THE MEDIASTINUM. 971 



the fourth costal cartilage. Here the line of reflection on the right side is con- 

 tinued onward in nearly a straight line to the lower end of the gladiolus and then 

 turns outward, while on the left side the line of reflection diverges outward, so that 

 opposite the seventh cartilage it is about three-quarters of an inch from the left 

 border of the sternum. It, however, always extends considerably farther over 

 the pericardium than the corresponding lung. The lower limit of the pleura is on 

 a considerably lower level than the lower limit of the lung, but does not extend to 

 the attachment of the Diaphragm, so that below the line of reflection of the pleura 

 from the chest wall on to the Diaphragm the latter is in direct contact with the 

 rib cartilages and the Internal intercostal muscles. Moreover, in ordinary inspira- 

 tion the thin margin of the base of the lung does not extend as low as the line of 

 pleural reflection, with the result that the costal and diaphragmatic pleura are here 

 in contact, the narrow slit between the two being termed the phrenico-costal sinus. 

 A similar condition 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 sometimes called the 

 costo-mediastinal sinus. 



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

 fluid ; its outer surface is intimately adherent to the surface of the lung, and to the 

 pulmonary vessels as they emerge from the pericardium ; it is also adherent to the 

 upper surface of the Diaphragm: throughout the rest of its extent it is somewhat 

 thicker, and may be separated from the adjacent parts with extreme facility. 



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

 the left. 



Vessels and Nerves. The arteries of the pleura are derived from the inter- 

 costal, the internal mammary, the musculo-phrenic, thymic, pericardiac, and 

 bronchial. The veins correspond to the arteries. The lymphatics are very 

 numerous. The nerves are derived from the phrenic and sympathetic (Luschka). 

 Kb'lliker states that nerves accompany the ramification of the bronchial arteries 

 in the pleura pulmonalis. 



Surgical Anatomy. In operations upon the kidney it must be borne in mind that the 

 pleura may sometimes extend below the level of the last rib, and may therefore be opened in 

 these operations, especially when the last rib is removed in order to give more room. 



THE MEDIASTINUM. 



The Mediastinum is the space left in the median portion of the chest by the non- 

 approximation of the two pleurae. It extends from the sternum in front to the 

 spine behind, and contains all the viscera in the thorax excepting the lungs. The 

 mediastinum 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 (Struthers) ; and a lower portion, below the upper level of 

 the pericardium. This lower portion is again subdivided into three that part 

 which contains the pericardium and its contents, the middle mediastinum ; that 

 part which is in front of the pericardium, the anterior mediastinum ; and that 

 part which is behind the pericardium, the posterior mediastinum. 



The superior mediastinum is that portion of the interpleural space which lies 

 above the upper level of the pericardium, between the manubrium sterni in front and 

 the upper dorsal vertebra behind. It is bounded below by a plane passing back- 

 ward from the junction of the manubrium and gladiolus sterni to the lower part 

 of the body of the fourth dorsal vertebra, and laterally by the lungs and pleurae. 

 It contains the origins of the Sterno-hyoid and Sterno-thyroid muscles and the 

 lower ends of the Longi colli muscles ; the arch of the aorta ; the innominate, the 

 thoracic portion of the left carotid and subclavian arteries ; the upper_half of the 

 superior vena cava and the innominate veins, and the left superior intercostal vein ; 

 the pneumogastric, cardiac, phrenic, and left recurrent laryngeal nerves ; the 

 trachea, ossophagus, and thoracic duct ; the remains of the thymus gland and some 

 lymphatic glands. 



