420 THE THORACIC AORTA. 



called the thoracic aorta ; it extends from the lower border of the 

 third dorsal A'ertebra on the left side, to the opening between the crura of 

 the diaphragm in front of the last dorsal vertebra. It lies in the l)ack 

 part of the interpleural space or posterior mediastinum, being placed 

 before the spine and behind the root of the left lung and the pericardium : 

 on the left side it is in contact with the corresponding pleura and lung, 

 and close on the right side are the azygos vein, the thoracic duct, and 

 the (]esophagus. The oesophagus, however, towards the lower part of 

 the thorax, is in front of the artery, and near the diaphragm gets some- 

 what to the left side. The left or small azygos vein crosses behind the 

 thoracic aorta. 



The hrancltes derived fi-om the thoracic aorta are numerous, Init 

 small. They are distributed to the walls of the thorax, and to the 

 viscera contained within it — the latter being much the smaller and.least 

 numerous branches. 



A. Branches to the Viscera : — 



The pericardiac brp-nches are some very small and irregular vessels 

 which pass forwards and ramify on the pericardium. 



The bronchial arteries are the proper nutritious arteries of the 

 substance of the lung : they accompany the bronchial tubes in their 

 ramifications through that organ, and they also supply the bronchial 

 glands, and in part the oesophagus. These vessels va]y frequently in 

 number, and in their mode of origin. The bronchial artery of the 

 right side arises from the first aortic intercostal artery, or by a common, 

 trunk with the left bronchial artery from the thoracic aorta ; on the 

 left side there are generally two bronchial arteries, both of which arise 

 from the thoracic aorta, one near the commencement of that trunk, and 

 the other, named inferior bronchial, lower down. Each artery is usually 

 directed to the back part of the corresponding bronchus, along which it 

 runs, dividing and subdividing with the successive bronchial ramifica- 

 tions in the substance of the lung. 



"Varieties. — The place of origin of the bronchial arteries is liable to much 

 variation. The artery of the right side has been found to arise singly from tlie 

 aorta, from the internal mammary, or from the inferior thjToid. The bronchial 

 arteries of the two sides have been seen to arise by a common trunk from the 

 subclavian. (Haller.) Two common triinks, each furnishing a branch to the 

 right and left lungs, have been observed in a single case to descend into the 

 thorax after arising, one from the internal mammaiy, and the other from the 

 superior intercostal artery. (R. Quain, pi. 2i>, f. ."i.) In some cases they arise 

 from the under surface of the arch, instead of from the thoracic aorta. Instances 

 occur of two distinct bronchial arteries for each lung. 



The oesophageal arteries are variable in size and number. There 

 are usually four or five, which arise from the fore part or right side of 

 the aorta, and run obliquely downwards upon the oesophagus, supplying 

 its coats. 



Their lower laranches anastomose with the ascending offsets of the coronary 

 artery of the stomach and with the phrenic artery, while their upper branches 

 communicate with those of the inferior thjToid artery. 



Posterior mediastinal branches of the aorta, small and irregular, 

 supply the glands and loose tissue of the posterior mediastinum. 

 B. Parietal Branches to the Thorax. 

 The intercostal arteries arise from the posterior part of the aorta. 



