332 THE ARTERIES.-AORTA. 



part of the arch. The pulmonary artery and the aorta are united together 

 by connective tissue and by the serous layer of the pericardium, which for 

 the space of about two inches forms a single tube around both vessels. 

 Rather to the left of its point of bifurcation it is connected to the under 

 side of the aortic arch by means of a short fibrous cord, which passes 

 obliquely upward?, backwards, and to the left. This is the remains of the 

 ductus arteriosus, a large vessel peculiar to the foetus, which has been 

 already described. 



The two branches of the pulmonary artery. The right branch, longer and 

 somewhat larger than the left, runs almost transversely outwards behind 

 the ascending aorta and the superior vena cava into the root of the right 

 lung, where it immediately begins to divide in the usual manner of arteries. 

 The left branch, shorter than the right, passes horizontally in front of the 

 descending aorta and left bronchus into the root of the left lung, to undergo 

 its ramification. 



The right and left pulmonary arteries, at the root of the lung, both lie 

 in front of the bronchus and behind the veins. On the right side the 

 bronchus is highest and the veins lowest, while on the left side the bronchus 

 sinks to a level between the artery and veins. 



Pulmonary Veins. The pulmonary veins, which convey the red blood 

 back from the lungs to the left side of the heart, ultimately converge into 

 four short venous trunks, which are found, two on each side, in the root of 

 the corresponding lung. The two veins of the right side, which are longer 

 than those of the left, pass below the right pulmonary artery, and behind 

 the superior vena cava, the right auricle, and the aorta, to enter the left 

 auricle. Not unfrequently a third smaller vein exists on the right side. 

 The two left pulmonary veins run a shorter course to reach the auricle, 

 passing in front of the descending aorta. 



SYSTEMIC VESSELS. 

 ARTERIES. 



THE AORTA. 



The aorta, the large main trunk of the systemic arteries, is situated partly 

 within the thorax and partly in the abdomen. It commences at the left 

 ventricle of the heart, and, after arching over the root of the left lung, 

 descends in front of the vertebral column, and, passing through the diaphragm 

 into the abdominal cavity, ends opposite the fourth lumbar vertebra, by 

 dividing into the right and left common iliac arteries. In this course the 

 aorta forms a continuous undivided trunk, which gradually diminishes in 

 size from its commencement to its termination, and gives off larger or 

 smaller branches at various points. Different parts of the vessel have 

 received particular names, derived from their position or direction : the 

 following are generally recognised, viz., the arch of the aorta, the thoracic 

 aorta, and the abdominal aorta. The short curved part, which reaches 

 from the ventricle of the heart to the side of the third dorsal vertebra, is 

 named the arch ; the straight part, which extends from that vertebra to the 

 diaphragm, is called the thoracic aorta ; and the remainder of the vessel, 

 down to its bifurcation, is designated the abdominal aorta. 



Arch of the Aorta. 



The arch of the aorta commences at the upper part or base of the left 

 ventricle of the heart, behind the pulmonary artery. At first it passes 

 upwards and to the right side, somewhat in the direction of the heart 



