76 THORAX 



part of the aorta, in a common sheath of the serous 

 pericardium. It commences at the upper end of the conus 

 arteriosus, posterior to the sternal extremity of the third left 

 costal cartilage. It is about two inches long, and it runs up- 

 wards and posteriorly into the concavity of the aortic arch, where 

 it bifurcates into two branches. The bifurcation takes place 

 posterior to the sternal end of the left second costal cartilage. 

 Relations. At its commencement it is placed anterior to 

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

 and posteriorly it passes to the left side of the latter vessel, 

 and lies anterior to 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, below, are the right coronary artery 

 and the apex of the right auricle, and above is the ascending 

 aorta. To its left side lie the left coronary artery and the 

 anterior end of the left auricle. 



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 transversely to the right, and that the left 

 branch runs posteriorly and to the left. 



The right pulmonary artery commences at the bifurcation 

 of the pulmonary stem, below the arch of the aorta. As 

 it runs to the right, towards the hilus of the right lung, along 

 the upper border of the left atrium and the transverse sinus 

 (Figs. 21 and 27), it passes posterior to the ascending aorta 

 and the superior vena cava, and anterior to the oesophagus 

 and the stem of the right bronchus. It enters the hilus 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 will be dissected at a later period 



( P . 9 8). 



Branches. As it enters the hilus 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. 98). 



The left pulmonary artery runs posteriorly and to the 



