THE SYSTEMIC ARTERIES 529 



To the left are the pericardium, the left pleura and lung, the left auricle, the left coronary 

 artery, and the cardiac nerves. 



The right pulmonary artery [ramus dexter] longer than the left, passes almost 

 horizontally under the arch of the aorta to the root of the right luDg, where it 

 divides, either directly or after repeated division, into three branches, one for 

 each lobe. These branches follow the course of the bronchi, dividing and sub- 

 dividing for the supply of the lobules of the lung. The terminal branches do 

 not anastomose with each other. 



Relations. — In its course to the lung it has in front of it the ascending aorta, the superior 

 vena cava, the phrenic nerve, the anterior pulmonary plexus, and the reflexion of the pleura. 

 Behind are the right bronchus and the termination of the azygos vein. Above is the arch of 

 the aorta, and below are the left atrium and the upper right pulmonary vein. 



At the root of the lung it has the right bronchus above and behind it; the pulmonary veins 

 below and in front. Crossing in front of it and the other structures forming the root of the lung 

 are the phrenic nerve and the anterior pulmonary plexus; behind are the azygos vein, the vagus 

 nerve, and the posterior pulmonary plexus. 



The left pulmonary artery, shorter and slightly smaller than the right, passes 

 in front of the descending aorta to the root of the left lung, where it divides into 

 two branches for the supply of the upper and lower lobes respectively. These 

 divide and subdivide as on the right side. 



Relations. — At the root of the lung it has the left bronchus behind and also below it, in 

 consequence of the more vertical direction taken by the left bronchus than by the right. Below 

 and in front are the pulmonary veins, while passing from the artery and the upper left pulmonary 

 vein is the hgament of the left superior cava. Crossing in front of it and the other structures 

 forming the root of the lung are the phrenic nerve, the anterior pulmonary plexus, and the reflex- 

 ion of the left pleura; crossing behind, are the descending aorta, the left vagus nerve, and the 

 posterior pulmonary plexus. 



The pulmonary veins [vv. pulmonales] (figs. 424, 441) return the aerated 

 blood from the lungs to the heart. They are usually four in number, superior 

 and inferior, of the right and left sides. Occasionally, however, there are three 

 pulmonary veins on the right side, the result of the vein from the middle lobe 

 of the right lung opening separately into the left atrium instead of joining as usual 

 the upper of the two right pulmonary veins. The relations of the pulmonary veins 

 to the pulmonary arteries and bronchi in the lungs are given with the anatomy 

 of the lungs (Section X). 



The pulmonary veins are about 15 mm. in length. In the pericardium the 

 right pulmonary veins [vv. pulmonales dextrae] both pass behind the superior 

 vena cava. The superior vein receives the vein from the right middle lobe and 

 runs below and in front of the right pulmonary artery. 



The left pulmonary veins [vv. pulmonales sinistrse] enter the left atrium about 

 3 cm. in front of the veins of the right side. The superior vein is below the left 

 pulmonary artery. 



2. THE SYSTEMIC ARTERIES 

 THE AORTA 



The aorta (fig. 442) is the main systemic arterial trunk, and from it all the 

 systemic arteries are derived. It begins at the left ventricle of the heart, and 

 ascends near the anterior thoracic wall as high as the second right chondro-sternal 

 articulation [aorta ascendens]. It then turns backward and to the left forming 

 an arch [arcus aortas] which reaches the posterior thoracic wall at the left side of 

 the fourth thoracic vertebra. From here it runs downward along the vertebral 

 column [aorta descendens] through the thorax and abdomen and ends by dividing, 

 opposite the fourth lumbar vertebra, into the right and left common iliac arteries. 

 From the point of bifurcation a small vessel, the middle sacral, is continued down 

 the middle line in front of the sacrum and coccyx. The middle sacral represents 

 the sacral and coccygeal aorta. 



The Ascending Aorta 



The ascending aorta [aorta ascendens] (fig. 442) begins at the upper part of 

 the left ventricle, on a level with the third intercostal space, and ascends behind 



