THORACIC CAVITY 109 



from the subclavian arteries ; the remaining nine pairs are 

 branches of the thoracic part of the descending aorta. 



The Aortic Intercostal Arteries. The nine pairs of aortic 

 intercostal arteries spring from the posterior surface of the 

 descending aorta, either separately or by a series of common 

 trunks, one for each pair. The right arteries are longer than 

 the left because the aorta lies to the left of the median plane ; 

 and, since the descending aorta commences only at the level 

 of the lower border of the fourth thoracic vertebra, the four 

 or five highest pairs have to ascend to gain the level of 

 the spaces to which they are distributed. 



The right aortic intercostal arteries run across the anterior 

 aspects of the bodies of the vertebrae, lying posterior to the 

 thoracic duct and the vena azygos; then they turn posteriorly, 

 between the sides of the bodies of the vertebrae and the parietal 

 pleura ; and, finally, immediately before they enter the inter- 

 costal spaces, they pass between the sides of the bodies of the 

 vertebrae medially and the sympathetic trunk laterally. The 

 shorter left aortic intercostal arteries run posteriorly, first 

 between the left pleura and the bodies of the vertebrae, and 

 then between the sympathetic trunk and the vertebral bodies. 

 As each artery enters the space to which it belongs it gives off 

 a dorsal branch, which passes posteriorly, between the vertebral 

 column medially and the anterior costo-transverse ligament 

 laterally ; it gives off a spinal twig, which enters the verte- 

 bral canal through the corresponding intervertebral foramen ; 

 then it divides into a medial and a lateral branch which 

 accompany the medial and lateral divisions of the posterior 

 branch of the corresponding thoracic nerve. After giving off 

 the dorsal branch, the trunk of the artery runs laterally, along 

 the upper border of the space to which it belongs, at first 

 anterior to the posterior intercostal membrane, and then 

 between the internal and external intercostal muscles. Its 

 further course has been described already (p. 6). As it passes 

 along the upper border of the intercostal space, in the shelter 

 of the subcostal groove of the rib, it is situated between 

 the intercostal vein above and the anterior branch of the 

 thoracic nerve below. 



The Subcostal Arteries. The subcostal arteries are the 

 last pair of branches which spring from the posterior aspect 

 of the thoracic part of the descending aorta. They enter 

 the abdomen, by passing beneath the lateral lumbo-costal 



