1114 THE ORGANS OF VOICE AND RESPIRATION. 



vertebrae, where it is separated by a narrow interval, the posterior mediastinum, 

 from the opposite pleura. From the vertebral column the pleura passes to the side 

 of the pericardium, which it covers to a slight extent ; it then covers the back part 

 of the root of the lung, from the lower border of which a triangular fold descends 

 vertically by the side of the posterior mediastinum to the Diaphragm. This fold 

 is the broad ligament of the lung, the ligamentum latum pulmonis, and serves to 

 retain the lower part of that organ in position. From the root the pleura may be 

 traced over the convex surface of the lung, the summit and base, and also over the 

 sides of the fissures between the lobes on to its anterior surface and the front part 

 of its root ; from this it is reflected upon the side of the pericardium to the inner 

 surface of the sternum. Below, it covers the upper surface of the Diaphragm, and 

 extends in front as low as the costal cartilage of the seventh rib ; at the side of the 

 chest, as low as the tenth rib on the left side and the ninth on the right side ; and 

 behind, it reaches as low as the twelfth rib, and sometimes even beyond it, as low 

 as the transverse process of the first lumbar vertebra. Above, its apex projects, in 

 the form of a cul-de-sac, through the superior opening of the thorax into the neck, 

 extending from one to two inches above the margin of tha first rib, and receives 

 the summit of the corresponding lung ; this sac is strengthened, according to Dr. 

 Sibson, by a dome-like expansion of fascia, attached in front to the posterior border 

 of the first rib, and behind to the anterior border of the transverse process of the 

 seventh cervical vertebra. This is covered and strengthened by a few spreading 

 muscular fibres derived from the Scaleni muscles. 



A little above the middle of the sternum, the contiguous surfaces of the two 

 pleurae are sometimes in contact for a slight extent ; but above and below this point 

 the interval left between them forms part of the mediastinum. 



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 easily separated from the adjacent parts. 



The right pleural sac is shorter and wider than the left. 



A portion of the Diaphagm below and behind, i. e. a narrow interval around 

 this part of its circumference, is not covered by pleura and is in direct contact with 

 the costal parietes. Furthermore, it is to be noted that there are certain localities 

 in the pleural sac in which the surfaces of two portions of parietal pleura are always 

 in contact, even when the lung is in a state of complete inspiration. These localities 

 are known as sinuses or complementary spaces. The largest and most distinct of 

 these is the costo-phrenic sinus, which follows the line of reflection of the costal 

 pleura on to the diaphragm. 



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). 

 Kolliker 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 



