THE ARCH OF THE AORTA 247 



arid backward, to end under the transverse aorta by dividing 

 into a right and a left branch. 



Relations. In front, second left intercostal space and cartilage, 

 left border of the sternum; behind, origin of the aorta, left 

 auricle; above, transverse aorta, remains of the ductus arteriosus; 

 to the right, right appendix and coronary artery, ascending aorta; 

 to the left, left appendix and coronary artery. 



This vessel, with the ascending aorta, is enclosed in a sheath 

 of pericardium. It winds around the aorta, being at first in 

 front, and later to the left side, of the ascending portion. In 

 fetal life the ductus arteriosus connects it a little to the left 

 of its bifurcation with the transverse aorta. 



Each branch enters the root of the corresponding lung; the 

 right, the larger, passing behind the ascending aorta and 

 superior vena cava; the left, in front of the descending aorta. 

 The left divides into two branches for the lobes of the left lung; 

 the right also divides into two primary branches for the upper 

 and lower lobes. From the lower one of these is sent a branch 

 to the middle lobe. 



THE CORPOREAL ARTERIAL SYSTEM 



The Aorta 



The aorta is the main trunk from which spring the systemic 

 arteries. From the base of the left ventricle it runs upward, 

 forward, and to the right as far as the second right cartilage; 

 then backward and to the left, over the root of the left lung, 

 to the fourth dorsal vertebra; thence, along the spine, it descends 

 through the thorax and abdomen, to divide, at the fourth lumbar, 

 into the common iliacs. 



It has been divided, for convenience of description, into the 

 arch and the descending aorta. The arch is subdivided into 

 the ascending, transverse, and descending parts; the descending 

 aorta, into the thoracic and abdominal portions. 



The Arch of the Aorta 



The ascending part of the arch runs upward, forward, and to 

 the right, from a point opposite the lower border of the third 



