970 



THE ORGANS OF VOICE AND RESPIRATION. 



Intercostal muscles, and at the back part of the thorax passes over the thoracic 

 ganglia and their branches, and is reflected upon the sides of the bodies of the 



TRIANGULARIS STERNI. 



Internal Mammary Vessels. 



Left Phrenic Nerve, 



Pleura Pulmonalis. 

 ra Costalis. 



Sympathetic Nerve'. 



Thoracic Duct. 



Vena Azygos Major 

 Piieiimogastric Nerves 



FIG. 535. A transverse section of the thorax, showing the relative position of the viscera and the reflections 

 of the oleurce. 



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 Avhich 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 inner surface and the front part of 

 its root ; from this it is reflected on to the pericardium, and from it to the back 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, to 

 the lower border of the tenth rib on the left side, and to the upper border of the 

 same rib on the right side; and behind, it reaches as low as the twelfth rib, and 

 sometimes even 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 the 

 first rib, and receives the summit of the corresponding lung ; this sac is strength- 

 ened, according to 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 strength- 

 ened by a few spreading muscular fibres derived from the Scaleni muscles. 



In the front of the chest, where the parietal layer of the pleura is reflected back- 

 ward to the pericardium, the two pleural sacs are in contact for a considerable extent. 

 At the upper part of the chest, behind the manubrium, they are not in contact ; 

 the point of reflection being represented by a line drawn from the sterno-clavicular 

 articulation to the mid-point of the junction of the manubrium to the body of the 

 sternum. From this point the two pleurae descend in close contact to the level of 



