34G THE AOETA. 



is the remains of the ductus arteriosus, a largo vessel peculiar to the 

 foetus, which has been already described. 



The right pulmonary artery, 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 imme- 

 diately begins to divide in the usual manner of arteries. 



The left pulmonary artery, shorter than the right, passes horizon- 

 tally 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 are four short venous trunks which convey the- 

 red blood back from the lungs to the left side of the heart, and which 

 are found, two on each side, in the root of the corresponding lung. 

 The tAvo veins of the ri/jht 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 unfrccpiently a third smaller vein exists on the right side in 

 connection with the tliird lobe of the right lung. The two left pul- 

 monary veins run a shorter course to reach the auricle, passing iu 

 fi-ont of the aorta. The distribution of the pulmonary capillaries is 

 exclusively to the membrane lining the air cells of the lungs. 



The varieties of the pulmonary arteries will be referred to along with those of 

 the aorta. 



SYSTEMIC ARTEEIES. 



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 ter- 

 mination, 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 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. 



AECH OF THE AOETA. 



The arch of the aorta commences at the upper part of the 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 itself, and crosses oblicjuely behind the sternum, approach- 



