588 THE BLOOD-VASCULAR SYSTEM 



Branches of the Thoracic Aorta 



The branches of the thoracic aorta may be divided into the visceral and the 

 parietal. The visceral are: — (1) The pericardiac; (2) the bronchial; and (3) the 

 oesophageal. The parietal are: — (1) The intercostal; (2) the superior phrenic; 

 and (3) the arteria aberrans. 



A. Visceral Branches 



(1) The pericardiac branches [rami pericardiac!] — two or three small branches, 

 irregular in their origin, course, and distribution — pass to the posterior surface of 

 the pericardium to supply that structure, and anastomose with the other peri- 

 cardiac branches. They give small twigs to the posterior mediastinal glands. 



(2) The bronchial arteries [aa. bronchiales] supply the bronchi and the lung 

 substance. They vary considerably in their origin, course, and distribution; 

 they are usually three in number — one on the right side, and two on the left. 



(a) The right bronchial generally arises either from the first right aortic intercostal, or else 

 as a common trunk witli the left upper bronchial from the front of the aorta just below the level 

 of the bifurcation of the trachea. It passes laterally on the back of the right bronchus, and is 

 distributed to the bronchi and lung substance. (6) The left upper bronchial arises from the front 

 of the aorta just below the bifurcation of the trachea, or as a common trunk with the right 

 bronchial, (c) The left lower bronchial arises from the front of the aorta just below the level of 

 the left bronchus. Like the corresponding artery on the right side, the left bronchial arteries 

 run laterally on the left bronchus, and, after dividing and subdividing on the back of the bronchi, 

 supply the bronchi themselves and the lung substance. Small twigs are given off from the 

 bronchial arteries to the bronchial glands and to the oesophagus. 



(3) The oesophageal arteries [aa. assophagece], four or sometimes five in 

 number, arise at intervals from the front of the descending thoracic aorta, the 

 first coming off just below the left lower bronchial. They usually increase in 

 size from above downward, the upper coming off more toward the right side of 

 the aorta, the lower more toward the left side. They pass forward to the 

 oesophagus, supplying that tube and anastomosing with each other and with the 

 descending oesophageal branches of the inferior thyreoid above, and with the 

 ascending oesophageal branches of the phrenic and gastric arteries below, thus 

 forming a chain of anastomoses along the whole length of the tube. 



B. Parietal Branches 



(1) The intercostal arteries [aa. intercostal es], usually ten in number on each 

 side, supply the lower intercostal spaces, the two upper spaces (occasionally the 

 first only) being supplied from the costo-cervical trunk of the subclavian artery. 

 The lowest artery accompanies the twelfth thoracic nerve below the last rib and 

 is therefore called the subcostal artery. Its distribution is similar to that of the 

 lumbar arteries (p. 593) except that it commonly crosses the anterior surface, 

 rather than the posterior, of the quadratus lumborum. 



The intercostals arise in pairs from the back part of the aorta, and at once 

 turning, the one to the right, the other to the left, wind backward over the 

 front and sides of the vertebral bodies to reach the intercostal spaces. In f(Ktal life 

 these arteries run almost transversely backward, or even with a slight inclination 

 downward, to the intercostal spaces; bust after the first year, in consequence of 

 the disproportionate growth of the aorta and vertebral column, the upper inter- 

 costals have to ascend to reach their respective spaces. 



The arteries in their course around tlio vertebrae differ on the two sides of the 

 body. On the right side the arteries — and especially the upper, in consequence 

 of the aorta lying a little to the left side of the spine in the upper part of its 

 course — are longer than the left. They wind over tiie front and right side of the 

 vertebra?, being crossed by the thoracic duct and vena azygos (major), and covered 

 by the right pleura and lung. The upper are also crossed by the oesophagus. 

 1'hey give off small branches to the boflies of the vertebra) and anterior longi- 

 tudinal ligament. On the left side, as the intercostals wind around the sides of 

 the bodies of the vertebra;, the lower are crossed by the vena hemiazygos (azygos 

 minor), the two u})per by the left superior intercostal vein, and the two next by 



