THORACIC CAVITY 29 



Dissection.^ When the study of the thoracic portion of the sympathetic 

 trunk and its branches is completed the posterior parts of the inter- 

 costal spaces should be cleaned and examined. The internal intercostal 

 muscles will be seen passing as far medially as the angles of the ribs. In 

 some cases fibres with the same direction as those of the internal intercostal 

 muscles will be found descending from one rib to the second or third 

 below, across the pleural surfaces of the intermediate ribs. Such fibres 

 constitute the subcostal muscles, which are very variably developed in 

 different subjects. Sometimes they form an almost complete lining for the 

 posterior part of the thoracic wall, and in other cases they are represented 

 by a few scattered fibres, or they are entirely absent. 



The Posterior Intercostal Membranes are medial to the 

 internal intercostal muscles and on a more posterior plane. 

 Each is attached, medially, to the anterior costo-transverse 

 ligament, which passes from the neck of the rib below to the 

 lower border of the transverse process of the vertebra above. 

 Laterally, it is continuous with the fascial layer between the 

 internal and external intercostal muscles, and above and 

 below, it is attached to the adjacent ribs. On the pleural 

 surface of the posterior intercostal membrane, in each space, 

 lie the corresponding intercostal nerve and vessels (see Figs. 

 12 and 13). These pass laterally, on the internal surface 

 of the membrane, and disappear posterior to the border of 

 the internal intercostal muscle. When the membranes are 

 removed the posterior fibres of the external intercostal 

 muscles will be exposed, passing medially as far as the 

 tubercles of the ribs. 



After the posterior parts of the intercostal spaces and their contents have 

 been fully considered the vena azygos should be studied. If it is traced 

 downwards, from above the root of the lung, it will be found to disappear 

 gradually posterior to the right margin of the oesophagus, which must be 

 raised to display its lower portion. 



The Vena Azygos. This vein enters the thorax through 

 the aortic aperture of the diaphragm, to the right of the aorta 

 and thoracic duct, the. lower parts of which will be displayed 

 as the vein is cleaned. After entering the thorax the vein 

 ascends, along the right side of the aorta, from which it is 

 separated by the thoracic duct. A short distance above the 

 diaphragm it passes more or less completely behind the right 

 border of the oesophagus. At the lower border of the root 

 of the lung it emerges from behind the oesophagus, passes 

 posterior to the lung root, turns anteriorly above its superior 

 border, at the level of the fifth thoracic vertebra, and terminates 

 in the posterior wall of the superior vena cava, immediately 

 above the point where the latter enters the pericardium, 



