THORACIC CAVITY 17 



In the mediastinal septum lie the heart, enveloped in a 

 fibro-serous sac called the pericardium ; the great vessels 

 passing to and from the heart, i.e. the pulmonary artery and 

 veins, the aorta, and the vena cava superior ; the oesophagus ; 

 the trachea and the commencements of the bronchi; the 

 thoracic duct; the azygos, hemiazygos and accessory hemi- 

 azygos veins ; the vagi and phrenic nerves ; numerous lymph 

 glands ; all the structures mentioned are embedded in the 

 areolar tissue of the septum which ensheaths them and binds 

 them together whilst, at the same time, on account of its 

 elasticity it allows the heart and vessels to dilate and con- 

 tract. In addition there are the remains of the thymus gland 

 which vary considerably in size at different periods of life. 

 For convenience of description the mediastinum is divided 

 into a superior and an inferior portion, by an imaginary plane 

 which passes from the lower border of the manubrium sterni 

 anteriorly, to the lower border of the fourth thoracic vertebra 

 posteriorly ; and the inferior mediastinum is subdivided into 

 anterior, middle, and posterior portions. The anterior 

 mediastinum is the part anterior to the pericardium ; the 

 posterior mediastinum is the part posterior to the pericardium ; 

 whilst the pericardium and the heart, with the great vessels, 

 and portions of the phrenic nerves, with their accompanying 

 vessels, lie in the middle mediastinum (Figs. 20, 21, pp. 



5 5 1 )- 



The lateral portions of the thoracic cavity are known as 

 the pleural spaces, though no such spaces exist, for each so- 

 called space contains and is completely filled by the corre- 

 sponding lung, which is surrounded by an invaginated serous 

 membrane called the pleural sac. There are therefore two 

 pleural sacs, and each is so disposed that it not only lines the 

 chamber in which the lung lies, but is also reflected over the 

 surface of the lung, so as to give it an external covering which 

 is intimately connected with the pulmonary substance. Con- 

 sequently, the wall of each pleural sac is separable into two 

 portions, an investing or visceral part which covers the surface 

 of the lung, and a lining or parietal part which clothes the 

 inner surfaces of the boundary of each lateral part of the 

 thoracic cavity. It must be clearly understood, however, that 

 the two terms are applied merely to indicate different portions 

 of a continuous membrane. 



Each lung lies free in the pleural space, except along its 



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