind the sternum and rib cartilages, where the anterior thin margin of 

 the lung falls short of the line of pleural reflection, and where the slit-like cavity 

 between the two layers of pleura forms what is called the costomediastinal sinus. 



The line along which the right pleura is reflected from the chest-wall to the 

 diaphragm starts in front, immediately below the seventh sternocostal joint, and 

 runs downward and backward behind the seventh costal cartilage so as to cross 

 the tenth rib in the mid-axillary line, from which it is prolonged to the spinous 

 process of the twelfth thoracic vertebra. The reflection of the left pleura follows 

 at first the ascending part of the sixth costal cartilage, and in the rest of its course 

 is slightly lower than that of the right side. 



The free surface of the pleura is smooth, polished, and moistened by a serous 

 fluid; its attached surface is intimately adherent to the lung, and to the pulmonary 

 vessels as they emerge from the pericardium; it is also adherent to the upper sur- 

 face of the diaphragm : throughout the rest of its extent it is easily separable from 

 the adjacent parts. 



The right pleural sac is shorter, wider, and reaches higher in the neck than the 

 left. 



Pulmonary Ligament (ligamentum pulmonale; ligamentum latum pulmonis). 

 From the above description it will be seen that the root of the lung is covered in 

 front, above, and behind by pleura, and that at its lower border the investing 

 layers come into contact. Here they form a sort of mesenteric fold, the pulmonary 

 ligament, which extends between the lower part of the mediastinal surface of the 

 lung and the pericardium. Just above the diaphragm the ligament ends in a free 

 falciform border. It serves to retain the lower part of the lung in position. 



Structure of Pleura. Like other serous membranes, the pleura is covered by a single layer 

 of flattened, nucleated cells, united at their edges by cement substance. These cells are modified 

 connective-tissue corpuscles, and rest on a basement membrane. Beneath the basement mem- 

 brane there are net-works of yellow elastic and white fibers, imbedded in ground substance which 

 also contains connective-tissue cells. Bloodvessels, lymphatics, and nerves are distributed in 

 the substance of the pleura. 



Vessels and Nerves. The arteries of the pleura are derived from the intercostal, internal 

 mammary, musculophrenic, thymic, pericardiac, and bronchial vessels. The veins correspond 

 to the arteries. The lymphatics are described on page 719. The nerves are derived from the 

 phrenic and sympathetic (Luschka). Kolliker states that nerves accompany the ramifications 

 of the bronchial arteries in the pulmonary pleura. 



THE MEDIASTINUM (INTERPLEURAL SPACE). 



The mediastinum lies between the right and left pleurae in and near the median 

 sagittal plane of the chest. It extends from the sternum in front to the vertebral 

 column behind, and contains all the thoracic viscera excepting the lungs. It may 

 be divided for purposes of description into two parts: an upper portion, above the 

 upper level of the pericardium, which is named the superior mediastinum; and a 

 lower portion, below the upper level of the pericardium. This lower portion is 

 again subdivided into three parts, viz., that in front of the pericardium, the 

 anterior mediastinum; that containing the pericardium and its contents, the middle 

 mediastinum ; and that behind the pericardium, the posterior mediastinum. 



The Superior Mediastinum (Fig. 967) is that portion of the interpleural space 

 which lies between the manubrium sterni in front, and the upper thoracic verte- 

 brae behind. It is bounded below by a slightly oblique plane passing backward 

 from the junction of the manubrium and body of the sternum to the lower part 

 of the body of the fourth thoracic vertebra, and laterally by the pleurae. It con- 

 tains the origins of the Sternohyoidei and Sternothyreoidei and the lower ends of 

 the Longi colli; the aortic arch; the innominate artery and the thoracic portions 

 of the left common carotid and the left subclavian arteries; the innominate veins 





