I 



THE ASCENDING AORTA 545 





of the ascending aorta, as far as the under surface of the aortic arch, where it 

 divides, about the level of the fibrocartilage between the fifth and sixth thoracic 

 vertebra?, into right and left branches of nearly equal size. 



Relations. The whole of this vessel is contained within the pericardium. It is enclosed with 

 the ascending aorta in a single tube of the visceral layer of the serous pericardium, which is con- 

 tinued upward upon them from the base of the heart. The fibrous layer of the pericardium is 

 gradually lost upon the external coats of the two branches of the artery. In front, the pulmonary 

 artery is separated from the anterior end of the second left intercostal space by the pleura and 

 left lung, in addition to the pericardium; it rests at first upon the ascending aorta, and higher 

 up lies in front of the left atrium on a plane posterior to the ascending aorta. On either side of 

 its origin is the auricula of the corresponding atrium and a coronary artery, the left coronary 

 artery passing, in the first part of its course, behind the vessel. The superficial part of the cardiac 

 plexus lies above its bifurcation, between it and the arch of the aorta. 



The right branch of the pulmonary artery (ramus dexter a. pulmonalis] , longer 

 and larger than the left, runs horizontally to the right, behind the ascending aorta 

 and superior vena cava and in front of the right bronchus, to the root of the right 

 lung, where it divides into two branches. The lower and larger of these goes to 

 the middle and lower lobes of the lung; the upper and smaller is distributed to the 

 upper lobe. 



The left branch of the pulmonary artery (ramus sinister a. pulmonalis}, shorter 

 and somewhat smaller than the right, passes horizontally in front of the descending 

 aorta and left bronchus to the root of the left lung, where it divides into two 

 branches, one for each lobe of the lung. 



Above, it is connected to the concavity of the aortic arch by the ligamentum 

 arteriosum, on the left of w r hich is the left recurrent nerve, and on the right the 

 superficial part of the cardiac plexus. Below, it is joined to the upper left pul- 

 monary vein by the ligament of the left vena cava. 



The terminal branches of the pulmonary arteries will be described with the 

 anatomy of the lungs. 



THE AORTA. 



The aorta is the main trunk of a series of vessels which convey the oxygenated 

 blood to the tissues of the body for their nutrition. It commences at the upper 

 part of the left ventricle, where it is about 3 cm. in diameter, and after ascending 

 for a short distance, arches backward and to the left side, over the root of the left 

 lung; it then descends within the thorax on the left side of the vertebral column, 

 passes into the abdominal cavity through the aortic hiatus in the diaphragm, 

 and ends, considerably diminished in size (about 1.75 cm. in diameter), opposite 

 the lower border of the fourth lumbar vertebra, by dividing into the right and left 

 common iliac arteries. Hence it is described in several portions, viz., the ascending 

 aorta, the arch of the aorta, and the descending aorta, which last is again divided into 

 the thoracic and abdominal aortee. 



THE ASCENDING AORTA (AORTA ASCENDENS) (Fig. 505). 





The ascending aorta is about 5 cm. in length. It commences at the upper part 

 of the base of the left ventricle, on a level with the lower border of the third costal 

 cartilage behind the left half of the sternum; it passes obliquely upward, forward, 

 and to the right, in the direction of the heart's axis, as high as the upper border 

 of the second right costal cartilage, describing a slight curve in its course, and being 

 situated, about 6 cm. behind the posterior surface of the sternum. At its origin 

 it presents, opposite the segments of the aortic valve, three small dilatations 

 called the aortic sinuses. At the union of the ascending aorta with the aortic arch 

 the caliber of the vessel is increased, owing to a bulging of its right wall. This 

 dilatation is termed the bulb of the aorta, and on transverse section presents a some- 

 35 



