THORACIC CAVITY 103 



the right and left pulmonary arteries. The bifurcation takes 

 place posterior to the sternal end of the second left costal 

 cartilage. 



Relations. At its commencement it is placed anterior to 

 the lower end of the ascending aorta, but as it runs backwards 

 and upwards it passes to the left side of the aorta, and 

 lies in front of the upper part of the anterior wall of the 

 left atrium, from which it is separated by the transverse sinus 

 of the pericardium. Anterior to it is the upper part of the 

 anterior wall of the pericardium, which separates it from the 

 anterior part of the mediastinal surface of the left pleura and 

 lung. To its right side are the right coronary artery and the 

 apex of the auricle of the right atrium, below, and the 

 ascending aorta, above. To its left side lie the left coronary 

 artery and the anterior end of the auricle of the left atrium. 



Dissection. Cut away the anterior wall of the pulmonary 

 artery up to the level of its bifurcation and pass probes into its 

 right and left branches. Note that the right branch runs nearly 

 transversely to the right, and that the left branch runs backwards 

 and to the left. 



The right pulmonary artery commences at the bifurcation 

 of the pulmonary stem, below the arch of the aorta. It at 

 once runs to the right, towards the hilum of the right lung, 

 along the upper border of the left atrium and the transverse 

 sinus of the pericardium (Figs. 28, 37, 59). It passes behind 

 the ascending aorta and the superior vena cava, and in front 

 of the oesophagus and the stem of the right bronchus. It 

 enters the hilum of the lung below the eparterial branch of 

 the bronchus, above and posterior to the upper right 

 pulmonary vein ; and it descends, in the substance of the 

 lung, on the postero-lateral side of the stem bronchus, and 

 between its ventral and its dorsal branches, where it has 

 already been dissected (p. 59). 



Branches. As it enters the hilum of the lung it gives off a 

 branch which accompanies the eparterial bronchus, and as it 

 descends in the substance of the lung it gives off branches 

 which correspond with the branches of the stem bronchus 

 (see p. 62). 



The left pulmonary artery runs to the left and slightly 

 backwards, across the anterior aspect of the descending aorta 

 and the left bronchus, to the hilum of the left lung (Figs. 

 20, 25). It is covered anteriorly and on the left by the 



