882 THE VASCULAE SYSTEM. 



each side ; the pulmonary veins anteriorly, above, and below ; and the inferior vena 

 cava anteriorly and on each side. The aorta and the pulmonary artery are enclosed 

 together in a complete sheath of the visceral layer. Therefore, when the pericardial 

 sac is opened from the front, it is possible to pass the fingers behind them and 

 in front of the atria, from the right to the left side, through a passage called the 

 great transverse sinus of the pericardium (Fig. 756). The spaces or pouches which 

 intervene between the vessels which receive partial coverings from the serous 

 pericardium are also called sinuses ; and the largest of them, which is bounded 

 below and on the right by the inferior vena cava, and above and on the left by 

 the left inferior pulmonary vein, is known as the great oblique sinus. It passes 

 upwards and to the right behind the left atrium, and lies anterior to the oesophagus 

 and the descending thoracic aorta. 



A small fold of the serous pericardium, the vestigial fold, passes from the left 

 pulmonary artery to the left superior pulmonary vein, posterior to the left extremity 

 of the transverse sinus. It merits special attention because it encloses a fibrous 

 strand, the ligame^itum vence cavce sinistrce. This is a remnant of the left superior 

 vena cava or duct of Cuvier, which atrophied at an early period of fcetal life. 



Structure. The fibrous pericardium consists of ordinary connective tissue fibres felted 

 together into a dense, unyielding membrane. The serous pericardium is covered on its inner 

 aspect by a layer of flat endothelial cells. The endothelium rest upon a basis of mixed white 

 and elastic fibres in which run numerous blood-vessels, lymphatics, and nerves. 



ARTERLE. 

 AETEEIA PULMONALIS. 



The pulmonary artery springs from the anterior and left angle of the base 

 of the right ventricle, at the termination of the conus arteriosus. It is slightly 

 larger at its commencement than the aorta, and is dilated, immediately above the 

 cusps of the valve, into three pouches, the sinuses of the pulmonary artery. It runs 

 upwards and posteriorly, towards the concavity of the aortic arch, curving from the 

 front round the left side of the ascending aorta to reach a plane posterior to the 

 latter ; and it terminates, by dividing into right and left branches, opposite the 

 fifth thoracic vertebrae. Its length is a little more than two inches. 



Relations. The pulmonary artery is enclosed within the fibrous pericardium, 

 and is enveloped, along with the ascending aorta, in a common sheath of the 

 visceral layer of the serous pericardium. It lies behind the anterior extremity 

 of the second left intercostal space, from which it is separated by the anterior 

 margins of the left lung and pleural sac. 



Its posterior relations are the bulb of the aorta, the anterior wall of the left 

 atrium, and the first part of the left coronary artery. To the right it is in relation 

 with the right coronary artery, the auricle of the right atrium, and the ascending 

 aorta, and to the left with the left coronary artery and the auricle of the left 

 atrium. Immediately above its bifurcation, between it and the aortic arch, is the 

 superficial cardiac plexus. 



The right branch of the pulmonary artery is longer and larger than the 

 left. It passes to the hilum of the right lung, forming one of the constituents of 

 the root of the lung, and, after entering the lung, it descends, with the main 

 bronchus, to the lower extremity of the organ. 



Relations. Before it enters the lung the right pulmonary artery passes posterior to 

 the ascending aorta, the superior vena cava, and the upper right pulmonary vein. At first, 

 it lies below the arch of the aorta and the right bronchus, anterior to the oesophagus, and 

 above the left atrium and the lower right pulmonary vein ; then it crosses anterior to the 

 right bronchus, immediately below the eparterial branch of that bronchus, and reaches the 

 hilum of the lung. After it has passed through the hilum the artery descends, in the lung, 

 posterior and lateral to the main bronchus and between its ventral and dorsal branches. 



Branches. Before entering the hilum it gives oif a large branch to the upper lob( 

 of the right lung which accompanies the eparterial bronchus, and in the substance o 



