58 THORAX 



the bronchus and artery, and the upper pulmonary vein is 

 placed in a more anterior plane than either the artery or the 

 bronchus. Before the right bronchus enters the hilum of 

 the lung it gives off a large branch, called the eparterial branch 

 because it arises above the level at which the pulmonary 

 artery crosses the front of the main bronchus. There is no 

 corresponding branch from the left bronchus. On both 

 sides the branches given off below the point where the 

 pulmonary artery crosses the main stem bronchus are called 

 hyparterial branches. It necessarily follows from what has 

 already been stated that in sections made through the root 

 of the lung, at right angles to its long axis, the relative 

 positions of the structures will vary slightly according to 

 whether the section is nearer to or farther from the median 

 plane ; but in all such sections the upper pulmonary vein 

 will lie in an anterior plane, the pulmonary artery in an 

 intermediate plane, and the bronchus in a posterior plane. 

 If the section is made close to the hilum of the lung, the 

 relationship from before backwards, on both sides, is upper 

 pulmonary vein, pulmonary artery, stem bronchus. The 

 relationship from above downwards, on the right side, is 

 eparterial bronchus, pulmonary artery, stem bronchus, pul- 

 monary vein ; and, on the left side, pulmonary artery, stem 

 bronchus, pulmonary vein; the difference being due to the 

 eparterial branch from the stem bronchus which is present 

 only on the right side. 



The Relations of the Roots of the Lungs. Anterior to the 

 root of each lung are the phrenic nerve, with its accompanying 

 vessels, and the anterior pulmonary plexus ; behind it, the 

 posterior pulmonary plexus; and below it, the ligamentum" 

 pulmonis. In addition, in front of the root of the right lung 

 is the superior vena cava ; and above and behind it, the vena 

 azygos; whilst above the root of the left lung is the aortic 

 arch, and behind it, the descending aorta (Figs. 18 and 19). 



It will be obvious to the dissector who has followed the 

 preceding descriptions that parts of the main stems of the 

 bronchi, the pulmonary arteries, and the pulmonary veins lie 

 medial to the lungs and outside of their substance. They 

 are the extra-pulmonary parts. They are only partially dis- 

 played at present, and the study of their special relations 

 must be deferred until a later period (see p. 126). The 

 portions of the bronchi and the pulmonary blood vessels 



