750 THE VASCULAR SYSTEM. 
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 auricles, and les in front of the 
cesophagus and the descending thoracic aorta. 
A small fold of the serous pericardium, the vestigial fold of Marshall (liga- 
mentum Vv. cavee sinistree), passes from the left pulmonary artery to the left superior 
pulmonary vein behind the left extremity of the transverse sinus. It merits 
special attention because it encloses the remains of the left superior vena cava, 
which atrophies 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 which rest upon a basis of mixed white and elastic fibres 
in which run numerous blood-vessels, lymphatics, and nerves. 
THE ARTERIES. 
THE PULMONARY ARTERY. 
The pulmonary artery (a. pulmonalis), which is shghtly larger at its com- 
mencement than the aorta, springs from the anterior and left angle of the 
base of the right ventricle, at the termination of the infundibulum. It runs 
upwards and backwards 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 
dise between the fifth and sixth dorsal vertebre. Its length is a little more than 
two inches. 
Relations.—Enclosed within the fibrous pericardium, and enveloped along 
with the ascending aorta in a common sheath of the visceral layer of the serous 
pericardium, the pulmonary artery les behind the inner 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 root of the aorta, the anterior wall of the lett 
auricle, and the first part of the left coronary artery. To the right it is in relation 
with the right coronary artery and the right auricular appendix, and to the 
left with the left coronary artery and the left auricular appendix. 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 hilus of the right lung, forming one of the constituents of 
its root, and, entering the lung, descends with the main bronchus to the lower 
extremity of the organ. 
Relations.—Before it enters the lung the right pulmonary artery passes behind 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, in front of the cesophagus, and 
above the left auricle and the lower right pulmonary vein ; then it crosses in front of the 
right bronchus immediately below the eparterial branch, and reaches the hilus of the 
lung. After entering the lung the artery descends, behind and to the outer side of the 
main bronchus and between its ventral and dorsal branches. 
Branches.—Before entering the hilus it gives off a large branch to the upper lobe 
which accompanies the eparterial bronchus, and in the substance of the lung it gives 
off numerous branches which correspond with and accompany the dorsal, ventral, and 
accessory branches of the right bronchus. 
The left branch of the pulmonary artery, shorter, smaller, and somewhat 
higher in position than the right, passes outwards and backwards from the bifurca- 
tion of the pulmonary stem, and runs in the root of the left lung to the hilus; it 
then descends in company with the main bronchus to the lower end of the lung. 
Relations. —Before it enters the lung it is crossed in front by the upper left 
