THE ABDOMINAL AOETA. 885 



border of the fourth thoracic vertebra. As its name implies, it forms an arch ; 

 i and the arch makes two curves, one with the convexity upwards, and the other 

 with the convexity forwards and to the left. From its origin it runs for a short 

 distance upwards, posteriorly, and to the left, anterior to the trachea ; then it 

 passes posteriorly, round the left side of the trachea to the left side of the body 

 of the fourth thoracic vertebra. Finally it turns downwards to become continuous 

 with the descending aorta. 



At its commencement it has the same diameter as the ascending aorta, 28 mm. 

 (1-J- inches), but after giving off three large branches, the diameter is reduced to 

 23 mm. (a little less than one inch). 



Relations. It is overlapped anteriorly and on the left side by the right and left 

 lungs and pleural sacs, but much more by the left than the right, and in the interval 

 between and posterior to the anterior borders of the pleural sacs it is covered by the 

 remains of the thymus. As it turns backwards it is crossed vertically, on the left side, 

 by four nerves in the following order from before backwards : the left phrenic, the 

 inferior cervical cardiac branch of the left vagus, the superior cardiac branch of the left 

 sympathetic, and the trunk of the left vagus. The left superior intercostal vein passes 

 1 obliquely upwards and to the right, across it, between the left vagus and left 

 phrenic nerves. 



Posterior to, and to the right side of the arch, are the trachea, the deep cardiac plexus, 

 the left recurrent nerve, the left border of the oesophagus, and the thoracic duct. Above 

 are its three large branches the innominate, the left common carotid, and the left 

 subclavian arteries ; and crossing anterior to their roots is the left innominate vein. 

 Below is the bifurcation of the pulmonary artery and the root of the left lung ; the 

 ligamentum arteriosum, which is also below, attaches it to the commencement of the left 

 pulmonary artery, whilst to the right of the ligament lies the superficial cardiac plexus, 



and to its left the left recurrent nerve. 



Branches. The three great vessels which supply the head and neck, part of the 

 thoracic wall, and the upper extremities viz. the innominate, the left common carotid, 

 and the left subclavian arteries arise from the aortic arch. 



Aorta descendens. The thoracic portion of the descending aorta lies in the 

 posterior mediastinum ; it extends from the termination of the arch, at the lower 

 border of the left side of the fourth thoracic vertebra, to the aortic opening in the 



: diaphragm, where, opposite the twelfth thoracic vertebra, it becomes continuous 

 with the abdominal portion. Its length is from 1*7*5 to 20 cm. (seven to eight 

 inches), and its diameter diminishes from 23 mm. at its commencement to 21 



i mm. at its termination. 



- Relations. Immediately posterior to it are the vertebral column and the anterior 

 i longitudinal ligament. It rests also on the accessory hemiazygos and the hemiazygos 

 veins, whilst from its posterior aspect the aortic intercostal branches are given off. 



Anteriorly it is in relation, from above downwards, with the root of the left lung, the 

 pericardium, which separates it from the back of the left atrium, the oesophagus with the 

 oesophageal plexus of nerves, and the crura of the diaphragm which separate it from the 

 caudate lobe of the liver. On the left side are the left lung and pleura. On the right side 

 the thoracic duct and the vena azygos form immediate relations along its whole length. 

 The oesophagus also lies to the right of the upper part of the descending aorta, whilst the 

 right lung and pleura are in relation below. 



Branches. Nine pairs of aortic intercostal arteries, two left bronchial arteries, four 

 1 or five oesophageal, some small pericardial, and a few posterior mediastinal and superior 



phrenic branches, usually arise from the thoracic part of the descending aorta. 



. 



AOETA ABDOMINALIS. 



The abdominal portion of the descending aorta lies in the epigastric and 

 umbilical regions of the abdomen. It extends from the middle of the lower border 

 of the last thoracic vertebra to the body of the fourth lumbar vertebra, where, to the 

 left of the median plane, it bifurcates into the right and left common iliac arteries. 

 The point of division is a little below and to the left of the umbilicus, opposite 



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