THE CIRCULATORY OR VASCULAR SYSTEM 97 



the thoracic aorta; after passing through the diaphragm 

 it is termed the abdominal aorta. 



The first or ascending portion of the aorta is about 2^ 

 inches in length, and is contained within the pericardium* 

 Anteriorly, it is in relation with the pulmonary artery ; on 

 the left side, with the left auricle and pulmonary artery; on 

 the right side, with the right auricle and superior vena cava; 

 and behind, with the right pulmonary artery and veins. 



The second or transverse portion gives off the innominate 

 artery, left common carotid, and left subclavian arteries. 

 It is in relation behind with the left recurrent nerve, tra- 

 chea, esophagus, thoracic duct, and nerves of the cardiac 

 plexus; in front, with the left phrenic, left superior car- 

 diac, and left pneumogastric nerves. 



The third or descending portion of the arch lies adjacent 

 to the third dorsal vertebra, and is covered with a portion 

 of the pleura. 



The descending aorta proper is divided into two great 

 portions, named the thoracic and the abdominal aorta. 



The thoracic aorta takes its origin at the lower border 

 and to the left of the third dorsal vertebra, curves forward 

 to the right, runs downward on the median line of the 

 vertebral column, and passes through the aortic opening 

 in the diaphragm. Relations: behind, with the vertebral 

 column and vena azygos minor; anteriorly, with the 

 esophagus and left pneumogastric nerve ; on the left, with 

 the pleura; on the right, with the thoracic duct. 



The abdominal portion (see Fig. 56) enters through the 

 opening in the diaphragm, proceeds downward in line with 

 the vertebral column, slightly to the left, and extends to 

 the fourth lumbar vertebra, where it divides into the two 

 common iliac arteries. Relations: in front, with the left 

 renal and splenic veins, pancreas, transverse duodenum, 

 and mesentery; behind, with the thoracic duct, receptacu- 

 lum chyli, and left lumbar veins ; on the left, with the left 

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