466 THE ARTERIES 



lung, where it divides into a larger upper braneh, which sujjplies the ui)i)er lobe 

 and gives off a descending branch to the middle or third lobe, and into a smaller 

 inferior branch for the supply of the inferior lol^e. These branches follow the 

 course of the bronchi, dividing and subdividing 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 (fig. 319) the 

 ascending aorta, the vena cava superior, the ])hrenic nerve, the anterior pulmonary 

 plexus, and the reflexion of the pleura. Behind are the right bronchus and 

 the termination of the vena azygos major. Above is the transverse portion 

 of the arch of the aorta, and below are the left auricle and the uj)per right 

 pulmonary vein. 



At the root of the lung it lias the right bronchus above and behind it; the 

 l)ulmonary veins below and in front. Crossing in front of it and the other struc- 

 ture forming the root of the lung are the phrenic nerve and the anterior ])ulmonary 

 plexus; behind are the vena azygos major, the pneumogastric nerve, and the 

 posterior pulmonary plexus (fig. 324). 



THE LEFT PULMONARY ARTERY 



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 (fig. 319). 



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 l)ronchus 

 than l)y the right. Below and in front are the pulmonary veins, while between 

 the artery and the ui)per left pulmonary vein the vestigial fold of Marshall is to be 

 seen. 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 reflexion of the left 

 pleura; crossing behind, are the descending aorta, the left pneumogastric nerve, 

 and the posterior pulmonary plexus (fig. 324). 



THE SYSTEMIC ARTERIES 



THE AOBTA 



The aorta 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, after running 

 a short distance upwards and to the right, turns backwards and to the left, and 

 then downwards, forming the arch of the aorta. It is thence continued through 

 the thorax as the thoracic aorta, and finally enters the abdomen at the aortic 

 opening in the dia])lirngm, and. passing through the abdominal cavity under the 

 name of the abdominal aorta, terminates opposite the foiu'th lum])ar verte1)ra in 

 the right and left coiumon iliac arteries. From the point of l)ifurcation a small 

 vessel is continued down the middle line in front of the sacrum and co(>cyx, and 

 ends in the coccygeal glomerulus. This vessel (known as the middle sacral) is 

 iisually regarded, mor])hologically, as the sacral and coccygeal aorta; while the 

 coccygeal glomerulus is believed to represent the rudiments of the caudal aorta, 

 with probably a persistent part of the post-anal gut. 



