PAEIETAL BEANCHES OF DESCENDING THOEACIC AOETA. 925 



contribute also to the supply of the spinal medulla and its membranes, and to that 

 of the vertebral column 'and of the upper part of the abdominal wall. The 

 branches, which are numerous and for the most part arranged in pairs, are as 

 follows : 



f Bronchial, flntercostal. 



Visceral ^^P 1 ^ 1 - Parietal Sllbcostal - 



aL 1 Pericardial. ' L 1 Superior phrenic. 



[Mediastinal. [The vas aberrans. 



VISCERAL BRANCHES OF THE DESCENDING THORACIC AORTA. 



1. Arterise Bronchiales. The bronchial branches of the thoracic aorta are usually 

 two in number an upper and a lower and both pass to the left lung. The upper 

 left bronchial artery arises from the front of the aorta opposite the fifth thoracic 

 vertebra; the inferior left bronchial artery usually takes origin near the lower 

 border of the left bronchus. Both vessels are directed downwards and laterally 

 to the back of the bronchus which they accompany, and, dividing similarly 

 they follow its ramifications in the lung. They not only supply the walls of the 

 bronchial tubes and the substance of the lungs, but also give branches to the 

 bronchial glands, the pulmonary vessels, the pericardium, and the oesophagus. 



As a rule there is only one right bronchial artery. It arises either from the 

 first right aortic intercostal artery or from the upper left bronchial artery. More 

 rarely it springs directly from the aorta. In its course and distribution it corre- 

 sponds to the bronchial arteries of the left side. 



2. Arterise CEsophagese. The cesophageal branches are variable ; usually four 

 or five small branches spring from the front of the aorta and pass forwards to the 

 oesophagus, in the walls of which they ramify, anastomosing above with branches 

 of the left bronchial and inferior thyreoid arteries, and below with cesophageal 

 branches of the left gastric and the phrenic arteries. 



3. The pericardial branches are three or four small irregular vessels which are 

 distributed on the surface of the pericardium. 



4. Small mediastinal branches pass to the areolar tissue and glands in the 

 posterior mediastinum, and to the posterior part of the diaphragm. 



PARIETAL BRANCHES OF THE DESCENDING THORACIC AORTA. 



1. Arterise Intercostales The Intercostal Arteries. There are nine pairs of 

 aortic intercostal arteries which usually arise separately from the back of the 

 aorta, but, not uncommonly, a pair may take origin by a common trunk. They 

 are distributed to the lower nine intercostal spaces, to the vertebral column, to 

 the contents of the vertebral canal, and to the muscles and skin of the back 

 The first three on each side give branches to the mammary gland also. 

 The arteries of opposite sides closely correspond, but, since the aorta, in the 

 thoracic region, lies on the left of the vertebral column, the right intercostal 

 arteries cross the front of the column, posterior to the oesophagus, the thoracic 

 duct, and the vena azygos, and are longer than the left arteries. In other 

 respects the courses of all the aortic intercostal arteries are almost identical. As 

 each artery runs postero-laterally, across the side of the vertebral column, to an 

 intercostal space, it passes posterior to the pleura, and is crossed, opposite the 

 head of a rib, by the sympathetic trunk. The lower arteries are crossed by the 

 splanchnic nerves also, and those on the left side are crossed by the hemiazygos 

 or accessory hemiazygos veins. 



As each artery passes laterally, between the necks of two adjacent ribs, it gives off 

 a posterior branch. Then it ascends to the upper border of the space to which it 

 belongs, and, passing either behind or in front of the corresponding intercostal 

 nerve, is continued along the space, in the costal groove. In the space, as far as the 

 angle of the rib, it lies between the pleura and the posterior intercostal membrane, 

 and, in that part of its course, it is embedded in the endothoracic fascia. Then it is 

 continued forward between the internal and external intercostal muscles. In the 





