178 HUMAN ANATOMY. 



In its course it is divided into the arch,, the thoracic aorta, and 

 the abdominal aorta, the arch being subdivided into the ascend- 

 ing, transverse, and descending portion. 



Relations. The ascending portion of the arch is in relation on the 

 right side with the superior cava and right auricle; on the left side 

 with the pulmonary artery; in front, with the pulmonary artery, the 

 pericardium, the right appendix auriculae, and the thymus gland; and 

 behind, with the root of the right lung and the right pulmonary vessels. 



The transverse portion of the arch is in relation in front with the 

 left lung and pleura, the left pneumogastric and phrenic nerves, the 

 superficial cardiac nerves, the left superior intercostal vein, and the 

 thymus gland; behind, with the trachea, the oesophagus, the thoracic 

 duct, the deep cardiac plexus, and the left recurrent nerve; above, with 

 the arteria innominata, the left innominate vein, the left subclavian, 

 and the left carotid; and below, with the left bronchus, the bifurcation 

 of the pulmonary artery, the left recurrent nerve, and the remains of 

 the ductus arteriosus. 



The descending portion of the arch is in relation in front with the 

 root of the left lung and the pleura; behind, with the left side of the 

 body of the fifth dorsal vertebra; on the right side with the thoracic 

 duct and oesophagus; and on the left side with the pleura. 



The branches of the aorta are, from the arch: 



Two coronary, Left common carotid, 



Innominate, Left subclavian. 



THE CORONARY ARTERIES supply the heart and are two in 

 number the right and the left. The right arises from the 

 aorta, above the right semi-lunar valve, and passes downward in a 

 groove between the right auricle and ventricle on its posterior 

 surface, dividing into two branches, one of which anastomoses 

 with the left coronary. The left coronary arises above the left 

 semi-lunar valve and descends in the anterior interventricular 

 groove, where it divides into two branches, one of which anas- 

 tomoses with the right coronary. 



ARTERIA INNOMINATA. This is the largest branch from the 

 arch of the aorta. It ascends to the upper border of the right 

 sterno-clavicular articulation, where it divides into the right 

 common carotid and the right subclavian arteries. It is a short 

 vessel, from one and a half to two inches long. 



Relations. It is in relation in front with the sternum, the sterno- 

 hyoid and sterno-thyroid muscles, the left innominate and right inferior 

 thyroid veins, the thymus gland, and the cardiac branch from the right 

 pneumogastric; behind, with the trachea" on the right side, with the 

 pleura and right vena innominata and right pneumogastric nerve; on 

 the left side, with the left carotid and the thymus gland. 



THE COMMON CAROTID ARTERIES. The left common ca- 

 rotid, arising directly from the arch of the aorta, is longer and 

 ascends more obliquely and is more deeply placed than the right. 



