454 THE DESCENDING THORACIC AORTA. 



DESCENDING THORACIC AORTA. 



From the point at which its arch is considered to terminate the lower margin 

 of the fourth dorsal vertebra, the aorta descends along the spine to the fourth lumbar 

 vertebra, where it divides into the common iliac arteries. The direction of this 

 part of the vessel is not vertical, for, as it follows the bend of the spine, upon which 

 it rests, it is necessarily concave forwards in the dorsal region, and convex forwards 

 in the lumbar. Again, as its commencement is on the left side of the bodies of the 

 vertebrae, and its termination nearly in the middle line, there is a general inclination 

 inwards in its whole length, but more marked in the upper half. Within the thorax, 

 where the offsets are small, the aorta diminishes only slightly in size (from 23 to 

 21 mm.) ; in the abdomen the diminution is considerable (from 21 to 17 mm.), in 

 consequence of large branches being furnished to the viscera of that cavity. 



That part of the descending aorta which is situated in the thorax is called the 

 descending thoracic aorta ; it extends from the lower border of the fourth dorsal 

 vertebra, on the left side, to the upper margin of the opening between the crura of 

 the diaphragm below the last dorsal vertebra, and has a length of from seven to 

 eight inches. It lies in the posterior mediastinum, resting against the spine, and 

 covered in front by the root of the left lung, the pericardium, and the vertebral 

 portion of the diaphragm ; on the left side it is in contact with the corresponding 

 pleura and lung ; and close to it on the right side are the large azygos vein, the 

 thoracic duct, and the oesophagus. The ossophagus, however, towards the lower 

 part of the thorax is in front of the artery, and near the diaphragm gets somewhat 

 to the left side. The left or small azygos veins lie behind the descending thoracic 

 aorta. 



The BRANCHES derived from the descending thoracic aorta are numerous, but 

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

 within it the parietal branches being the larger and more numerous. 



A. Branches to the viscera : 



The pericardial branches are some very small and irregular vessels which pass 

 forwards and ramify on the pericardium. 



The bronchial arteries are the nutrient arteries of the substance of the lung, 

 and they supply also the bronchial glands, and in part the pericardium and oeso- 

 phagus.- These vessels vary frequently in number, and in their mode of origin. On 

 the right side there is usually one bronchial artery which arises from the first aortic 

 intercostal artery, or by a common trunk with the upper left bronchial artery from 

 the descending thoracic aorta ; on the left side there are generally two arteries, both 

 of which arise from the descending thoracic aorta, one near the commencement of 

 that trunk, and the other, named inferior bronchial, lower down. Each artery is 

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

 dividing and subdividing with the successive bronchial ramifications 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 the aorta, from the internal mam- 

 mary, or from the inferior thyroid. The bronchial arteries of the two sides have been seen to 

 arise by a common trunk from the subclavian (Haller). Two common trunks, each furnish- 

 ing 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 mammary, and the other from the superior 

 intercostal artery (R. Quain). In some cases they arise from the under surface of the arch, 

 instead of from the descending thoracic aorta. Instances also occur of two distinct bronchial 

 arteries for each lung. 



The cesophageal 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 down- 

 wards upon the oesophagus, supplying its coats. Their lower branches anastomose 



